THE LIBRARY 
 
 OF 
 
 THE UNIVERSITY 
 OF CALIFORNIA 
 
 PRESENTED BY 
 
 PROF. CHARLES A. KOFOID AND 
 MRS. PRUDENCE W. KOFOID 
 
 ASTRONOMY 
 LIBRARY 
 
 
Essays in Astronomy 
 by Great Astronomers 
 
 The World's Great Books 
 
 Committee of Selection 
 Thomas B. Reed William R. Harper 
 
 Speaker of the House President of the 
 
 of Representatives University of Chicago 
 
 Edward Everett Hale Ainsworth R. Spofford 
 
 Author of The Man Of the Congressional 
 
 Without a Country Library 
 
 Rossiter Johnson 
 
 Editor of Little Classics and Editor-in-Chief of this Series 
 
 Edition de Grand Luxe 
 
Go-uplijr 
 
Essays in Astronomy 
 
 B 
 
 Ball, Harkness, Herschel, Huggins, 
 
 Laplace, Mitchel, Proctor, 
 
 Schiaparelli, and Others 
 
 i a Critical Introduction by 
 Edward Singleton Holden 
 
 
 i'WjbjnrfuoH doDJs^ vd gnivsTgns tin moH 3iir/jn 
 i^I vailboO ii2 vd nbaijs 
 
 D. Appleto 
 
Photogravure from an engraving by Jacob Houbracken after 
 a painting by Sir Godfrey Kneller. 
 
Essays in Astronomy 
 
 By 
 
 Ball, Harkness, Herschel, Huggins, 
 
 Laplace, Mitchel, Proctor, 
 
 Schiaparelli, and Others 
 
 With a Critical Introduction by 
 Edward Singleton Holden 
 
 Illustrated 
 
 New York 
 D. Appleton and Company 
 
 1900 
 
COPYRIGHT, 1900, 
 BY D. APPLETON AND COMPANY. 
 
QB3 
 
 ASTRONOMf 
 
 LIBRARY 
 
 ESSAYS ON ASTRONOMY 
 
 {T is the design of the series of which this volume is a 
 part to give in complete form the greatest masterpieces 
 in every department of literature, and in science, so far 
 as practicable. In natural science the object may be at- 
 tained by presenting, in their entirety, the World's Great 
 Books. The case is not the same for the more exact sci- 
 ences. There is no single book on astronomy, for example, 
 that displays the amazing achievements of the past century 
 in untechnical form. On the other hand, there are many 
 scattered addresses and memoirs that present a single aspect 
 of the advance in an authoritative and, at the same time, 
 in a popular manner. A volume might now be written that 
 would set forth the present state of scientific opinion on 
 vital astronomical questions and exhibit the history of the 
 successive steps by which our actual vantage-ground has 
 been reached. A single volume of the sort would have the 
 great advantage of symmetrical presentation. The whole 
 of astronomy might be viewed from our present standpoint, 
 and its different parts displayed in their just historical pro- 
 portions. But there is no such book. 
 
 It has seemed that the special ends in view could be 
 attained by extracting from the writings of the astronomers 
 of the past and present generations mostly from those of 
 acknowledged masters of science their own accounts of 
 great discoveries made by themselves, or their luminous 
 reviews of the discoveries of others, their colleagues. No 
 account of the nebular hypothesis of Laplace from the 
 hand of another could give the vital essence of his great 
 
 iii 
 
 
iv ESSAYS ON ASTRONOMY 
 
 generalization so vividly and succinctly as his own note, 
 first printed in 1796 and subsequently amended in its de- 
 tails until it took its final shape. Here we have the matured 
 reflections of a great philosopher precisely in the form in 
 which they were marshalled in his own mind. His problem 
 is to account for the present state of the solar system how 
 it came to be that which it is. We are bound to preserve 
 the substance of his thoughts. Can they be better pre- 
 sented than in his very words? A paraphrase of Laplace's 
 original would be as inappropriate as a prose version of one 
 of Milton's immortal sonnets. The present volume gives 
 Laplace's note in full. 
 
 Again, it would not be difficult to write out afresh a 
 satisfactory account of the achievements of the spectro- 
 scope in determining the motions and the physical constitu- 
 tions of the sun, the stars, the comets, and the nebulae. It 
 was a novel and little understood instrument when Dr. 
 Huggins (now Sir William Huggins) first devoted it to 
 researches on the heavenly bodies. In 1866, in the first 
 flush of successful investigation, Dr. Huggins paused to 
 make a report on the extraordinary results that he had 
 obtained from the application of spectrum analysis to the 
 heavens. Again, in 1891, a quarter of a century later, the 
 veteran observer presented to the British Association for 
 the Advancement of Science, at Cardiff, an elaborate ac- 
 count of the achievements of spectroscopists up to that 
 time. Finally, in 1897, in a paper on the " New Astron- 
 omy," Sir William takes a retrospective view of the whole 
 subject. All these papers from the same skilled hand are 
 printed in this volume. Taken together, they exhibit, as no 
 other writings can, the history of the immense advances 
 that have been made in this novel department of astro- 
 nomical research. The earlier paper sets forth the first 
 results, and shows what was even then, a generation ago, 
 to be looked for in the future. The later papers review the 
 progress that has actually been made, and indicate the re- 
 sults that are safely ours, and the further advances that 
 may be confidently expected. Here we have what we 
 
ESSAYS ON ASTRONOMY V 
 
 are seeking presented to us by a master hand, and we be- 
 come, in some sense, witnesses of the processes of research, 
 and appreciate the historical relations of its different parts. 
 
 It is to be noted that all the papers reprinted in this 
 volume are given in their original forms and in their en- 
 tirety (except those of the present writer, which have been 
 condensed, and except that a single paragraph of Sir John 
 Herschel's has been added to supplement an address that 
 he delivered in 1841, which is here printed in full). Sub- 
 stance and form are both scrupulously preserved. 
 
 The successive chapters in this book have the great ad- 
 vantage, then, of setting forth the state of the questions of 
 which they treat exactly as they were conceived at the time 
 of writing by those best qualified to report upon them. In 
 general, they accurately represent the present state of opin- 
 ion, or, at least, they are suited to lead up to it, and to 
 exhibit the historic development of the science. 
 
 This series of chapters is by no means addressed to pro- 
 fessional astronomers, although it collects into a single vol- 
 ume many papers that astronomers will be glad to possess. 
 On the contrary, the selections have been made especially 
 to meet the wants of the general reader, and to furnish him 
 with the best judgment on those questions of astronomy 
 that appeal to us all: What is to be the future of our sys- 
 tem? By what processes of evolution has it come to be 
 what it is? How long may we expect the sun's heat to 
 endure? What is the actual physical condition of the 
 planets? Are they suited to maintain human life? Along 
 with these fundamental questions others, less far-reaching, 
 but not less interesting, are treated. 
 
 The dimensions of the solar system are discussed in an 
 address delivered before the American Association for the 
 Advancement of Science in 1894 by its president, Prof. 
 William Harkness, Director of the United States Naval 
 Observatory at Washington. The results of the great 
 mathematical astronomers Lagrange, Laplace, and others 
 upon the question of the mechanical stability of our 
 system are set forth in a chapter by Mitchel, founder of 
 
vi ESSAYS ON ASTRONOMY 
 
 the Cincinnati Observatory. The lectures and addresses of 
 this remarkable man had the greatest influence in creating 
 the universal and intelligent interest in astronomy that is 
 so distinctive a note in our country. His lectures were 
 attended by thousands, and are still remembered. The Cin- 
 cinnati Observatory was built and equipped by voluntary 
 contributions from his hearers, and it is largely to the im- 
 petus given by him that we owe the establishment of so 
 many college and university observatories in the twenty 
 years preceding our civil war. The chapter here repro- 
 duced from his addresses is an excellent example of his elo- 
 quent manner of presenting subjects difficult in themselves 
 and foreign to ordinary thinking. 
 
 The papers upon " Atoms and Sunbeams," by Sir Rob- 
 ert Stawell Ball, formerly Astronomer-Royal for Ireland, 
 and now Professor of Astronomy in the University of Cam- 
 bridge, England, and upon the " Age of the Sun's Heat," 
 by Sir William Thomson, now Lord Kelvin, professor in 
 the University of Glasgow, present two aspects in the study 
 of the central ruler of our system. In the latter chapter 
 we have the reasons for Lord Kelvin's conclusions upon 
 the past and future duration of the sun, and of the solar 
 system and of our earth. The sun can not have ex- 
 isted in the past more than fifteen million or twenty million 
 years, nor can its heat support life on the globe more than 
 ten million years in the future. 
 
 The immense periods of past time called for by the ge- 
 ologist, and especially by the biologist, to account for ob- 
 served changes in strata and in the species of animals, are 
 thus denied to them by the mathematical astronomer. The 
 history of the earth must be compressed into a comparatively 
 few millions of years. On the other hand, the future of our 
 planet, so far as it depends upon the solar heat, is strictly 
 limited. There is no escape from the general conclusions 
 of Lord Kelvin's paper. The number of hundreds of thou- 
 sands of years during which the sun will furnish heat to 
 the earth and to its populations can not be fixed with ex- 
 actness. But it appears to be certain that our system is not 
 
ESSAYS ON ASTRONOMY vii 
 
 self-sustaining, and that its life is not to be indefinitely long. 
 It is an organism that, like every other organism known to 
 us, must come to an end in consequence of the very laws that 
 now keep it active. It must have had a beginning within a 
 period that can not be fixed with certainty, but which can 
 not, in any event, much exceed twenty million years; and it 
 must end, so far as life is concerned, in a chaos of cold and 
 dead globes at a calculable time in the future, when the 
 sun has radiated away its store of heat ; unless that heat is re- 
 created by the operation of forces now totally unknown to us. 
 
 At the annual meeting of the ancient Accademia dei 
 Lined in Rome (Galileo's Academy), in 1889, in the pres- 
 ence of the tting and Queen of Italy, the famous astrono- 
 mer of Milan Schiaparelli was called upon to report on 
 his remarkable conclusions regarding the rotation and the 
 physical constitution of the planet Mercury. His paper is 
 printed as a chapter in this volume. In it he announced 
 that the planet Mercury revolved about the sun, turning 
 always one face to it, as the moon always turns one face 
 to the earth. In the one case as in the other, the efficient 
 cause has been the action of tides, which operate to reduce 
 the rate of axial rotation. Working always in one direction 
 to retard and never to accelerate the tides may, in time, 
 retard the motion of rotation of a planet (as they have the 
 motion of our own satellite) until it presents a single face 
 to its central body. This face of Mercury alone receives 
 solar light and heat; the other is condemned to perpetual 
 darkness. The conditions of life (if so be that life exists) 
 upon such a planet are as different as possible from those 
 that we experience upon our own earth; and Schiaparelli 
 has lucidly expounded the consequences of such condi- 
 tions as exist on Mercury. 
 
 As early as 1882 M. Schiaparelli confided to one of his 
 correspondents his discovery in the verses that follow: 
 
 " Cynthiae ad exemplum versus Cyllenius axe 
 Sternum noctem sustinet, atque diem: 
 Altera perpetuo facies comburitur sestu, 
 Abdita pars tenebris altera Sole caret." 
 
viii ESSAYS ON ASTRONOMY 
 
 A little later he announced, after a comparison between 
 observations made at Milan, Washington, and Brussels, 
 that the planet Venus was in like case. Venus, too, revolves 
 like our moon, turning once on its axis during one revolu- 
 tion about its central body (in 225 days for Venus, 88 days 
 for Mercury, and 29 days for the moon). 
 
 The observations of Mr. Percival Lowell, in Arizona, 
 seem to confirm Schiaparelli's conclusions. There is noth- 
 ing in the observations made at the Lick Observatory to 
 throw doubt upon them. The question is very difficult. If 
 the results of M. Schiaparelli are finally conclusively estab- 
 lished, we shall have an entirely novel set of circumstances 
 in the family of planets. A mathematical investigation of 
 the effects of tidal retardation in the case of the moon shows 
 that certain effects are possible, and that the rotation of a 
 satellite to the earth (the moon), or to the sun (a planet), 
 may go through strange alterations during its life history. 
 Not only may the period of rotation be changed, but the 
 distance from the central body may vary enormously. Our 
 moon now turns a single face to the earth. Schiaparelli 
 interprets his observations as showing that two out of the 
 eight planets of our system are likewise in this stage of 
 evolution. These and other planets will in the future pass 
 through very various circumstances. 
 
 A chapter is also devoted to the presentation of Pro- 
 fessor Schiaparelli's conclusions as to the planet Mars. 
 The views of Schiaparelli, who is an observer of the highest 
 competence, and a skilled mathematician and physicist, 
 must not be confounded with the reports of his work that 
 have been made from time to time by sanguine popular 
 writers who have decided a priori that the Universe was 
 created to be peopled by human beings, and who have dis- 
 torted Schiaparelli's guarded and cautious words into defi- 
 nite pronouncements that Mars, at least, is inhabited by 
 a race of high intelligence. Nor does the mass of writ- 
 ing in newspapers and ephemeral publications upon the 
 question of life in Mars deserve any scientific consideration 
 whatever. Weight of a certain kind it has. There is no 
 
ESSAYS ON ASTRONOMY il 
 
 doubt that thousands of readers get, and must get, their 
 acquaintance with scientific conclusions from newspaper 
 paragraphs compiled at third and fourth hand by persons 
 without accurate and sufficient technical knowledge. Many 
 such persons are now fully satisfied that somehow and some- 
 where, some one, probably Schiaparelli, has decided in a 
 scientific manner, first, that Mars is a planet very much 
 like our own earth, with a sufficient atmosphere, abundant 
 water, and all the conditions for habitability; second, that 
 the " canals " on the planet are without doubt the work 
 of human hands, and a convincing proof of intelligent en- 
 gineering; and, third, that we are now on the point of 
 communicating by some kind of electric signalling with 
 our cosmic brothers. An ardent desire that all this should 
 be true is taken as convincing proof that it has veritably 
 been proved. 
 
 The judgment of the majority of competent astrono- 
 mers on these points is very different. They know how 
 very far we still are from a determination of the question 
 whether Mars, or any other planet, is, in fact, inhabited. 
 They know that this question, universally interesting as it 
 is, by no means represents the present inquiry of science, 
 which is: Are any of the planets habitable? Are they fit 
 to be inhabited? Even this limited inquiry is still so far 
 from an answer that it is almost idle to speculate on the 
 larger question. Unless it can be proved as it certainly 
 has not been that Mars is a planet where conditions pre- 
 vail that are favourable to human existence, it is a simple 
 waste of effort to inquire whether such life is now in exist- 
 ence. No one can say with any kind of scientific certainty 
 whether a human being transported to Mars could live 
 even for an instant. It is more than probable that he 
 could not. 
 
 On account of the intense interest that such questions 
 excite, a few remarks are here devoted to a review of 
 the case as it stands. In the first place, we may safely say 
 that Mars has little or no atmosphere, basing this conclu- 
 sion, at the outset, upon its capacity of reflecting the solar 
 
X ESSAYS ON ASTRONOMY 
 
 light that falls upon its surface, as compared with the like 
 capacity of other bodies of the solar system. The moon 
 reflects ^V of the light falling upon it about as much as 
 sandstone rocks. Mercury reflects -^j-. These bodies have 
 little or no atmosphere. Venus reflects (from the outer sur- 
 face of its envelope of clouds) -ffo of the incident light. 
 Jupiter d%), Saturn (ffr), Uranus ( T Vir)> Neptune (rW)> 
 are all surrounded by extensive atmospheres and all have 
 high reflecting powers. The corresponding number for 
 Mars (TI&-) is so small as to indicate that this planet has 
 little atmosphere, if any. Again, the planet's surface has 
 been under careful scrutiny for many years, and observers 
 are all but unanimous in their report that no clouds are 
 visible over the surface. The centre of the disks of bodies 
 with extensive atmospheres (the sun, Jupiter, Saturn, 
 etc.) is always brighter than the edges. The centre of 
 the moon, which has no atmosphere, is not so bright as 
 the edge. Mars is like the moon in this respect, and 
 not like Jupiter. Finally, the only satisfactory spectro- 
 scopic observations of the planet (made independently 
 at the Lick Observatory and at the Allegheny Observa- 
 tory) show no evidence whatever of an atmosphere to 
 Mars and no sign of water-vapour about the planet. 
 If there is any atmosphere at all, it can hardly be more 
 dense than the earth's atmosphere at the high summits 
 of the Himalaya Mountains not enough to support 
 human life, therefore. As there is no evidence of the pres- 
 ence of water-vapour and of clouds, etc., it follows that there 
 is little or no water on the planet's surface. The spectrum 
 of Mars and the spectrum of the moon are identical in every 
 respect. This could not be true if Mars had any consid- 
 erable atmosphere. 
 
 The important and long-continued observations of 
 Schiaparelli on Mars led him to announce that the planet 
 was provided with an elaborate system of water-courses 
 (" oceans, seas, lakes, canals, etc."), and the authority of 
 this distinguished observer is the chief support of those who 
 maintain that the planet is fit for human habitation. 
 
ESSAYS ON ASTRONOMY xi 
 
 Complete explanations of all the phenomena presented 
 by the planet can not be given in the light of our present 
 knowledge. This is not to be wondered at, in spite of 
 the industry and ability of the observers who have spent 
 years in studying its surface. The case is much the same 
 for the planets Mercury, Venus, Jupiter, Saturn, Uranus, 
 Neptune. We know very little of the real conditions that 
 prevail on their surfaces. We know comparatively little 
 of the interior of the earth on which we live, and next to 
 nothing about the interior of other planets. There is every 
 reason to believe that complete explanations will be forth- 
 coming in time. It is, at any rate, certain that the conclu- 
 sions of Schiaparelli, named above, can not be accepted 
 without serious modification. In the telescope the main 
 body of the planet is reddish, and there are many perma- 
 nent dark markings of a grayish-blue colour. The polar 
 caps are sometimes dazzlingly white. 
 
 When Sir William Herschel was examining Mars in 
 the eighteenth century he called the red areas of Mars 
 " land " and the greenish and bluish areas " water." It 
 was a general opinion in his day that all the planets were 
 created to be useful to man. Astronomers of the eighteenth 
 century set out with this belief very much as the phi- 
 losophers of Ptolemy's time set out with the fundamental 
 theorem that the earth was the centre of the motions of the 
 planets. For example, Herschel maintained that the sun 
 was cool and habitable underneath its envelope of fire. He 
 says (1795), "The sun appears to be nothing else than a 
 very eminent, large, and lucid planet, most probably also 
 inhabited by beings whose organs are adapted to the pe- 
 culiar circumstances of that vast globe." It is certain 
 that the sun is not inhabited by any beings with organs. 
 This conclusion is now as obvious as that no beings inhabit 
 the carbons of an electric street lamp. Herschel's guess 
 that the red areas on Mars were land and the blue areas 
 water had no more foundation than his guess that the sun 
 might be inhabited. 
 
 The next careful studies of Mars were made by Maedler 
 
xii ESSAYS ON ASTRONOMY 
 
 about 1840. He also called the red areas of the disk land 
 and the dark areas water. In this he followed Herschel. 
 There was no reason why he should not have called the 
 red areas water and the dark areas land. He had absolutely 
 no evidence on the point. The same is true of later ob- 
 servers down to the first observations of Schiaparelli about 
 1877. Schiaparelli gave reasons for these names, though 
 his reasons are not convincing. He pointed out that the 
 narrow dark streaks (" canals ") generally ended in large 
 dark areas (" oceans ") or in smaller dark areas (" lakes "). 
 The narrow, dark streaks (very seldom less than sixty miles 
 wide) are quite straight. They can not be rivers, then. If 
 they are water at all, the name " canal " is not inappro- 
 priate, though sixty or one hundred miles is a very wide 
 canal. If they are water, then the large dark areas must be 
 seas. The narrow, dark streaks are not water, however, 
 because it was discovered by Dr. Schaeberle at the Lick 
 Observatory that the so-called " seas " sometimes had so- 
 called " canals " crossing them. A sea traversed by a canal 
 is an absurdity. It is maintained by a few recent observers 
 of Mars that some of the dark areas are water, and some 
 are not so. The bluish-green colour of the dark spots is 
 said to " suggest vegetation." But who can know what 
 colours the vegetation on Mars may have? 
 
 The foregoing very brief abstract of a long history 
 proves that the dark areas on Mars are not water. The red 
 areas are not known to be land. The spectroscopic and 
 other evidence proves that Mars has little or no atmosphere 
 little or no water-vapour no clouds. It is not yet known 
 what is the real nature of the red areas and of the dark areas. 
 It is one of the many unsolved problems of astronomy to 
 discover the answer to this fundamental question. There 
 is no doubt that the red areas and the large dark areas have 
 a real existence, since some of the markings on Mars have 
 been seen for more than two centuries. 
 
 It is not certain that all the " canals " that have been 
 mapped really exist. Some of them are probably mere 
 optical illusions. If they were real streaks on the planet's 
 
ESSAYS ON ASTRONOMY xiii 
 
 surface (like wide fissures, broad water-courses, etc.) they 
 would always appear broadest when they were at the centre 
 of the disk and would always be narrower when they were 
 at the edges. The laws of perspective demand this. It is 
 found by observation that the reverse is frequently true. 
 
 The distance of Mars from the sun is one and a half 
 times the earth's distance. The heat received by the earth 
 from the sun is to the heat received by Mars as (1.5)2 = 
 2.25 to i. Mars receives less than half as much sun heat 
 as the earth. If the earth had no more atmosphere than 
 the moon, the earth's temperature would be like that of the 
 moon. If the earth had no denser atmosphere than that on 
 the summits of the Himalayas, the temperature of the earth 
 would always be below zero. Human life could not exist 
 here. The case is the same with Mars. The temperature 
 of the whole surface of the planet must be extremely low, 
 even in its equatorial regions. The temperature at the 
 poles of Mars must be several hundred degrees (Fahren- 
 heit) below zero when the pole is turned away from the 
 sun, and below zero even when the pole is turned toward 
 the sun. 
 
 Before going further it is worth while to consider the 
 circumstances under which Mars is seen by an observer on 
 the earth. The mean distance of the moon from the earth 
 is 240,000 miles. If it is viewed through a field glass mag- 
 nifying four times, it is virtually brought within 60,000 
 miles of the observer. The nearest approach of Mars to 
 the earth is 35,000,000 miles. The planet can very seldom 
 be viewed to advantage with a magnifying power so high 
 as five hundred. If such a power is employed when Mars 
 is nearest, the planet is virtually brought within 70,000 
 miles. It follows, therefore, that we never see Mars so ad- 
 vantageously even with the largest telescopes as we may 
 see the moon in a common field glass. If the reader will 
 examine the moon with a field glass magnifying four times, 
 he will have a realizing sense of the best conditions under 
 which it is possible to see Mars, and he will be surprised 
 that so much is known of the planet. The industry and 
 
xiv ESSAYS ON ASTRONOMY 
 
 fidelity of observers can only be appreciated after such an 
 experiment. 
 
 Observations upon the polar caps of Mars must be 
 interpreted in the light of the foregoing facts namely, that 
 Mars has little or no water-vapour, and that its temperature 
 is appallingly low. The main facts of observation are as 
 follows: Cassini, the royal astronomer of France, discov- 
 ered in 1666 that Mars sometimes had dazzling white cir- 
 cular patches near his poles. In 1783 Sir William Herschel 
 observed these patches to wax and wane, and he called 
 them " snow " caps, thus begging the question as to their 
 real nature. Herschel's observations and those of all later 
 observers show that these caps wax and wane with the Mar- 
 tian seasons. In the Martian polar summer they are small- 
 est, or they even vanish; in the Martian polar winter they 
 are largest. As Herschel began with the conviction that all 
 planets were analogous to the earth and were meant to be 
 inhabited, his conclusion was that the polar winter con- 
 densed water-vapour into snow, and that the polar summer 
 melted this snow, and so on. A more scientific conclusion 
 would have been that some vapour was condensed and sub- 
 sequently dissipated by the solar heat. It is practically 
 certain that the phenomena of the waxing and waning of 
 the caps depend on solar heat. 
 
 If the caps are snow condensed from water-vapour, the 
 layer of snow must be exceedingly thin, because when these 
 caps are melted no clouds appear. When snow melts on 
 the earth, clouds are formed and our atmosphere is charged 
 with the vapour of water. No clouds are seen on Mars, 
 and no water-vapour is to be found above its surface by any 
 spectroscopic test. 
 
 The polar caps must be formed by the vapour of some 
 other substance than water. It is worth while to inquire 
 whether they may not be carbon dioxide in a solid state. 
 This substance is a heavy gas (carbonic-acid gas) at ordi- 
 nary temperatures. It would lie at the bottom of valleys 
 and fill canons or ravines. At a temperature of about one 
 hundred Fahrenheit below zero it is a colourless liquid. 
 
ESSAYS ON ASTRONOMY XV 
 
 At temperatures such as must obtain at the pole of Mars 
 turned away from the sun it becomes a snowlike solid. 
 Caps of carbon dioxide would wax and wane at the poles of 
 Mars under variations of solar heat such as obtain at these 
 poles, very much as caps of snow and ice wax and wane in 
 our arctic regions, which, under all circumstances, are at 
 a higher temperature than the poles of Mars. 
 
 There is so far no observational proof that the polar 
 caps of Mars are formed of carbon dioxide. There is con- 
 vincing proof that they are not formed of water. The ques- 
 tion as to the nature of the polar caps is still open. 
 
 The question of life in other worlds is vitally and pro- 
 foundly interesting to all mankind, and it is discussed in 
 several chapters of this volume. But the inquiries of sci- 
 ence must at present be limited to a determination of more 
 humble questions. Are the planets habitable? is the 
 present problem; later, we may perhaps be able to attack 
 the question whether they are, in fact, inhabited. 
 
 A chapter on " Sidereal Astronomy, Old and New," 
 gives an account of researches that led to an inventory of the 
 stellar universe, and of some of the main results attained, 
 and serves as an introduction to two memorable addresses 
 by Sir John Herschel on the occasion of awarding the 
 gold medal of the Royal Astronomical Society to Bessel 
 and to Francis Baily for their great catalogues of stars. 
 The true significance of the work of the observing astrono- 
 mer is not grasped until it is understood to how many 
 and various uses the results of his patient and laborious 
 watchings may be applied. These addresses of Herschel 
 are, moreover, models of lucid and eloquent exposition. 
 
 Two papers on the " Mathematical Theories of the 
 Earth," by Professor Robert Simpson Woodward, of Co- 
 lumbia University, and on the " Wanderings of the North 
 Pole," by Sir Robert Ball, exhibit the astronomical rela- 
 tions of our planet in an unfamiliar light, and describe one 
 of the capital discoveries of modern times the variability 
 of terrestrial latitudes. Incidentally, they serve to display 
 the vitality of the " old " exact astronomy. The " new " 
 
xvi ESSAYS ON ASTRONOMY 
 
 astronomy, which avails itself of photographic and spectro- 
 scopic aids, appeals more directly to popular imagination, 
 and its results can be presented to a popular audience 
 through a series of striking photographic pictures, but it is 
 helpless unless it employs the rigid methods of its elder 
 sister science; and new triumphs in many fields await the 
 votary of the old astronomy. The recent discovery of 
 variable terrestrial latitudes is not the least of them. 
 
 The papers on the " History of the Astronomical Tele- 
 scope," by Professor Charles Hastings, of Yale University; 
 on " An Astronomer's Life in a Modern Observatory," by 
 Dr. David Gill, royal astronomer at the Cape of Good 
 Hope; on " Photography as the Servant of Astronomy," 
 and on the " Beginnings of American Astronomy," by the 
 writer, present the history of astronomy in this country 
 and elsewhere, and exhibit certain little-understood aspects 
 of modern astronomical activity. These chapters of actu- 
 alities will, it is believed, be especially welcome to readers 
 who are familiar with the results of astronomical work, but 
 who are not acquainted with its details. 
 
 Laplace's Nebular Hypothesis a household word 
 was first proposed in 1796. He mentioned the suggestions 
 of Buffon with respect. It is an interesting fact that Buf- 
 fon possessed a copy of the " Principia " of Swedenborg, 
 in which the Swedish philosopher had expounded (1734) 
 a hypothesis to account for the genesis of the solar system. 
 The cosmologies of Swedenborg, Buffon, and Laplace 
 stand in a kind of historic relation to each other. The 
 theory propounded by Laplace must, of course, be regarded 
 as an independent and original generalization of a master 
 mind. His obligations to his predecessors were vanish- 
 ingly small. 
 
 The astronomical question that touches us most inti- 
 mately and deeply is that of life in other worlds. We know 
 our own life here in its thousand forms. We see the firma- 
 ment studded with stars, each star a sun, and each sun pre- 
 sumably accompanied by its retinue of planets. Our own 
 sun, a fairly insignificant star, has eight planets in its train, 
 
ESSAYS ON ASTRONOMY xvii 
 
 and one of them we know to swarm with life. How is 
 it with the planets that accompany those millions of other 
 suns? The instructed mind refuses to accept their shining 
 as accessory to our little existence. Man was the king and 
 centre of the universe. The telescope discrowned him. 
 To the eye of reason he became a mere ephemeral being, 
 living out a short and troubled life on the surface of one of 
 the smaller bodies of a vast universe. And the universe 
 itself is limited. We have seen that it is to end in dark- 
 ness, as it began. The immensity of man's fall in dignity has 
 often been insisted upon. It is sometimes forgotten that, 
 after all, it was by a man that man was discrowned. If he 
 is not the master of his own fate, at least he has foreseen 
 it. Has he companions of like nature on the planets of 
 his own system, or in the worlds that accompany each of 
 the brilliant stars of a winter's night? The subject is of 
 immense intellectual interest, and it is treated in two chap- 
 ters of this volume: by the late Richard Proctor, on a 
 " New Theory of Life in Other Worlds," and by the Rev- 
 erend Father Searle, some time Professor of Astronomy in 
 the Catholic University of America, on the " Habitability 
 of the Planets." 
 
 A glance over the table of contents will show that the 
 present volume contains essays on a great variety of sub- 
 jects, written at first hand, mostly by the great astronomers. 
 Place has been found for chapters on the sun, the planets, 
 meteorites, stellar systems, and on our own earth; on the 
 magnitude and stability of the solar system; on the nebular 
 hypothesis; on sidereal astronomy; on photography and 
 spectroscopy in their astronomical relations; on the history 
 of American astronomy, and of the telescope in all lands; on 
 the special aims and methods of the modern astronomer; 
 and, finally, on the theory of life in other worlds than ours. 
 Taken together, they present a vivid picture of the present 
 state of scientific opinion, and they afford a historical per- 
 spective that is of high value. 
 
 EDWARD S. HOLDEN. 
 
LOOK II J'STRATIONS 
 
 of fac- g- the development of manuscript and 
 
 book ne during four thousand 
 
 * ^^ JB^l*" rm^"^.^* 
 
 .HELM AND LUDWR 
 
 OF BAVARIA. 
 
 fi. I^^WB^^^vSS T , - 
 
 rac-simile of a page in the " 
 
 'KES 
 
G/A T1LS108UMAM 3UpIHU tT/lA 8UOMA^ 
 .8TCOITA.HT8UJJ1 
 
 bn JqhDaunBfn lo Jn^mqoiavab sri; gniworie ,23limie-Diil lo 39032 A 
 bnguoriJ 
 
 '* sboO w&I rij ni 3Bq "io 
 .a .A dii 
 
CONTENTS 
 
 ATOMS AND SUNBEAMS PAGK 
 
 By Sir Robert Stawell Ball . . ", . . . . . I 
 
 THE WANDERINGS OF THE NORTH POLE 
 
 By Sir Robert Stawell Ball . . . . ' . " l . .' ' . 22 
 
 THE AGE OF THE SUN'S HEAT 
 
 By Sir William Thomson (Lord Kelvin) . . . .> 4* 
 
 THE PAST AND FUTURE OF OUR EARTH 
 
 By Richard Anthony Proctor . . 9 ._, -.,.?. 55 
 
 A NEW THEORY OF LIFE IN OTHER WORLDS 
 
 By Richard Anthony Proctor . . . . . . .83 
 
 MATHEMATICAL THEORIES OF THE EARTH 
 
 By Robert Simpson Woodward . . . . . , t , f -. 103 
 
 THE ROTATION AND PHYSICAL CONSTITUTION OF THE 
 PLANET MERCURY 
 
 By Giovanni Virginio Schiaparelli .. , .' . . . 131 
 THE PLANET MARS 
 
 By Giovanni Virginio Schiaparelli . . . . . . 143 
 
 METEORITES AND STELLAR SYSTEMS 
 
 By George Howard Darwin ....... 163 
 
 MAGNITUDE OF THE SOLAR SYSTEM 
 
 By William Harkness . . . . . . . . .183 
 
 THE STABILITY OF THE SOLAR SYSTEM 
 
 By Ormsby McKnight Mitchel . . . . . . .213 
 
 xix 
 
xx ESSAYS IN ASTRONOMY 
 
 THE NEW PLANET, EROS PAGE 
 
 By Edmund Ledger . . . . . , ^ 239 
 
 SIDEREAL ASTRONOMY : OLD AND NEW 
 
 By Edward Singleton Holden .253 
 
 PHOTOGRAPHY THE SERVANT OF ASTRONOMY 
 
 By Edward Singleton Holden 290 
 
 THE BEGINNINGS OF AMERICAN ASTRONOMY 
 
 By Edward Singleton Holden .'*.*'''""' 309 
 
 STELLAR PARALLAX 
 
 By Sir John Frederick William Herschel . . . . . 321 
 
 THE HISTORY OF THE TELESCOPE 
 
 By Charles S. Hastings . . '.,,,., ... n . 339 
 
 RESULTS OF SPECTRUM ANALYSIS APPLIED TO HEAVENLY 
 
 BODIES 
 By William Huggins . .'' . J . ''' J ""'** . 363 
 
 CELESTIAL SPECTROSCOPY 
 
 By William Huggins . . . . ' , l ?^r "''*' 39 1 
 THE NEW ASTRONOMY 
 
 By William Huggins . . . . / . V . . 441 
 
 AN ASTRONOMER'S WORK IN A MODERN OBSERVATORY 
 
 ByDavidGill / . . . ''''.' <; . ^. . 473 
 
 THE SYSTEM OF THE WORLD THE NEBULAR HYPOTHESIS 
 
 By Pierre Simon, Marquis de Laplace . ... . 497 
 
 ARE THE PLANETS HABITABLE? 
 
 By George M. Searle .. ' ." . 513 
 
ILLUSTRATIONS 
 
 FACING PAGE 
 
 SIR ISAAC NEWTON . . . . . Frontispiece 
 
 Photogravure from an engraving by Jacob Houbracken after a 
 painting by Sir Godfrey Kneller 
 
 WlLHELM AND LUDWIG, DUKES OF BAVARIA .... XVlii 
 
 Fac-simile of a page in the " Law Code" folio, printed at Ingol- 
 stadt, 1516 A. D. 
 
 THE NEBULA IN ORION . . . . . . . . 60 
 
 Photogravure from a photograph taken at the Lick Observatory 
 
 NICOLAS COPERNICUS , 188 
 
 Photogravure from a painting 
 
 FIRST OBSERVATION OF THE TRANSIT OF VENUS . . . 244 
 
 Photogravure from an old etching 
 
 THE LICK OBSERVATORY . . . . . ... 306 
 
 Photogravure from a photograph 
 
 FRAUNHOFER DEMONSTRATING THE SPECTROSCOPE . ... 446 
 Photogravure from a painting by Richard Wimmer 
 
 THE MOON'S SURFACE 518 
 
 Photogravure from a photograph taken at the Lick Observatory 
 
 xxi 
 
ATOMS AND SUNBEAMS 
 
 AND 
 
 THE WANDERINGS OF THE 
 NORTH POLE 
 
 BY 
 
 SIR ROBERT STAWELL BALL 
 
ATOMS AND SUNBEAMS 1 
 
 IN recent years an important change has taken place 
 in the manner in which many physical problems are 
 approached. The philosopher who now seeks an ex- 
 planation of great natural phenomena not unfrequently 
 finds much assistance from certain remarkable discoveries 
 as to the ultimate constitution of matter. Many an obscure 
 question in physics has been rendered clear when some of 
 the properties of molecules have been brought to light. 
 No doubt our knowledge of the natural history of the 
 molecule is still vastly wanting in detail. It must, however, 
 be admitted that we have traced an outline of that won- 
 derful chapter in Nature which is specially serviceable in 
 the question which I now propose to discuss. 
 
 The problem before us may be stated in the following 
 terms: We have to illustrate how the sun is enabled to 
 maintain its tremendous expenditure of light and heat with- 
 out giving any signs of approaching exhaustion. It will 
 be found that the atomic theory of the constitution of mat- 
 ter exhibits the mechanism of the process by which that 
 capacity of the great luminary for supplying the radiation 
 so vital to the welfare of mankind is sustained from age 
 to age. 
 
 Let me here anticipate an objection which may not im- 
 probably be raised. Those who have paid attention to this 
 subject are aware that the remarkable doctrine first pro- 
 pounded by Helmholtz removed all real doubt from the 
 matter. It is to this eminent philosopher we owe an ex- 
 planation of what at first seemed to be a paradox. He ex- 
 1 From the " Fortnightly Review," October, 1893. 
 3 
 
4 BALL 
 
 plained how, notwithstanding that the sun radiates its heat 
 so profusely, no indications of the inevitable decline of heat 
 can be as yet discovered. If the sun had been made of 
 solid coal from centre to surface, and if that coal had been 
 burned for the purpose of sustaining the radiation, it can 
 be demonstrated that a few thousand years of solar expendi- 
 ture at the present rate would suffice to exhaust all the 
 heat which the combustion of that great sphere of fuel 
 could generate. We know, however, that the sun has been 
 radiating heat, not alone for thousands of years, but for mil- 
 lions of years. The existence of fossil plants and animals 
 would alone suffice to demonstrate this fact. We have thus 
 to account for the extremely remarkable circumstance that 
 our great luminary has radiated forth already a thousand 
 times as much heat as could be generated by the combus- 
 tion of a sphere of coal as big as the sun is at present, and 
 yet, notwithstanding this expenditure in the past, physics 
 declares that for millions of years to come the sun may con- 
 tinue to dispense light and heat to its attendant worlds 
 with the same abundant prodigality. To have shown how 
 the apparent paradox could be removed is one of the most 
 notable achievements of the great German philosopher. 
 
 What Helmholtz did was to refer to the obvious fact 
 that the expenditure of heat by radiation must necessarily 
 lead to shrinkage of the solar volume. This shrinkage has 
 the effect of abstracting from a store of potential energy 
 in the sun and transforming what it takes into the active 
 form of heat. The transformation advances pari passu with 
 the radiation, so that the loss of heat arising from the radia- 
 tion is restored by the newly produced heat derived from 
 the latent reservoir. Such is an outline of the now famous 
 doctrine universally accepted among physicists. It fulfils 
 the conditions of the problem, and when tested by arith- 
 metical calculation it is not found wanting. 
 
 But the genuine student of Nature loves to get to the 
 heart of a great problem like this; he loves to be able to 
 follow it, not through mere formulae or abstract principles, 
 but so as to be able to visualize its truth and feel its cer- 
 
ATOMS AND SUNBEAMS 5 
 
 tainty. He will, therefore, often desire something in addi- 
 tion to the bare presentation of the theory as above stated. 
 It may be no doubt sufficient for the mathematician to 
 know that the total potential energy in the sun, due to the 
 dispersed nature of its materials, is so vast that as contrac- 
 tion brings the materials, on the whole, somewhat nearer 
 together, the potential energy thus surrendered is trans- 
 formed into a supply of heat quite adequate to compensate 
 for the losses arising from the radiation by which the con- 
 traction was produced. The student who admits and who 
 is there that does not admit? the doctrine of the conserva- 
 tion of energy knows that in this argument he is on thor- 
 oughly reliable ground. At the same time the argument 
 does not actually offer any very clear conception, or indeed 
 any conception at all, of the precise modus operandi by 
 which, as the active potential energy vanishes, its equivalent 
 in available heat appears. I have always felt that this was 
 the unsatisfactory part of an otherwise perfect theory. It 
 was, therefore, with much interest that I became acquainted 
 a short time ago with a development of the molecular 
 theory of gases which afforded precisely what seemed 
 wanted to make every link in the chain of the great argu- 
 ment distinctly perceptible. I make no doubt that the no- 
 tions which have occurred to me on this subject must have 
 presented themselves to others also. I have, however, not 
 read in print or heard in conversation any use made of the 
 illustration that I am going to set forth. I feel, therefore, 
 confident that even if it be known at all, it is certainly not 
 generally known among the large and ever-increasing circle 
 of readers to whom the great questions of physics are of 
 interest. 
 
 The division of matter into the three forms of solids, 
 liquids, and gases has acquired in these days a special sig- 
 nificance now that the constitution of matter is becoming 
 in some degree understood. First let it be noted that, 
 though matter is capable of subdivision to a certain extent, 
 yet that there is a limit beyond which subdivision could not 
 be carried. This statement touches upon the ancient con- 
 
6 BALL 
 
 troversy as to the infinite divisibility of matter. Even still 
 we can find the statement in some of our old text-books 
 that there is no particle of matter so small that it could not 
 be again subdivided into half. No doubt, so far as most 
 ordinary experience goes, this statement may be unques- 
 tionable. It is quite true that we do not often reduce mat- 
 ter to fragments so small that each of them shall be insus- 
 ceptible of further conceivable division. But, to illustrate 
 the natural principle now under consideration, let us take 
 the example of a body which is itself composed of but a 
 single element. Think, for instance, of a diamond, which 
 is, as we all know, a portion of crystallized carbon. It is 
 true that the reduction of diamonds to powder is a labori- 
 ous process. Still, diamond dust has to be produced in the 
 finishing of the rough stone, and this element will serve the 
 purpose of our present argument better than a substance 
 of a composite nature. Each particle of the diamond dust 
 is, of course, as much a particle of carbon as was the 
 original crystal. We may, however, suppose that by a repe- 
 tition of the process a reduction of the diamond dust to 
 powder still finer is accomplished. The grains thus ob- 
 tained may have become so minute that they have ceased 
 to be visible to the unaided eye, and require a microscope 
 to render them perceptible; but even after this comminu- 
 tion each of these particles is still a veritable diamond. It 
 possesses the properties, optical, chemical, and mechanical, 
 of the original gem, from which it differs merely in the 
 attribute of size. Even when the disintegration has been 
 carried to such a point that each individual particle can be 
 only just perceived by the keenest power of the most pow- 
 erful microscope, there is still no indication that the par- 
 ticles cease to possess the characteristics of the original 
 body. These facts being undoubted, it was perhaps not 
 unnatural to suppose that the reduction could be carried on 
 indefinitely, and that even if the smallest fragment of dia- 
 mond which could be seen in a powerful microscope were 
 reduced to a millionth part, and each of those to a million 
 more, yet that the ultimate particles thus reached would be 
 
ATOMS AND SUNBEAMS 7 
 
 diamonds still. Now, however, we know that that is not 
 the case. The smallest particle visible under a microscope 
 might indeed be crushed into a thousand parts, and each 
 one of those parts, though wholly inappreciable to our 
 sense of touch or vision, would nevertheless be a genuine 
 diamond. If, however, the subdivision be carried on until 
 the particles produced are, roughly speaking, one-millionth 
 part of the bulk of the smallest objects which could be seen 
 in the microscope, we then approach the limits of partition 
 of which the diamond would be susceptible. We now 
 know that there is an atom of diamond so small that it must 
 refuse to undergo any further division. This ultimate atom, 
 be it observed, is not an infinitely small quantity. It has 
 definite dimensions; it possesses a definite weight. All such 
 diamond atoms are precisely alike in weight, and probably in 
 other characteristics. It might be thought that if this atom 
 has finite dimensions, it is, at all events, conceivable that it 
 should admit of further subdivision. In a certain sense this 
 is, no doubt, the case. The diamond atom is made up of 
 parts and, being so made, it is, of course, conceivable that 
 those parts could be separated. The important point to 
 notice is, that no means known to us could produce this 
 separation, while it is perfectly certain that if the decom- 
 position of the atom of diamond into distinct parts could 
 be effected, those parts would not be diamonds at all, nor 
 anything in the least resembling diamonds. 
 
 What we have said as regards the element carbon may 
 be extended to every other elementary substance. Sulphur 
 is familiarly known in a form of extreme subdivision, and 
 each little particle of sulphur could be further comminuted 
 to a certain point beyond which any further partition would 
 be impossible. So, too, any composite body such, for ex- 
 ample, as a lump of sugar admits of being decomposed 
 into molecules so small that any further separation would 
 be impossible if the molecule were still to remain sugar. 
 No doubt, a separation of the molecule of any composite 
 body into constituent atoms of other elements is not alone 
 possible, but is incessantly taking place. 
 
8 BALL 
 
 The first step in our knowledge of the constitution of 
 matter has been taken when we have come to recognise 
 that every body is composed of a multitude of extremely, 
 but not infinitely, small molecules. The next point relates 
 to the condition in which these molecules are found. At 
 first it might be thought that in a solid, at all events, the 
 little particles must be clustered together in a compact 
 mass. If we depended merely on sensible evidence it would 
 seem that a lump of iron, if constituted from molecules at 
 all, must be simply a cohering mass of particles, just as a 
 multitude of particles of sand unite to form a lump of sand- 
 stone. But the truth is far more wonderful than such a 
 belief would imply. Were the sensibility of our eyes so 
 greatly increased as to make them a few million times more 
 powerful than our present organs, then, indeed, the display 
 of the texture of solid matter would be an astonishing reve- 
 lation. It would be seen that the diamond atoms, which, 
 when aggregated in sufficient myriads, form the perfect 
 gem, were each in a condition of rapid movement of the 
 most complex description; each molecule would be seen 
 swinging to and fro with the utmost violence among the 
 neighbouring molecules. It would be seen quivering all 
 over under the influence of the shocks which it would re- 
 ceive from the vehement encounters with other molecules 
 which occur millions of times in each second. Such would 
 be the minute anatomy of the diamond. The well-known 
 properties of such gems seem, at first sight, wholly at 
 variance with the curious structure we have assigned to 
 them. Surely, it may be said that the hardness and the 
 impenetrability so characteristic of the diamond refute at 
 once the supposition that it is no more than a cluster of 
 rapidly moving particles. But the natural philosopher now 
 knows that his explanation of the qualities of the diamond 
 holds the field against all other explanations. The well- 
 known impenetrability of the diamond seems to arise from 
 the fact that when you try to press a steel point into the 
 stone you fail to do so because the rapidly moving mole- 
 cules of the gem batter the end of the steel point with such 
 
ATOMS AND SUNBEAMS 9 
 
 extraordinary vehemence that they refuse to allow it to 
 penetrate or even to mark the crystallized surface. When 
 you cut glass with a diamond it is quite true that the edge, 
 which seems so intensely hard, is really composed of rapidly 
 moving atoms. But the glass which is submitted to the 
 operation is also merely a mass of moving molecules, and 
 what seems to happen is, that, as the diamond is pressed 
 forward, its several particles, by their superior vigour, drive 
 the little particles of glass out of the way. We do not see 
 the actual details of the myriad encounters in which the 
 diamond atoms are victorious over the glassy molecules; 
 we only discern the broad result that the diamond has done 
 its work, and that the glass has been cut. 
 
 It may well be asked how we know that matter is con- 
 stituted of molecules in intensely rapid movement. The 
 statement seems at the first glance to be so utterly at vari- 
 ance with our ordinary experience that we demand, and 
 rightly demand, some convincing proof on the matter. 
 There are many arguments by which the required demon- 
 stration can be forthcoming. The one which I shall give 
 is not perhaps the most conclusive, but it has the advantage 
 of being one of the simplest and the most readily intel- 
 ligible. 
 
 Let us see if we can not prove at once that the mole- 
 cules in, let us say, a piece of iron must be in movement. 
 Suppose that the iron is warmed so that it radiates heat to 
 a perceptible extent. We know that the heat which, in 
 this case, affects our nerves has been transmitted from its 
 origin by ethereal undulations. Those undulations have, 
 undoubtedly, been set in motion by the iron, and yet the 
 parts of the metal seem quite motionless relatively to each 
 other, notwithstanding that they possess the power of set- 
 ting the ether into vibration. It is impossible that such 
 vibrations could be produced were it not that there is in 
 the iron a something which vibrates in such a manner as to 
 communicate the necessary pulses to the ether. It there- 
 fore follows that in the texture of the solid iron there must 
 be some molecular movement, timed in such a way as to 
 
10 BALL 
 
 impart to the ether the actual vibrations which we find it 
 to possess. The argument in this case may be illustrated 
 by the analogous phenomena presented in the case of 
 sound. As we listen to the notes of a violin, what we 
 actually perceive are vibrations communicated through the 
 air to the auditory apparatus. We can trace these aerial 
 vibrations back to their source, and we find they originate 
 from the quivering of the violin under the influence of the 
 bow of the performer. Were it not for these vibrations of the 
 instrument the aerial vibrations would not be produced, and 
 the corresponding sounds would not be heard. Far more 
 delicate than the atmospheric waves of sound are the ethe- 
 real waves corresponding to light or to heat, but none the 
 less must these latter also originate from the impulse of 
 some vibrating mass. It is thus apparent that a hot piece 
 of iron, however still it may seem, must be animated by an 
 excessively rapid molecular movement. Nor is the validity 
 of this conclusion impaired even if the iron be at ordinary 
 temperature. We know that a body which is no hotter 
 than the surrounding bodies is still incessantly radiating 
 heat to them and receiving heat from them in return. Thus 
 we are led to the conviction that a piece of iron, whatever 
 be its temperature, must consist of atoms in a state of lively 
 movement. The important conclusion thus drawn with 
 regard to iron may be equally stated with respect to every 
 other solid, or, indeed, every other body, whether solid, 
 liquid, or gaseous. All matter of every description is not 
 only known to be composed of molecules, but it is also now 
 certain that those molecules are incessantly performing 
 movements of a very complex type. 
 
 A closer study of this subject will be necessary for our 
 present purpose, and it will be convenient to examine 
 matter in that state in which it is exhibited in its very sim- 
 plest type from the molecular point of view. This condi- 
 tion is not presented, as might at first be supposed, when 
 the matter is solid, like a diamond, or like a piece of iron. 
 Even in a liquid the complexity of molecular constitution, 
 though somewhat less than in the case of a solid, is still 
 
ATOMS AND SUNBEAMS II 
 
 notably greater than in matter which has the gaseous form. 
 The air that we breathe is matter almost of the most simple 
 kind, so far as ^molecular constitution is concerned. It 
 should, however, be noted that, as air consists of a mixture, 
 it would be better for our purpose to think of a gas isolated 
 from any other element. Let us take the case of oxygen, 
 the most important constituent of our atmosphere. 
 
 Like every other element, oxygen is composed of mole- 
 cules, and those molecules are in a state of rapid motion. 
 It might be expected that the affinity by which the different 
 molecules were allied in the case of a gas should be of the 
 simplest nature, and this is indeed found to be the case. 
 Notwithstanding that oxygen is an invisible body, and not- 
 withstanding that the molecules are so excessively minute 
 as to be severally quite inappreciable to our senses, yet we 
 have been able to learn a great deal with regard to the con- 
 stitution of the molecules of this gas. The mental eye of 
 the philosopher shows him that, though the oxygen with 
 which a jar is filled appears to be perfectly quiescent, yet 
 that quiescence has there no real existence. He knows that 
 oxygen consists of myriads of molecules identical in weight 
 and in other features, and darting about one among the 
 other with velocities which vary perhaps between those of 
 express trains and those of rifle bullets. He sees that each 
 little molecule hurries along quite freely for a while until 
 it happens to encounter some other molecule equally bent 
 on its journey, and then a collision takes place. Perhaps it 
 would be more correct to say that what usually happens is 
 that the two impinging molecules make a very close ap- 
 proach; then each of them so vehemently attracts the other 
 as to make it swerve out of its course and start it off along 
 a path, inclined, it may be, even at a right angle to that 
 which it previously pursued. The molecules in a gas at 
 ordinary pressures are so contiguous that these encounters 
 take place incessantly; in fact, we are able to show that 
 each individual molecule will probably experience such ad- 
 ventures some millions of times in the course of each sec- 
 ond. We are able to calculate the average velocity with 
 
12 BALL 
 
 which the several molecules move when the gas has a 
 certain temperature. We know how to determine the 
 average length of the free path which each molecule trav- 
 erses in the interval between two consecutive encounters. 
 We are able to trace how all these circumstances would 
 vary if, instead of oxygen gas, we took nitrogen, or hydro- 
 gen, or any other body in the same molecular state. It is, 
 in fact, characteristic of every gas that each molecule wan- 
 ders freely, subject only to those incessant encounters with 
 other similar wanderers by which its path is so frequently 
 disturbed. If two gases be placed in the same vessel, one 
 being laid over the other, it will presently be found that the 
 two gases begin to blend; ere long one gas will have dif- 
 fused uniformly through the latter, so that the two will have 
 become a perfect mixture just as the oxygen and nitrogen 
 have done in our own atmosphere. The molecular theory 
 of gases explains at once the actual character of the opera- 
 tion by which diffusion is effected. Across the boundary 
 which initially separates the two gases certain molecules are 
 projected from either side, and this process of interchange 
 goes on until the molecules become uniformly distributed 
 throughout. 
 
 There is, indeed, nothing more remarkable than the fact 
 that information so copious and so recondite can be ob- 
 tained in a region which lies altogether beyond the direct 
 testimony of the senses. Just as the astronomer staggers 
 our powers of conception by the description of appalling 
 distances and stupendous periods of time, and relies with 
 confidence on the evidence which convinces him of the 
 reality of his statements, so the physicist avails himself of 
 a like potent method of research to study distances so 
 minute and times so brief that the imagination utterly fails 
 to realize them. 
 
 In the case of a liquid, the freedom enjoyed by the mole- 
 cules is considerably more restricted than in the case of a 
 gas. It would seem that in the denser fluid there can be 
 no intervals of undisturbed travel permitted to a molecule; 
 it is almost incessantly in a state of encounter with some 
 
ATOMS AND SUNBEAMS 13 
 
 other similar object. When a molecule in a liquid breaks 
 away from its association with one group, it is only because 
 it has entered into alliance with another. As, however, two 
 liquids will very frequently blend if so placed that diffusion 
 be possible, we have a proof that, though the transference 
 of a particular molecule through the liquid may be com- 
 paratively slow, yet it will gradually exchange association 
 with one group for association with another, and may in 
 this way travel throughout any distance to which the liquid 
 extends. 
 
 In the case of a solid there is still further limitation im- 
 posed on the mobility of each separate molecule. It is now 
 no longer permitted to make excursions throughout the 
 entire volume of the body. Each molecule is in rapid 
 motion, it is true, but those movements are confined to 
 gyrations within minutely circumscribed limits. Two solids 
 placed in contact do not generally diffuse one into the 
 other, the incapacity for diffusion being the direct conse- 
 quence of the inferior degree of mobility possessed by the 
 molecules in this condition of matter. 
 
 It is known that the immediate effect of the application 
 of heat is to increase the velocities with which the molecules 
 move. Apply heat, for instance, to the water in a kettle; 
 the moving molecules of water are thereby stimulated to 
 even greater activity, and it will occasionally happen that 
 the velocity thus acquired by a molecule becomes so 
 great that the little particle will swing clear away from the 
 influence of the other molecules with which it had been 
 associated. When this takes place in the case of a suf- 
 ficient number of molecules, they dart freely from the sur- 
 face of the liquid, thus producing the effect which in our 
 ordinary language we describe as giving off steam. If, 
 therefore, a volume of gas be heated, the velocities with 
 which its molecules are animated will be in general in- 
 creased. As the molecular velocities throughout the ex- 
 tent of the gas are, on the whole, augmented, it is quite 
 plain that the intensities of the shocks experienced by the 
 molecules in their several encounters will be also accentu- 
 

 I4 BALL 
 
 ated. The more rapidly moving particles will strike each 
 against the other with increased violence, and the contem- 
 plation of this single fact leads us close to one of Nature's 
 greatest secrets. 
 
 Let us think of the abounding heat which is dispensed 
 to us from the sun. That heat comes, as we know, in the 
 form of undulations imparted to the ether by the heated 
 matter in the sun, and transmitted thence across space for 
 the benefit of the earth and its inhabitants. I have already 
 explained that these vibrations in the ether must take their 
 rise from molecular movements, and it is important to no- 
 tice that the character of the vibrations in the ether enable 
 us to learn to some extent the precise description of mo- 
 lecular movements which alone would be competent to 
 produce the particular vibrations corresponding to radiant 
 heat. At first it might be thought that it was the rapid 
 movements of translation of the molecules themselves, as 
 entire if extremely minute bodies, which caused the ethe- 
 real vibration, but this is not so. We must carefully ob- 
 serve that there is another kind of molecular motion be- 
 sides that which the molecule possesses as a whole. We 
 have hitherto been occupied only with the movements of 
 each molecule as a little projectile pursuing its zigzag 
 course, each turn of the zigzag being the result of an en- 
 counter with some similar molecule belonging to the same 
 medium. But we have now to observe that the molecule 
 itself is by no means to be regarded as a simple rigid par- 
 ticle; indeed, if it were so, it is certain that we should re- 
 ceive no heat at all from the sun. We have the best reasons 
 for believing that the molecule of matter, so far from re- 
 sembling a simple rigid particle, is an elaborate structure, 
 whose parts are in some degree capable of independent 
 movement. It will not, indeed, be necessary for us to adopt 
 the splendid hypothesis of Lord Kelvin, which supposes 
 that molecules of matter are merely vortex rings in that 
 perfect fluid, the ether. It seems difficult to doubt that this 
 doctrine represents the facts, but if any one should reject it, 
 then I have only to say that its assumption is not required 
 
ATOMS AND SUNBEAMS 15 
 
 for our present argument. All that is necessary for us is 
 to regard each molecule as somewhat resembling an elastic 
 structure made of parts which can quiver like springs, and 
 so arranged as to be susceptible of many different modes 
 of vibration. We are to suppose that each molecule, in 
 addition to the energy which it possesses in virtue of its 
 movement of translation as a whole, has also a store of 
 energy corresponding to the oscillations of its electric 
 springs. We can, in fact, in some cases determine the ratio 
 which exists between the amount of energy which is, on 
 the average, possessed by molecules in consequence of their 
 velocities of translation, and the amount of energy which 
 they possess in consequence of the vibrations by which their 
 several parts are animated. It is these internal molecular 
 vibrations which are of essential importance in our present 
 inquiry. It is believed that the radiation of light, or of heat, 
 generally takes rise in the impulses given to ether by the 
 internal molecular vibrations. Do we not know that the 
 essential characteristic of those ethereal movements which 
 correspond to radiant light and heat is that they have the 
 nature of oscillations? Such could not be imparted by 
 mere rectilinear movements of the molecules as a whole. 
 They must be due to those internal oscillations by which 
 the actual molecules are animated. 
 
 No doubt it is difficult to realize that much can be 
 learned with regard to the performances that actually go 
 on in the internal parts of a molecule, especially when it is 
 remembered that each molecule in its entirety is so ex- 
 tremely minute as to be entirely beyond the reach of our 
 organs of sense. It is, nevertheless, impossible to doubt 
 that the statements just made correspond to the veritable 
 facts of Nature. It would be impracticable here to go into 
 any complete detail with regard to the evidence on this sub- 
 ject; I can only sketch an outline of it. Let us take, per- 
 haps as the simplest case, that presented by hydrogen. 
 
 At the ordinary temperature of the air hydrogen is, of 
 course, invisible; this means that the vibrations in the in- 
 terior of the molecules are not sufficiently vehement to im- 
 
1 6 BALL 
 
 " 
 
 part pulses to the ether with the energy that would be re- 
 ^__ quired to produce visual effects. Now, let us suppose that 
 the hydrogen is heated. The effect of heating is to impart 
 additional speed to the molecules of the gas, and conse- 
 quently when the molecules happen to come together their 
 encounter is more violent. The effect of such an occur- 
 rence on one of these little elastic bodies is to set it quiver- 
 ing with greater vehemence in those particular modes of 
 vibration for which it is tuned. If the temperature of the 
 gas has been raised sufficiently high, as it can be by the aid 
 of electricity, then the internal energy acquired by the mole- 
 cules, in consequence of the increased vehemence of their 
 collisions, has become so great that they are able to impart 
 pulses to the ether with sufficient intensity to affect our 
 nerves of vision; thereupon we declare that the hydrogen 
 is now so hot as to have become luminous. Suppose we 
 employ a spectroscope for the purpose of studying the par- 
 ticular character of the light which the glowing hydrogen 
 dispenses. It will appear that the spectrum consists of a 
 definite number of bright lines. We know that each one of 
 these lines corresponds to a particular period of vibration 
 of the ether, and hence we see that the light emitted by the 
 hydrogen does not consist of vibrations of all periods indis- 
 criminately, but only of certain particular waves which are 
 in unison with the oscillations to which the internal parts of 
 the molecule of hydrogen are adapted. Had we examined 
 the spectrum of some other gas in a state of incandescence 
 we should have found a wholly different system of lines 
 from those pertaining to hydrogen. This demonstrates 
 that the molecules of one gas differ essentially from those 
 of another in respect to the character of the internal vibra- 
 tions which they are adapted to perform. The extraordi- 
 nary activity of the movements which take place within the 
 molecules may be appreciated from the following facts: 
 We know that the wave corresponding to one of the hydro- 
 gen lines has a length of about the forty-thousandth of an 
 inch; we also know that in a single second of time light 
 travels over a space of a hundred and eighty-six thousand 
 
ATOMS AND SUNBEAMS 17 
 
 miles; a simple calculation will, therefore, assure us that 
 certain vibrations in the molecules of hydrogen correspond- 
 ing to this particular undulation must take place with such 
 an extraordinary frequency that about four hundred and 
 sixty millions of millions of them are performed in each 
 second of time. 
 
 Provided with these conceptions, we shall now, I think, 
 be able to see without difficulty how it is that the sun's heat 
 is sustained. We may, for our present purpose, think of the 
 great luminary as a mass of glowing gas. It is quite true 
 that the physical condition of the matter in the interior of 
 the tremendous globe can hardly be that which we ordi- 
 narily consider as gaseous. But this need not affect our 
 argument. It is undoubtedly true that those portions of 
 the solar atmosphere from which the light and heat are 
 mainly dispensed are gaseous in their character, or, at all 
 events, come sufficiently near to matter in the gaseous state 
 to permit the application of the line of argument with which 
 we have hitherto been engaged. In consequence of the 
 vast mass of the sun the gravitation with which it draws all 
 bodies toward it is very much greater than the gravitation 
 on the surface of the earth. On our globe we know that 
 the effect of gravitation is to impart to any body near the 
 surface velocity directed toward the earth's centre at the 
 rate of thirty-two feet per second. The sun is more than 
 three hundred thousand times as massive as the earth; we 
 can not, however, assert that the gravitation is increased in 
 the same proportion, because, on account of the vast size 
 of the sun, a particle at its surface is more than a hundred 
 times farther away from the solar centre than a body on 
 the surface of the earth is from the terrestrial centre. It 
 can, however, be shown that, taking these various matters 
 into account, the actual intensity of gravitation at the solar 
 surface is sufficient to tend to impart to all objects an in- 
 crease of velocity toward the sun's centre at the rate of four 
 hundred and fifty-seven feet per second. This would apply 
 not only to a meteorite, or other considerable mass, which 
 is falling into the sun; it would be equally true of an object 
 
!8 BALL 
 
 as small as a molecule. Every one of the myriads of gase- 
 ous molecules in the outer regions of the solar atmosphere 
 must be constantly acted upon by this attractive force, 
 which tends in the course of each second to add to them a 
 downward velocity at that rate per second which has already 
 been stated. It is quite true that to a great extent the effect 
 of this attraction is masked by counteracting tendencies. 
 In particular we may mention that, inasmuch as the den- 
 sity of the solar atmosphere increases as the sun's centre 
 is approached, the flying molecule generally finds itself 
 more obstructed by encounters with other molecules when 
 it is descending than when it is ascending. We may here 
 contrast the condition of the atmosphere on the earth with 
 the condition of the solar atmosphere. Each molecule in 
 our air, being acted upon by terrestrial gravitation, has 
 thereby a tendency to fall downward with a velocity con- 
 tinually increasing at the rate of thirty-two feet per second. 
 As, however, the terrestrial atmosphere has long since 
 reached a stable condition, in which it undergoes no further 
 contraction, the effect of gravitation in adding velocity to 
 the molecules is so completely masked by the counteracting 
 tendencies that, on the whole, there is no continual increase 
 of molecular velocities downward due to gravitation. Were 
 such an increase at present going on, we should necessarily 
 find that the terrestrial atmosphere was decreasing in vol- 
 ume, and ever becoming more condensed in its lower strata. 
 It is, however, well known that no such changes as are here 
 implied are taking place. The essential difference between 
 the earth and the sun, so far as the matter now before us is 
 concerned, is to be found in the fact that, as the sun has not 
 yet passed into the form of a rigid body, it is still contract- 
 ing at a rate very much greater than that at which a body 
 grown so cold as the earth draws its particles closer to- 
 gether. The molecules in the solar photosphere accord- 
 ingly yield to a certain extent to the gravitation which 
 constantly seeks to draw them down. The counteracting 
 tendencies can not in the sun, as they do in the earth, mask 
 the direct and obvious effect of gravitation. The conse- 
 
ATOMS AND SUNBEAMS 19 
 
 quence is that the intense attraction which is capable of 
 adding velocity to the molecules at the phenomenal rate of 
 four hundred and fifty-seven feet per second is permitted to 
 accomplish something, and thus increase the average speeds 
 with which the molecules hurry along. To express the 
 matter a little more accurately, we should say that the 
 downward velocity imparted by gravitation, being com- 
 pounded with the velocities otherwise possessed by the 
 molecules, tends, on the whole, to increase the rate at which 
 they move. 
 
 We shall now be able to discern what actually takes 
 place as the sun contracts by dispersing heat, and in con- 
 sequence of its decline in bulk finds a store of energy lib- 
 erated which it is permitted to use for the purpose of sus- 
 taining its radiating capacity. Owing to the intense heat 
 which prevails in the photosphere, the molecules must there 
 be in very rapid movement; their mutual encounters must 
 be of the utmost vehemence, and their internal vibrations, 
 which are the consequences of the shocks in the encoun- 
 ters, must be correspondingly energetic. It is, as we have 
 seen, these internal molecular vibrations which set the ether 
 in motion, and thus dispense solar heat and light far and 
 wide through the universe. But this the molecules can 
 only do at the expense of the energy which they possess 
 in virtue of their internal vibrations. Unless, therefore, the 
 internal molecular energy were to be in some way recuper- 
 ated from time to time, the radiating power must neces- 
 sarily flag. It is now plain that the necessary recuperation 
 takes place in the successive encounters. A molecule 
 whose internal energy of vibration is becoming exhausted 
 by the effort of setting the ether into vibration presently 
 impinges against some other molecule, and in consequence 
 of the blow is again set into active vibration which permits 
 it to carry on the work of radiation anew, until its declining 
 energies have again to be sustained by some similar addi- 
 tion arising from a fresh collision. Of course, we know that 
 the internal molecular energy thus acquired can not be 
 created out of nothing. If the molecule receives such ac- 
 
20 BALL 
 
 cessions of internal energy, it must be at the expense of 
 the energy which is elsewhere. Obviously the only pos- 
 sible source of such energy must be found in the movement 
 of the molecule as a whole that is to say, in the velocity 
 of translation with which it rushes about among the other 
 molecules. Thus we see that the immediate effect of ex- 
 penditure of heat or light by radiation is to diminish the 
 internal energies of the molecules. These energies are re- 
 stored by the transference of energy obtained from the 
 general velocities of the molecules regarded as moving 
 projectiles. It follows that the velocities of the several 
 particles must on the whole tend to decline; in other words, 
 that the temperature tends to fall. What we have to dis- 
 cover is the agent which at present prevents the solar tem- 
 perature from falling. We want, therefore, to ascertain 
 the means by which the molecular velocities are preserved 
 at the same average value, notwithstanding that there is a 
 constant tendency for these velocities to abate in conse- 
 quence of the losses of light and heat by radiation. We 
 have already explained how the gravitation of the sun con- 
 stantly tends to impart additional downward velocity to the 
 molecules in its atmosphere. This is precisely the action 
 which we now require. The contraction of the sun tends 
 to an augmentation of the molecular velocities, and this 
 augmentation just goes to supply the loss of velocities 
 which is the consequence of the radiation. A complete ex- 
 planation of the maintenance of the sun's heat is thus 
 afforded. Observation, no doubt, seems to show that the 
 capacity for radiation is at present sensibly constant, and this 
 being so, we see that the gain of molecular velocities from 
 gravitation and their losses from radiation are at present 
 just adapted to neutralize each other. Nothing, however, 
 that has as yet been said demonstrates that the efficiency 
 of the sun for radiating light and heat must always be pre- 
 served exactly at its present value. 
 
 It is quite possible that if we had the means of studying 
 the sun's heat for a hundred thousand years, we might find 
 that the capacity for radiation was slightly decreasing, or, 
 
ATOMS AND SUNBEAMS 21 
 
 it may be, that it would be slightly increasing, for it is at 
 least conceivable that the gain of molecular velocity due 
 to gravitation may, on the whole, exceed the loss due to 
 the dispersal of energy by radiation. On the other hand, 
 it is, of course, possible that the acquisition of velocity 
 by gravitation, though nearly sufficient to countervail the 
 expenditure by radiation, may not be quite enough, in 
 which case the sun's temperature would be slowly declining. 
 It must not, however, be supposed that the argument 
 which we have been here following attributes eternal vigour 
 to the great luminary. It will be noted that it is of the 
 essence of the argument that the contraction is still in 
 progress. If the contraction were to cease, then the resti- 
 tution of velocity by gravitation would cease also, and the 
 speedy dispersal of the existing heat by radiation would 
 presently produce bankruptcy in the supply of sun- 
 beams. Indeed, such bankruptcy must arrive in due time, 
 when, after certain millions of years, the sun has so 
 far contracted that it ceases to be a gaseous mass. The 
 vast accumulated store of energy which is now being 
 drawn upon, to supply the current radiation, will then yield 
 such supplies no longer. Once this state has been reached, 
 a few thousand years more must witness the extinction of 
 the sun altogether as a source of light, and the great orb, 
 at present our splendid luminary, will then pass over into 
 the ranks of the innumerable host of bodies which were 
 once suns, but are now suns no longer. 
 
THE WANDERINGS OF THE NORTH 
 POLE 1 
 
 ON a recent visit to Cambridge, Professor Barnard, 
 the discoverer of the fifth satellite of Jupiter, exhib- 
 ited at the Cavendish Laboratory his most interest- 
 ing collection of photographs made at the Lick Observatory. 
 These pictures were obtained by a six-inch photographic 
 lens of three feet focus, attached to an ordinary equatorial, 
 the telescope of which was used as a guider when it was 
 desired to obtain a picture of the stars with a long exposure. 
 Among the advantages of this process may be reckoned 
 the large field that is thereby obtained, many of the plates 
 that he exhibited being as much as four degrees on the 
 edge. I am, however, not now going to speak of Barnard's 
 marvellous views of the Milky Way, nor of the plate on 
 which a comet was discovered, nor of the vicissitudes of 
 Holmes's comet, nor of that wonderful picture in which 
 Swift's comet actually appears to be producing, by a pro- 
 cess of gemmation, an offshoot which is evidently adapted 
 for an independent cometary existence. The picture to 
 which I wish specially to refer in connection with our 
 immediate subject is one in which the instrument was 
 directed toward the celestial pole. In this particular case 
 the clockwork which is ordinarily employed to keep the 
 stars acting at the same point of the plate was dispensed 
 with. The telescope, in fact, remained fixed while the 
 heavens rotated in obedience to the diurnal motion. Under 
 these circumstances each star, as minute after minute 
 passed by, produced an image on a different part of the 
 1 From the " Fortnightly Review," August, 1893. 
 
 22 
 
WANDERINGS OF THE NORTH POLE 23 
 
 plate; the consequence of which was that the record which 
 the star was found to have left, when the picture was de- 
 veloped, was that of a long trail instead of a sharply de- 
 fined point. As each star appears to describe a circle in the 
 sky around the pole, and as, in the vicinity of the pole, these 
 circles were small enough to be included in the plate, this 
 polar photograph exhibits a striking spectacle. It dis- 
 played a large number of concentric circles, or rather, I 
 should say, of portions of circles, for the exposures having 
 lasted for about four hours, about one sixth of each circum- 
 ference was completed during that time. The effect thus 
 produced was that of a number of circular arcs of varying 
 sizes, and of different degrees of brightness. Most con- 
 spicuous among them was the trail produced by the actual 
 Pole Star itself. It is well known, of course, that though 
 the situation of the pole is conveniently marked by the for- , 
 tunate circumstance that a bright star happened during the 
 present century to lie in the immediate vicinity of the 
 veritable pole, yet, of course, this star is not actually at the 
 pole, and consequently, like all the other stars, Polaris itself 
 must be revolving in a circle whereof the centre lies at the 
 true pole. The brighter the star the brighter is the trail 
 which it produces, so that the circle made by Polaris is 
 much more conspicuous than the circles produced by the 
 other stars of inferior lustre. It is, however, to be noted 
 that some of the faint stars lie much closer to the pole than 
 Polaris itself. There is, indeed, one very minute object so 
 close to the pole that the circle in which its movements 
 are performed seems very little more than a point when 
 represented on the screen on which the slide was projected. 
 The interesting circumstance was noted that there ap- 
 peared to be occasional interruptions to the continuity of 
 the circular arcs. This was due to the fact that clouds had 
 interposed during the intervals represented by the interrup- 
 tions. A practical application is thus suggested, which has 
 been made to render useful service at Harvard College Ob- 
 servatory. Every night, and all night long, a plate is there 
 exposed to this particular part of the sky, and the degree 
 
24 BALL 
 
 in which the Pole Star leaves a more or less complete trail 
 affords an indication of the clearness or cloudiness of the 
 sky throughout the course of the night. From the posi- 
 tions of the parts where the trail has been interrupted it is 
 possible not only to learn the amount of cloudiness that 
 has prevailed, but the particular hours during which it has 
 lasted. This interesting system of concentric polar circles 
 affords us perhaps the most striking visual representation 
 that could possibly be obtained of the existence of that 
 point in the heavens which we know as the pole. The pic- 
 ture thus exhibited was a striking illustration of the Coper- 
 nican doctrine that the diurnal stellar movement was indeed 
 only apparent, being, of course, due to the rotation of the 
 earth on its axis. 
 
 Suppose that a photograph, like that I have been de- 
 scribing, were to be taken at intervals of a century, it would 
 be found that the centre of the system of circles that is to 
 say, the veritable pole itself was gradually changing on 
 the heavens. I do not by this mean that the stars them- 
 selves would be found to have shifted their places relatively 
 to each other. No doubt there is some effect of this kind, 
 but it is an insignificant one, and need not at present con- 
 cern us. The essential point to be noticed is, that the 
 stars which happen to lie in the vicinity of the pole would 
 have a changed relation to the pole in consequence of the 
 fact that this latter point is itself in incessant movement. 
 At the present time the pole is advancing in such a direc- 
 tion that it is getting nearer to the Pole Star, so that the 
 actual circle which the Pole Star is describing is becoming 
 less and less. The time will come when the circle which this 
 star performs will have reached its lowest dimensions, but 
 still the pole will be moving on its way, and then, of course, 
 the dimensions of the circle traversed by the Pole Star will 
 undergo a corresponding increase. As hundreds of years 
 and thousands of years roll by the pole will retreat farther 
 and farther from the Pole Star, so that in the course of 
 a period as far on in the future as the foundation of Rome 
 was in the past, the pole will be no longer sufficiently near 
 
WANDERINGS OF THE NORTH POLE 25 
 
 the Pole Star to enable the latter to render to astronomers 
 the peculiar services which it does at present. 
 
 Looking still further ahead, we find that in the course 
 of about twelve thousand years the pole will have gained a 
 position as remote as it possibly can from that position 
 which it now occupies. This most critical point in the 
 heavens will then lie not far from the star Vega, the bright- 
 est point in the northern sky, and then it will commence 
 to return, so that after the lapse of about twenty-five thou- 
 sand years the pole will be found again in the same celes- 
 tial neighbourhood where it is to-night, having, in the 
 meantime, traversed a mighty circle through the constel- 
 lations. In all this there is no novelty; these movements 
 of the pole are so conspicuous that they were detected long 
 before the introduction of accurate instruments. They 
 were discovered so far back as the time of Hipparchus, and 
 the cause of them was assigned by Newton as one of the 
 triumphs of his doctrine of universal gravitation. In giv- 
 ing the title of " The Wanderings of the North Pole " to 
 this paper I did not, however, intend to discourse of the 
 movements to which I have hitherto referred. They are so 
 familiar that every astronomer has to attend to them prac- 
 tically in the reduction of almost every observation of the 
 place of a celestial body. It was, however, necessary to 
 make the reference which I have done to this subject in 
 order that the argument on which we are presently to 
 enter should be made sufficiently clear. It must be noted 
 that the expression " the north pole " is ambiguous. It 
 may mean either of two things, which are quite distinct. 
 In the case we have already spoken of, I understand by the 
 north pole that point on the celestial sphere which is the 
 centre of the system of concentric circles described by the 
 circumpolar stars. The other sense in which the north pole 
 is used is the terrestrial one; it denotes that point on this 
 earth which has been the goal of so many expeditions, and 
 to reach which has been the ambition of so many illustrious 
 navigators. We have a general notion that the terrestrial 
 north pole lies in a desolate region of eternal ice, somewhat 
 
26 BALL 
 
 relieved by the circumstance that for six months of the 
 year the frozen prospect is brightened by perpetual day, 
 though, on the other hand, during the remaining six months 
 of the year this region is the abode of perpetual night. 
 The north pole is that hitherto unattainable point on our 
 globe on which, if an observer could take his station, he 
 would find that the phenomena of the rising and the set- 
 ting of the stars, so familiar elsewhere, were non-existent. 
 Each star viewed from the coign of vantage offered by the 
 north pole would move round and round in a horizontal 
 circle; and the system of concentric circles would be 
 directly overhead. In midsummer the sun would seem to 
 revolve around, remaining practically at the same eleva- 
 tion above the horizon for a few days, until it slowly began 
 to wend its way downward in a spiral. In a couple of 
 months it would draw near the horizon, and as day after 
 day passed by the luminary would descend lower and lower 
 until its edge grazed the horizon all round. The setting 
 of the sun for the long winter would then be about to com- 
 mence, and gradually less and less of the disk would re- 
 main perceptible. Finally, the sun would disappear alto- 
 gether, though for many days afterward a twilight glow 
 would travel round the whole hemisphere, ever getting less 
 and less, until at last all indications of the sun had vanished. 
 The utter darkness of winter would then ensue for months, 
 mitigated only so far as celestial luminaries were concerned 
 by starlight or occasional moonlight. Doubtless, however, 
 the fitful gleams of the aurora would often suffice to render 
 the surrounding desolation visible. Then as the spring 
 drew near, if, indeed, such a word as spring be at all ap- 
 plicable to an abode of utter dreariness, a faint twilight 
 would be just discernible. The region thus illuminated 
 would move round and round the horizon each twenty-four 
 hours, gradually becoming more and more conspicuous, 
 until at last the edge of the sun appeared. Then, by a spiral 
 movement inverse to that with which its descent was ac- 
 complished, the great luminary would steal above the hori- 
 zon, there to continue for a period of six months until the 
 
WANDERINGS OF THE NORTH POLE 27 
 
 beginning of the ensuing winter. Indeed, the actual 
 duration of apparent summer would be somewhat pro- 
 tracted in consequence of the effect of refraction in raising 
 the sun visually above the horizon when in reality it was 
 still below. The result would be to lengthen the summer 
 at one end and to anticipate it at the other. Such would 
 be the astronomical conditions at the north pole; that 
 anomalous point, from whence every other locality on the 
 globe lies due south, that mysterious point which up to 
 the present seems never to have been approached by man 
 within a distance less than four hundred miles, unless, in- 
 deed, as is not improbably the case, the preglacial man 
 who lived in the last genial period found a temperate cli- 
 mate, and enjoyable conditions even at the latitude of 90. 
 For our present purpose it will be necessary to get a 
 very clear idea as to the precise point on the earth which we 
 mean when we speak of the north pole. As our knowledge 
 of it is almost entirely derived from astronomical phe- 
 nomena it is necessary to assign the exact locality of the 
 pole by a strict definition depending on astronomical facts. 
 Supposing that Nansen does succeed in his expedition, 
 as every one hopes that he will, and does penetrate within 
 that circle of four hundred miles' radius where the foot of 
 civilized man has never yet trod, how is he to identify that 
 particular spot on this globe which is to be defined by the 
 north pole? It was for this purpose that at the begin- 
 ning of this paper I referred to that photograph of the con- 
 centric circles which illustrated so forcibly the position of 
 the pole in the heavens. Imagine that your eye was placed 
 at the centre of the earth, and that you had a long, slender 
 tube from that centre to the surface through which you 
 could look out at the celestial sphere; if that tube be placed 
 in such a way that, when looking from the centre of the 
 earth through this tube your vision was directed exactly to 
 that particular point of the heavens which is the centre of 
 the circle now described by the Pole Star and the other 
 circumpolar stars, then that spot in which the end of the 
 tube passes out through the surface of the earth is the north 
 
28 BALL 
 
 pole. Imagine a stake to be driven into the earth at the 
 place named, then the position of that stake is the critical 
 spot on our globe which has been the object of so much sci- 
 entific investigation and of so much maritime enterprise. 
 The reader must not think that I am attempting to be hy- 
 peraccurate in this definition of the north pole; no doubt, in 
 our ordinary language we often think of the pole as some- 
 thing synonymous with the polar regions, an ill-defined 
 and most vaguely known wilderness of ice. For scientific 
 purposes it is, however, essential to understand that the pole 
 is a very definitely marked point, and we must assign its 
 position accurately, not merely to within miles, but even to 
 within feet. Indeed, it is a truly extraordinary circum- 
 stance that, considering no one, with the possible excep- 
 tion just referred to, has ever yet been within so many 
 hundreds of miles of the pole, we should be able to locate 
 it so precisely that we are absolutely certain of its position 
 to within an area not larger than that covered by a small 
 town, or even by a good-sized drawing-room. 
 
 We have seen that the north pole in the sky is in inces- 
 sant movement, and that the travels which it accomplishes 
 in the course of many centuries extend over a wide sweep 
 of the heavens; this naturally suggests the question, Does 
 the pole in the earth move about in any similar manner, 
 and if so, what are the nature and extent of its variation? 
 Here is the point about which those modern researches 
 have been made which it is my special object to discuss 
 in this paper. Let us first see clearly the issue that is 
 raised. At the time of the building of the Pyramids the 
 pole in the heavens was in quite a different place from its 
 present position; the Pole Star had not at that time the 
 slightest title to be called a pole star; in fact, the point 
 around which the heavens revolved lay in a wholly differ- 
 ent constellation. It was certainly not far from the star 
 Alpha Draconis about 3000 B. c., and we could indicate its 
 position quite definitely if we had any exact knowledge as 
 to the date of the Pyramids' erection. It is, however, plain 
 that the difference was so patent between the celestial pole 
 
WANDERINGS OF THE NORTH POLE 29 
 
 at the time of the Pyramids and the celestial pole of later 
 centuries, that it could not be overlooked in attentive ob- 
 servation of the heavens. As the north pole in the sky 
 was, therefore, so different in the time of the Pharaohs from 
 the north pole in the time of Victoria, it is proper to ask 
 whether there was a like difference, or any difference at all, 
 between the terrestrial pole at the time of the building of 
 the Pyramids and that terrestrial pole in whose quest Nan- 
 sen is just setting off. If Pharaoh had despatched a suc- 
 cessful expedition to the north pole and driven a post in 
 there to mark it, and if Nansen were now successful, would 
 he find that the north pole in the earth which he was to 
 mark occupied the same position or a different position 
 from that which had been discovered thousands of years 
 previously? At first one might hastily say that there must 
 be such a difference, for it will be remembered that I have 
 defined the north pole in the earth as that point through 
 which the tube passes which would permit an eye placed at 
 the centre of the earth to view the north pole in the sky. 
 If, therefore, the north pole in the sky had undergone a 
 great change in its position, it might seem obvious that the 
 tube from the earth's centre to its surface which would now 
 conduct the vision from that centre to the north celestial 
 pole would emerge at a different point of the earth's crust 
 from that which it formerly occupied. We have here to deal 
 with the case that arises not unfrequently in astronomy, in 
 which a fact of broad general truth requires a minute degree 
 of qualification; indeed, it is not too much to say that it 
 is in this qualification of broad general truths that many of 
 the greatest discoveries in physical science have consisted. 
 And such is the case in the present instance. There is a 
 broad general truth and there is the qualification of it. It 
 is the qualification that constitutes the essential discovery 
 which it is my object herein to set forth. But before doing 
 so it will be necessary for me to lay down the broad general 
 truth that the north pole of the earth as it existed in the 
 time of the Pharaohs appears to be practically the same as 
 the north pole of the earth now. It seems perfectly certain 
 
30 BALL 
 
 that at any time within the last ten thousand years the 
 north pole might have been found within a region on the 
 earth's surface not larger than Hyde Park. Indeed, the 
 limits might be drawn much more closely. It is quite pos- 
 sible that many an edifice in London occupies an area suf- 
 ficiently great to cover the holes that would be made by 
 all the posts that might be driven to mark the precise sites 
 of the north pole on the earth not only for the last five 
 thousand or ten thousand years, but probably for periods 
 much more ancient still. It is very likely that the north 
 pole at the time of the Glacial epoch was practically in- 
 distinguishable from the north pole now; in fact, the con- 
 stancy, or sensible constancy I should, perhaps, rather 
 say, of the situation of this most critical point in our globe 
 is one of the most astonishing facts in terrestrial physics. 
 
 Let us, then, assume this broad, general fact of the per- 
 manency in the position of the north pole, and deduce the 
 obvious consequences it implies with regard to the earth's 
 movement. At this point we find the convenience of the 
 time-honoured illustration in our geography books which 
 likens the earth to an orange. Let us thrust a knitting- 
 needle through the orange along its shortest diameter to 
 represent the axis about which the earth rotates. Not only 
 does the earth perform one revolution about this axis in 
 the space of each sidereal day, but the axis itself has a 
 movement. If the earth's axis always remained fixed, or 
 never had any motion except in a direction parallel to itself, 
 then the point to which it was directed on the sky wotild 
 never change. We have, however, seen that the pole in 
 the sky is incessantly altering its position; we are therefore 
 taught that the direction of the earth's axis of rotation is 
 constantly changing. To simulate the movement by the 
 orange and knitting-needle we must imagine the orange to 
 rotate around its axis once in that period of twenty-three 
 hours and fifty-six minutes, which is well known as the 
 length of the sidereal day; while at the same time the knit- 
 ting-needle, itself bearing, of course, the orange with it, 
 performs a conical movement with such extreme slowness 
 
WANDERINGS OF THE NORTH POLE 31 
 
 that not less than twenty-five thousand years is occupied 
 in making the circuit. The movement, as has often been 
 pointed out, is like that of a peg-top which rotates rapidly 
 on its axis while at the same time the axis itself has a slow 
 revolving motion. Thus the phenomena which are pre- 
 sented in the rotation of the earth demonstrate that the 
 axis about which the earth rotates occupies what is, at all 
 events, approximately a fixed position in the earth, though 
 not a fixed position in space. We can hardly be surprised 
 at this result; it merely implies that the earth acts like a 
 rigid body on the whole, and does not permit the axis 
 about which it is turning to change its position. 
 
 It will now be easily understood how it comes to pass 
 that the position of the north pole upon the earth has not 
 appreciably changed in the course of thousands of years. 
 The axis around which the earth rotates has retained a per- 
 manent position relative to the earth itself; it has, however, 
 continuously changed, it is at this moment changing, and it 
 will continue to change with regard to its direction in space. 
 So far our knowledge extended up to within the last few 
 years, but in these modern days a closer inquiry has been 
 made into this, as into so many other physical subjects, and 
 the result has been to disclose the important fact that, 
 though the phenomena as just described are very nearly 
 true, they must receive a certain minute qualification. 
 Complete examination of this subject is desirable, not only 
 on account of its natural importance, but also because 
 it illustrates the refinements of which modern astronomical 
 processes are susceptible. I have stated the broad, general 
 fact that the position of the terrestrial pole undergoes no 
 large or considerable fluctuation. But while we admit that 
 no large fluctuation is possible, it is yet very proper to 
 consider whether there may not be a small fluctuation. It 
 is certain that the position of the pole as it would be 
 marked by a post driven into the earth to-day can not differ 
 by a mile from the position in which the same point would 
 be marked last year or next year. But does it differ at all? 
 Is it absolutely exactly the same? Would there be a dif- 
 
32 BALL 
 
 ference not indeed of miles, but of yards or of feet, between 
 the precise position of the pole on the earth determined 
 at successive intervals of time? Would it be the same if 
 we carried out our comparisons not merely between one 
 year and another, but day after day, week after week, 
 month after month? No doubt the more obvious phe- 
 nomena proclaim in the most unmistakable manner that 
 the position of the pole is substantially invariable. If, 
 therefore, there be any fluctuations in its positions, those 
 could only be disclosed by careful scrutiny of minute phe- 
 nomena which were too delicate to be detected in the 
 coarser methods of observation. There is indeed a cer- 
 tain presumption in favour of the notion that absolute con- 
 stancy in the position of the pole need not be expected. 
 Almost every statement of astronomical doctrine requires 
 its qualification, and it would seem indeed unlikely that 
 when sufficient refinement was introduced into the meas- 
 urements the position of the pole in the earth should appear 
 to be absolutely unalterable. Until a very recent period the 
 evidence on the subject was almost altogether negative; it 
 was no doubt recognised that there might be some fluctua- 
 tions in the position of the pole, but it was known that 
 they would only be extremely small, and it was believed 
 that in all probability those fluctuations must be comprised 
 within those slender limits which are too much affected 
 by inevitable errors of observation to afford any reliable 
 result. Perseverance in this interesting inquiry has been 
 at last rewarded; and, as in so many similar cases, we are 
 indebted to the labours of many independent workers for 
 the recent extension of our knowledge. We are, however, 
 at present most interested by the labours of Mr. Chandler, 
 a distinguished American astronomer, who has made an 
 exhaustive examination into the subject. The result has 
 been to afford a conclusive proof that the terrestrial pole 
 does undergo movement. Mr. Chandler has been so suc- 
 cessful as to have determined the law of those polar move- 
 ments, and he has found that when they are taken into con- 
 sideration an important improvement in certain delicate 
 
WANDERINGS OF THE NORTH POLE 33 
 
 astronomical inquiries is the result. These valuable in- 
 vestigations merit, in the highest degree, the attention, not 
 only of those who are specially devoted to astronomical 
 and mathematical researches, but of that large and ever- 
 increasing class who are anxious for general knowledge 
 with regard to the physical phenomena of our globe. 
 
 At first sight it might seem difficult indeed to conduct 
 the investigation of this question. Here is a point on the 
 earth's surface, this wonderful north pole, which, so far as 
 we certainly know, has never yet been approached to within 
 four hundred miles, and yet we are so solicitous about the 
 position of this pole and about its movement that we de- 
 mand a knowledge of its whereabouts with an accuracy 
 which at first apears wholly unattainable. It sounds almost 
 incredible when we are told that a shift in the position of 
 the north pole to the extent of twenty yards, or even of 
 twenty feet, is appreciable, notwithstanding that we have 
 never been able to get nearer to it than from one end of 
 England to the other. Indeed, as a matter of fact, our 
 knowledge of the movements of the pole are derived from 
 observations made not alone hundreds but even many 
 thousands of miles distant. It is in such observatories as 
 those at Greenwich or Berlin, Pulkowa or Washington, 
 that the determinations have been made by which changes 
 in the position of the pole can be ascertained with a deli- 
 cacy and precision for which those would hardly be pre- 
 pared who were not aware of the refinement of modern 
 astronomical methods. I do not, however, imply that the 
 observations conducting to the discoveries now about to 
 be considered have been exclusively obtained at the ob- 
 servatories I have named. There are a large number of 
 similar institutions over the globe which have been made 
 to bear their testimony. Tens of thousands of different 
 observations have been brought together, and by discuss- 
 ing them it has been found possible to remove a large part 
 of the errors by which such work is necessarily affected, 
 and to elicit from the vast mass those grains of truth which 
 could not have been discovered had it not been for the 
 
34 BALL 
 
 enormous amount of material that was available. Mr. 
 Chandler has discussed these matters in a remarkable series 
 of papers, and it will be necessary for me now to enter into 
 some little detail, both as regards the kind of observations 
 that have been made, and the results to which astronomers 
 have been thereby conducted. 
 
 Greenwich Observatory lies more than two thousand 
 miles from the north pole, and yet if the pole were to shift 
 by as much as the width of Regent Street, the fact that it 
 had done so would be quite perceptible at Greenwich. Let 
 me endeavour to explain how such a measurement could 
 be achieved. In finding the latitude at any locality we 
 desire, of course, to know the distance between the locality 
 and the equator, expressed in angular magnitude. But 
 though this is distinctly the definition of latitude, it does 
 not at once convey the idea as to how this element can be 
 ascertained. How, for instance, would an astronomer at 
 Greenwich be able to learn the angular distance of the 
 observatory from the equator? The equator is not marked 
 on the sky, and it is obvious that the observer must employ 
 a somewhat indirect process to ascertain what he wants. 
 Here, again, we have to invoke the aid of that celestial 
 pole to which I have so often referred. Think of that point 
 on the sky which is the common centre of the circles ex- 
 hibited on Professor Barnard's photograph. That point 
 is not indeed marked by any special star, but it is com- 
 pletely defined by the circumstance that it is the centre of 
 the track performed by the circumpolar stars. We thus 
 obtain a clear idea of this definite point in the sky, and 
 the horizon is a perfectly definite line, at all events from 
 any station where the sea is visible. It is not difficult to 
 imagine that by suitable measurements we can ascertain 
 the altitude of this point in the heavens above the horizon. 
 That altitude is the latitude of the place; it is, in fact, the 
 very angle which lies between the locality on the earth and 
 the equator. It is quite true that as the pole is implied by 
 these circles rather than directly marked by them, the meas- 
 urement of the altitude can not be effected quite directly. 
 
WANDERINGS OF THE NORTH POLE 35 
 
 The actual process is to take the Pole Star, or some one of 
 the other circumpolar stars, and to measure the greatest 
 height to which it ascends above the horizon and the lowest 
 altitude to which it declines about twelve hours later. The 
 former of these is as much above the pole as the latter is 
 below it, so between them we are able to ascertain the 
 altitude of the pole with a high degree of accuracy. It is 
 true that in a fixed observatory such as Greenwich there is 
 no visible sea horizon, and even if there were it would not 
 provide so excellent a method as is offered by the equiva- 
 lent process of first observing the star directly and then 
 observing its reflection from a dish of mercury. In this 
 way the altitude of the star above the horizon is determined 
 with the utmost precision. The practical astronomer will, 
 however, remember that, of course, he has to attend to the 
 effects of atmospheric refraction, which invariably shows a 
 star higher up than it ought to be. This can be allowed 
 for, and in this way the latitude of the observatory is ascer- 
 tained with all needful accuracy. When the highest degree 
 of precision is sought for, and it is only observations with 
 a very high degree of precision which are available for our 
 present purpose, a considerable number of stars have to be 
 employed, and very many observations have to be taken 
 at different seasons of the year, so as to eliminate as far as 
 possible all sources of casual error. When, however, due 
 attention has been paid to those precautions which the 
 experience of astronomers suggests, the result that is ob- 
 tained is characterized by extraordinary precision. How 
 great that precision may be I must endeavour to ex- 
 plain. The latitude of every important observatory is 
 obtained from a large number of observations, and it 
 would be unlikely that it was more than one or two 
 tenths of a second different from the actual mean value. 
 Now, a tenth of a second on the surface of the earth cor- 
 responds to a distance of about ten feet, and this means 
 that the latitude of the observatory or, as we must now 
 speak very precisely, the latitude of the centre of the 
 meridian circle in the observatory, is known to a degree 
 
3 6 BALL 
 
 of precision represented by a few paces. It will thus be 
 seen that, with the accuracy attainable in our modern ob- 
 servations, it would often be an appreciable blunder to mis- 
 take the latitude of one wall of the observatory for that 
 of the opposite wall; in other words, we know accurately to 
 within the tenth of a second, or within not much more 
 than the tenth of a second, the distance from the centre of 
 the transit circle at Greenwich down to the earth's equator. 
 But, of course, the distance from the pole to the equator is 
 90, and this being so, it follows that the distance from the 
 north pole of the earth to the centre of the transit circle 
 at Greenwich Observatory has been accurately ascertained 
 to within one or two tenths of a second. If any change 
 took place in the distance between the pole and the 
 meridian circle at Greenwich, then it must be manifested 
 by the changes of latitude. We shall now be able to under- 
 stand how any movement of the pole, or rather of the posi- 
 tion which it occupies in the earth, would be indicated at 
 Greenwich. Suppose, for instance, that the pole actually 
 advanced toward Great Britain, and that it moved to a dis- 
 tance of, let us say, thirty feet, the effect of this would be 
 to produce a diminution of the distance between the pole 
 and Greenwich that is to say, there must be an increase 
 in the distance from Greenwich to the equator. This would 
 correspond to a change in the latitude of Greenwich; that 
 latitude would diminish by three tenths of a second, which 
 is a magnitude quite large enough to be recognisable by 
 the observations I have already indicated as proper for 
 the determination of latitude. A shift of the pole to a 
 distance of sixty feet would be a conspicuous alteration 
 announced in every observatory in Europe provided with 
 instruments of good modern construction. 
 
 Until the last few years there was not much reason to 
 think that the pole exhibited any unequivocal indications 
 of movement. No doubt, displacements resembling those 
 which have now been definitely ascertained have existed 
 for many years, but they were too small to produce any 
 appreciable effect, except with instruments of a more re- 
 
WANDERINGS OF THE NORTH POLE 37 
 
 fined description than those with which the earlier observa- 
 tories were equipped. It was obvious that the pole did 
 not make movements of anything like a hundred yards in 
 extent; had it done so the resulting variations in latitude 
 would have been conspicuous enough to have obtained 
 notice many years ago. The actual movements which the 
 pole does make are of that small character which require 
 very minute discussion of the observations to establish 
 them beyond reach of cavil. There is, however, one striking 
 method of confirming such observations as have been made 
 which leaves no doubt of the accuracy of the results to 
 which they point. Suppose, for instance, that the great ob- 
 servatories in Europe indicate at a certain time that their 
 latitudes have all increased; this necessarily implies that the 
 equator has receded from them, and that, therefore, the 
 north pole has approached Europe. If, however, the north 
 pole has approached Europe it must have retreated from 
 those regions on the opposite side of the world say, for 
 instance, the Sandwich Islands. Observations in the Sand- 
 wich Islands should, therefore, indicate, if our reasoning 
 has been correct, that the pole has retreated from them, 
 and that the equator has, therefore, advanced in such a 
 way that the latitudes of localities in the Sandwich Islands 
 have diminished. The various observations which have 
 been brought together by the diligence of Mr. Chandler, 
 including those which he has himself made with an in- 
 genious apparatus of his own design, have been submitted 
 to this test, and they have borne it well. The result has 
 been that it is now possible to follow the movements of 
 the pole with a considerable degree of completeness. Pro- 
 fessor Chandler has tracked the pole month after month, 
 year after year, through a period of more than a century 
 of exact observations, and he has succeeded in determining 
 the movements which this point undergoes. Let me here 
 endeavour to describe the result at which he has arrived. 
 
 In that palaeocry stic ocean which arctic travellers have 
 described, where the masses of ice lie heaped together in 
 the wildest confusion, lies this point which is the object 
 
3 8 BALL 
 
 of so much speculation. Let us think of this tract, or a 
 portion of it, to be levelled to a plain, and at a particular 
 centre let a circle be drawn the radius of which is about 
 thirty feet; it is in the circumference of this circle that the 
 pole of the earth is constantly to be found. In fact, if at 
 different times, month after month and year after year, the 
 position of the pole was ascertained as the extremity of 
 that tube from which an eye placed at the centre of the 
 earth would be able to see the pole of the heavens, and if 
 the successive positions of this pole were marked by pegs 
 driven into the ground, then the several positions in which 
 the pole would be found must necessarily trace out the 
 circumference of the circle that has been thus described. 
 The period in which each revolution of the pole around 
 the circle takes place is about four hundred and twenty- 
 seven days; the result, therefore, of these investigations 
 shows, when the observations are accurate, that the north 
 pole of the earth is not, as has been so long supposed, a 
 fixed point, but that it revolves around in the earth, ac- 
 complishing each revolution in about two months more 
 than the period that the earth requires for the performance 
 of each revolution around the sun. 
 
 The discovery of the movement of the pole which I 
 have here described must be regarded as a noteworthy 
 achievement in astronomy, nor is the result to which it 
 leads solely of interest in consequence of the lesson it 
 teaches us with regard to the circumstances of the earth's 
 rotation. It has a higher utility, which the practical 
 astronomer will not be slow to appreciate, and of which he 
 has, indeed, already experienced the benefit. There are 
 several astronomical investigations in which the latitude 
 of the observatory enters as a significant element. Lati- 
 tude is, in fact, at every moment employed as an important 
 factor in many astronomical determinations: to take one 
 of the most simple cases, suppose that we are finding the 
 place of a planet in the observatory, we deduce its posi- 
 tion by measuring its zenith distance, and then to obtain 
 the declination the latitude of the observatory has, of 
 
WANDERINGS OF THE NORTH POLE 39 
 
 course, to be considered. Now, astronomers have hitherto 
 been in the habit of accepting the determination of their 
 latitude which had been established by a protracted series 
 of observations, and treating it as if it were 'a constant. 
 This method will be no longer admissible when astro- 
 nomical work of the highest class is demanded. No doubt, 
 from the sailor's point of view, an alteration in latitude 
 which at most amounts to a shift of sixty feet, not a quarter, 
 perhaps, of the length of his vessel, is immaterial. But in 
 the more refined parts of astronomical work these discov- 
 eries can no longer be overlooked; indeed, Mr. Chandler 
 has shown that many discrepancies by which astronomers 
 had been baffled can be removed when note is taken of 
 the circumstance that the latitude of the observatory is in 
 an incessant condition of transformation in accordance with 
 the law which his labours have expounded. It will ere 
 long be necessary in every observatory where important 
 work is being done to obtain for every day the correction 
 to the mean value of the latitude, in order to obtain the 
 value appropriate for that day. 
 
 There are also other grounds of a somewhat profounder 
 character on which the discoveries now made are eminently 
 instructive. Those who are interested in the physics of our 
 globe often discuss the question as to whether the internal 
 heat, which the earth certainly possesses, is sufficiently in- 
 tense to render the deep-seated portions of our globe more 
 or less fluid. On the other hand, the effects of pressure, 
 especially of such pressures as are experienced in the 
 depths hundreds and thousands of miles below the surface, 
 must go far to consolidate the materials to form what must 
 be sensibly a rigid body. The question, therefore, arises, 
 Is the earth to be regarded as a rigid mass, or is it not? 
 The phenomena of the tides had already to some extent 
 afforded information on this subject, and now Mr. Chan- 
 dler's investigation adds much further light, for it is certain 
 from his result that the earth can not be a rigid body. It 
 is quite true that, even though the earth were rigid, the 
 pole might go round in a circle, and that circle might have 
 
40 BALL 
 
 a thirty-feet radius, but in such a case the period would be 
 only about three quarters of the four hundred and twenty- 
 seven days which he has found. In the interest, therefore, 
 of the theoretical astronomer, as well as on the other 
 grounds which I have set forth, Mr. Chandler's investiga- 
 tions must be regarded as a most important contribution 
 to modern astronomy. 
 
THE AGE OF THE SUN'S HEAT 
 
 BY 
 
 SIR WILLIAM THOMSON (LORD KELVIN) 
 
THE AGE OF THE SUN'S HEAT 1 
 
 THE second great law of thermodynamics involves a 
 certain principle of irreversible action in Nature. It 
 is thus shown that, although mechanical energy is 
 indestructible, there is a universal tendency to its dissipa- 
 tion, which produces gradual augmentation and diffusion 
 of heat, cessation of motion, and exhaustion of potential 
 energy through the material universe. 2 The result would 
 inevitably be a state of universal rest and death, if the uni- 
 verse were finite and left to obey existing laws. But it is 
 impossible to conceive a limit to the extent of matter in 
 the universe; and therefore science points rather to an 
 endless progress, through an endless space, of action in- 
 volving the transformation of potential energy into pal- 
 pable motion and thence into heat, than to a single finite 
 mechanism, running down like a clock, and stopping for- 
 ever. It is also impossible to conceive either the begin- 
 ning or the continuance of life, without an overruling 
 creative power; and, therefore, no conclusions of dynam- 
 ical science regarding the future condition of the earth can 
 be held to give dispiriting views as to the destiny of the 
 race of intelligent beings by which it is at present inhabited. 
 The object proposed in the present article is an ap- 
 plication of these general principles to the discovery of 
 probable limits to the periods of time, past and future, 
 during which the sun can be reckoned on as a source of 
 
 1 From " Macmillan's Magazine," March, 1862. 
 
 3 See " On a Universal Tendency in Nature to the Dissipation of 
 Mechanical Energy," " Proceedings of the Royal Society of Edin- 
 burgh," April 19, 1852; or the "Philosophical Magazine," October, 
 1852; also "Mathematical and Physical Papers," vol. i, art. 59. 
 
 43 
 
44 THOMSON 
 
 heat and light. The subject will be discussed under three 
 heads: 
 
 I. The secular cooling of the sun. 
 
 II. The present temperature of the sun. 
 
 III. The origin and total amount of the sun's heat. 
 
 I. THE SECULAR COOLING OF THE SUN. How much 
 the sun is actually cooled from year to year, if at all, we 
 have no means of ascertaining, or scarcely even of esti- 
 mating in the roughest manner. In the first place we do 
 not know that he is losing heat at all. For it is quite 
 certain that some heat is generated in his atmosphere by 
 the influx of meteoric matter; and it is possible that the 
 amount of heat so generated from year to year is sufficient 
 to compensate the loss by radiation. It is, however, also 
 possible that the sun is now an incandescent liquid mass, 
 radiating away heat, either primitively created in his sub- 
 stance, or, what seems far more probable, generated by the 
 falling in of meteors in past times, with no sensible com- 
 pensation by a continuance of meteoric action. 
 
 It has been shown 3 that, if the former supposition were 
 true, the meteors by which the sun's heat would have been 
 produced during the last 2,000 or 3,000 years must have 
 been all that time much within the earth's distance from the 
 sun, and must therefore have approached the central body 
 in very gradual spirals; because, if enough of matter to 
 produce the supposed thermal effect fell in from space out- 
 side the earth's orbit, the length of the year would have 
 been very sensibly shortened by the additions to the sun's 
 mass which must have been made. The quantity of mat- 
 ter annually falling in must, on that supposition, have 
 amounted to ^ of the earth's mass, or to irmjVuw of the 
 sun's; and therefore it would be necessary to suppose 
 the " zodiacal light " to amount to at least 7 -gVir of the 
 sun's mass, to account in the same way for a future supply 
 of 3,000 years' sun-heat. When these conclusions were 
 
 * " On the Mechanical Energies of the Solar System," " Transac- 
 tions of the Royal Society of Edinburgh," April, 1854, and " Philo- 
 sophical Magazine," December, 1854. 
 
THE AGE OF THE SUN'S HEAT 45 
 
 first published it was pointed out that " disturbances in the 
 motions of visible planets " should be looked for, as afford- 
 ing us means for estimating the possible amount of matter 
 in the zodiacal light; and it was conjectured that it could 
 not be nearly enough to give a supply of 30,000 years' heat 
 at the present rate. These anticipations have been to some 
 extent fulfilled in Le Verrier's great researches on the 
 motion of the planet Mercury, which have recently given 
 evidence of a sensible influence attributable to matter cir- 
 culating, as a great number of small planets, within his 
 orbit round the sun. But the amount of matter thus indi- 
 cated is very small; and, therefore, if the meteoric influx 
 taking place at present is enough to produce any appre- 
 ciable portion of the heat radiated away, it must be sup- 
 posed to come from matter circulating round the sun, 
 within very short distances of his surface. The density of 
 this meteoric cloud would have to be supposed so great 
 that comets could scarcely have escaped as comets actu- 
 ally have escaped, showing no discoverable effects of re- 
 sistance, after passing his surface within a distance equal 
 to j- of his radius. All things considered, there seems 
 little probability, in the hypothesis that solar radiation 
 is at present compensated, to any appreciable degree, by 
 heat generated by meteors falling in; and, as it can be 
 shown that no chemical theory is tenable, 4 it must be con- 
 cluded as most probable that the sun is at present merely 
 an incandescent liquid mass cooling. 
 
 How much he cools from year to year becomes there- 
 fore a question of very serious import, but it is one which 
 we are at present quite unable to answer. It is true we 
 have data on which we might plausibly found a probable 
 estimate, and from which we might deduce, with at first 
 sight seemingly well-founded confidence, limits, not very 
 wide, within which the present true rate of the sun's cool- 
 ing must lie. For we know, from the independent but con- 
 cordant investigations of Herschel and Pouillet, that the 
 sun radiates every year from his whole surface about 
 4 " Mechanical Energies of the Solar System." 
 
46 THOMSON 
 
 6 x io 30 (six million million million million million) times 
 as much heat as is sufficient to raise the temperature of one 
 pound of water by i C. We also have excellent reason 
 for believing that the sun's substance is very much like the 
 earth's. Stokes's principles of solar and stellar chemistry 
 have been for many years explained in the University of 
 Glasgow, and it has been taught as a first result that sodium 
 does certainly exist in the sun's atmosphere, and in the 
 atmospheres of many of the stars, but that it is not discov- 
 erable in others. The recent application of these principles 
 in the splendid researches of Bunsen and Kirchhof (who 
 made an independent discovery of Stokes's theory) has 
 demonstrated with equal certainty that there are iron and 
 manganese, and several of our other known metals, in the 
 sun. The specific heat of each of these substances is less 
 than the specific heat of water, which indeed exceeds that 
 of every other known terrestrial body, solid or liquid. It 
 might, therefore, at first sight seem probable that the mean 
 specific heat 5 of the sun's whole substance is less, and very 
 certain that it can not be much greater, than that of water. 
 If it were equal to the specific heat of water we should 
 only have to divide the preceding number (6 x io 30 ), de- 
 rived from Herschel's and Pouillet's observations, by the 
 number of pounds (4.3 x io 30 ) in the sun's mass, to find 
 1.4 C. for the present annual rate of cooling. It might 
 therefore seem probable that the sun cools more, and 
 almost certain that he does not cool less, than a centigrade 
 degree and four tenths annually. But, if this estimate were 
 well founded, it would be equally just to assume that the 
 
 6 The " specific heat " of a homogeneous body is the quantity of heat 
 that a unit of its substance must acquire or must part with, to rise or 
 to fall by i in temperature. The main specific heat of a heterogeneous 
 mass, or of a mass of homogeneous substance, under different pres- 
 sures in different parts, is the quantity of heat which the whole body 
 takes or gives in rising or in falling i in temperature, divided by the 
 number of units in its mass. The expression, " mean specific heat " of 
 the sun, in the text, signifies the total amount of heat actually radiated 
 away from the sun, divided by his mass, during any time in which the 
 average temperature of his mass sinks by i, whatever physical or 
 chemical changes any part of his substance may experience. 
 
THE AGE OF THE SUN'S HEAT 47 
 
 sun's expansibility 6 with heat does not differ greatly from 
 that of some average terrestrial body. If, for instance, it 
 were the same as that of solid glass, which is about TirSinj 
 on bulk, or oihnnr on diameter, per i C. (and for most 
 terrestrial liquids, especially at high temperatures, the ex- 
 pansibility is much more), and if the specific heat were the 
 same as that of liquid water, there would be in 860 years 
 a contraction of one per cent on the sun's diameter, which 
 Could scarcely have escaped detection by astronomical ob- 
 servation. There is, however, a far stronger reason than 
 this for believing that no such amount of contraction could 
 have taken place, and therefore for suspecting that the 
 physical circumstances of the sun's mass render the con- 
 dition of the substances of which it is composed, as to ex- 
 pansibility and specific heat, very different from that of the 
 same substances when experimented on in our terrestrial 
 laboratories. Mutual gravitation between the different 
 parts of the sun's contracting mass must do an amount of 
 work, which can not be calculated with certainty, only be- 
 cause the law of the sun's interior density is not known. 
 The amount of work performed on a contraction of one 
 tenth per cent of the diameter, if the density remained uni- 
 form throughout the interior, would, as Helmholtz showed, 
 be equal to 20,000 times the mechanical equivalent of the 
 amount of heat which Pouillet estimated to be radiated 
 from the sun in a year. But in reality the sun's density 
 must increase very much toward his centre, and probably 
 in varying proportions, as the temperature becomes lower 
 and the whole mass contracts. We can not, therefore, say 
 whether the work actually done by mutual gravitation dur- 
 ing a contraction of one tenth per cent of the diameter 
 
 ' The " expansibility in volume," or the " cubical expansibility," of 
 a body, is an expression technically used to denote the proportion 
 which the increase or diminution of its bulk, accompanying a rise or 
 fall of i in its temperature, bears to its whole bulk at some stated 
 temperature. The expression, " the sun's expansibility," used in the 
 text, may be taken as signifying the ratio which the actual contrac- 
 tion, during a lowering of his mean temperature by i C., bears to 
 his present volume. 
 
4 8 THOMSON 
 
 would be more or less than the equivalent of 20,000 years' 
 heat; but we may regard it as most probably not many 
 times more or less than this amount. Now, it is in the 
 highest degree improbable that mechanical energy can in 
 any case increase in a body contracting in virtue of cool- 
 ing. It is certain that it really does diminish very notably 
 in every case hitherto experimented on. It must be sup- 
 posed, therefore, that the sun always radiates away in heat 
 something more than the Joule-equivalent of the work 
 done on his contracting mass, by mutual gravitation of 
 its parts. Hence, in contracting by one tenth per cent in 
 his diameter, or three tenths per cent in his bulk, the 
 sun must give out something either more, or not greatly 
 less, than 20,000 years' heat; and thus, even without 
 historical evidence as to the constancy of his diameter, it 
 seems safe to conclude that no such contraction as that 
 calculated above (one per cent in 860 years) can have 
 taken place in reality. It seems, on the contrary, probable 
 that, at the present rate of radiation, a contraction of one 
 tenth per cent in the sun's diameter could not take place 
 in much less than 20,000 years, and scarcely possible that 
 it could take place in less than 8,600 years. If, then, the 
 mean specific heat of the sun's mass, in its actual con- 
 dition, is not more than ten times that of water, the ex- 
 pansibility in volume must be less than ^Vir P er IOO C. 
 (that is to say, less than & of that of solid glass), which 
 seems improbable. But although from this consideration 
 we are led to regard it as possible that the sun's specific 
 heat is considerably more than ten times that of water (and, 
 therefore, that his mass cools considerably less than 100 C. 
 in 700 years, a conclusion which, indeed, we could scarcely 
 avoid on simply geological grounds), the physical prin- 
 ciples we now rest on fail to give us any reason for sup- 
 posing that the sun's specific heat is more than 10,000 
 times that of water, because we can not say that his ex- 
 pansibility in volume is probably more than ^J-g- per i C. 
 And there is, on other grounds, very strong reason for 
 believing that the specific heat is really much less than 
 
THE AGE OF THE SUN'S HEAT 49 
 
 10,000. For it is almost certain that the sun's mean tem- 
 perature is even now as high as 14,000 C; and the great- 
 est quantity of heat that we can explain, with any proba- 
 bility, to have been by natural causes ever acquired by the 
 sun (as we shall see in the third part of this article), could 
 not have raised his mass at any time to this temperature, 
 unless his specific heat were less than 10,000 times that of 
 water. 
 
 We may therefore consider it as rendered highly 
 probable that the sun's specific heat is more than ten times, 
 and less than 10,000 times, that of liquid water. From 
 this it would follow with certainty that his temperature 
 sinks 1 00 C. in some time from 700 years to 700,000 
 years. 
 
 What, then, are we to think of such geological esti- 
 mates as 300,000,000 years for the " denudation of the 
 Weald"? Whether is it more probable that the physical 
 conditions of the sun's matter differ 1,000 times more than 
 dynamics compel us to suppose they differ from those of 
 matter in our laboratories; or that a stormy sea, with pos- 
 sibly Channel tides of extreme violence, should encroach 
 on a chalk cliff 1,000 times more rapidly than Mr. Dar- 
 win's estimate of one inch per century? 
 
 II. THE PRESENT TEMPERATURE OF THE SUN. At his 
 surface the sun's temperature can not, as we have many 
 reasons for believing, be incomparably higher than tem- 
 peratures attainable artificially in our terrestrial labora- 
 tories. 
 
 Among other reasons it may be mentioned that the 
 sun radiates out heat from every square foot of his surface 
 at only about 7,000 horse power. 7 Coal, burning at a rate 
 of a little less than a pound per two seconds, would gen- 
 erate the same amount; and it is estimated (Rankine, 
 
 7 One horse power in mechanics is a technical expression (follow- 
 ing Watt's estimate) used to denote a rate of working in which energy 
 is involved at the rate of 33,000 foot pounds per minute. This, according 
 to Joule's determination of the dynamical value of heat, would, if spent 
 wholly in heat, be sufficient to raise the temperature of 23! pounds of 
 water by i C. per minute. 
 
 4 
 
50 THOMSON 
 
 " Prime Movers," p. 285, edition 1852) that, in the fur- 
 naces of locomotive engines, coal burns at from one pound 
 in thirty seconds to one pound in ninety seconds per 
 square foot of grate-bars. Hence heat is radiated from the 
 sun at a rate not more than from fifteen to forty-five times 
 as high as that at which heat is generated on the grate- 
 bars of a locomotive furnace, per equal areas. 
 
 The interior temperature of the sun is probably far 
 higher than that at his surface, because direct conduction 
 can play no sensible part in the transference of heat be- 
 tween the inner and outer portions of his mass, and there 
 must be an approximate convective equilibrium of heat 
 throughout the whole, if the whole is fluid. That is to say, 
 the temperatures, at different distances from the centre, 
 must be approximately those which any portion of the 
 substance, if carried from the centre to the surface, would 
 acquire by expansion without loss or gain of heat. 
 
 III. THE ORIGIN AND TOTAL AMOUNT OF THE SUN'S 
 HEAT. The sun being, for reasons referred to above, 
 assumed to be an incandescent liquid now losing heat, the 
 question naturally occurs, How did this heat originate? It 
 is certain that it can not have existed in the sun through 
 an infinity of past time, since, as long as it has so existed, 
 it must have been suffering dissipation, and the finiteness 
 of the sun precludes the supposition of an infinite primitive 
 store of heat in his body. 
 
 The sun must, therefore, either have been created as 
 an active source of heat at some time of not immeasurable 
 antiquity, by an overruling decree; or the heat which he 
 has already radiated away, and that which he still possesses, 
 must have been acquired by a natural process, following 
 permanently established laws. Without pronouncing the 
 former supposition to be essentially incredible, we may 
 safely say that it is in the highest degree improbable, if 
 we can show the latter to be not contradictory to known 
 physical laws. And we do show this and more, by merely 
 pointing to certain actions going on before us at present, 
 which, if sufficiently abundant at some past time, must haye 
 
THE AGE OF THE SUN'S HEAT 51 
 
 given the sun heat enough to account for all we know of 
 his past radiation and present temperature. 
 
 It is not necessary at present to enter at length on de- 
 tails regarding the meteoric theory, which appears to have 
 been first proposed in a definite form by Mayer, and after- 
 ward independently by Waterston; or regarding the modi- 
 fied hypothesis of meteoric vortices, which the writer of 
 the present article showed to be necessary, in order that 
 the length of the year, as known for the last 2,000 years, 
 may not have been sensibly disturbed by the accessions 
 which the sun's mass must have had during that period, if 
 the heat radiated away has been always compensated by 
 heat generated by meteoric influx. 
 
 For reasons mentioned in the first part of the present 
 article, we may now believe that all theories of complete, or 
 nearly complete, contemporaneous meteoric compensation 
 must be rejected; but we may still hold that "meteoric 
 action ... is ... not only proved to exist as a cause 
 of solar heat, but it is the only one of all conceivable causes 
 which we know to exist from independent evidence." 8 
 
 The form of meteoric theory which now seems most 
 probable, and which was first discussed on true thermo- 
 dynamic principles by Helmholtz, 9 consists in supposing 
 the sun and his heat to have originated in a coalition of 
 smaller bodies, falling together by mutual gravitation, and 
 generating, as they must do according to the great law 
 demonstrated by Joule, an exact equivalent of heat for the 
 motion lost in collision. 
 
 That some form of the meteoric theory is certainly the 
 true and complete explanation of solar heat can scarcely 
 be doubted, when the following reasons are considered: 
 
 1. No other natural explanation, except by chemical 
 action, can be conceived. 
 
 2. The chemical theory is quite insufficient, because the 
 most energetic chemical action we know, taking place be- 
 
 " Mechanical Energies of the Solar System." Note, p. 351. 
 * Popular lecture delivered on February 7, 1854, at Konigsberg, on 
 the occasion of the Kant commemoration. 
 
5 2 THOMSON 
 
 tween substances amounting to the whole sun's mass, 
 would only generate about 3,000 years' heat. 10 
 
 3. There is no difficulty in accounting for 20,000,000 
 years' heat by the meteoric theory. 
 
 It would extend this article to too great a length, and 
 would require something of mathematical calculation, to 
 explain fully the principles on which this last estimate is 
 founded. It is enough to say that bodies, all much smaller 
 than the sun, falling together from a state of relative rest, 
 at mutual distances all large in comparison with their di- 
 ameters, and forming a globe of uniform density equal in 
 mass and diameter to the sun, would generate an amount 
 of heat which, accurately calculated according to Joule's 
 principles and experimental results, is found to be just 
 20,000,000 times Pouillet's estimate of the annual amount 
 of solar radiation. The sun's density must, in all proba- 
 bility, increase very much toward his centre, and therefore 
 a considerably greater amount of heat than that must be 
 supposed to have been generated if his whole mass was 
 formed by the coalition of comparatively small bodies. On 
 the other hand, we do not know how much heat may have 
 been dissipated by resistance and minor impacts before the 
 final conglomeration; but there is reason to believe that 
 even the most rapid conglomeration that we can conceive 
 to have probably taken place could only leave the finished 
 globe with about half the entire heat due to the amount of 
 potential energy of mutual gravitation exhausted. We 
 may, therefore, accept, as a lowest estimate for the sun's 
 initial heat, 10,000,000 times a year's supply at the present 
 rate, but 50,000,000 or 100,000,000 as possible, in conse- 
 quence of the sun's greater density in his central parts. 
 
 The considerations adduced above, in this paper, re- 
 garding the sun's possible specific heat, rate of cooling, and 
 superficial temperature, render it probable that he must 
 have been very sensibly warmer 1,000,000 years ago than 
 now; and, consequently, if he has existed as a luminary for 
 10,000,000 or 20,000,000 years, he must have radiated away 
 
 10 " Mechanical Energies of the Solar System." Note, p. 351. 
 
THE AGE OF THE SUN'S HEAT 53 
 
 considerably more than the corresponding number of times 
 the present yearly amount of loss. 
 
 It seems, therefore, on the whole most probable that the 
 sun has not illuminated the earth for 100,000,000 years, and 
 almost certain that he has not done so for 500,000,000 
 years. As for the future, we may say, with equal certainty, 
 that inhabitants of the earth can not continue to enjoy the 
 light and heat essential to their life for many million years 
 longer unless sources now unknown to us are prepared 
 in the great storehouse of creation. 
 
THE PAST AND FUTURE OF 
 OUR EARTH 
 
 AND 
 
 A NEW THEORY OF LIFE IN 
 OTHER WORLDS 
 
 BY 
 
 RICHARD ANTHONY PROCTOR 
 (Born 1837 ; died 1888) 
 

THE PAST AND FUTURE OF OUR EARTH 1 
 
 " Ut his exordia primis 
 
 Omnia, et ipse tener Mundi concreverit orbis. 
 Turn durare solum, et discludere Nerea ponto 
 Coeperit, et rerum paullatim sumere formas." 
 
 VIRGIL. 
 
 THE subject with which I am about to deal 2 is asso- 
 ciated by many with questions of religion. Let me 
 premise, however, that I do not thus view it myself. 
 It seems to me impossible to obtain from science any clear 
 ideas respecting the ways or nature of the Deity, or even 
 respecting the reality of an Almighty personal God. Sci- 
 ence deals with the finite, though it may carry our thoughts 
 to the infinite. Infinity of space and of matter occupying 
 space, of time and of the processes with which time is occu- 
 pied, and infinity of energy as necessarilv implied by the 
 infinities of matter and of the operations affecting matter 
 these infinities science brings clearly before us. For sci- 
 ence directs our thoughts to the finites to which these in- 
 finites correspond. It shows us that there can be no con- 
 ceivable limits to space or time, and though finiteness of 
 matter or of operation may be conceivable, there is mani- 
 fest incongruity in assuming an infinite disproportion be- 
 tween unoccupied and occupied space, or between void 
 
 1 From " Our Place among Infinities," D. Appleton and Company. 
 
 * This essay presents the substance of a lecture delivered in New 
 York on April 3, 1874, being the first of a subsidiary series in which, 
 of set purpose (and in accordance with the request of several esteemed 
 friends), I dealt less with the direct teachings of astronomy, which 
 had occupied me in a former series, than with ideas suggested by 
 astronomical facts, and more particularly by the discoveries made dur- 
 ing the last quarter of a century. 
 
 57 
 
58 PROCTOR 
 
 time and time occupied with the occurrence of events of 
 what sort soever. So that the teachings of science bring 
 us into the presence of the unquestionable infinities of 
 time and of space, and the presumable infinities of mat- 
 ter and of operation hence, therefore, into the presence 
 of infinity of energy. But science teaches us nothing about 
 these infinities, as such. They remain none the less incon- 
 ceivable, however clearly we may be taught to recognise 
 their reality. Moreover, these infinites, including the in- 
 finity of energy, are material infinities. Science tells us 
 nothing of the infinite attributes of an Almighty Being; it 
 presents to us no personal infinites, whether of Power, 
 Beneficence, or Wisdom. Science may suggest some ideas 
 on these points, though we perceive daily more and more 
 clearly that it is unsafe to accept as her teaching ideas 
 which commonly derive their colouring from our own pre- 
 possessions. And assuredly, as respects actual facts, Sci- 
 ence in so far as she presents personal infinity to us at all, 
 presents it as an inconceivable, like those other inconceiv- 
 able infinities, with the finites corresponding to which her 
 operations are alone directly concerned. To speak in plain 
 terms so far as science is concerned, the idea of a per- 
 sonal God is inconceivable, 3 as are all the attributes which 
 religion recognises in such a Being. On the other hand, 
 it should be admitted as distinctly, that Science no more 
 disproves the existence of infinite personal power or wis- 
 dom than she disproves the existence of infinite material 
 
 * I mean these words to be understood literally. To the man of 
 science, observing the operation of second causes in every process with 
 which his researches deal, and finding no limit to the operation of 
 such causes however far back he may trace the chain of causation, the 
 idea of a first cause is as inconceivable in its relation to observed sci- 
 entific facts as is the idea of infinite space in its relation to the finite 
 space to which the observations of science extend. Yet infinite space 
 must be admitted; nor do I see how even that man of science who 
 would limit his thoughts most rigidly to facts, can admit that all things 
 are of which he thinks, without having impressed upon him the feeling 
 that, in some way he can not understand, these things represent the 
 operation of Infinite Purpose. Assuredly we do not avoid the incon- 
 ceivable by assuming as at least possible that matter exists only as it 
 affects our perceptions. 
 
THE PAST AND FUTURE OF OUR EARTH 59 
 
 energy (which on the contrary must be regarded as prob- 
 able) or the existence of infinite space or time (which must 
 be regarded as certain). 
 
 So much premised, we may proceed to inquire into the 
 probable past and future of our earth, as calmly as we 
 should inquire into the probable past and future of a 
 pebble, a weed, or an insect, of a rock, a tree, or an animal, 
 of a continent, or of a type whether of vegetable or of 
 animal life. The beginning of all things is not to be 
 reached, not appreciably to be even approached, by a few 
 steps backward in imagination, nor the end of all things by 
 a few steps forward. Such a thought is as unfounded as 
 was the fear of men in old times that by travelling too far 
 in any direction they might pass over the earth's edge and 
 be plunged into the abyss beyond, as unreasonable as was 
 the hope that by increase of telescopic range astronomers 
 could approach the imagined " heavens above the crys- 
 talline." 
 
 In considering the probable past history of the earth, 
 we are necessarily led to inquire into the origin of the 
 solar system. I have already sketched two theories of the 
 system, and described the general facts on which both 
 theories are based. The various planets circle in one direc- 
 tion around the sun, the sun rotating in the same direction, 
 the satellite families (with one noteworthy but by no means 
 inexplicable exception) travelling round their primaries in 
 the same direction, and all the planets whose rotation has 
 been determined still preserving the same direction of cir- 
 culation (so to speak). These relations seem to point, in 
 a manner there is no mistaking, to a process of evolution 
 by which those various parts of the solar system which 
 now form discrete masses were developed from a former 
 condition characterized by a certain unity as respects the 
 manner of its circulation. One theory of this process of 
 evolution, Laplace's, implies the contraction of the solar 
 system from a great rotating nebulous mass; according to 
 the other theory, the solar system, instead of contracting to 
 its present condition, was formed by a process of accre- 
 
60 PROCTOR 
 
 tion, due to the indrawing of great flights of meteoric and 
 cometic matter. 
 
 I need not here enter at length, for I have already done 
 so elsewhere, into the astronomical evidence in favour of 
 either theory; but it will be well to present briefly some 
 of the more striking facts. 
 
 Among the various forms of nebulae (or star-cloudlets) 
 revealed by the telescope, we find many which seem to 
 accord with our ideas as to some of the stages through 
 which our solar system must have passed in changing from 
 the nebulous condition to its present form. The irregular 
 nebulae such, for instance, as that wonderful nebula in the 
 Sword of Orion show by their enormous extension the 
 existence of sufficient quantities of gaseous matter to form 
 systems as large and as massive as our own, or even far 
 vaster. We know from the teachings of the spectroscope 
 that these irregular nebulae do really consist of glowing 
 gas (as Sir W. Herschel long since surmised), hydrogen 
 and nitrogen being presumably present, though the spec- 
 trum of neither gas appears in its complete form (one line 
 only of each spectrum being shown, instead of the sets of 
 lines usually given by these gases). An American physicist 
 has suggested that hydrogen and nitrogen exist in the gase- 
 ous nebulae in an elementary condition, these gases really be- 
 ing compound, and he suggests further that all our so-called 
 elements may have been derived from those elementary 
 forms of hydrogen and nitrogen. In the absence of any 
 evidence from observation or experiment, these ideas must 
 be regarded as merely speculative; and I think that we 
 arrive here at a point where speculation helps us as little 
 as it does in attempting to trace the evolution of living 
 creatures across the gap which separates the earliest forms 
 of life from the beginning itself of life upon the earth. 
 Since we can not hope to determine the real beginning of 
 the earth's history, we need not at present attempt to pass 
 back beyond the earliest stage of which we have any clear 
 information. 
 
 Passing from the irregular nebulae, in which we see 
 
THE XEBULA IN ORIOX. 
 Photogravure from a photograph taken at the Lick Observatory 
 
n, due to the c flights of meteoric and 
 cometic matter. 
 
 I need not ^h, for I have already done 
 
 so elsewher" d evidence in favour of 
 
 either t veil to present briefly some 
 of the n. 
 
 Am .ebulae (or star-cloudlets) 
 
 revt we find many \vhich seem to 
 
 eas as to some of the stages through 
 
 n changing from 
 
 The irregular 
 
 nebula in the 
 
 sion the 
 
 lorm 
 
 far 
 
 .^flrarwbma w'Tm- 
 
 and nitrogen being presumably present, though the spec- 
 
 trum of neither gas appears in its complete form (one line 
 
 cctrum being shown, instead of the sets of 
 
 An American physicist 
 
 r in the gase- 
 
 be- 
 
 !!ed 
 
 any 
 must 
 
 that we 
 arrh ; tt j e 
 
 as it do< ,f Hying 
 
 creature^ earliest forms 
 
 of life from ,>on the earth. 
 
 Since we ca mine the real beginning of 
 
 the earth's histc not at present attempt to pass 
 
 beyond the eu; ge of which we have any clear 
 
 mation. 
 
 Passing from the irregular nebulas, in which we see 
 

THE PAST AND FUTURE OF OUR EARTH 6l 
 
 chaotic masses of gaseous matter occupying millions of 
 millions of cubic miles and scattered as wildly through 
 space as clouds are scattered in a storm-swept air, we come 
 to various orders of nebulae in which we seem to find clear 
 evidence of a process of evolution. We see first the traces 
 of a central aggregation. This aggregation becomes more 
 and more clearly defined, until there is no possibility of mis- 
 taking its nature as a centre having power (by virtue of the 
 quantity of matter contained in it) to influence the motions 
 of the matter belonging to the rest of the nebula. Then, 
 still passing be it remembered from nebula to nebula, and 
 only inferring, not actually witnessing, the changes de- 
 scribed we see a subordinate aggregation, wherein, after 
 a while, the greater portion of the mass of the nebula out- 
 side the central aggregation becomes gathered, even as 
 Jupiter contains the greater portion of the mass of the solar 
 system outside the central sun. 4 Next we see a second sub- 
 ordinate aggregation, inferior to the first, but comprising, 
 if we judge from its appearance, by far the greater portion 
 of what remained after the first aggregation had been 
 formed even as Saturn's mass far exceeds the combined 
 mass of all the planets less than himself, and so comprises 
 far the greater portion of the solar system after account has 
 been taken of Jupiter and the sun. 5 And we may infer that 
 the other parts of nebulae contain smaller aggregations not 
 perceptible to us, out of which the smaller planets of the 
 developing system are hereafter to be formed. 
 
 Side views of some of these nebulae indicate a flatness 
 of figure agreeing well with the general tendency of the 
 members of the solar system toward the medial plane of 
 that system. For the solar system may be described as 
 flat, and if the nebulae I have been dealing with (the spiral 
 nebulae with aggregations) were globular we could not rec- 
 ognise in them the true analogues of our solar system in the 
 
 4 The mass of Jupiter exceeds, in the proportion of five to two, the 
 combined mass of all the remaining planets. 
 
 8 The mass of Saturn exceeds, in the proportion of nearly three to 
 one, the combined mass of all the planets smaller than himself. 
 
62 PROCTOR 
 
 earlier stages of its history. But the telescope reveals 
 nebulae manifestly corresponding in appearance to the 
 great whirlpool nebula of Lord Rosse, as it would appear 
 if it is a somewhat flattened spiral and could be viewed 
 nearly edgewise. 
 
 And here I may pause to note that, although, in thus 
 inferring progressive changes where in reality we have but 
 various forms of nebulae, I have been adopting an assump- 
 tion and one which no one can hope either to verify or to 
 disprove, yet it must be remembered that these nebulae by 
 their very figure indicate that they are not at rest. If they 
 consist of matter possessing the attribute of gravitation 
 and it would be infinitely more daring to assert that they 
 do not than that they do then they must be undergoing 
 processes of change. Nor can we conceive that discrete 
 gaseous masses in whorls spirally arranged around a great 
 central aggregation (taking one of the earlier stages) could 
 otherwise change than by aggregating toward their centre, 
 unless we admit motions of revolution (in orbits more or 
 less eccentric) the continuance of which would necessarily 
 lead, through collisions, to the rapid growth of the central 
 aggregation, and to the formation and slower growth of 
 subordinate gatherings. 
 
 I have shown elsewhere how the formation of our solar 
 system, in the manner supposed, would explain what La- 
 place admitted that he could not explain by his theory 
 the peculiar arrangement of the masses forming the solar 
 system. The laws of dynamics tell us that no matter what 
 the original configuration or motion of the masses, prob- 
 ably gaseous, forming the nebula, the motions of these 
 masses would have greater and greater velocity the nearer 
 the masses were to the central aggregation, each distance 
 indicating certain limits between which the velocities must 
 inevitably lie. For example, in our solar system, supposing 
 the central sun had already attained very nearly his full 
 growth as respects quantity of matter, then the velocity of 
 any mass whatever belonging to the system would at 
 Jupiter's distance be less than twelve miles per second, 
 
THE PAST AND FUTURE OF OUR EARTH 63 
 
 whereas at the distance of the earth, the largest planet 
 travelling inside the orbit of Jupiter, the limit of the velocity 
 would be more than twice as great. Hence we can see with 
 what comparative difficulty an aggregation would form close 
 to the central one, and how the first subordinate aggregation 
 would lie at a distance where the quantity of matter was 
 still great but the average velocity of motion not too great. 
 Such an aggregation once formed, the next important 
 aggregation would necessarily lie far outside, for within 
 the first there would now be two disturbing influences pre- 
 venting the rapid growth of these aggregations. The third 
 and fourth would be outside the second. Between the 
 first aggregation and the sun only small planets, like the 
 earth and Venus, Mars, Mercury, and the asteroids, could 
 form; and we should expect to find that the largest of the 
 four small planets would be in the middle of the space be- 
 longing to the family (as Venus and the earth are actually 
 placed), while the much smaller planets Mercury and Mars 
 travel next on either side, one close to the sun and the other 
 next to Jupiter, the asteroids indicating the region where 
 the combined disturbing influences of Jupiter and the sun 
 prevented any single planet from being developed. 
 
 But I should require much more time than is now at 
 my command to present adequately the reasoning on which 
 the theory of accretion is based. And we are not concerned 
 here to inquire whether this theory, or Laplace's theory of 
 contraction, or (which I hold to be altogether more prob- 
 able than either) a theory involving combined processes 
 of accretion and contraction, be the true hypothesis of the' 
 evolution of the solar system. Let it suffice that we recog- 
 nise as one of the earliest stages of our earth's history her 
 condition as a rotating mass of glowing vapour, capturing 
 then as now, but far more actively then than now, masses 
 of matter which approached near enough, and growing by 
 these continual indraughts from without. From the very 
 beginning, as it would seem, the earth grew in this way. 
 This firm earth on which we live represents an aggregation 
 of matter not from one portion of space, but from all space. 
 
64 PROCTOR 
 
 All that is upon and within the earth, all vegetable forms 
 and all animal forms, our bodies, our brains, are formed of 
 materials which have been drawn in from those depths of 
 space surrounding us on all sides. This hand that I am 
 now raising contains particles which have travelled hither 
 from regions far away amid the northern and southern con- 
 stellations, particles drawn in toward the earth by processes 
 continuing millions of millions of ages, until after multi- 
 tudinous changes the chapter of accidents has so combined 
 them, and so distributed them in plants and animals, that 
 after coming to form portions of my food they are here 
 present before you. Passing from the mere illustration of 
 the thought, is not the thought itself striking and sugges- 
 tive, that not only the earth on which we move, but every- 
 thing we see or touch, and every particle in body and brain, 
 has sped during countless ages through the immensity of 
 space? 
 
 The great mass of glowing gas which formed our earth 
 in the earliest stage of its history w r as undergoing two note- 
 worthy processes first, the process of cooling by which 
 the mass was eventually to become at least partially solid, 
 and secondly a process of growth due to the gathering in 
 of meteoric and cometic matter. As respects the latter 
 process, which will not hereafter occupy our attention, I 
 must remark that many astronomers appear to me to give 
 far less consideration to the inferences certainly deducible 
 from recent discoveries than the importance of these dis- 
 coveries would fairly warrant. It is now absolutely certain 
 that hour by hour, day by day, and year by year, the earth 
 is gathering in matter from without. On the most mod- 
 erate assumption as to the average weight of meteors and 
 shooting stars, the earth must increase each year in mass 
 by many thousands of tons. And when we consider the 
 enormous, one may almost say the awful, time-intervals 
 which have elapsed since the earth was in a gaseous condi- 
 tion, we can not but perceive that the process of accretion 
 now going on indicates the existence of only the merest 
 residue of matter (ungathered) compared with that which 
 
THE PAST AND FUTURE OF OUR EARTH 65 
 
 at the beginning of those time-intervals was freely moving 
 around the central aggregation. The process of accretion 
 which now does not sensibly increase the earth's mass was 
 then a process of actual growth. Jupiter and Saturn might 
 then no longer be gathering in matter appreciably increasing 
 their mass, although the quantity of matter gathered in by 
 them must have been far larger than all that the other form- 
 ing earth could gather in equal times. For those planets 
 were then as now so massive that any possible increment 
 from without was as nothing compared with the mass they 
 had already attained. We have to throw back into yet 
 more awful time-depths the birth and growth of those giant 
 orbs. And even those depths of time are as nothing com- 
 pared with the intervals which have elapsed since the sun 
 himself began to be. Yet it is with time-intervals measur- 
 able by hundreds of millions of years that we have to deal 
 in considering only our earth's history nay, two or three 
 hundred millions of years only carry us back to a period 
 when the earth was in a stage of development long sequent 
 to the gaseous condition we are now considering. That the 
 supply of meteoric and cometic matter not gathered in 
 was then enormously greater than that which still exists 
 within the solar domain, appears to me not a mere fanciful 
 speculation, nor even a theoretical consideration, but as 
 nearly a certainty as anything not admitting of mathe- 
 matical demonstration can possibly be. That the rate of 
 ingathering at that time enormously exceeded the present 
 rate may be regarded as certain. That the increase result- 
 ing from such ingathering during the hundreds of millions 
 of years that it has been in operation since the period when 
 the earth first existed as a gaseous mass, must have resulted 
 in adding a quantity of matter forming no inconsiderable 
 aliquot part of the earth's present mass, seems to me a 
 reasonable inference, although it is certain that the present 
 rate of growth continued even for hundreds of millions of 
 years would not appreciably affect the earth's mass. 6 And 
 
 c It is, perhaps, hardly necessary to explain that I refer here not to 
 absolute but to relative increase. The absolute increase of mass would 
 5 
 
66 PROCTOR 
 
 it is a thought worthy of consideration, in selecting be- 
 tween Laplace's theory of contraction and the theory of 
 accretion, that accretion being a process necessarily ex- 
 haustive, we are able to trace it back through stages of 
 gradually increasing activity without limit until we reach 
 that stage when the whole of the matter now forming our 
 solar system was as yet unformed. Contraction may 
 alternate with expansion, according to the changing con- 
 dition of a forming system; but accretion is a process 
 which can only act in one direction; and as accretion is cer- 
 tainly going on now, however slowly, we have but to trace 
 back the process to be led inevitably, in my judgment, to 
 regard our system as having its origin in processes of accre- 
 tion though it seems equally clear that each individual 
 orb of the system, if not each subordinate scheme within 
 .it, has also undergone a process of contraction from a for- 
 mer nebulous condition. 
 
 In this early gaseous stage our earth was preparing, as 
 it were, to become a sun. As yet her gaseous globe proba- 
 bly extended beyond the smaller aggregation out of which 
 the moon was one day to be formed. This may be inferred, 
 I think, from the law of the moon's rotation. It is true 
 that a moon independently created, and started on the 
 moon's present course, with a rotation-period nearly equal- 
 ling its period of revolution, would gradually have acquired 
 a rotation-period exactly equalling the mean period of 
 revolution. But there is no reason in Nature why there 
 should have been any such near approach; whereas, if we 
 suppose the moon's gaseous globe to have been originally 
 entangled within the outskirts of the earth's, we see that 
 the peculiar relation in question would have prevailed from 
 the beginning of the moon's existence as a separate body. 
 The laws of dynamics show us, moreover, that although 
 the conditions under which the moon moved and rotated 
 must have undergone considerable changes since her first 
 formation, yet that since those changes took place very 
 
 amount to many millions of tons, but the earth would not be increased 
 by the billionth part of her present mass. 
 
THE PAST AND FUTURE OF OUR EARTH 67 
 
 slowly, the rotation of the moon would be gradually modi- 
 fied, pari passu, so that the peculiar relation between the 
 moon's rotation and revolution would continue unim- 
 paired. 7 
 
 In her next stage, our earth is presented to us as a 
 sun. It may be that at that time the moon was the abode 
 of life, our earth affording the supplies of light and heat 
 necessary for the wants of creatures living on the moon. 
 But whether this were so or not, it may be safely assumed 
 that when the earth's contracting gaseous globe first began 
 to have liquid or solid matter in its constitution, the earth 
 must have been a sun so far as the emission of heat and 
 light were concerned. I must warn you, however, against 
 an undue regard for analogy which has led some astrono- 
 mers to say that all the members of the solar system have 
 passed or will pass through exactly similar stages. That 
 our earth once gave out light and heat, as the sun does 
 now, may be admitted as probable; and we may believe 
 that later the earth presented the characteristics which we 
 now recognise in Jupiter; while hereafter it may pass 
 through a stage comparable with that through which our 
 moon is now passing. But we must remember that -the 
 original quantity of matter in any orb passing through such 
 stages must very importantly modify the actual condition 
 of the orb in each of those stages, as well, of course, as the 
 duration of each stage; and it may even be that no two 
 orbs in the universe were ever in the same or very nearly 
 the same condition, and that no change undergone by one 
 has corresponded closely with any change undergone by 
 another. 
 
 We know so little respecting the sun's actual condition, 
 that even if we could be assured that in any past stages 
 of her history the earth was nearly in the same state, we 
 
 7 On the theory of evolution some such view of the origin of the 
 moon's rotation must be adopted, unless the matter be regarded as 
 the result of a strange chance. If we believe, on the contrary, that the 
 arrangement was specially ordained by the Creator, we are left to won- 
 der what useful purpose a relation so peculiar and so artificial can have 
 been intended to subserve. 
 
68 PROCTOR 
 
 should nevertheless remain in almost complete ignorance 
 as to the processes to which the earth's orb was at that time 
 subject. In particular we have no means of forming an 
 opinion as to the manner in which the elementary con- 
 stituents of the earth's globe were situated when she was 
 in the sunlike stage. We may adopt some general theory 
 of the sun's present condition; for example, we may 
 accept the ingenious reasoning by which Professor Charles 
 A. Young has supported his theory that the sun is a 
 gigantic bubble; 8 but we should be far from having any 
 exact idea of the processes actually taking place within 
 the solar globe, even if we were absolutely certain that that 
 or some other general theory were the true one. 
 
 Assuming that our earth, when in the sunlike stage, was 
 a gaseous mass within a liquid non-permanent shell, we 
 can see that as the process of cooling went on the showers 
 forming the shell would attain a greater and greater depth, 
 the shell thus becoming thicker, the space within the shell 
 
 8 " The eruptions which are all the time " (Anglice, " always ") 
 M occurring on the sun's surface," says Professor Young, " almost 
 compel the supposition that there is a crust of some kind which re- 
 strains the imprisoned gases, and through which they force their way 
 with great violence. This crust may consist of a more or less con- 
 tinuous sheet of rain not of water, of course, but of materials whose 
 vapours are shown by means of the spectroscope to exist in the solar 
 atmosphere, and whose condensations and combinations are supposed 
 to furnish the solar heat. The continuous outflow of the solar heat is 
 equivalent to the supply that would be developed by the condensation 
 from steam to vapour of a layer about five feet thick over the whole 
 surface of the sun per minute. As this tremendous rain descends, the 
 velocity of the falling drops would be increased by the resistance of the 
 dense gases underneath, the drops would increase until continuous 
 sheets would be formed; and the sheets would unite and form a sort 
 of bottomless ocean, resting upon the compressed vapours beneath and 
 pierced by innumerable ascending jets and bubbles. It would have 
 nearly a constant depth in thickness, because it would re-evaporate at 
 the bottom nearly as fast as it would grow by the descending rains 
 above, though probably the thickness of this sheet would continually 
 increase at some slow rate, and its whole diameter diminish. In other 
 words, the sun, according to this view, is a gigantic bubble, whose 
 walls are gradually thickening and its diameter diminishing at a rate 
 determined by its loss of heat. It differs, however, from ordinary 
 bubbles in the fact that its skin is constantly penetrated by blasts and 
 jets from within." 
 
THE PAST AND FUTURE OF OUR EARTH 69 
 
 becoming less, the whole earth contracting until it became 
 entirely liquid; or rather these changes would progress 
 until no considerable portion of the earth would be gaseous, 
 for doubtless long before this stage was reached large por- 
 tions of the earth would have become solid. As to the 
 position which the solid parts of the earth's globe would 
 assume when the first processes of solidification took place, 
 we must not fall into the mistake of judging from the for- 
 mation of a crust of ice on freezing water that these solid 
 parts would form a crust upon the earth. Water presents 
 an exception to other substances, in being denser in the 
 liquid form than as a solid. Some metals and alloys are 
 like water in this respect; but with most earthy substances, 
 " and notably," says Dr. Sterry Hunt, " the various min- 
 erals and earthy compounds like those which may be sup- 
 posed to have made up the mass of the molten globe, the 
 case is entirely different. The numerous and detailed ex- 
 periments of Sainte-Claire Deville, and those of Delesse, be- 
 sides the earlier ones of Bischof, unite in showing that the 
 density of fused rocks is much less than that of the crystal- 
 line products resulting from their slow cooling, these being, 
 according to Deville, from one seventh to one sixteenth 
 heavier than the fused mass, so that if formed at the surface 
 they would, in obedience to the laws of gravity, tend to 
 sink as soon as formed." 9 
 
 Nevertheless, inasmuch as solidification would occur at 
 the surface, where the radiation of heat would take place 
 most rapidly, and as the descending solid matter would 
 be gradually liquefied, it seems certain that for a long time 
 the solid portions of the earth, though not forming a solid 
 crust, would occupy the exterior parts of the earth's globe. 
 After a time, the whole globe would have so far cooled that 
 a process of aggregation of solid matter around the centre 
 of the earth would take place. The matter so aggregated 
 consisted probably of metallic and metalloidal compounds 
 denser than the material forming the crust of the earth. 
 
 9 It is as yet doubtful how far the recent experiments of Mallet 
 affect this reasoning. 
 
70 PROCTOR 
 
 Between the solid centre and the solidifying crust there 
 would be a shell of uncongealed matter, gradually dimin- 
 ishing in amount, but a portion probably retaining its liquid 
 condition even to the present time, whether existing in 
 isolated reservoirs, or whether, as Scrope opines, it forms 
 still a continuous sheet surrounding the solid nucleus. One 
 strange fact of terrestrial magnetism may be mentioned in 
 partial confirmation of the theory that the interior of the 
 earth is of this nature a great solid mass, separated from 
 the solid crust by a viscous plastic ocean: the magnetic 
 poles of the earth are changing in position in a manner 
 which seems only explicable on the supposition that there 
 is an interior solid globe rotating under the outer shell, but 
 at a slightly different rate, gaining or losing one complete 
 rotation in the course of about six hundred and fifty years. 
 Be this as it may, we find in this theory an explanation 
 of the irregularities of the earth's surface. The solid crust, 
 contracting at first more rapidly than the partially liquid 
 mass within, portions of this liquid matter would force 
 their way through and form glowing oceans outside the 
 crust. Geology tells us of regions which, unless so formed, 
 must have been produced in the much more startling man- 
 ner conceived by Meyer, who attributed them to great 
 meteoric downfalls. 10 At a later stage, when the crust, 
 
 10 There is very little new under the sun. In dealing with the mul- 
 titudinous lunar craters, which were certainly formed in ages when un- 
 attached meteors were enormously greater in number and size than 
 at present, I mentioned as a consideration not to be overlooked the 
 probability that some of the meteoric matter falling on the moon when 
 she was plastic with intensity of heat might be expected to leave traces 
 which we could discern; and although none of the larger lunar craters 
 could be so formed, yet some of the smaller craters in these lunar re- 
 gions where craters overlap like the rings left by raindrops which have 
 fallen on a plastic surface, might be due to meteoric downfall. I find 
 that Meyer had far earlier advanced a similar idea in explanation of 
 those extensive regions of our earth which present signs of having 
 been in a state of igneous fluidity. Again, two or three years ago, Sir 
 W. Thomson startled us all by suggesting the possibility that vegetable 
 life might have been introduced upon our earth by the downfall of 
 fragments of old worlds. Several years before, Dr. Sterry Hunt had 
 pointed to evidence which tends to show that large meteoric globes 
 had fallen on the earth, and he showed further that some meteors con- 
 
THE PAST AND FUTURE OF OUR EARTH 71 
 
 having hitherto cooled more rapidly than the interior, be- 
 gan to have a slower rate of cooling, the retreating nucleus 
 left the crust to contract upon it, corrugating in the process, 
 and so forming the first mountain ranges upon the sphe- 
 roidal earth, which preceding processes had left partially 
 deformed and therefore ready to become in due time 
 divided into oceans and continents. 
 
 At this stage the earth must have been surrounded by 
 an atmosphere much denser than that now existing, and 
 more complex in constitution. We may probably form 
 the most trustworthy opinion of the nature of the earth's 
 atmosphere and the probable condition of the earth's sur- 
 face at this early epoch by following the method of reason- 
 ing employed by Dr. Sterry Hunt. It will be remembered 
 that he conceives an intense heat applied to the earth as at 
 present existing, and infers the chemical results. It is evi- 
 dent that such a process would result in the oxidation of 
 every form of carbonaceous matter; all carbonates, chlo- 
 rides, and sulphates would be converted into silicates 
 carbon, chlorine, and sulphur being separated in the form 
 of acid gases. These gases, with nitrogen, an excess of 
 oxygen, and enormous quantities of aqueous vapour, would 
 form an atmosphere of great density. In such an atmos- 
 phere condensation would only take place at a temperature 
 far above the present boiling point; and the lower level of 
 the slowly cooling crust would be drenched with a heated 
 solution of hydrochloric acid, whose decomposing action, 
 aided by its high temperature, would be exceedingly rapid. 
 The primitive igneous rock on which these heavy showers 
 fell probably resembled in composition certain furnace- 
 slags or basic volcanic glasses. Chlorides of the various 
 
 tain hydrocarbons and certain metallic compounds indicating processes 
 of vegetation. Dr. Hunt tells me that, in his opinion, some of the 
 meteors whose fragments have fallen on the earth in historic times 
 were once covered with vegetation, since otherwise, according to our 
 present chemical experience, the actual condition of these meteoric 
 fragments would be inexplicable. He does not regard them as frag- 
 ments of a considerable orb comparable even with the least of the 
 planets, but still, whatever their dimensions may have been, he con- 
 siders that vegetable life must have formerly existed upon them. 
 
72 PROCTOR 
 
 bases would be formed, and silica would be separated under 
 the decomposing action of the heated showers until the 
 affinities of the hydrochloric acid were satisfied. Later, sul- 
 phuric acid would be formed in large quantities by the com- 
 bination of oxygen with the sulphurous acid of the primeval 
 atmosphere. After the compounds of sulphur and chlorine 
 had been separated from the air, carbonic acid would still 
 continue to be an important constituent of the atmosphere. 
 This constituent would gradually be diminished in quantity, 
 during the conversion of the complex aluminous silicates 
 into hydrated silicate of alumina, or clay, while the sepa- 
 rated lime, magnesia, and alkalies would be changed into 
 bicarbonates, and carried down to the sea in a state of 
 solution. 
 
 Thus far the earth was without life; at least no forms 
 of life, vegetable or animal, with which we are familiar, 
 could have existed while the processes hitherto described 
 were taking place. The earth during the long series of ages 
 required for these changes was in a condition comparable 
 with the condition through which Jupiter and Saturn are 
 apparently at present passing. A dense atmosphere con- 
 cealed the surface of the earth, even as the true surface of 
 Jupiter is now concealed. Enormous cloud-masses were 
 continually forming and continually pouring heavy showers 
 on the intensely heated surface of the planet, throughout 
 the whole of the enormous period which elapsed between 
 the time when first the earth had a surface, and the time 
 when the atmosphere began to resemble in constitution the 
 air we breathe. Even when vegetable life, such as we are 
 familiar with, was first possible, the earth was still intensely 
 heated, and the quantity of aqueous vapour and cloud 
 always present in the air must have been far greater than at 
 present. 
 
 It has been in vain, thus far, that men have attempted 
 to lift the veil which conceals the beginning of life upon 
 the earth. It would not befit me to express an opinion on 
 the controversy whether the possibility of spontaneous 
 generation has, or has not, been experimentally verified. 
 
THE PAST AND FUTURE OF OUR EARTH 
 
 73 
 
 That is a question on which experts alone can give an 
 opinion worth listening to; and all that can here be noted 
 is that experts are not agreed upon the subject. As a mere 
 speculation it may be suggested that, somewhat as the ele- 
 ments when freshly released from chemical combination 
 show for a short time an unusual readiness to enter into 
 new combinations, so it may be possible that, when the 
 earth was fresh from the baptism of liquid fire to which her 
 primeval surface had for ages been exposed, certain of the 
 substances existing on her surface were for the time in a 
 condition fitting them to pass to a higher order of exist- 
 ence, and that then the lower forms of life sprang spontane- 
 ously into existence on the earth's still throbbing bosom. In 
 any case, we need not feel hampered by religious scruples in 
 considering the possibility of the spontaneous generation of 
 life upon the earth. It would be straining at a gnat and swal- 
 lowing a camel if we found a difficulty of that sort here, 
 after admitting, as we are compelled by clearest evidence 
 to admit, the evolution of the earth itself and of the system 
 to which the earth belongs, by purely natural processes. 
 The student of science should view these matters apart 
 from their supposed association with religious questions, 
 apart in particular from interpretations which have been 
 placed upon the Bible records. We may be perfectly satis- 
 fied that the works of God will teach us aright if rightly 
 studied. Repeatedly it has been shown that ideas respect- 
 ing creation which had come to be regarded as sacred be- 
 cause they were ancient, were altogether erroneous, and it 
 may well be so in this matter of the creation of life. 11 
 
 u It is not for me to undertake to reconcile the Bible account of 
 creation with the results which science is bringing gradually more 
 clearly before us. It seems to me unfortunate, in fact, that such 
 reconciliation should be thought necessary. But it must be conceded, 
 I suppose, by all, that it is not more difficult to reconcile modern bio- 
 logical theories of evolution with the Bible record than it is to recon- 
 cile with that record the theory of the evolution of the solar system. 
 Yet strangely enough many oppose the biological theories (not without 
 anger), who readily admit that some form or other of the nebular 
 hypothesis of the solar system must be adopted in order to explain the 
 peculiarities of structure presented by that system. 
 
74 PROCTOR 
 
 Whatever opinion we form on these points, it seems 
 probable that vegetable life existed on the earth before ani- 
 mal life, and also that primeval vegetation was far more 
 luxuriant than the vegetation of our own time. Vast for- 
 ests were formed, of which our coal-fields, enormous as is 
 their extent, represent merely a small portion preserved in 
 their present form through a fortuitous combination of ex- 
 ceptional conditions. By far the greater portion of those 
 forest masses underwent processes of vegetable decay 
 effectually removing all traces of their existence. What 
 escaped, however, suffices to show the amazing luxuriance 
 with which vegetation formerly throve over the whole 
 earth. 
 
 In assuming the probability that vegetable life preceded 
 animal life, I may appear to be opposing myself to an ac- 
 cepted paleontological doctrine, according to which animal 
 and vegetable life began together upon the earth. But I 
 would remind you that the actual teaching of the ablest, 
 and therefore the most cautious, paleontologists on this 
 point, amounts merely to this, that if the geological record 
 as at present known be assumed to be coeval with the 
 commencement of life upon the globe, then animals and 
 plants began their existence together. In a similar way the 
 teachings of geology and paleontology as to the nature of 
 the earliest known forms of life and as to the succession 
 of faunae and florae, depend on an admittedly imperfect 
 record. Apart, however, from this consideration, I do not 
 think it would serve any useful purpose if I were to at- 
 tempt, I will not say to discuss, for that is out of the ques- 
 tion, but to speak of the geological evidence respecting 
 that portion of the past history of our earth which belongs 
 to the interval between the introduction of life upon the 
 surface and the present time. In particular, my opinion 
 on the interesting question, whether all the forms of life 
 upon the earth, including the various races of man, came 
 into being by processes of evolution, could have no weight 
 whatever. I may remark that, even apart from the evi- 
 dence which the most eminent biologists have brought to 
 
THE PAST AND FUTURE OF OUR EARTH 75 
 
 bear on this question, it seems to me illogical to accept 
 evolution as sufficient to explain the history of our earth 
 during millions of years prior to the existence of life, and 
 to deny its sufficiency to explain the development of life 
 (if one may so speak), upon the earth. It seems even more 
 illogical to admit its operation up to any given stage in the 
 development of life, and there to draw a hard-and-fast line 
 beyond which its action can not be supposed to have ex- 
 tended. 12 Nor can I understand why it should be con- 
 sidered a comforting thought, that at this or that epoch 
 in the history of the complex machine of life, some imper- 
 fection in the machinery compelled the intervention of 
 God thus presented to our contemplation as Almighty, 
 but very far from being All-wise. 
 
 There is, however, one aspect in which the existence of 
 life has to be considered as intimately associated with the 
 future history of our earth. We perceive that the abun- 
 dance of primeval vegetation during long ages, aided by 
 other processes tending gradually to reduce the amount 
 of carbonic-acid gas in the air, must have led to a gradual 
 change in the constitution of the atmosphere. At a later 
 epoch, when animal life and vegetable life were more 
 equally proportioned, a state of things existed which, so 
 far as can be judged, might have lasted many times as long 
 as it has already lasted had not man appeared upon the 
 scene. But it seems to me impossible to consider what is 
 actually taking place on the earth at present, without per- 
 ceiving that within periods, short indeed by comparison 
 with geological eras, and still shorter compared with the 
 intervals to which the astronomical history of our earth has 
 introduced us, the condition of the earth as an abode of 
 
 13 Since I thus spoke, a new and as it seems to me an even more 
 illogical limit has been suggested for the operation of the process of 
 evolution as affecting the development of life, and this by an advocate 
 of the general doctrine of evolution. I refer to the opinion advanced 
 by Mr. J. Fiske, of Harvard University, that " no race of organisms 
 can in future be produced through the agency of natural selection 
 and direct adaptation, which shall be zoologically distinct from, and 
 superior to, the human race." 
 
76 PROCTOR 
 
 life will be seriously modified by the ways and works of 
 man. It is only in the savage state that man is content to 
 live upon the produce of the earth, taking his share, as it 
 were, of what the earth (under the fruitful heat of the sun, 
 which is her life) brings forth day by day, month by 
 month, year by year, and century by century. But civilized 
 man is not content to take his share of the earth's income, 
 he uses the garnered wealth which is the earth's capital 
 and this at a rate which is not only ever increasing, but is 
 increasing at an increasing rate. The rapid consumption 
 of coal is but a single instance of his wasteful expenditure 
 of the stores which during countless ages have been 
 gathered together, seemingly for the use of man. In this 
 country (America), I need not dwell upon the fact that, 
 in many other ways, man is consuming, if not wasting, 
 supplies of earth-wealth which can not be replaced. It is 
 not merely what is found within the earth, but the store of 
 wealth which clothes the earth's surface, which is thus be- 
 ing exhausted. Your mighty forests seem capable of sup- 
 plying all the timber that the whole race of man could 
 need for ages; yet a very moderate computation of the rate 
 at which they are being cut down, and will presumably 
 continue to be, by a population increasing rapidly in num- 
 bers and in the destructive capabilities which characterize 
 modern civilization, would show that America will be de- 
 nuded of its forest-wealth in about the same period which 
 we in England have calculated as probably limiting the 
 effective duration of our stores of coal. That period a 
 thousand or twelve hundred years may seem long com- 
 pared with the life of individual men, long even compared 
 with the duration of any nation in the height of power; 
 but though men and nations pass away, the human race 
 continues, and a thousand years are as less than a day in 
 the history of that race. Looking forward to that future 
 day, seemingly so remote, but (on the scale upon which 
 we are at present tracing our earth's history) in reality the 
 to-morrow of our earth, we see that either a change in 
 their mode of civilization will be forced on the human race, 
 
THE PAST AND FUTURE OF OUR EARTH 77 
 
 or else it will then have become possible, as your Ericsson 
 has already suggested, to make the sun's daily heat the 
 mainspring of the machinery of civilization. 
 
 But turning from those portions of the past and future 
 of our earth which, by comparison with the astronomical 
 eras of her history, may be regarded as present, let us con- 
 sider, so far as known facts permit, the probable future of 
 the earth after astronomical eras comparable with those 
 which were presented to us when we considered her past 
 history. 
 
 One of the chief points in the progression of the earth 
 toward her present condition was the gradual passing away 
 of the heat with which formerly her whole globe was in- 
 stinct. We have now to consider whether this process of 
 cooling is still going on, and how far it is likely to ex- 
 tend. In this inquiry we must not be misled by the prob- 
 able fact, for such it seems, that during hundreds of thou- 
 sands of years the general warmth of the surface of the 
 earth has not appreciably diminished. In the first place, 
 hundreds of thousands of years are the seconds of the time- 
 measures we have now to deal with; and next, it is known 
 that the loss of temperature which our earth is at present 
 undergoing chiefly affects the interior parts of her globe. 
 The inquiries of Mallet and others show that the present 
 vulcanian energies of the earth are due in the main to the 
 gradual withdrawal of the earth's nuclear parts from the 
 surface crust, because of the relatively more rapid loss of 
 heat by the former. The surface crust is thus left to con- 
 tract under the action of gravity, and vulcanian phenomena 
 that is, volcanoes and earthquakes represent the me- 
 chanical equivalent of this contraction. Here is a process 
 which can not continue forever, simply because it is in its 
 very nature exhaustive of the energy to which it is due. It 
 shows us that the earth's nuclear regions are parting with 
 their heat, and as they can not part with their heat without 
 warming the surface-crust, which nevertheless grows no 
 wanner, we perceive that the surface heat is maintained 
 from a source which is being gradually exhausted. The 
 
78 PROCTOR 
 
 fitness of the earth to be the abode of life will not only 
 be affected directly in this way, but will be indirectly 
 affected by the loss of that vulcanian energy which appears 
 to be one of its necessary conditions. At present, the sur- 
 face of the earth is like the flesh clothing the living body; 
 it does not wear out because (through the life which is 
 within it) it undergoes continual change. But even as the 
 body itself is consumed by natural processes so soon as life 
 has passed from it, so, when the internal heat of the earth, 
 which is its life, shall have passed away, her surface will 
 " grow old as doth a garment "; and with this inherent ter- 
 restrial vitality will pass away by slow degrees the life 
 which is upon the earth. 
 
 In dealing with the past history of our earth, we recog- 
 nised a time when she was a sun, rejoicing as a giant in 
 the strength of youth; and later we considered a time when 
 her condition resembled that of the planets Jupiter and 
 Saturn, whose dense atmospheres seem to be still loaded 
 with the waters which are to form the future oceans of those 
 noble orbs. In considering our earth's future, we may 
 recognise in the moon's actual condition a stage through 
 which the earth will hereafter have to pass. When the earth's 
 inherent heat has passed away and long ages have elapsed 
 since she had been the abode of life, we may believe that her 
 desert continents and frost-bound oceans will in some de- 
 gree resemble the arid wastes which the astronomer rec- 
 ognises in the lunar surface. And yet it is not to be sup- 
 posed that the appearance of the earth will ever be closely 
 similar to that presented by the moon. The earth may 
 part, as completely as the moon has, with her internal heat; 
 the rotation of the earth may in hundreds of millions of 
 years be slowed down by tidal action into agreement with 
 the period in which the moon completes her monthly orbit; 
 and every form of animal and vegetable life may perish 
 from off the face of the earth: yet ineffaceable traces of the 
 long ages during which her surface was clothed with life 
 and instinct with inherent vitality, will distinguish her 
 from the moon, where the era of life was incomparably 
 
THE PAST AND FUTURE OF OUR EARTH 79 
 
 shorter. Even if the speculations of Stanislas Meunier be 
 just, according to which the oceans will gradually be with- 
 drawn beneath the surface crust and even the atmosphere 
 almost wholly disappear, there would forever remain the 
 signs of changes brought about by rainfall and snowfall, by 
 wind and storm, by river and glacier, by ocean waves and 
 ocean currents, by the presence of vegetable life and of 
 animal life during hundreds of millions of years, and even 
 more potently by the fiery deluge poured continually on 
 the primeval surface of our globe. By all these causes the 
 surface of the earth has been so wrought upon as no longer 
 to resemble the primary igneous rock which we seem to 
 recognise in the scarred surface of our satellite. 
 
 Dare we look onward to yet later stages in the history 
 of our earth? Truly it is like looking beyond death; for 
 now imagination presents our earth to us as an inert mass, 
 not only lifeless as at the beginning, but no longer pos- 
 sessing that potentiality of life which existed in her sub- 
 stance before life appeared upon her surface. We trace her 
 circling year after year around the sun, serving no useful 
 purpose according to our conceptions. The energy repre- 
 sented by her motions of rotation and revolution seems to 
 be as completely wasted as are those parts (the whole save 
 only one 23O,ooo,oooth portion) of the sun's light and heat, 
 which, falling on no planet, seem to be poured uselessly into 
 desert space. Long as has been, and doubtless will be, the 
 duration of life upon the earth, it seems less than a second 
 of time compared with those two awful time-intervals one 
 past, when as yet life had not begun, the other still to come, 
 when all life shall have passed away. 
 
 But we are thus led to contemplate time-intervals of a 
 yet higher order to consider the eras belonging to the 
 lifetime of the solar system itself. Long after the earth 
 shall have ceased to be the abode of life other and nobler 
 orbs will become in their time fit to support millions of forms 
 as well of animal as of vegetable existence; and the later 
 each planet is in thus " putting on life," the longer will be 
 the duration of the life-supporting era of its own existence. 
 
80 PROCTOR 
 
 Even those time-intervals will pass, however, until every 
 orb in turn has been the scene of busy life, and has then, 
 each after its due life-season, become inert and dead. One 
 orb alone will then remain, on which life will be possible 
 the sun, the source whence life had been sustained in all 
 those worlds. And then, after the lapse, perchance, of a 
 lifeless interval, compared with which all the past eras of the 
 solar system were utterly insignificant, the time will arrive 
 when the sun will be a fit abode for living creatures. There- 
 after, during ages infinite to our conceptions, the great 
 central orb will be (as now, though in another sense) the 
 life of the solar system. We may even look onward to 
 still more distant changes, seeing that the solar system is 
 itself moving "on an orbit, though the centre round which 
 it travels is so distant that as yet it remains unknown. 
 We see in imagination change after change, cycle after 
 cycle, till 
 
 " Drawn on paths of never-ending duty, 
 
 The worlds eternity begun 
 Rest, absorbed in ever-glorious beauty, 
 On the Heart of the All-Central Sun." 
 
 But in reality it is only because our conceptions are 
 finite that we thus look forward to an end even as we seek 
 to trace events back to a beginning. The notion is incon- 
 ceivable to us that absolutely endless series of change may 
 take place in the future and have taken place in the past; 
 equally inconceivable is the notion that series on series of 
 material combinations, passing onward to ever-higher 
 orders from planets to suns, from suns to sun-systems, 
 from sun-systems to galaxies, from galaxies to systems of 
 galaxies, from these to higher and higher orders, abso- 
 lutely without end may surround us on every hand. And 
 yet, as I set out by saying, these things are not more in- 
 conceivable than infinity of time and infinity of space, while 
 the idea that time and space are finite is not merely incon- 
 ceivable, but opposed directly to what the mind conceives 
 of space and time. It has been said that progression neces- 
 sarily implies a beginning and an end; but this is not so 
 
THE PAST AND FUTURE OF OUR EARTH 8l 
 
 where the progression relates to absolute space or time. 
 No one can indeed doubt that progression in space is of its 
 very nature limitless. But this is equally true, though not 
 less inconceivable, of time. Progression implies only rela- 
 tive beginning and relative ending; but that there should 
 be an absolute beginning or an absolute end is not merely 
 inconceivable, like absolute eternity, but is inconsistent 
 with the necessary conditions of the progression of time as 
 presented to us by our conceptions. Those who can may 
 find relief in believing in absolutely void space and abso- 
 lutely unoccupied time before some very remote but not 
 infinitely remote epoch, which may in such belief be called 
 the beginning of all things; but the void time before that be- 
 ginning can have had no beginning, unless it were preceded 
 by time not unoccupied by events, which is inconsistent 
 with the supposition. We find no absolute beginning if we 
 look backward; and looking forward we not only find an 
 absolute end inconceivable by reason, but revealed religion 
 as ordinarily interpreted teaches that on that side lies 
 an eternity not of void but of occupied time. The time- 
 intervals, then, which have presented themselves to our con- 
 templation in dealing with the past and future of our earth, 
 being in their nature finite, however vast, are less than the 
 shortest instant in comparison with absolute time, which 
 endless itself is measured by endless cycles of change. 
 And in like manner, the space seemingly infinite from which 
 our solar system has drawn its materials in other words, 
 the universe as partially revealed to us in the study of the 
 star-depths is but the merest point by comparison with 
 absolute space. The end, seemingly so remote, to which 
 our earth is tending, the end infinitely more remote to 
 which the solar system is tending, the end of our galaxy, 
 the end of systems of such galaxies as ours all these end- 
 ings (each one of which presents itself in turn to our con- 
 ceptions as the end of the universe itself) are but the begin- 
 nings of eras comparable with themselves, even as the be- 
 ginnings to which we severally trace back the history of 
 our planet, of the planetary system, and of galaxies of such 
 6 
 
82 PROCTOR 
 
 systems, are but the endings of prior conditions which 
 have followed each other in infinite succession. The wave 
 of life which is now passing over our earth is but a ripple 
 in the sea of life within the solar system; this sea of life 
 is itself but as a wavelet on the ocean of eternal life through- 
 out the universe. Inconceivable, doubtless, are these in- 
 finities of time and space, of matter, of motion, and of 
 life. Inconceivable that the whole universe can be for all 
 time the scene of the operation of infinite personal power, 
 omnipresent, all-knowing. Utterly incomprehensible how 
 Infinite Purpose can be associated with endless material 
 evolution. But it is no new thought, no modern discovery, 
 that we are thus utterly powerless to conceive or compre- 
 hend the idea of an Infinite Being, Almighty, All-knowing, 
 Omnipresent, and Eternal, of whose inscrutable purpose 
 the material universe is the unexplained manifestation. 
 Science is in presence of the old, old mystery; the old, old 
 questions are asked of her: " Canst thou by searching find 
 out God? canst thou find out the Almighty unto perfec- 
 tion? It is as high as heaven; what canst thou do? deeper 
 than hell; what canst thou know? " And Science answers 
 these questions, as they were answered of old, " As touch- 
 ing the Almighty, we can not find him out." 
 
A NEW THEORY OF LIFE IN OTHER 
 WORLDS l 
 
 TWO opposite views have been entertained respecting 
 life in other worlds. One is the theory which Brew- 
 ster somewhat strangely described as the creed of 
 the philosopher and the hope of the Christian, that nearly 
 all the orbs which people space are the abode of life. 
 Brewster, Chalmers, Dick, and a host of other writers, have 
 adopted and enforced this view, Brewster going so far 
 as to maintain the probability that life may exist upon 
 the moon, dead though her surface seems, or beneath the 
 glowing photosphere of the sun. But even where so ex- 
 treme an opinion has not been entertained, the believers 
 in the theory of a plurality of worlds have maintained that 
 all the celestial orbs have been created to be, and are at 
 this present time, the abodes of life, or else minister to 
 the wants of creatures living in other orbs. It is worthy 
 of notice that this view has been entertained even by 
 astronomers, who, like the Herschels, have devoted their 
 lives to the scientific study of the heavens. So completely 
 has the theory been identified, as it were, with modern 
 astronomy, that we find the astronomer passing from a 
 statement respecting some observed fact about a planet, to 
 the consideration of the bearing of the fact on the require- 
 ments of living creatures on the planet's surface, without 
 expressing any doubt whatever as to the existence of such 
 creatures. For example, Sir John Herschel, writing about 
 the rings of Saturn, after discussing Lardner's supposed 
 demonstration that the eclipses caused by the rings would 
 1 From " Our Place among Infinities," D. Appleton and Company. 
 
 83 
 
84 PROCTOR 
 
 last but for a short time, 2 says : " This will not prevent, 
 however, some considerable regions of Saturn from suf- 
 fering very long total interception of the solar beams, 
 affording to our ideas but an inhospitable asylum to ani- 
 mated beings, ill compensated by the feeble light of the 
 satellites; but we shall do wrong to judge of the fitness 
 or unfitness of their condition from what we see around 
 us, when perhaps the very combinations which convey to 
 our minds only images of horror may be, in reality, thea- 
 tres of the most striking and glorious displays of benefi- 
 cent contrivance." And many other such cases might 
 be cited. 
 
 Before passing to the opposite view of life in other 
 worlds, a view commonly associated with the name of the 
 late Dr. Whewell, I shall venture to quote a few passages 
 from his " Bridgewater Treatise on Astronomy and Gen- 
 eral Physics," in which he writes very much like a sup- 
 porter of the theory he subsequently opposed in his " Plu- 
 rality of Worlds." Thus, speaking of the satellites in the 
 solar system, he says: "There is one fact which imme- 
 diately arrests our attention; the number of these attendant 
 bodies appears to increase as we proceed to planets farther 
 and farther from the sun. Such, at least, is the general rule. 
 Mercury and Venus, the planets near the sun, have no at- 
 tendants; the earth has but one. Mars, indeed, who is still 
 farther removed, has none, nor have the minor planets, so 
 that the rule is only approximately verified. But Jupiter, 
 who is at five times the earth's distance, has four satellites; 
 and Saturn, who is again at a distance nearly twice as great, 
 has seven " (now eight) " besides that most extraordinary 
 phenomenon, his ring, which for purposes of illumination is 
 equivalent to many thousand satellites. Of Uranus it is 
 
 * This is disproved, and the justice of Herschel's views demon- 
 trated in Chapter VII of my treatise on Saturn, in which work I give 
 a table of the climatic relations in Saturn (for I also once adopted the 
 theory criticised above), the time and place of sunrise and sunset in 
 Saturnian latitudes in Saturnian spring, summer, autumn, and winter, 
 and so on. Labour wasted, I fear, except as practice in Geometrical 
 Astronomy. 
 
A NEW THEORY OF LIFE IN OTHER WORLDS 85 
 
 difficult to speak, for his great distance renders it almost 
 impossible to observe the smaller circumstances of his con- 
 dition. It does not appear at all probable that he has a 
 ring like Saturn; but he has at least four satellites which 
 are visible to us, at the enormous distance of nine hundred 
 millions of miles, and I believe that the astronomer will 
 hardly deny that he may possibly have thousands of smaller 
 ones circulating about him. But leaving conjecture, and 
 taking only the ascertained cases of Venus, the earth, 
 Jupiter, and Saturn, we conceive that a person of common 
 understanding will be strongly impressed with the persua- 
 sion that the satellites are placed in the system with a view 
 to compensate for the diminished light of the sun at greater 
 distances." Then he presently adds, after considering the 
 exceptional case of Mars, " No one familiar with such con- 
 templations will, by one anomaly, be driven from the per- 
 suasion that the end which the arrangements of the satel- 
 lites seem suited to answer is really one of the ends of 
 their creation." Here is the theory of life in other worlds 
 definitely adopted, and moreover presented in company 
 with the extremest form of the teleological argument, and 
 that, too, by Whewell, whose name afterward became asso- 
 ciated with the extremest development of the doctrine of 
 the paucity of worlds ! 
 
 The Whewellite theory is tolerably well known, though 
 certainly it is not held in very great favour. For my own 
 part, I used, at one time, to think that Whewell only ad- 
 vanced it in jest; but now (perhaps because my own re- 
 searches and study have led me to regard the Brewsterian 
 theory as untenable) I recognise in Whewell's later views 
 the result of longer and more careful study than he had 
 given to the subject, when (nearly a quarter of a century 
 earlier) he wrote his " Bridgewater Treatise." 
 
 Whatever opinion we form as to the theory advanced 
 in the " Plurality of Worlds," we must admit that Whewell 
 did good service to science in breaking the chains of old- 
 fashioned ideas, and inaugurating freedom of discussion. 
 The stock writers on astronomy had been repeating so 
 
86 PROCTOR 
 
 often the imperfect analogies on which astronomers had 
 earlier insisted, that the suggestions based on such analogies 
 had come to be regarded as so many scientific facts. The 
 earth is a planet, and Mars is a planet, therefore what we 
 know about the earth may be inferred respecting Mars, no 
 account being taken of the known difference in the con- 
 dition of the two planets: accordingly, not only are the 
 white spots at the Martian poles to be regarded as snow- 
 covered regions, and the blue markings on his surface as 
 seas, but we are to infer a similarity of climatic conditions 
 and other habitudes, without entering into any close con- 
 sideration of the probable extent of the planet's atmos- 
 phere, the heat received from the sun by Mars, and a 
 variety of other relations respecting which we are at least 
 as well informed as we are respecting the analogies in ques- 
 tion. Jupiter, again, is a planet, and though he is so much 
 larger than the earth that we might be disposed at the out- 
 set to regard him as a body of another order, we must be 
 so guided by analogies (which, after all, may be imaginary) 
 as to consider that his size only renders him so much the 
 nobler an abode for such life as we are familiar with: and 
 instead of being struck by the fact that Jupiter, unlike 
 Mars, shows no polar snow-caps, we are to direct our atten- 
 tion to his belts, and to regard them as cloud-belts analo- 
 gous to the tropical cloud-zone of the earth. Nor are 
 we to inquire too closely whether the aspect of his equa- 
 torial belt, to say nothing of his other belts, corresponds 
 in any degree with that which the cloud zone of our earth 
 would present to observers on another planet. Let it suffice 
 to note a few analogies, as thus: "The earth is a planet, 
 Jupiter is a planet; the earth rotates and therefore has a 
 day, Jupiter rotates and has a day; the earth has a year, 
 Jupiter has a year; the earth has clouds, Jupiter has clouds; 
 the earth has a moon, Jupiter has four moons; this done, 
 every other consideration may be conveniently overlooked, 
 and we may proceed to descant on the wonderful extent 
 and dignity of this distant world, with as little question of 
 its being inhabited as though we had seen with our own 
 
A NEW THEORY OF LIFE IN OTHER WORLDS 87 
 
 eyes the creatures which exist upon the planet's surface. 
 So with Saturn, and the rest." 
 
 Whewell broke through all these old-fashioned meth- 
 ods. He dealt with the several planets on the true scientific 
 principle long since enunciated by Descartes, taking noth- 
 ing for granted that had not been proved. He showed how 
 unlike the conditions prevailing in the other planets must 
 be to those existing on the earth, and without pretending 
 to demonstrate absolutely that none of the higher forms of 
 life can exist on certain planets, he showed that at any rate 
 the probabilities are in favour of that hypothesis. Passing 
 on to the stars, he did good service by showing how much 
 had been taken for granted by astronomers in their assump- 
 tions respecting those orbs; nor is the value of his work, 
 in this field, by any means diminished by the circumstance 
 that during recent years evidence which was wanting when 
 Whewell wrote has been obtained, and the stars have been 
 shown demonstratively to be suns. And lastly, he dealt 
 in an independent and therefore instructive manner with 
 the star-cloudlets or nebulae, giving many strong reasons 
 for doubting the views which were at that time repeated in 
 every text-book of astronomy. 
 
 The conclusions to which Whewell was led were: (i) 
 that no sufficient reason exists for believing in other worlds 
 than ours; and (2) if the other planets are inhabited, it 
 can only be, in all probability, by creatures belonging to 
 the lowest orders of animated existence. He somewhat 
 softened the harshness of these inferences by pointing out 
 that our conceptions of the glories of God's kingdom need 
 not be enfeebled by our doubts as to the existence of life 
 in the planets of our own system, or of systems circling 
 around other suns. " However destitute," he wrote, 
 " planets, moon, and rings may be of inhabitants, they are 
 at least vast scenes of God's presence, and of the activity 
 with which he carries into effect everywhere the laws of 
 Nature; and the glory of creation arises from its being, not 
 only the product but the constant field of God's activity 
 and thought, wisdom, and power." And, in passing, I 
 
88 PROCTOR 
 
 may note that Sir David Brewster, when commenting 
 somewhat angrily and contemptuously on this remark, 
 failed really to grasp Whewell's meaning. Brewster was 
 at great pains to show how large a portion of the glories of 
 the heavens is invisible and useless to man; but Whewell 
 was manifestly not referring to the glories of God as re- 
 vealed to man, but as they exist in themselves. It must 
 be admitted, indeed, even by those who prefer Brewster's 
 theory, that he maintained it with much more warmth than 
 was necessary in such a discussion. In presence of Whew- 
 ell's philosophic, calm, and dispassionate force of rea- 
 soning, there was something almost ludicrous in the im- 
 passioned outbursts of the great physicist who took the 
 doctrine of life in other worlds under his protection. 
 " Where," says he, " is the grandeur, where the utility, 
 where the beauty, where the poetry, of the two almost in- 
 visible stars which usurp the celestial names of Uranus and 
 Neptune, and which have been seen by none but a very 
 few even of the cultivators of astronomy? The seaman in 
 the trackless ocean never seeks their guidance; to him they 
 have not even the value of the Pole Star; they contribute 
 nothing to the arts of terrestrial life: they neither light the 
 traveller on his journey, nor mark by their feeble ray the 
 happy hours which are consecrated to friendship and to 
 love." All this is very pretty writing, but it is very little 
 to the purpose, and while it has no bearing whatever on 
 what Whewell had urged, it is a very long way from estab- 
 lishing what Brewster desired to prove viz., that " Uranus 
 and Neptune must have been created for other and nobler 
 ends; to be the abodes of life and intelligence, the colossal 
 temples where their Creator is recognised and worshipped; 
 the remotest watch-towers of our system, from which his 
 works may be better studied, and his distant glories more 
 readily described." 
 
 Here, however, are two theories opposed to each 
 other, and not admitting of being reconciled. If we are 
 to make a selection between them, to which shall we turn 
 in preference? The balance of evidence is on the whole in 
 
A NEW THEORY OF LIFE IN OTHER WORLDS 89 
 
 favour of Whewell's (so at least the matter presents itself 
 to me after careful and long-continued study); but certainly 
 Brewster's is the theory which commends itself most fa- 
 vourably to the mind which would believe that God " hath 
 done all things well," and that nothing that he has made 
 was made in vain. Even those who, like myself, are indis- 
 posed to admit that the ways and works of God are to be 
 judged by our conceptions of the fitness of things (though 
 we may be altogether certain that all things are made in 
 wisdom and fitness), would prefer to accept the Brewsterian 
 theory, if decision were to be made between the two. For, 
 what amount of evidence could reconcile us to the belief 
 (even though it forced this belief upon us) that our earth 
 alone of all the countless orbs which people space, is the 
 abode of reasoning creatures, capable of recognising the 
 glories of the universe, and of lauding the Creator of those 
 wonders and of their own selves? Nevertheless we must be 
 guided in these matters by evidence, not by sentiment 
 by facts, not by our feelings. It is well, therefore, to 
 note that the decision does not lie between the two theories 
 which have just been dealt with. Another theory, holding 
 a position intermediate between those two, and combining 
 in my judgment the evidence which favours one theory 
 with the fitness characterizing the other, remains yet to 
 be presented. 
 
 I propose to take, as the basis of the new theory of life 
 in other worlds, the analogy which has commonly been 
 regarded as affording the strongest evidence in favour of 
 the Brewsterian theory only I shall take a more extended 
 view of the subject than has been customary. 
 
 Before introducing that Brewsterian argument, I may 
 remark that the mere fact that our earth is an inhabited 
 world is not in itself sufficient even to render probable the 
 theory that there is life in other worlds than ours. An 
 equally strong argument might be derived against that 
 theory from the study of our moon the only other planet 
 of which we have obtained reliable information for few 
 can suppose that the moon is fit to be the abode of life. 
 
90 PROCTOR 
 
 Since, then, of the two planets we can examine, one the 
 earth is inhabited, while the other the moon is proba- 
 bly not inhabited, the only evidence we have is almost 
 equally divided between the Whewellite and Brewsterian 
 theories, whatever balance remains in favour of the latter 
 being too slight to afford any sufficient basis for a con- 
 clusion. 
 
 But while this reasoning is just, as applied to the mere 
 fact that the earth is inhabited, it is by no means capable 
 of overthrowing the evidence which is derived from the 
 manner in which life exists on the earth. When we con- 
 sider the various conditions under which life is found to 
 prevail, that no difference of climatic relations 6r of eleva- 
 tion, of land or of air or of water, of soil in land, of fresh- 
 ness or saltness in water, of density in air, appears (so far 
 as our researches have extended) to render life impossible, 
 we are compelled to infer that the power of supporting life 
 is a quality which has an exceedingly wide range in Nature. 
 I refrain, it will be noticed, from using here the usual ex- 
 pression, and saying, as of yore, that " the great end and 
 aim of all the workings of Nature is to afford scope and 
 room for the support of life," because this mode of speak- 
 ing may be misunderstood. We can see what Nature 
 actually does, and we may infer, if we so please, that 
 such or such is the end and aim of the God of Nature; 
 nevertheless we must remember that the evidence 
 we have belongs to the former relation, not to the 
 latter. I am careful to dwell on this point because the 
 longer I study such matters the more clearly do I rec- 
 ognise the necessity of most studiously limiting our state- 
 ments to that which the evidence before us really estab- 
 lishes. 
 
 Passing beyond the evidence which the earth at present 
 affords, we find that during many ages the earth has pre- 
 sented a similar scene. " Geology," I wrote four years 
 ago, " teaches us of days when this earth was peopled with 
 strange creatures such as now are not found upon its sur- 
 face. We turn our thoughts to the epochs when these 
 
A NEW THEORY OF LIFE IN OTHER WORLDS 91 
 
 monsters throve and multiplied, and picture to ourselves 
 the appearance which our earth then presented. Strange 
 forms of vegetation clothe the scene which the mind's eye 
 dwells upon. The air is heavily laden with moisture to 
 nourish the abundant flora; hideous reptiles crawl over 
 their slimy domain, battling with each other, or with the 
 denizens of the forest; huge batlike creatures sweep 
 through the dusky twilight which constituted the primeval 
 day; w r eird monsters pursue their prey amid the depths of 
 ocean: and we forget, as we dwell upon the strange forms 
 which existed in those long-past ages, that the scene now 
 presented by the earth is no less wonderful, and that the 
 records of our time may, perhaps, seem one day as per- 
 plexing as we now find those of the geological eras." In 
 the past, then, as in the present, this earth was inhabited 
 by countless millions of living creatures, and during the 
 enormous period which has elapsed since life first appeared 
 on the surface of the earth, myriads, if not millions, of orders 
 of living creatures have appeared, have lived the life ap- 
 pointed to their order, and have vanished, or exist only 
 under modified forms. As each individual has had its 
 period of life, so also has each race, and we may say with 
 the poet (noting always that the personification of Nature 
 is but a poetical idea, and does not present any real sub- 
 stantive truth) : 
 
 " Are God and Nature then at strife, 
 That Nature lends such evil dreams? 
 So careful of the type she seems, 
 So careless of the single life. 
 
 " ' So careful of the type? ' but no, 
 
 From scarped cliff and quarried stone 
 She cries, ' A thousand types are gone ; 
 I care for nothing, all shall go.' " 
 
 Abundant life, in ever-varying forms, and under all- 
 various conditions, continuing age after age during hun- 
 dreds of thousands of years, such is what our earth presents 
 to us when we turn our thoughts to its past history. And 
 looking forward, a similar scene is presented to our con- 
 
9 2 
 
 PROCTOR 
 
 templation. For many a long century, probably for hun- 
 dreds of thousands of years, life will continue on the earth, 
 unless some catastrophe (the occurrence of which we have 
 as yet no reason to anticipate) should destroy life suddenly 
 from off her surface. 
 
 So viewing this earth, we seem to find forced upon us 
 the belief that the support of life is the object for which 
 the earth was created, and thus we are led to regard the 
 other orbs which, like her, circle around a central sun, as 
 intended to be the abode of life. The only object which, 
 so far as we can see, the earth has fulfilled during an in- 
 definitely long period has been to present a field, so to 
 speak, for the support of life, nor can we recognise any 
 other purpose which she will fulfil in the future. If we 
 admit this, and if we also believe that God made nothing 
 without some purpose, of course we have no choice but 
 to admit that the purpose with which the earth was made 
 was the support of life. And reasoning from analogy, we 
 infer that the other planets, as well those of our own sys- 
 tem as those which we believe to exist, " wheeling in per- 
 petual round," as attendants upon other suns, were simi- 
 larly created to be the abode of life. 3 
 
 * I shall venture to quote here the once celebrated argument ad- 
 vanced by Dr. Bentley in favour of the plurality of worlds: " Consid- 
 ering," he says, " that the soul of one virtuous and religious man is 
 of greater worth and excellency than the sun and all his planets, and all 
 the stars in the heavens, their usefulness to man might be the sole end 
 of their creation if it could be proved that they were as beneficial to 
 us as the Pole Star formerly was for navigation, or as the moon is for 
 producing the tides and lighting us on winter nights. But we dare not 
 undertake to show what advantage is brought to us by those innumerable 
 stars in the galaxy of other parts of the firmament, not discernible by 
 naked eyes, and yet each many thousand times bigger than the whole 
 body of the earth. If you say they beget in us a great idea and 
 veneration of the mighty Author and Governor of such stupendous 
 bodies, and excite and devote our minds to his adoration and praise, 
 you say very truly and well. But would it not raise in us a higher 
 apprehension of the infinite majesty and boundless beneficence of God, 
 to suppose that those remote and vast bodies were formed, not merely 
 upon our account, to be peeped at through an optic glass, but for dif- 
 ferent ends and nobler purposes? And yet who will deny that there 
 are great multitudes of lucid stars even beyond the reach of the best 
 
A NEW THEORY OF LIFE IN OTHER WORLDS 93 
 
 But, before we infer from the strength of this reason- 
 ing that the other planets are inhabited worlds, let us look 
 somewhat more closely into the circumstances, or rather, 
 instead of examining only a portion of the evidence, let 
 us take a wider survey and examine all the evidence we 
 possess. It may appear, at a first view, that already we 
 are dealing with periods which, to our conceptions, are 
 practically infinite. How long, compared with the brief 
 span of human life, are the eras with which history deals! 
 how enormous, even by comparison with these eras, appears 
 the range of time (tens of thousands, if not hundreds of 
 thousands, of years) since man first appeared upon this 
 earth! and, according to the teachings of geology, we 
 have to deal with a yet higher order of time in passing 
 to the beginning of life upon our globe. From one 
 million of years to ten millions! It is between such lim- 
 its, say the most experienced geologists, that the choice 
 lies. Surely we may be content with periods such as 
 these, periods as utterly beyond our powers of conception 
 as the duration of the Pyramids would be to creatures 
 like the ephemeron, did such creatures possess the power 
 of reason! 
 
 And yet, why should we stop at the beginning of life 
 upon this earth? We have passed to higher and higher 
 orders of time-intervals, but the series has no limit that 
 we know of, while it possesses terms, recognisable by us, 
 
 telescopes ; and that every visible star may have opaque planets revolv- 
 ing about them which we can not discover? Now, if they were not 
 created for our sakes it is certain and evident that they were not made 
 for their own; for matter has no life or perception, is not conscious 
 of its own existence, nor capable of happiness, nor gives the sacrifice of 
 praise and worship to the author of its being. It remains, therefore, 
 that all bodies were formed for the sake of intelligent minds; and as 
 the earth was principally designed for the being and service and con- 
 templation of men, why may not all other planets be created for the 
 like uses, each for its own inhabitants which have life and under- 
 standing? " The objection to Dr. Bentley's argument resides, not in 
 the belief which he expresses in the wisdom and beneficence of the 
 Creator, but in the confidence with which he assumes that the Creator 
 had such and such purposes and not perhaps others such as we not 
 only can not discover, but can not even conceive. 
 
94 
 
 PROCTOR 
 
 of higher order than those we have been dealing with. We 
 know that in the far-off times before life appeared, 
 
 " The solid Earth whereon we tread 
 In tracts of fluent heat began, 
 And grew to seeming-random forms, 
 The seeming prey of cyclic storms." 
 
 Let us look back at that part of the earth's history, and 
 see whether the long periods which we have contemplated 
 may not be matched and more than matched by the aeons 
 which preceded them. When we thus 
 
 " Contemplate all this work of Time 
 The giant labouring in his youth," 
 
 we see how far we have been from recognising the true 
 breadth of the mighty waves on one of which the life upon 
 this earth has been borne, we see that as yet we have not 
 
 " Come on that which is, and caught 
 The deep pulsations of the world 
 ^Eonian music measuring out 
 The steps of time." 
 
 Taking as the extremest span of the past existence of life 
 upon the earth ten millions of years, we learn from the re- 
 searches of physicists that the age preceding that of life 
 (the age during which the world was a mass of molten 
 rock) lasted more than thirty-five times as long, since 
 Bischof has shown that the earth would require three hun- 
 dred and fifty millions of years to cool down from a tem- 
 perature of 2,000 C. to 200 C. But far back beyond the 
 commencement of that vast era, our earth existed as a 
 nebulous mass, nor can we form even a conjecture as yet 
 respecting the length of time during which that earlier 
 stage of the earth's existence continued. 
 
 So much for the past. Of the future we know less. 
 But still we recognise, not indistinctly, a time when all life 
 will have ceased upon the earth. Whether by the process 
 of refrigeration which is going on, or by the gradual ex- 
 haustion of the forces which at present reside in the earth, 
 
A NEW THEORY OF LIFE IN OTHER WORLDS 95 
 
 or by the change in the length of the day which we know 
 to be slowly taking place, a time must come when the con- 
 dition of our earth will no longer be suited for the support 
 of life. Or it may be that Stanilas Meunier is right in 
 his theory that as a planet grows older, the oceans, and 
 even the atmosphere, are gradually withdrawn into the in- 
 terior of the planet's globe, where space is formed for them 
 by the cooling and contracting of the solid frame of the 
 planet. But apart from all such considerations, we know 
 that a process of exhaustion is taking place, even in the 
 sun himself, whence all that exists upon the earth derives 
 its life and daily nourishment. So that indirectly by the 
 dying out of the source of life, if not directly by the dying 
 out of life, this earth must one day become as bleak and 
 desolate a scene as we believe the moon to be at this 
 present time. 
 
 It is easy to recognise the bearing of these considera- 
 tions upon the question of life in other worlds. We had 
 been led, by the contemplation of the long continuance of 
 life upon this earth, to regard the support of life as in 
 a sense the object of planetary existence, and therefore to 
 view the other planets as the abode of life. But we now 
 see that the time during which life has existed on the earth 
 has been a mere wavelet in the sea of our earth's lifetime, 
 this sea itself being but a minute portion of the infinite 
 ocean of time, while, as Tyndall has well remarked, in that 
 infinite ocean, the history of man (the sole creature known 
 to us that can appreciate the wonders of creation) is but 
 the merest ripple. We learn, then, from the earth's his- 
 tory, a lesson the very reverse of that which before we had 
 seemed so clearly to read there. It is not the chief, but 
 only a minute portion of the earth's existence which has 
 been characterized by the existence of life upon our globe; 
 and if we adopted the teaching now brought before us, as 
 readily as before we learned that other lesson, we should 
 say, " It is not the chief, but only an utterly subordinate 
 part of Nature's purpose, to provide for the existence and 
 support of life." 
 
96 PROCTOR 
 
 We have been led by the study of the probable past 
 history of the earth, and by the consideration of her prob- 
 able future fortunes, to the conclusion that although life 
 has existed on her surface for an enormously long period, 
 and will continue for a corresponding period in the future, 
 yet the whole duration of life must be regarded but as a 
 wave on the vast ocean of time, while the duration of the 
 life of creatures capable of reasoning upon the wonders 
 which surround them, is but as a ripple upon the surface 
 of such a wave. It matters little then whether we take life 
 itself, without distinction of kind or order, or whether we 
 take only the life of man, we still find a disproportion which 
 must be regarded as practically infinite, between the dura- 
 tion of such life, and the duration of the preceding and 
 following periods when there has been and will be no such 
 life upon the earth. 
 
 But yet, in passing, I can not but point to the fact that 
 in considering the usual arguments of life in other worlds, 
 I might limit myself to the existence of rational beings. 
 It would be difficult to show that mere life, without the 
 power which man possesses of appreciating the wonders 
 of the universe, is a more fitting final purpose in creation 
 than the existence of lifeless but moving masses like the 
 suns and their attendant planets. The insect or the fish, 
 the bird or the mammal, the minutest miscroscopic animal- 
 cule or the mightiest cetacean, may afford suggestive indica- 
 tions of what we describe as beneficent contrivance; yet it 
 is hard to see in what essential respect a universe of worlds 
 beyond our own, inhabited only by such animals, would 
 accord better w r ith those ideas which the believers in the 
 plurality of worlds entertain respecting the purpose of the 
 Almighty, than a universe with none but vegetable life, or 
 a universe with no life at all, yet replete with wonderful and 
 wonderfully moving masses of matter. It is rational life 
 alone to which the arguments of our Brewsters and Chal- 
 mers really relate. Nor would it be difficult to raise here 
 another perplexing consideration, by inquiring what de- 
 gree of cultivation of the intellect in human races accords 
 
A NEW THEORY OF LIFE IN OTHER WORLDS 97 
 
 with the " argument from admiration " which the followers 
 of Brewster delight to employ. The savage engaged in 
 the mere effort to support life or to combat his foes, knows 
 nothing of the glories whereof science tells us. The won- 
 ders of Nature, so far as they affect him at all, tend to give 
 ignoble and debasing ideas of the being or beings to whose 
 power he attributes the occurrence of natural phenomena. 
 Nor, as we advance in the scale of civilization, do we 
 quickly arrive at the stage where the admiration of Nature 
 begins to be an ordinary exercise even of a few minds. 
 Still less do we arrive quickly, even in reviewing the 
 progress of the most civilized races, at the stage when the 
 generality of men give much of their thoughts to the 
 natural wonders which surround them. Is it saying too 
 much to assert that this stage has never yet been attained 
 by any nation, even the most advanced and the most cul- 
 tured? If we limit ourselves, however, to the existence 
 merely of some few nations, among whom the study of 
 Nature has been more or less in vogue, how brief in the 
 history of this earth has been the period when such nations 
 have existed! how brief the continuance of those among 
 such nations which belong to the past, and whose whole 
 history is thus known to us! how few even in such nations 
 the men who have been so deeply impressed with the won- 
 ders of Nature as to be led to the utterance of their 
 thoughts! If the life of man is but as a ripple where life 
 itself is as a wave on the ocean of time, surely the life 
 of man as the student and admirer of Nature is but as the 
 tiniest of wave-crests upon the ripple of human life. 
 
 How, then, does all this bear upon the question of life 
 in other worlds? The answer will be manifest if we apply 
 to these considerations the same argument which Brewster 
 and Chalmers have applied to the evidence which indicates 
 the enormous duration of life upon the earth. Since this 
 enormous duration, taking life even in its most general 
 aspect, has been shown to be as a mere nothing by com- 
 parison with the practically infinite duration of the earth 
 without life, the argument as respects life in any other 
 7 
 
98 PROCTOR 
 
 world (at least, in any world of which antecedently we know 
 nothing) must be directly reversed. It is far more probable 
 that that world is now passing through a part of the stage 
 preceding the appearance of life, or of the stage following 
 the appearance of life, than that this particular epoch be- 
 longs to the period when that particular world is inhabited. 
 If, indeed, we had some special reason for believing that 
 this epoch to which terrestrial life belongs has some special 
 importance as respects the whole universe, we might feel 
 unwilling to consider the question of life in any other world 
 independently of preconceptions derived from our experi- 
 ence in this world. But I apprehend that we have no rea- 
 son whatever for so believing. It appears to me that such 
 a belief that is, the belief that life in this earth corre- 
 sponds with a period special for the universe itself is as 
 monstrous as the old belief that our earth is the centre of 
 the universe. It is, in fact, a belief which bears precisely the 
 same relation to time that the last-mentioned belief bears 
 to space. According to one belief, the minute space occu- 
 pied by our earth was regarded as the central and most 
 important part of all space, and the only part which the 
 Creator had specially in his plans, so to speak, in creating 
 the universe; according to the other, the minute time 
 occupied by the existence of life on the earth is the central 
 and most important part of all time, and the only part 
 during which the Creator intended that living creatures 
 should exist anywhere. Both ideas are equally untenable, 
 though one only has been formally discarded. 
 
 This present time, then, is a random selection, so to 
 speak, regarded with reference to the existence of life in 
 any other world, and being a random selection, it is much 
 more likely to belong to the period when there is no life 
 there. Let me illustrate my meaning by an example. Sup- 
 pose I know that a friend of mine, living at a distance, will 
 be at home for six minutes exactly, some time between 
 noon and ten on any given day, but that I have no means 
 of forming any opinion as to when the six minutes will be. 
 Then, if at any given moment, say at three, I ask myself 
 
A NEW THEORY OF LIFE IN OTHER WORLDS 99 
 
 the question, " Is my friend at home? " although I can not 
 know, I can form an opinion as to the probability of his 
 being so. There are six hundred minutes between noon 
 and ten, and he is to be at home only six minutes, or the 
 one-hundredth part of the time; accordingly, the chance 
 that he is at home is one in a hundred, or speaking in a 
 general way it is much more likely that he is not at home 
 than that he is. And so precisely with any given planet, 
 apart from any evidence we may have as to its condition 
 what we know about life on our earth teaches us that the 
 probability is exceedingly minute that that planet is in- 
 habited. The argument is the favourite argument from 
 analogy. Thus: life on our earth lasts but a very short 
 time compared with the duration of the earth's existence; 
 therefore life in any given planet lasts but a very short time 
 compared with the planet's existence; accordingly, the 
 probability that that planet is inhabited at this present mo- 
 ment of time is exceedingly small, being, in fact, as the 
 number of years of life to the number of years without life, 
 or as one chance in many hundreds at the least. 
 
 This applies to the planets of our solar system only in 
 so far as we are ignorant of their condition. We may know 
 enough about some of them to infer either a much higher 
 probability that life exists, or almost certainly that life can 
 not exist. Thus we may view the condition of Venus or 
 Mars as perchance not differing so greatly from that of 
 our earth as to preclude the probability that many forms 
 of life may exist on those planets. Or, on the other hand, 
 we may believe from what we know about Jupiter and 
 Saturn that both these planets are still passing through the 
 fiery stages which belong to the youth of planet life; while 
 in our moon we may see a world long since decrepit, and 
 now utterly unfit to support any forms of animated exist- 
 ence. But even in the case of our solar system, though the 
 evidence in some cases against the possibility of life is ex- 
 ceedingly strong, we do not meet with a single instance 
 in which evidence of the contrary kind is forcible, still less 
 decisive. So that in the solar system the evidence is almost 
 
100 PROCTOR 
 
 as clear in favour of the conclusion above indicated as where 
 we reason about worlds of whose actual condition we know 
 nothing. As respects such worlds that is, as respects the 
 members of those systems of worlds which circle, as we 
 believe (from analogy), around other suns than ours the 
 probability that any particular world is inhabited at this 
 present time is exceedingly small. 
 
 But let us next consider what is the probability that 
 there is life on some member or other of a scheme of worlds 
 circling around any given sun. Here, again, the argument 
 is from analogy, being derived from what we have learned 
 or consider probable in the case of our own system. And I 
 think we may adopt as probable some such view as I shall 
 now present. Each planet, according to its dimensions, 
 has a certain length of planetary life, the youth and age 
 of which include the following eras: a sunlike state; a state 
 like that of Jupiter or Saturn, when much heat but little 
 light is evolved; a condition like that of our earth; and 
 lastly, the stage through which our moon is passing, which 
 may be regarded as planetary decrepitude. In each case of 
 world existences the various stages may be longer or 
 shorter, as the whole existence is longer or shorter, so that, 
 speaking generally, the period of habitability bears the same 
 proportion in each world to the whole period of its exist- 
 ence; or perhaps there is no such uniform proportion, 
 while, nevertheless, there exists in all cases that enormous 
 excess of the period when no life is possible over the period 
 of habitability. In either case, it is manifest that regarding 
 the system as a whole, now one, now another planet (or 
 more generally, now one, now another member of the sys- 
 tem) would be the abode of life, the smaller and shorter- 
 lived having their turn first, then larger and larger mem- 
 bers, until life has existed on the mightiest of the planets, 
 and even at length upon the central sun himself. We need 
 not concern ourselves specially with the peculiarities affect- 
 ing the succession of life in the case of subordinate systems, 
 or of the members of the asteroidal family, or in other cases 
 where we have little real knowledge to guide us: the gen- 
 
A NEW THEORY OF LIFE IN OTHER WORLDS ioi 
 
 eral conclusion remains the same, that life would appear 
 successively in planet after planet, step by step from the 
 smaller to the larger, until the approach of the last scene 
 of all, when life would have passed from all the planets, and 
 our sun would alone remain to be in due time inhabited, 
 and then in turn to pass (by time intervals to us practically 
 infinite) to decrepitude and death. 
 
 During all this progression, the intervals without life 
 would in all probability be far longer than those when one 
 or other planet was inhabited. In fact, the enormous ex- 
 cess of the lifeless periods for our earth over the period 
 of habitability renders the conclusion all but certain that 
 the lifeless gaps in the history of the solar system must last 
 very much longer than the periods of life (in this or that 
 planet) with which they would alternate. 
 
 If we apply this conclusion to the case of any given star 
 or sun with its scheme of dependent worlds, we see that 
 even for a solar system so selected at random the proba- 
 bility of the existence of life is small. It is, of course, 
 greater than for a single world taken at random just as 
 if I had ten friends who were to be at home each for six 
 minutes between noon and ten, the chance would be greater 
 that some one of the number would be at home at a given 
 moment of that interval than would be the chance that a 
 given one of the number would be then at home; while 
 yet even taking all the ten it would still be more likely than 
 not that at that moment not one would be at home. 
 
 Thus when we look at any star, we may without im- 
 probability infer that at the moment that star is not sup- 
 porting life in any one of those worlds which probably 
 circle round it. 
 
 Have we then been led to the Whewellite theory that 
 our earth is the sole abode of life? Far from it. For not 
 only have we adopted a method of reasoning which teaches 
 us to regard every planet in existence, every moon, every 
 sun, every orb in fact in space, as having its period as the 
 abode of life, but the very argument from probability which 
 leads us to regard any given sun as not the centre of a 
 
102 PROCTOR 
 
 scheme in which at this moment there is life, forces upon 
 us the conclusion that among the millions on millions, nay, 
 the millions of millions of suns which people space, mil- 
 lions have orbs circling round them which are at this pres- 
 ent time the abode of living creatures. If the chance is 
 one in a thousand in the case of each particular star, then 
 in the whole number (practically infinite) of stars, one in 
 a thousand has life in the system which it rules over: and 
 what is this but saying that millions of stars are life-sup- 
 porting orbs? There is then an infinity of life around us, 
 although we recognise infinity of time as well as infinity of 
 space as an attribute of the existence of life in the universe. 
 And remembering that as life in each individual is finite, in 
 each planet finite, in each solar system finite, and in each sys- 
 tem of stars finite, so (to speak of no higher orders) the infin- 
 ity of life itself demonstrates the infinity of barrenness, the 
 infinity of habitable worlds implies the infinity of worlds not 
 as yet habitable, or which have long since passed their 
 period of inhabitability. Yet is there no waste, whether of 
 time, of space, of matter, or of force; for waste implies a 
 tending toward a limit, and therefore of these infinities, 
 which are without limits, there can be no waste. 
 
MATHEMATICAL THEORIES 
 OF THE EARTH 
 
 BY 
 
 ROBERT SIMPSON WOODWARD 
 
THE MATHEMATICAL THEORIES OF 
 THE EARTH 1 
 
 THE name of this section, which by your courtesy it 
 is my duty to address to-day, implies a community 
 of interest among astronomers and mathematicians. 
 This community of interest is not difficult to explain. We 
 can, of course, imagine a considerable body of astronomical 
 facts quite independent of mathematics. We can also im- 
 agine a much larger body of mathematical facts quite inde- 
 pendent of and isolated from astronomy. But we never 
 think of astronomy in the large sense without recognising 
 its dependence on mathematics, and we never think of 
 mathematics as a whole without considering its capital 
 applications in astronomy. 
 
 Of all the subjects and objects of common interest to 
 us, the earth will easily rank first. The eart'h furnishes us 
 with a stable foundation for instrumental work and a fixed 
 line of reference, whereby it is possible to make out the 
 orderly arrangement and procession of our solar system 
 and to gain some inkling of other systems which lie within 
 telescopic range. The earth furnishes us with a most at- 
 tractive store of real problems; its shape, its size, its mass, 
 its precession and nutation, its internal heat, its earthquakes 
 and volcanoes, and its origin and destiny, are to be classed 
 with the leading questions for astronomical and mathe- 
 matical research. We must of course recognise the claims 
 of our friends the geologists to that indefinable something 
 
 1 Vice-presidential address before the Section of Mathematics and 
 Astronomy of the American Association for the Advancement of Sci- 
 ence at the Toronto meeting, August, 1889. (From the " Proceedings 
 of the American Association for the Advancement of Science," vol. 
 xxxviii.) 
 
 105 
 
I0 6 WOODWARD 
 
 called the earth's crust, but considered in its entirety and 
 in its relations to similar bodies of the universe, the earth 
 has long been the special province of astronomers and 
 mathematicians. Since the times of Galileo and Kepler 
 and Copernicus it has supplied a perennial stimulus to 
 observation and investigation, and it promises to tax the 
 resources of the ablest observers and analysts for some cen- 
 turies to come. The mere mention of the names of New- 
 ton, Bradley, D'Alembert, Laplace, Fournier, Gauss, and 
 Bessel, calls to mind not only a long list of inventions and 
 discoveries, but the most important parts of mathematical 
 literature. In its dynamical and physical aspects the earth 
 was to them the principal object of research, and the thor- 
 oughness and completeness of their contributions toward 
 an explanation of the " system of the world " are still a 
 source of wonder and admiration to all who take the 
 trouble to examine their works. 
 
 A detailed discussion of the known properties of the 
 earth, and of the hypotheses concerning the unknown 
 properties, is no fit task for a summer afternoon; the in- 
 tricacies and delicacies of the subject are suitable only for 
 another season and a special audience. But it has seemed 
 that a somewhat popular review of the state of our mathe- 
 matical knowledge of the earth might not be without in- 
 terest to those already familiar with the complex details, 
 and might also help to increase that general interest in sci- 
 ence, the promotion of which is one of the most important 
 functions of this association. 
 
 As we look back through the light of modern analysis, 
 it seems strange that the successors of Newton, who took 
 up the problem of the shape of the earth, should have 
 divided into hostile camps over the question whether our 
 planet is elongated or flattened at the poles. They agreed 
 in the opinion that the earth is a spheroid, but they de- 
 bated, investigated, and observed for nearly half a century 
 before deciding that the spheroid is oblate rather than 
 oblong. This was a critical question, and its decision marks 
 perhaps the most important epoch in the history of the 
 
MATHEMATICAL THEORIES OF THE EARTH 
 
 107 
 
 figure of the earth. The Newtonian view of the oblate form 
 found its ablest supporters in Huygens, Maupertuis, and 
 Clairaut, while the erroneous view was maintained with 
 great vigour by the justly distinguished Cassinian school 
 of astronomers. Unfortunately for the Cassinians, defect- 
 ive measures of a meridional arc in France gave colour 
 to the false theory and furnished one of the most conspicu- 
 ous instances of the deterring effect of an incorrect obser- 
 vation. As you well know, the point was definitely settled 
 by Maupertuis's measurement of the Lapland arc. For 
 this achievement his name has become famous in literature 
 as well as in science, for his friend Voltaire congratulated 
 him on having " flattened the poles and the Cassinis "; and 
 Carlyle has honoured him with the title of " Earth-flat- 
 tener." 2 
 
 Since the settlement of the question of the form 
 progress toward a knowledge of the size of the earth has 
 been consistent and steady, until now it may be said that 
 there are few objects with which we have to deal whose 
 dimensions are so well known as the dimensions of the 
 earth. But this is a popular statement, and, like most such, 
 needs to be explained in order not to be misunderstood. 
 Both the size and shape of the earth are defined by the 
 lengths of its equatorial and polar axes; and, knowing the 
 fact of the oblate spheroidal form, the lengths of the axes 
 may be found within narrow limits from simple measure- 
 ments conducted on the surface quite independently of any 
 knowledge of the interior constitution of the earth. It is 
 evident, in fact, without recourse to mathematical details, 
 that the length of any arc, as a degree of latitude or longi- 
 tude on the earth's surface, must depend on the lengths of 
 those axes. Conversely, it is plain that the measurement 
 of such an arc and the determination of its geographical 
 position constitute an indirect measurement of the axes. 
 Hence it has happened that scientific as distinguished from 
 practical geodesy has been concerned chiefly with such 
 
 s Todhunter, " History of the Theories of Attraction and the Figure 
 of the Earth," London, 1873, vol. i, art. 195. 
 
I0 g WOODWARD 
 
 linear and astronomical measurements, and the zeal with 
 which the work has been pursued is attested by triangula- 
 tions on every continent. Passing over the earlier deter- 
 minations as of historical interest only, all of the really 
 trustworthy approximations to the lengths of the axes have 
 been made within the half century just passed. The first 
 to appear of these approximations were the well-founded 
 values of Airy, 3 published in 1830. These, however, were 
 almost wholly overshadowed and supplanted eleven years 
 later by the values of Bessel, 4 whose spheroid came to 
 occupy a most conspicuous place in geodesy for more than 
 a quarter of a century. Knowing as we now do that Bes- 
 sel's values were considerably in error, it seems not a little 
 remarkable that they should have been so long accepted 
 without serious question. One obvious reason is found 
 in the fact that a considerable lapse of time was essential for 
 the accumulation of new data, but two other possible rea- 
 sons of a different character are worthy of notice because 
 they are interesting and instructive, whether specially ap- 
 plicable to this particular case or not. It seems not im- 
 probable that the close agreement of the values of Airy and 
 Bessel, computed independently and by different methods 
 the greatest discrepancy being about one hundred and 
 fifty feet may have been incautiously interpreted as a con- 
 firmation of Bessel's dimensions, and hence led to their 
 too ready adoption. It seems also not improbable that the 
 weight of Bessel's great name may have been too closely 
 associated in the minds of his followers with the weights 
 of his observations and results. The sanction of eminent 
 authority, especially if there is added to it the stamp of 
 an official seal, is sometimes a serious obstacle to real 
 progress. We can not do less than accord to Bessel the 
 first place among the astronomers and geodesists of his 
 day, but this is no adequate justification for the exaggerated 
 estimate long entertained of the precision of the elements 
 of his spheroid. 
 
 The next step in the approximation was the important 
 
 8 " Encyclopedia Metropolitana." 
 
 4 " Astronomische Nachrichten," No. 438, 1841. 
 
MATHEMATICAL THEORIES OF THE EARTH 109 
 
 one of Clarke 5 in 1866. His new values showed an in- 
 crease over Bessel's of about half a mile in the equatorial 
 semi-axis and about three tenths of a mile in the polar 
 semi-axis. Since 1866, General Clarke has kept pace with 
 the accumulating data and given us so many different ele- 
 ments for our spheroid that it is necessary to affix a date 
 to any of his values we may use. The later values, how- 
 ever, differ but slightly from the earlier ones, so that the 
 spheroid of 1866, which has come to be pretty generally 
 adopted, seems likely to enjoy a justly greater celebrity 
 than that of its immediate predecessor. The probable error 
 of the axes of this spheroid is not much greater than the 
 hundred thousandth part, 6 and it is not likely that new data 
 will change their lengths by more than a few hundred feet. 
 In the present state of science, therefore, it may be said 
 that the first order of approximation to the form and 
 dimensions of the earth has been successfully attained. 
 The question which follows naturally and immediately is, 
 How much further can the approximation be carried? 
 The answer to this question is not yet written, and the 
 indications are not favourable for its speedy announcement. 
 The first approximation, as w r e have seen, requires no knowl- 
 edge of the interior density and arrangement of the earth's 
 mass; it proceeds on the simple assumption that the sea sur- 
 face is closely spheroidal. The second approximation, if it 
 be more than a mere interpolation formula, requires a knowl- 
 edge of both density and arrangement of the constituents of 
 the earth's mass, and especially of that part called the crust. 
 " All astronomy/' says Laplace, " rests on the stability of 
 the earth's axis of rotation." 7 In a similar sense we may 
 say all geodesy rests on the direction of the plumb line. 
 The simple hypothesis of a spheroidal form assumes that 
 
 8 " Comparison of Standards of Length," made at the ordnance 
 office, Southampton, England, by Captain A. R. Clarke, R. E. Pub- 
 lished by order of the Secretary of State for War, 1866. 
 
 a Clarke, Colonel A. R., " Geodesy," Oxford. 1880, p. 319. 
 
 7 " Toute 1'Astronomie repose sur rinvariabilite de 1'axe de rotation 
 de la Terre a la surface du spheroide terrestre et sur 1'uniformite de 
 cette rotation." (" Mecanique Celeste," Paris, 1882, tome v, p. 22.) 
 
1 10 WOODWARD 
 
 the plumb line is everywhere coincident with the normal to 
 the spheroid, or that the surface of the spheroid coincides 
 with the level of the sea. But this is not quite correct. 
 The plumb line is not in general coincident with the normal, 
 and the actual sea level or geoid must be imagined to be 
 an irregular surface lying partly above and partly below the 
 ideal spheroidal surface. The deviations, it is true, are rela- 
 tively small, but they are in general much greater than the 
 unavoidable errors of observation and they are the exact 
 numerical expression of our ignorance in this branch of 
 geodesy. It is well known, of course, that deflections of 
 the plumb line can sometimes be accounted for by visible 
 masses, but on the whole it must be admitted that we pos- 
 sess only the vaguest notions of their cause and a most 
 inadequate knowledge of their distribution and extent. 
 
 What is true of plumb-line deflections is about equally 
 true of the deviations of the intensity of gravity from what 
 may be called the spheroidal type. Given a closely sphe- 
 roidal form of the sea level and it follows from the law of 
 gravitation, as a first approximation, without any knowl- 
 edge of the distribution of the earth's mass, that the in- 
 crease of gravity varies as the square of the sine of the 
 latitude in passing from the equator to the poles. This 
 is the remarkable theorem of Stokes, 8 and it enables us to 
 determine the form or ellipticity of the earth by means of 
 pendulum observations alone. It must be admitted, how- 
 ever, that the values of the ellipticity recently obtained in 
 this way by the highest authorities, Clarke 9 and Helmert, 10 
 are far from satisfactory, whether we regard them in the 
 light of their discrepancy or in the light of the different 
 methods of computing them. In general terms we may 
 say that the difficulty in the way of the use of pendulum 
 observations still hinges on the treatment of local anomalies 
 and on the question of reduction to sea level. At present, 
 
 8 Stokes, G. G., " Mathematical and Physical Papers," Cambridge 
 University Press, 1880, vol. ii. 
 
 '"Geodesy," chap. xiv. 
 
 10 Helmert, Dr. F. R., " Die Mathematischen und Physikalischen 
 Theorieen der Hoheren Geodasie," Leipsic, 1880, 1884, ii Teil. 
 
MATHEMATICAL THEORIES OF THE EARTH m 
 
 the case is one concerning which the doctors agree neither 
 in their diagnosis nor in their remedies. 
 
 Turning attention now from the surface toward the in- 
 terior, what can be said of the earth's mass as a whole, of 
 its laws of distribution, and of the pressures that exist at 
 great depths? Two facts namely, the mean density and 
 the surface density are roughly known; a third fact 
 namely, the precession constant, or the ratio of the differ- 
 ence of the two principal moments of inertia to the greater 
 of them is known with something like precision. These 
 facts lie within the domain of observation and require only 
 the law of gravitation for their verification. Certain in- 
 ferences, also, from these facts and others, have long been 
 and still are held to be hardly less cogent and trustworthy, 
 but before stating them it will be well to recall briefly the 
 progress of opinion concerning this general subject during 
 the past century and a half. 
 
 The conception of the earth as having been primitively 
 fluid was the prevailing one among mathematicians before 
 Clairaut published his " Theorie de la Figure de la Terre," 
 in 1743. By the aid of this conception Clairaut proved 
 the celebrated theorem which bears his name, and probably 
 no idea in the mechanics of the earth has been more sug- 
 gestive and fruitful. It was the central idea in the elaborate 
 investigations of Laplace and received at his hands a de- 
 velopment which his successors have found it about equally 
 difficult to displace or to improve. From the idea of 
 fluidity spring naturally the hydrostatical notions of pres- 
 sure and level surfaces, or the arrangement of fluid masses 
 in strata of uniform density. Hence follows, also, the no- 
 tion of continuity of increase in density from the surface 
 toward the centre of the earth. All of the principal me- 
 chanical properties and effects of the earth's mass viz., 
 the ellipticity, the surface density, the mean density, the 
 precession constant, and the lunar inequalities were cor- 
 related by Laplace 11 in a single hypothesis, involving only 
 one assumption in addition to that of original fluidity and 
 11 " Mecanique Celeste," tome v, livre xi. 
 
112 WOODWARD 
 
 the law of gravitation. This assumption relates to the com- 
 pressibility of matter and asserts that the ratio of the in- 
 crement of pressure to the increment of density is propor- 
 tional to the density. Many interesting and striking con- 
 clusions follow readily from this hypothesis, but the most 
 interesting and important are those relative to density and 
 pressure, especially the latter, whose dominance as a factor 
 in the mechanics of celestial masses seems destined to 
 survive whether the hypothesis stands or falls. The hy- 
 pothesis requires that, while the density increases slowly 
 from something less than three at the surface to about 
 eleven at the centre of the earth, the pressure within the 
 mass increases rapidly below the surface, reaching a value 
 surpassing the crushing strength of steel at the depth of 
 a few miles and amounting at the centre to no less than 
 three million atmospheres. The inferences, then, as dis- 
 tinguished from facts, are that the mass of the earth is very 
 nearly symmetrically disposed about its centre of gravity, 
 that pressure and density except near the surface are 
 mutually dependent, and that the earth in reaching this 
 stage has passed through the fluid or quasi-fluid state. 
 
 Later writers have suggested other hypotheses for a 
 continuous distribution of the earth's mass, but none of 
 them can be said to rival the hypothesis of Laplace. Their 
 defects lie either in not postulating a direct connection be- 
 tween density and pressure or in postulating a connection 
 which implies extreme or impossible values for these and 
 other mechanical properties of the mass. 
 
 It is clear, from the positiveness of his language in 
 frequent allusions to this conception of the earth, that La- 
 place was deeply impressed with its essential correctness. 
 " Observations/' he says, " prove incontestably that the 
 densities of the strata (couches) of the terrestrial spheroid 
 increase from the surface to the centre," 12 and " the regu- 
 
 u " Enfin il (Newton) regarde la terre comme homogene, ce qui est 
 contraire aux observations, qui prouvent. incontestablement que les 
 densites des couches du spheroide terrestre croissent de la surface 'au 
 centre." (" Mecanique Celeste," tome v, p. 9.) 
 
MATHEMATICAL THEORIES OF THE EARTH 
 
 larity with which the observed variation in length of a 
 second's pendulum follows the law of squares of the sines 
 of the latitudes proves that the strata are arranged sym- 
 metrically about the centre of gravity of the earth." 13 
 The more recent investigations of Stokes, to which allu- 
 sion has already been made, forbid our entertaining any- 
 thing like so confident an opinion of the earth's primi- 
 tive fluidity or of a symmetrical and continuous arrange- 
 ment of its strata. But, though it must be said that 
 the sufficiency of Laplace's arguments has been seriously 
 impugned, we can hardly think the probability of the 
 correctness of his conclusions has been proportionately 
 diminished. 
 
 Suppose, however, that we reject the idea of original 
 fluidity. Would not a rotating mass of the size of the earth 
 assume finally the same aspects and properties presented by 
 our planet? Would not pressure and centrifugal force suf- 
 fice to bring about a central condensation and a sym- 
 metrical arrangement of strata similar at least to that re- 
 quired by the Laplacian hypothesis? Categorical answers 
 to these questions can not be given at present. But, what- 
 ever may have been the antecedent condition of the earth's 
 mass, the conclusion seems unavoidable that at no great 
 depth the pressure is sufficient to break down the struc- 
 tural characteristics of all known substances, and hence 
 to produce viscous flow whenever and wherever the stress 
 difference exceeds a certain limit, which can not be large 
 in comparison with the pressure. Purely observational evi- 
 dence, also, of a highly affirmative kind in support of this 
 conclusion, is afforded 6y the remarkable results of Tresca's 
 experiments on the flow of solids and by the abundant 
 proofs in geology of the plastic movements and viscous 
 flow of rocks. With such views and facts in mind the fluid 
 stage, considered indispensable by Laplace, does not appear 
 
 " " La regularite avec laquelle la variation observee des longueurs 
 du pendule a secondes suit la loi du carre du sinus de la latitude prouve 
 que ces couches sont disposees regulierement autour du centre de 
 gravite de la terre et que leur forme est a peu pres elliptique et de revo- 
 lution." (" Mecanique Celeste," tome v, p. 17.) 
 8 
 
H 4 WOODWARD 
 
 necessary to the evolution of a planet, even if it reach the 
 extreme refinement of a close fulfilment of some such 
 mathematical law as that of his hypothesis. If, as is here 
 assumed, pressure be the dominant factor in such large 
 masses, the attainment of a stable distribution would be 
 simply a question of time. The fluid mass might take on 
 its normal form in a few days or a few months, whereas the 
 viscous mass might require a few thousand or a few mil- 
 lion years. 
 
 Some physicists and mathematicians, on the other hand, 
 reject both the idea of existence of great pressures within 
 the earth's mass, and the notion of an approach to con- 
 tinuity in the distribution of density. As representing this 
 side of the question the views of the late M. Roche, who 
 wrote much on the constitution of the earth, are worthy of 
 consideration. He tells us that the very magnitude of the 
 central pressure computed on the hypothesis of fluidity is 
 itself a peremptory objection to that hypothesis. 14 Accord- 
 ing to his conception, the strata of the earth from the 
 centre outward are substantially self-supporting and un- 
 yielding. It does not appear, however, that he had sub- 
 mitted this conception to the test of numbers, for a simple 
 calculation will show that no materials of which we have 
 any knowledge would sustain the stress in such shells or 
 domes. If the crust of the earth were self-supporting, its 
 crushing strength would Have to be about thirty times that 
 of the best cast steel, or five hundred to one thousand times 
 that of granite. The views of Roche on the distribution of 
 the terrestrial densities appear equally extreme. 15 He pre- 
 fers to consider the mass as made up of two distinct parts, 
 an outer shell or crust whose thickness is about one sixth 
 of the earth's radius, and a solid nucleus having little or no 
 central condensation. The nucleus is conceived to be 
 purely metallic, and to have about the same density as 
 iron. To account for geological phenomena, he postulates 
 
 14 " Memoire stir 1'etat interieur du globe terrestre," par M. douard 
 Roche; " Memoires de la section des sciences de 1' Academic des Sci- 
 ences et Lettres de Montpellier," 1880-1884, tome x. " Ibid. 
 
MATHEMATICAL THEORIES OF THE EARTH 115 
 
 a zone of fusion separating the crust from the nucleus. 
 The whole hypothesis is consistently worked out in con- 
 formity with the requirements of the ellipticity, the super- 
 ficial density, the mean density, and precession; so that to 
 one who can divest his mind of the notion that pressure 
 and continuity are important factors in the mechanics of 
 such masses, the picture which Roche draws out of the 
 constitution of our planet will present nothing incon- 
 gruous. 
 
 In a field so little explored and so inaccessible, though 
 hedged about as we have seen by certain sharply limiting 
 conditions, there is room for a wide range of opinion and 
 for great freedom in the play of hypothesis; and although 
 the preponderance of evidence appears to be in favour of 
 a terrestrial mass in which the reign of pressure is well- 
 nigh absolute, we should not be surprised a few decades 
 or centuries hence to find many of our notions on this sub- 
 ject radically defective. 
 
 If the problem of the constitution and distribution of 
 the earth's mass is yet an obscure and difficult one after 
 two centuries of observation and investigation, can we re- 
 port any greater degree of success in the treatment of that 
 still older problem of the earth's internal heat; of its origin 
 and effects? Concerning phenomena always so impressive 
 and often so terribly destructive as those intimately con- 
 nected with the terrestrial store of heat, it is natural that 
 there should be a considerable variety of opinion. The 
 consensus of such opinion, however, has long been in fa- 
 vour of the hypothesis that heat is the active cause of many 
 and a potent factor in most of the grander phenomena 
 which geologists assign to the earth's crust; and the pre- 
 vailing interpretation of these phenomena is based on the 
 assumption that our planet is a cooling sphere whose outer 
 shell or crust is constantly cracked and crumpled in adjust- 
 ing itself to the shrinking nucleus. 
 
 The conception that the earth was originally an in- 
 tensely heated and molten mass appears to have first taken 
 something like definite form in the minds of Leibnitz and 
 
Il6 WOODWARD 
 
 Descartes. 16 But neither of these philosophers was armed 
 with the necessary mathematical equipment to subject this 
 conception to the test of numerical calculation. Indeed, 
 it was not fashionable in their day, any more than it is with 
 some philosophers in ours, to undertake the drudgery of 
 applying the machinery of analysis to the details of a 
 hypothesis. Nearly a century elapsed before an order of 
 intellects capable of dealing with this class of questions ap- 
 peared. It was reserved for Joseph Fournier to lay the 
 foundation and build a great part of the superstructure of 
 our modern theory of heat diffusion, his avowed desire 
 being to solve the great problem of terrestrial heat. " The 
 question of terrestrial temperatures," he says, " has always 
 appeared to us one of the grandest objects of cosmological 
 studies, and we have had it principally in view in estab- 
 lishing the mathematical theory of heat." 17 This ambition, 
 however, was only partly realized. Probably Fournier 
 underestimated the difficulties of his problem, for his most 
 ingenious and industrious successors in the same field have 
 made little progress beyond the limits he attained. But 
 the work he left is a perennial index to his genius. Though 
 quite inadequately appreciated by his contemporaries, the 
 " Analytical Theory of Heat," which appeared in 1820, is 
 now conceded to be one of the epoch-making books. In- 
 deed, to one who has caught the spirit of the extraordinary 
 analysis which Fournier developed and illustrated by nu- 
 merous applications in this treatise, it is evident that he 
 opened a field whose resources are still far from being ex- 
 hausted. A little later Poisson took up the same class of 
 questions and published another great work on the mathe- 
 matical theory of heat. 18 Poisson narrowly missed being 
 
 18 Protegee, ou de la formation et des revolutions du globe, par 
 Leibnitz, ouvrage traduite . . . avec tine introduction et des notes 
 par le Dr. Bertrand de Saint-Germain, Paris, 1859. 
 
 17 " La question des temperatures terrestres nous a toujours paru 
 un des plus grands objets des etudes cosmologiques, et nous 1'avions 
 principalement en vue en etablissant la theorie mathematique de la 
 chaleur." (" Annales de Chimie et de Physique," 1824, tome xxvii, 
 p. 159.^ 
 
 18 " Theorie Mathematique de la Chaleur," Paris, 1835. 
 
MATHEMATICAL THEORIES OF THE EARTH 
 
 the foremost mathematician of his day. In originality, in 
 wealth of mathematical resources, and in breadth of grasp 
 of physical principles he was the peer of the able'st of his 
 contemporaries. In lucidity of exposition it would be 
 enough to say that he was a Frenchman, but he seems to 
 have excelled in this peculiarly national trait. His contri- 
 butions to the theory of heat have been somewhat over- 
 shadowed in recent times by the earlier and perhaps more 
 brilliant researches of Fournier, but no student can afford 
 to take up that enticing though difficult theory without the 
 aid of Poisson as well as Fournier. 
 
 It is natural, therefore, that we should inquire what 
 opinions these great masters in the mathematics of heat 
 diffusion held concerning the earth's store of heat. I say 
 opinions, for unhappily this whole subject is still so largely 
 a matter of opinion that, in discussing it, one may not inap- 
 propriately adopt the famous caution of Marcus Aurelius, 
 " Remember that all is opinion." It does not appear that 
 Fournier reached any definite conclusion on this question, 
 though he seems to have favoured the view that the earth 
 in cooling from an earlier state of incandescence reached 
 finally through convection a condition in which there was 
 a uniform distribution of heat throughout its mass. This 
 is the consistentior status of Leibnitz, and it begins with 
 the formation of the earth's crust, if not with the consolida- 
 tion of the entire mass. It thus affords an initial distribu- 
 tion of heat and an epoch from which analysis may start, 
 and the problem for the mathematician is to assign the sub- 
 sequent distribution of heat and the resulting mechanical 
 effects. But no great amount of reflection is necessary to 
 convince one that the analysis can not proceed without 
 making a few more assumptions. The assumptions which 
 involve the least difficulty, and which for this reason partly 
 have met with most favour, are that the conductivity and 
 thermal capacity of the entire mass remain constant, and 
 that the heat conducted to the surface of the earth passes 
 off by the combined process of radiation, convection, and 
 conduction, without producing any sensible effect on sur- 
 
Il8 WOODWARD 
 
 rounding space. These or similar assumptions must be 
 made before the application of theory can begin. In addi- 
 tion, two data are essential to numerical calculations 
 namely, the diffusivity, or ratio of the conductivity of the 
 mass to its thermal capacity, and the initial uniform tem- 
 perature. The first of these can be observed, approxi- 
 mately, at least; the second can only be estimated at pres- 
 ent. With respect to these important points which must 
 be considered after the adoption of the consistentior status, 
 the writings of Fournier afford little light. He was content, 
 perhaps, to invent and develop the exquisite analysis requi- 
 site to the treatment of such problems. 
 
 Poisson wrote much on the whole subject of terrestrial 
 temperatures, and carefully considered most of the trouble- 
 some details which lay between his theory and its applica- 
 tion. While he admitted the nebular hypothesis and an 
 initial fluid state of the earth, he rejected the notion that 
 the observed increase of underground temperature is due 
 to a primitive store of heat. If the earth was originally 
 fluid by reason of its heat, a supposition which Poisson re- 
 garded quite gratuitous, he conceived that it must cool and 
 consolidate from the centre outward; 19 so that according 
 to this view the crust of our planet arrived at a condition 
 of stability only after the supply of heat had been exhausted. 
 But Poisson was not at a loss to account for the observed 
 temperature gradient in the earth's crust. Always fertile in 
 hypotheses, he advanced the idea that there exists, by rea- 
 son of interstellar radiations, great variations in the tem- 
 perature of space, some vast regions being comparatively 
 cool and others intensely hot, and that the present store of 
 terrestrial heat was acquired by a journey of the solar sys- 
 tem through one of the hotter regions. " Such is," he says, 
 " in my opinion, the true cause of the augmentation of 
 temperature which occurs as we descend below the surface 
 of the globe." 20 This hypothesis was the result of Pois- 
 
 19 " Theorie Mathematique de la Chaleur," Supplement de, Paris, 
 1837. 
 
 20 " Telle est, dans mon opinion, la cause veritable de 1'augmentation 
 
MATHEMATICAL THEORIES OF THE EARTH 
 
 son's mature reflection, and as such is well worthy of atten- 
 tion. The notion that there exist hot foci in space was 
 advanced also in another form in 1852 by Rankine, in his 
 interesting speculation on the reconcentration of energy. 
 But whatever we may think of the hypothesis as a whole, 
 it does not appear to be adequate to the case of the earth 
 unless we suppose the epoch of transit through the hot 
 region exceedingly remote and the temperature of that 
 region exceedingly high. The continuity of geological and 
 paleontological phenomena is much better satisfied by the 
 Leibnitzian view of an earth long subject to comparatively 
 constant surface conditions, but still active with the energy 
 of its primitive heat. 
 
 Notwithstanding the indefatigable and admirable la- 
 bours of Fournier and Poisson in this field, it must be 
 admitted that they accomplished little more than the prepa- 
 ration of the machinery with which their successors have 
 sought and are still seeking to reap the harvest. The dif- 
 ficulties which lay in their way were not mathematical but 
 physical. Had they been able to make out the true condi- 
 tions of the earth's store of heat, they would undoubtedly 
 have reached a high grade of perfection in the treatment of 
 the problem. The theory as they left it was much in 
 advance of observation, and the labours of their successors 
 have therefore necessarily been directed largely toward the 
 determination of the thermal properties of the earth's crust 
 and mass. 
 
 Of those who in the present generation have con- 
 tributed to our knowledge and stimulated the investigation 
 of this subject, it is hardly necessary to say that we owe 
 most to Sir William Thomson. He has made the question 
 of terrestrial temperatures highly attractive and instructive 
 to astronomers and mathematicians, and not less warmly in- 
 teresting to geologists and paleontologists. Whether we 
 are prepared to accept his conclusions or not, we must all 
 
 de temperature qui a lieu sur chaque verticale a mesure que Ton 
 s'abaisse au-dessous de la surface du globe." (" Theorie Mathematique 
 de la Chaleur," Supplement de, p. 15.) 
 
I20 WOODWARD 
 
 acknowledge our indebtedness to the contributions of his 
 master hand in this field, as well as in most other fields of 
 terrestrial physics. The contribution of special interest to 
 us in this connection is his remarkable memoir on the 
 secular cooling of the earth. 21 In this memoir he adopts 
 the simple hypothesis of a solid sphere whose thermal 
 properties remain invariable while it cools by conduction 
 from an initial state of uniform temperature, and draws 
 therefrom certain striking limitations on geologic time. 
 Many geologists were startled by these limitations, and 
 geologic thought and opinion have since been widely in- 
 fluenced by them. It will be of interest, therefore, to state 
 a little more fully and clearly the grounds from which his 
 arguments proceed. Conceive a sphere having a uniform 
 temperature initially, to cool in a medium which instantly 
 dissipates all heat brought by conduction to its surface, 
 thus keeping the surface at a constant temperature. Sup- 
 pose we have given the initial excess of the sphere's tem- 
 perature over that of the medium. Suppose also that the 
 capacity of the mass of the sphere for the diffusion of heat 
 is known, and known to remain invariable during the 
 process of cooling. This capacity is called diffusivity, and 
 is a constant which can be observed. Then from these data 
 the distribution of temperature at any future time can be 
 assigned, and hence also the rate of temperature increase, 
 or the temperature gradient, from the surface toward the 
 centre of the sphere can be computed. It is tolerably cer- 
 tain that the heat conducted from the interior to the surface 
 of the earth does not set up any reaction which in any 
 sensible degree retards the process of cooling. It escapes 
 so freely that, for practical purposes, we may say it is in- 
 stantly dissipated. Hence, if we can assume that the earth 
 had a specified uniform temperature at the initial epoch, 
 and can assume its diffusivity to remain constant, the whole 
 history of cooling is known so soon as we determine the 
 diffusivity and the temperature gradient at any point. 
 
 81 " Transactions of the Royal Society of Edinburgh," 1862. Thom- 
 son and Tait's " Natural Philosophy," vol. i, part ii, Appendix D. 
 
MATHEMATICAL THEORIES OF THE EARTH 121 
 
 Now, Sir William Thomson determined a value for the 
 diffusivity from measurements of the seasonal variations 
 of underground temperatures, and numerous observations 
 of the increase of temperature with depth below the earth's 
 surface gave an average value for the temperature gradient. 
 From these elements, and from an assumed initial tem- 
 perature of 7,000 Fahr., he infers that geologic time is 
 limited to something between 20,000,000 and 400,000,000 
 years. He says: " We must allow very wide limits in such 
 an estimate as I have attempted to make; but I think we 
 may with much probability say that the consolidation can 
 not have taken place less than 20,000,000 years ago, or 
 we should have more underground heat than we actually 
 have, nor more than 400,000,000 years ago, or we should 
 not have so much as the least observed underground incre- 
 ment of temperature. That is to say, I conclude that 
 Leibnitz's epoch of emergence of the consistentior status 
 was probably between those dates." These conclusions 
 were announced twenty-seven years ago and were repub- 
 lished without modification in 1883. Recently, also, Pro- 
 fessor Tait, reasoning from the same basis, has insisted 
 with equal confidence on cutting down the upper limit of 
 geologic time to some such figures as 10,000,000 or 
 15,000,000 years. 22 As mathematicians and astronomers, 
 we must all confess to a deep interest in these conclusions 
 and the hypothesis from which they flow. They are very 
 important if true. But what are the probabilities? Hav- 
 ing been at some pains to look into this matter, I feel 
 bound to state that, although the hypothesis appears to be 
 the best which can be formulated at present, the odds are 
 against its correctness. Its weak links are the unverified 
 assumptions of an initial uniform temperature and a con- 
 stant diffusivity. Very likely these are approximations, but 
 of what order we can not decide. Furthermore, if we ac- 
 cept the hypothesis, the odds appear to be against the pres- 
 ent attainment of trustworthy numerical results, since the 
 data for calculation, obtained mostly from observations 
 K " Recent Advances in Physical Science," London, 1876. 
 
122 WOODWARD 
 
 on continental areas, are far too meagre to give satisfac- 
 tory average values for the entire mass of the earth. In 
 short, this phase of the case seems to stand about where 
 it did twenty years ago, when Huxley warned us that the 
 perfection of our mathematical mill is no guarantee of 
 the quality of the grist, adding that, " as the grandest 
 mill will not extract wheat flour from peascods, so pages 
 of formulae will not get a definite result out of loose 
 data." 23 
 
 When we pass from the restricted domain of quantita- 
 tive results concerning geologic time to the freer domain 
 of qualitative results of a general character, the contrac- 
 tional theory of the earth may be said still to lead all others, 
 though it seems destined to require more or less modifica- 
 tion if not to be relegated to a place of secondary impor- 
 tance. Old, however, as is the notion that the great sur- 
 face irregularities of the earth are but the outward evidence 
 of a crumpling crust, it is only recently that this notion has 
 been subjected to mathematical analysis on anything like 
 a rational basis. About three years ago Mr. T. Mellard 
 Reade 24 announced the doctrine that the earth's crust 
 from the joint effect of its heat and gravitation should be- 
 have in a way somewhat analogous to a bent beam, and 
 should possess at a certain depth a " level of no strain " 
 corresponding to the neutral surface in a beam. Above 
 the level of no strain, according to this doctrine, the strata 
 *will be subjected to compression and will undergo crum- 
 pling, while below that level the tendency of the strata to 
 crack and part is overcome by pressure which produces 
 what Reade calls " compressive extension," thus keeping 
 the nucleus compact and continuous. A little later the 
 same idea was worked out independently by Mr. Charles 
 Davison, 25 and it has since received elaborate mathematical 
 
 n " Geological Reform " (" The Anniversary Address to the Geo- 
 logical Society for 1869 "). 
 
 24 Reade, T. Mellard, " Origin of Mountain Ranges," London, 1886. 
 
 25 " On the Distribution of Strain in the Earth's Crust resulting 
 from Secular Cooling, with Special Reference to the Growth of Con- 
 tinents and the Formation of Mountain Chains." By Charles Davison, 
 
MATHEMATICAL THEORIES OF THE EARTH 123 
 
 treatment at the hands of Darwin, 28 Fisher, 27 and others. 
 The doctrine requires for its application a competent theory 
 of cooling, and hence can not be depended on at present 
 to give anything better than a general idea of the me- 
 chanics of crumpling and a rough estimate of the magni- 
 tudes of the resulting effects. Using Thomson's hypothe- 
 sis, it appears that the stratum of no strain moves down- 
 ward from the surface of the earth at a nearly constant rate 
 during the earlier stages of cooling, but more slowly dur- 
 ing later stages; its depth is independent of the initial tem- 
 perature of the earth; and if we adopt Thomson's value of 
 the diffusivity, it will be about two and a third miles below 
 the surface in 100,000,000 years from the beginning of 
 cooling, and a little more than fourteen miles below the 
 surface in 700,000,000 years. The most important infer- 
 ence from this theory is that the geological effects of secu- 
 lar cooling will be confined for a very long time to a com- 
 paratively thin crust. Thus if the earth is 100,000,000 
 years old, crumpling should not extend much deeper than 
 two miles. A test to which the theory has been subjected, 
 and one which some 2S consider crucial against it, is the 
 volumetric amount of crumpling shown by the earth at 
 the present time. This is a difficult quantity to estimate, 
 but it appears to be much greater than the theory can 
 account for. 
 
 The opponents of the contractional theory of the earth, 
 believing it quantitatively insufficient, have recently re- 
 vived and elaborated an idea first suggested by Babbage 29 
 and Herschel in explanation of the greater folds and move- 
 ments of the crust. This idea figures the crust as being in 
 a state bordering on hydrostatic equilibrium, which can 
 
 with a note by G. H. Darwin. " Philosophical Transactions," vol. 
 clxxviii (1887), A, pp. 231-249. 
 
 19 Ibid. 
 
 " Fisher, Rev. Osmond, " Physics of the Earth's Crust," second 
 edition, London, 1889, chap. viii. 
 
 M Notably, Rev. Osmond Fisher. See his " Physics of the Earth's 
 Crust," chap. viii. 
 
 w Appendix to the " Ninth Bridgewater Treatise " (by C. Babbage), 
 second edition, London, 1838. 
 
124 WOODWARD 
 
 not be greatly disturbed without a readjustment and con- 
 sequent movement of the masses involved. According to 
 this view, the transfer of any considerable load from one 
 area to another is followed sooner or later by a depression 
 over the loaded area and a corresponding elevation over 
 the unloaded one, and in a general way it is inferred that 
 the elevation of continental areas tends to keep pace with 
 erosion. The process by which this balance is maintained 
 has been called isostasy, 30 and the crust is said to be in an 
 isostatic state. The dynamics of the superficial strata with 
 the attendant phenomena of folding and faulting are thus 
 referred to gravitation alone, or to gravitation and what- 
 ever opposing force the rigidity of the strata may offer. 
 In a mathematical sense, however, the theory of isostasy 
 is in a less satisfactory state than the theory of con- 
 traction. As yet we can see only that isostasy is an 
 efficient cause if once set in action, but how it is started 
 and to what extent it is adequate remain to be deter- 
 mined. Moreover, isostasy does not seem to meet the 
 requirements of geological continuity, for it tends rapidly 
 toward stable equilibrium, and the crust ought therefore to 
 reach a state of repose early in geologic time. But there 
 is no evidence that such a state has been attained, and but 
 little if any evidence of diminished activity in crustal move- 
 ments during recent geologic time. Hence we infer that 
 isostasy is competent only on the supposition that it is 
 kept in action by some other cause tending constantly 
 to disturb the equilibrium which would otherwise result. 
 Such a cause is found in secular contraction, and it is not 
 improbable that these two seemingly divergent theories are 
 really supplementary. 
 
 Closely related to the questions of secular contraction 
 and the mechanics of crust movements are those vexed 
 questions of earthquakes, volcanism, the liquidity or solid- 
 ity of the interior, and the rigidity of the earth's mass as 
 
 30 Dutton, Captain C. E., " On some of the Greater Problem's of 
 Physical Geology," " Bulletin Philosophical Society of Washington," 
 vol. xi, pp. 51-64. 
 
MATHEMATICAL THEORIES OF THE EARTH 
 
 125 
 
 a whole; all questions of the greatest interest, but still lin- 
 gering on the battlefields of scientific opinion. Many of 
 the " thrice-slain " combatants in these contests would fain 
 risk being slain again; and whether our foundation be 
 liquid or solid, or, to speak more precisely, whether the 
 earth may not be at once highly plastic under the action of 
 long-continued forces and highly rigid under the action 
 of periodic forces of short period, it is pretty certain that 
 some years must elapse before the arguments will be con- 
 vincing to all concerned. The difficulties appear to be due 
 principally to our profound ignorance of the properties of 
 matter subject to the joint action of great pressure and 
 great heat. The conditions which exist a few miles be- 
 neath the surface of the earth are quite beyond the reach of 
 laboratory tests as hitherto developed, but it is not clear 
 how our knowledge is to be improved without resort to 
 experiments of a scale in some degree comparable with 
 the facts to be explained. In the mean time, therefore, we 
 may expect to go on theorizing, adding to the long list of 
 dead theories which mark the progress of scientific thought 
 with the hope of attaining the truth not so much by di- 
 rect discovery as by the laborious process of eliminating 
 error. 
 
 When we take a more comprehensive view of the prob- 
 lems presented by the earth, and look for light on their 
 solution in theories of cosmogony, the difficulties which 
 beset us are no less numerous and formidable than those 
 encountered along special lines of attack. Much progress 
 has recently been made, however, in the elaboration of such 
 theories. Roche, 31 Darwin, 32 and others have done much 
 to remove the nebulosity of Laplace's nebular hypothesis. 
 
 81 " Essai sur la Constitution et 1'origine du systeme solaire," par 
 M. Edouard Roche. " Memoires de 1' Academic des Sciences et Lettres 
 de Montpellier," tome viii, 1873. 
 
 82 " On the Precession of a Viscous Spheroid and on the Remote 
 History of the Earth," " Phil. Trans.," part ii, 1879. " On the Secular 
 Changes in the Elements of the Orbit of a Satellite revolving about a 
 Tidally Distorted Planet," "Phil. Trans.," part ii, 1880. "On the 
 Tidal Friction of a Planet attended by Several Satellites, and on the 
 Evolution of the Solar System," " Phil. Trans.," part ii, 1881. 
 
126 WOODWARD 
 
 Poincare 83 and Darwin 84 have gone far toward bridging 
 the gaps which have long rendered the theory of rotating 
 fluid masses incomplete. Poincare has, in fact, shown us 
 how a homogeneous rotating mass might, through loss of 
 heat and consequent contraction, pass from the spheroidal 
 form to the Jacobian ellipsoidal form, and thence, by rea- 
 son of its increasing speed of rotation, separate into two un- 
 equal masses. Darwin, starting with a swarm of meteorites 
 and gravitation as a basis, has reached many interesting and 
 instructive results in the endeavour to trace out the laws of 
 evolution of a planetary system. 35 But notwithstanding 
 the splendid researches of these and other investigators in 
 this field, it must be said that the real case of the solar 
 system, or of the earth and moon, still defies analysis; and 
 that the mechanics of the segregation of a planet from the 
 sun, or of a satellite from a planet, if such an event has 
 ever happened, or the mechanics of the evolution of a solar 
 system from a swarm of meteorites, are still far from being 
 clearly made out. 
 
 Time does not permit me to make anything but the 
 briefest allusion to the comparatively new science of mathe- 
 matical meteorology with its already considerable list of 
 well-defined theories pressing for acceptance or rejection. 
 Nor need I say more with reference to those older mathe- 
 matical questions of the tides and terrestrial magnetism than 
 that they are still unsettled. These and many other ques- 
 tions, old and new, might serve equally well to illustrate the 
 principal fact that this address has been designed to em- 
 phasize, namely, that the mathematical theories of the 
 earth already advanced and elaborated are by no means 
 complete, and that no mathematical Alexander need yet 
 pine for other worlds to conquer. 
 
 Speculations concerning the course and progress of sci- 
 
 83 Sur 1'equilibre d'une masse fluide animee d'un mouvement de rota- 
 tion. ("Acta Mathematical vol. vii, 1885.) 
 
 " On Figures of Equilibrium of Rotating Masses of Fluid," " Phil. 
 Trans./' vol. clxxviii, 1887. 
 
 " On the Mechanical Conditions of a Swarm of Meteorites and 
 on Theories of Cosmogony," " Phil. Trans.," vol. clxxx, 1889. 
 
MATHEMATICAL THEORIES OF THE EARTH 127 
 
 ence are usually untrustworthy if not altogether fallacious. 
 But, being delegated for the hour to speak to and for 
 mathematicians and astronomers, it may be permissible to 
 offer, in closing, a single suggestion, which will perhaps 
 help us to orient ourselves aright in our various fields of 
 research. If the curve of scientific progress in any domain 
 of thought could be drawn, there is every reason to believe 
 that it would exhibit considerable irregularities. There 
 would be marked maxima and minima in its general tend- 
 ency toward the limit of perfect knowledge; and it seems 
 not improbable that the curve would show throughout some 
 portions of its length a more or less definitely periodic suc- 
 cession of maxima and minima. Races and communities 
 as well as individuals, the armies in pursuit of truth as well 
 as those in pursuit of plunder, have their periods of cul- 
 minating activity and their periods of placid repose. It is 
 a curious fact that the history of the mathematical theories 
 of the earth presents some such periodicity. We have the 
 marked maximum of the epoch of Newton near the end 
 of the seventeenth century, with the equally marked maxi- 
 mum of the epoch of Laplace near the end of the eighteenth 
 cntury; and, judging from the recent revival of geodesy 
 and astronomy in Europe, and from the well-nigh general 
 activity in mathematical and geological research, we may 
 hope, if not expect, that the end of the present century will 
 signalize a similar epoch of productive activity. The 
 minima periods which followed the epochs of Newton and 
 Laplace are less definitely marked but not less noteworthy 
 and instructive. They were not periods of placid repose; 
 to find such one must go back into the night of the middle 
 ages; but they were periods of greatly diminished energy, 
 periods during which those who kept alive the spirit of in- 
 vestigation were almost as conspicuous for their isolation 
 as for their distinguished abilities. Many causes, of course, 
 contributed to produce these minima periods, and it would 
 be an interesting study in philosophic history to trace out 
 the tendency and effect of each cause. It is desired here, 
 however, to call attention to only one cause which con- 
 
128 WOODWARD 
 
 tributed to the somewhat general apathy of the periods 
 mentioned, and which always threatens to dampen the 
 ardour of research immediately after the attainment of any 
 marked success or advance. I refer to the impression of 
 contentment with and acquiescence in the results of science, 
 which seems to find easy access to trained as well as un- 
 trained minds before an investigation is half completed or 
 even fairly begun. That some such tacit persuasion of the 
 completeness of the knowledge of the earth has at times 
 pervaded scientific thought, there can be no doubt. This 
 was notably the case during the period which followed the 
 remarkable epoch of Laplace. The profound impression 
 of the sufficiency of the brilliant discoveries and advances 
 of that epoch is aptly described by Carlyle in the half 
 humorous, half sarcastic language of Sartor Resartus. 
 " Our theory of gravitation," he says, " is as good as per- 
 fect: Lagrange, it is well known, has proved that the plan- 
 etary system, on this scheme, will endure forever; Laplace, 
 still more cunningly, even guesses that it could hot have 
 been made on any other scheme. Whereby, at least, our 
 nautical log-books can be better kept; and water transport 
 of all kinds has grown more commodious. Of geology and 
 geognosy we know enough; what with the labours of our 
 Werners and Huttons, what with the ardent genius of their 
 disciples, it has come about that now, to many a royal so- 
 ciety, the creation of a world is little more mysterious than 
 the cooking of a dumpling; concerning which last, indeed, 
 there have been minds to whom the question How the 
 apples were got in presented difficulties." This was writ- 
 ten nearly sixty years ago, about the time the sage of 
 Ecclefechan abandoned his mathematics and astronomy for 
 literature to become the sees of Chelsea; but the force of 
 its irony is still applicable, for we have yet to learn, essen- 
 tially, " How the apples were got in," and what kind 
 they are. 
 
 As to the future, we can only guess, less or more vaguely, 
 from our experience in the past and from our knowledge of 
 present needs. Though the dawn of that future is certainly 
 
MATHEMATICAL THEORIES OF THE EARTH 129 
 
 not heralded by rosy tints of overconfidence among those 
 acquainted with the difficulties to be overcome, the prospect, 
 on the whole, has never been more promising. The con- 
 verging lights of many lines of investigation are now 
 brought to bear on the problems presented by our planet. 
 There is ample reason to suppose that our day will witness 
 a fair average of those happy accidents in science which 
 lead to the discovery of new principles and new methods. 
 We have much to expect from the elaborate machinery and 
 perfected methods of the older and more exact sciences 
 of measuring and weighing astronomy, geodesy, physics, 
 and chemistry. We have more to expect, perhaps, from 
 geology and meteorology, with their vast accumulation of 
 facts not yet fully correlated. Much, also, may be antici- 
 pated from that new astronomy which looks for the se- 
 crets of the earth's origin and history in nebulous masses 
 or in swarms of meteorites. We have the encouraging 
 stimulus of a very general and rapidly growing popular 
 concern in the objects of our inquiries, and the freest 
 avenues for the dissemination of new information; so that 
 we may easily gain the advantage of a concentration of 
 energy without centralization of personal interests. To 
 those, therefore, who can bring the prerequisites of endless 
 patience and unflagging industry, who can bear alike the 
 remorseless discipline of repeated failure and the prosperity 
 of partial success, the field is as wide and as inviting as 
 it ever was to a Newton or a Laplace. 
 
THE ROTATION 
 
 AND PHYSICAL CONSTITUTION 
 
 OF THE PLANET MERCURY 
 
 AND 
 
 THE PLANET MARS 
 
 BY 
 
 GIOVANNI VIRGINIO SCHIAPARELLI 
 
THE ROTATION AND PHYSICAL CONSTI- 
 TUTION OF THE PLANET MERCURY 1 
 
 A1ONG the older planets no one is so difficult to 
 observe as Mercury; and none presents so many 
 obstacles to the investigation of its orbit as well as 
 to the study of its physical nature. With respect to the 
 orbit it is enough to say that Mercury is the only planet 
 whose motions it has been declared to be impossible up 
 to the present time to subject to the laws of universal 
 gravitation; and the theory of whose orbit, though elab- 
 orated by the sagacious mind of a Leverrier, still presents 
 notable discrepancies with observations. As to its phys- 
 ical nature, very little is known, and of that little it 
 may be said that nearly all of it rests upon observations 
 now a century old, made at Lilienthal by the famous 
 Schroeter. 
 
 The telescopic examination of this planet is, in fact, 
 most difficult. So close is its orbit to the sun that Mer- 
 cury never appears in the sky far enough away from that 
 great luminary to admit of its examination in complete 
 darkness at least not in our latitudes. Observations 
 which are made in the period of twilight, before the rising 
 or after the setting of the sun, are rarely successful, because 
 under such circumstances the planet is always near to the 
 horizon, and so subject to disturbances and unequal re- 
 fraction in the lowest atmospheric strata, as to present for 
 the most part in the telescope that uncertain and flaming 
 
 X A discourse delivered at the meeting of the Royal Academy of 
 the Lincei, December 8, 1889, in the presence of the King and Queen 
 of Italy. Translated by SARA CARR UPTON. 
 
 i33 
 
134 
 
 SCHIAPARELLI 
 
 aspect which strikes the naked eye as a bright scintillation; 
 for that very reason the ancients called it Stilbon, which 
 means the scintillating. Observations by night being 
 therefore impossible, and twilight observations being rarely 
 successful, no other way remains but to make the attempt 
 in full daylight, in the continual presence of the sun, and 
 through an atmosphere constantly illuminated. 
 
 Certain trials made in 1881 persuaded me that it 
 would be possible not only to see the markings on Mer- 
 cury in full daylight, but also to obtain a series of suf- 
 ficiently connected and continuous observations of these 
 spots. In the beginning of 1882 I determined to make a 
 regular study of the planet; and in the eight following 
 years I have had the telescope directed upon Mercury 
 several hundreds of times, usually to little purpose, and 
 with the loss of much time; sometimes on account of at- 
 mospheric disturbance, which during the day is often very 
 great (especially in the summer months); and again on 
 account of insufficient transparency of the air. Neverthe- 
 less, by employing the necessary patience, I succeeded in 
 seeing the spots on the planet with greater or less pre- 
 cision on more than one hundred and fifty occasions, and 
 also in making at such times some rather satisfactory draw- 
 ings. For this purpose I used at first the smaller telescope 
 of our observatory, made by Merz, which was often found 
 to be inadequate for observations so difficult as these. But 
 in the mean time the new great equatorial refractor had 
 been installed in the observatory of Milan. It may be 
 called the most perfect work which has yet come from 
 the workshops of Munich. By its aid I was enabled to 
 pursue the work with greater success, and to attain more 
 complete and more certain results. And in regard to this 
 refractor I can not recall without lively emotions of grati- 
 tude the warm interest shown by your Majesties, now 
 eleven years ago, when it was a question of providing that 
 noble instrument for the Milan Observatory. Nor is it 
 possible for me to forget the generous eagerness with which 
 this academy, and Quintius Sella of glorious memory at 
 
THE PLANET MERCURY 135 
 
 the head of it, supported the proposition with an authorita- 
 tive vote, and the large majority by which it was honoured 
 in both branches of Parliament. The new facts concern- 
 ing the planet Mercury which this telescope has revealed, 
 I consider as the most important and most precious results 
 which have been so far obtained by its aid; so that to give 
 the first account of these new things at this time and in 
 this place seems to me the fulfilment of a duty. 
 
 I will first speak of the rotation of the planet, which I 
 have found to be very different from what has been be- 
 lieved up to the present time, on the faith of the few and 
 insufficient observations made a hundred years ago with 
 imperfect telescopes. The manner and chief peculiarities 
 of this rotation, which it has taken me many years of ob- 
 servation to establish, may be described in few words, by 
 saying that Mercury revolves around the sun in a manner 
 similar to that in which the moon revolves around the 
 earth. As the moon describes its orbit around the earth, 
 showing to us always very nearly the same face and the 
 same spots, so Mercury in its orbit around the sun con- 
 stantly presents to that great luminary very nearly the same 
 hemisphere of its surface. 
 
 I have said almost the same hemisphere, and not exactly 
 the same hemisphere. Mercury, in fact, like the moon, 
 presents the phenomenon of libration. Observing the full 
 moon with a small telescope at very different epochs, we 
 shall find that, in general, the same spots occupy the cen- 
 tral region of its disk; but, studying more minutely these 
 central spots, and the relations of their distances from the 
 eastern border of the moon, and from the western border, 
 we shall soon ascertain (as did Galileo, now two hundred and 
 fifty years ago, for the first time) that they oscillate by sen- 
 sible amounts, now toward the right hand and again toward 
 the left. This phenomenon is named the libration in longi- 
 tude, and arises chiefly because the point toward which the 
 moon perpetually and almost exactly 2 directs one of its 
 
 'That is, taking no account of the slight inclination of the lunar 
 equator with respect to the plane of its orbit, and supposing the 
 
136 SCHIAPARELLI 
 
 diameters is not the centre of the earth, neither is it the 
 centre of the lunar elliptical orbit, but that one of the two 
 foci of that orbit which is not occupied by the earth. This 
 point is called by astronomers the upper focus. To any one 
 stationed at this point the moon would therefore show 
 always the same aspect. To us, who are, instead, on the 
 average forty-two thousand kilometres distant from the 
 point, the moon shows itself in slightly different aspects at 
 different times, turning toward us now more of its eastern 
 regions, now more of its western. 
 
 Exactly similar is the way in which Mercury presents 
 itself to the sun during the various phases of its revolution 
 about that body. One of the diameters of the planet is 
 constantly directed not toward that focus of its elliptical 
 orbit which is occupied by the sun, but toward the other 
 focus toward the upper focus. Now, these two foci being 
 distant from each other not less than one fifth of the whole 
 diameter of the orbit of Mercury, the libration of the planet 
 is very great; and that point of Mercury which receives 
 the solar rays vertically is projected on the surface of the 
 planet, and oscillates along its equator in an arc which has 
 an amplitude of forty-seven degrees that is, more than 
 an eighth of the whole circumference of the equator itself. 
 The complete period of going and returning is equal to the 
 time employed by Mercury in moving around its orbit 
 that is, almost eighty-eight terrestrial days. Mercury, there- 
 fore, is continually oriented to the sun as a magnetic needle 
 to a piece of iron; but it does not point thither so con- 
 stantly as not to permit of a certain oscillatory motion of 
 the planet eastward and westward, similar to the moon's 
 oscillation with respect to the earth. 
 
 This oscillatory motion is of the greatest importance in 
 respect to the physical condition of the planet. Let us sup- 
 pose for a moment that this motion did not exist in fact, 
 and that Mercury always presented the same hemisphere 
 
 moon's motion in this orbit to be the so-called simple elliptic motion, 
 in which the perturbations of the true anomaly are disregarded as well 
 as certain terms that are of the order of the square of the eccentricity. 
 
THE PLANET MERCURY 137 
 
 to the light and heat of the sun, the other hemisphere re- 
 maining wrapped in perpetual night. That point of the 
 planet's surface which lies at the central pole of the illu- 
 minated hemisphere would eternally have the sun vertically 
 above it; all other places upon Mercury which are reached 
 by the sun's rays would always see the sun above the same 
 point of the horizon, at the same altitude, without any 
 apparent motion or sensible change of place whatever. 
 Therefore such places would have no alternation of day 
 and night, and no vicissitude of seasons. 
 
 Remaining thus perpetually in presence of the sun, 
 with the stars always invisible, Mercury having no moon, 
 it is difficult to understand how the dwellers in the regions 
 of perpetual day could make any determinations of time 
 (or have any chronology). 
 
 Upon Mercury things are nearly in this case, but not 
 entirely so. The oscillating motion to which we have seen 
 the body of the planet is subjected with respect to the sun 
 would be attributed by an observer on the planet to the 
 sun itself, just as we are used to attribute to the sun the 
 diurnal motion of rotation which actually belongs to the 
 earth. While, therefore, to us the sun seems to revolve 
 continually from east to west, and thus determines the 
 period of night and day to be twenty-four hours, an ob- 
 server on Mercury would see the sun describe an arc of 
 forty-seven degrees, with an alternating motion to and fro, 
 upon the celestial vault; and this arc would remain always 
 in the same position with respect to the horizon of the 
 observer. A complete cycle of such double oscillations 
 of the sun would last almost exactly eighty-eight terrestrial 
 days; and according as the arc of the solar oscillatory 
 motion aforesaid is all above the spectator's horizon, or 
 all below that horizon, or partly above and partly below it, 
 there would be different appearances and a different dis- 
 tribution of heat and of light. Accordingly, in those re- 
 gions of Mercury where the arc of solar oscillation remains 
 entirely below the local horizon, the sun will never be 
 seen, and there will be continual darkness. In such re- 
 
1 38 SCHIAPARELLI 
 
 gions, which occupy nearly three eighths of all the planet, 
 the dense and eternal night can never be abated except 
 by occasional sources of light, such as refraction and 
 atmospheric twilights, by polar auroras, or similar phe- 
 nomena, to which may be added the faint light afforded by 
 the stars and planets. Another region of Mercury which 
 also comprises three eighths of the whole surface will have 
 the entire arc of solar oscillation above the horizon, and 
 it will be continually exposed to the rays of the sun, with- 
 out any variation other than that of their greater or less 
 obliquity during the various phases of the period of eighty- 
 eight days: for such a region no night will be possible. 
 And, lastly, there are other regions, comprising in all a 
 fourth part of the whole planet, for which the arc of the 
 apparent oscillation of the sun is in part above the horizon, 
 and part below. For these places alone alternations of 
 light and darkness will be possible. In these privileged 
 regions the entire period of eighty-eight terrestrial days 
 will be divided into two intervals: one all light, the other 
 all darkness; the duration of each will be equal at certain 
 places; in others, instead, light or darkness will prevail in 
 greater or less degree, according to the position of the 
 place upon the surface of Mercury, and according as a 
 larger or smaller portion of the arc before described re- 
 mains above its horizon. 
 
 Upon a planet where affairs are so ordered the possi- 
 bility of organic life depends upon the existence of an 
 atmosphere capable of distributing the solar heat over dif- 
 ferent regions so as to modify the extraordinary excesses 
 of heat and of cold. The presence of such an atmosphere 
 upon Mercury was conjectured by Schroeter a century ago; 
 in my observations I find more evident indications of it, 
 which concur in making the probability of its existence 
 almost a certainty. 
 
 One of the first proofs springs from the continually 
 observed fact that the markings on the planet, visible for 
 the most part when they are found in the central regions 
 of the disk, are less visible or even disappear as they 
 
THE PLANET MERCURY 139 
 
 approach its circular borders. I have been able to con- 
 vince myself that this does not occur simply from the 
 greater obliquity of the line of sight due to perspective, 
 but from the fact that in that perimetral position there is 
 actually a greater hindrance to the vision, and this seems 
 to be due to nothing else but the greater length of the 
 path which the visual rays coming from the non-central 
 spot must pass through in the atmosphere of Mercury to 
 reach the eye. And from this I conclude that the atmos- 
 phere of Mercury is less transparent than that of Mars, 
 and that it more nearly resembles, in that regard, that of 
 the earth. 
 
 In addition to this, the circular border of the planet, 
 where the spots become less visible, always appears more 
 luminous than the rest of the disk. It is often irregularly 
 luminous, in certain parts more so, in others less; and 
 sometimes along this edge rather brilliant white areas may 
 be seen which remain visible for several consecutive days, 
 but which nevertheless ace, in general, changeable, now in 
 one portion and now in another. I attribute this fact to 
 condensations in the interior of the atmosphere of Mer- 
 cury, which must reflect the solar rays outward toward 
 celestial space, and more strongly as they become more 
 opaque. Such white areas are also often seen in the more 
 central parts of the disk, but in that case they are not so 
 brilliant as upon the border. 
 
 But there is still more. The dark spots of the planet, 
 although permanent in form and arrangement, are not 
 always equally apparent, but are sometimes more intense 
 and sometimes more faint; and it also happens that some 
 of these markings occasionally become entirely invisible. 
 This I can not attribute to any more obvious cause than to 
 atmospheric condensations similar to our terrestrial clouds, 
 which prevent more or less completely any view of the true 
 surface of Mercury in any portion. The clouded regions of 
 the earth must present an absolutely identical appearance 
 to a person viewing them from the depths of celestial space. 
 
 Concerning the nature of the surface of Mercury very 
 
140 
 
 SCHIAPARELLI 
 
 little can be ascertained from the observations so far made. 
 Thus we have to note that three eighths of its surface re- 
 main inaccessible to the solar rays, and hence to our vision 
 also; and there is very little hope of ever knowing anything 
 about it with certainty. But, nevertheless, it will be easy 
 to reach precise and certain knowledge of the portion 
 visible to us. The dark spots, even when they are not ob- 
 scured by atmospheric condensation in the manner men- 
 tioned above, appear always under the form of bands of 
 extremely light shadings, which under ordinary circum- 
 stances can only be observed with much difficulty and great 
 attention. Upon more favourable occasions these shadings 
 have a warm brown tint like sepia, which nevertheless is 
 never greatly different from the general colour of the 
 planet. This is usually of a light rose tint, tending toward 
 a copper colour. It is most difficult to give a satisfactory 
 graphic representation of such vague and diffused forms 
 or bands, specially from the want of fixity of the edges 
 which always leaves room for a certain choice. Such inde- 
 terminate edges, however, I have reason to believe, in 
 most cases are mere appearances arising from insufficient 
 optical power of the instrument; because the more beau- 
 tiful is the image and the more perfect the vision, the more 
 manifest is the tendency of the shadings to dissolve into a 
 number of minute particles. And there is no doubt that 
 by using a more powerful telescope all would appear re- 
 solved into minuter forms; exactly as with a simple opera- 
 glass we may see those irregular and indistinct masses of 
 shading which every one can see with the naked eye upon 
 the moon resolved into much smaller parts. 
 
 Considering the difficulty of making a proper study of 
 the dark spots of Mercury, it is not easy to express a well- 
 founded opinion on their nature. They might simply de- 
 pend upon the diverse material and structure of the solid 
 superficial strata, as we know to be the case with the moon. 
 But if any one, taking into account the fact that there exists 
 an atmosphere upon Mercury capable of condensation and 
 perhaps also of precipitation, should hold the opinion that 
 
THE PLANET MERCURY 141 
 
 there was something in those dark spots analogous to our 
 seas, I do not think that a conclusive argument to the con- 
 trary could be advanced. And as those spots are not 
 grouped in great masses, but are dispersed about in areas 
 and zones of no great width, much ramified, and alternated 
 with clear spaces with some uniformity, it might be con- 
 cluded that no vast oceans and great continents exist upon 
 Mercury, but that the liquid and solid areas mingle with 
 each other in reciprocal ways and with frequent vicissitudes, 
 thus giving rise to a condition of things very different from 
 that which exists upon the earth, and one which perhaps 
 we might envy. 
 
 At any rate, we have in the case of Mercury (as in 
 Mars), a world which is sufficiently diverse from our own; 
 which receives light and heat from the sun, not only in a 
 greater amount but in a different manner than the earth; 
 and where life, if so be life exists there, finds conditions so 
 different from those to which we are accustomed that we 
 can scarcely imagine them. The perpetual presence of the 
 sun almost vertically above certain regions, and its per- 
 petual absence from other regions, appears to us to be 
 something intolerable. But we must recollect that such a 
 contrast should produce an atmospheric circulation which 
 is at the same time stronger, more rapid, and more regular 
 than that which sows the elements of life on the earth; and 
 that on this account it may come about that an equilibrium 
 of temperature is produced quite as complete as ours, and 
 possibly even more so. 
 
 Mercury is conspicuously distinguished from the other 
 planets by the manner of its revolution around the sun, 
 turning always the same face toward it. All the other 
 planets (at least so far as is ascertained in the cases which 
 it has been possible to determine) revolve rapidly around 
 their axes in the space of a few hours. Mercury's manner 
 of revolution, however, which is unique among the planets, 
 seems, on the other hand, quite usual for the satellites; 
 such at least is true in all the cases where it has been pos- 
 sible to investigate the rotation motion of a satellite. That 
 
I 4 2 SCHIAPARELLI 
 
 our own satellite has always in the memory of man shown 
 to the earth the same hemisphere, is certain also from his- 
 torical testimony, because Dante speaks of " Cain and the 
 Thorns," and among the smaller works of Plutarch there is 
 one entitled " Of the Face to be seen in the Disk of the 
 Moon." That the satellites of Jupiter show always the 
 same face to their chief planet is probable as to the first 
 three, and for the fourth it is absolutely proved by the 
 observations of Auwers and Engelmann. The same fact 
 had been discovered by William Herschel in the case of 
 Japetus, the eighth and most distant satellite of Saturn. 
 That which would seem to be the general rule for the 
 satellites is therefore, as exemplified in the case of Mercury, 
 the exception among the planets. 
 
 Such an exception, however, seems not without cause, 
 and it is probably connected with the fact of Mercury's 
 great proximity to the sun, and perhaps also with the other 
 fact that Mercury is without satellites. In my opinion, it 
 depends also upon the way in which Mercury was gener- 
 ated at the time when the solar system took its present 
 form. The singularity of Mercury constitutes, therefore, a 
 new document to add to those which must be considered 
 in the study of the solar and planetary cosmogony. 
 
THE PLANET MARS 1 
 
 MANY of the first astronomers who studied Mars 
 with the telescope had noted on the outline of its 
 disk two brilliant white spots of rounded form and 
 of variable size. In process of time it was observed that 
 while the ordinary spots upon Mars were displaced rapidly 
 in consequence of its daily rotation, changing in a few 
 hours both their position and their perspective, the two 
 white spots remained sensibly motionless at their posts. 
 It was concluded rightly from this that they must oc- 
 cupy the poles of rotation of the planet, or at least must 
 be found very near to them. Consequently they were 
 given the name of polar caps or spots. And not without 
 reason is it conjectured that these represent upon Mars 
 that immense mass of snow and ice which still to-day pre- 
 vents navigators from reaching the poles of the earth. 
 We are led to this conclusion not only by the analogy 
 of aspect and of place, but also by another important 
 observation. 
 
 As things stand, it is manifest that if the above-men- 
 tioned white polar spots of Mars represent snow and ice 
 they should continue to decrease in size with the approach 
 of summer in those places and increase during the winter. 
 Now this very fact is observed in the most evident man- 
 ner. In the second half of the year 1892 the southern 
 polar cap was in full view; during that interval, and espe- 
 cially in the months of July and August, its rapid diminu- 
 tion from week to week was very evident to those observ- 
 
 1 From " Natura ed Arte," February 15, 1893. Translated by WILLIAM 
 H. PICKERING. 
 
 143 
 
SCHIAPARELLI 
 
 ing with common telescopes. This snow (for we may well 
 call it so), which in the beginning reached as far as latitude 
 70, and formed a cap of over 2,000 kilometres (1,200 miles) 
 in diameter, progressively diminished, so that two or three 
 months later little more of it remained than an area of per- 
 haps 300 kilometres (180 miles) at the most, and still less 
 was seen in the last days of 1892. In these months the 
 southern hemisphere of Mars had its summer, the summer 
 solstice occurring upon October I3th. Correspondingly 
 the mass of snow surrounding the northern pole should 
 have increased; but this fact was not observable, since that 
 pole was situated in the hemisphere of Mars which was 
 opposite to that facing the earth. The melting of the 
 northern snow was seen in its turn in the years 1882, 1884, 
 and 1886. 
 
 These observations of the alternate increase and de- 
 crease of the polar snows are easily made even with tele- 
 scopes of moderate power, but they become much more 
 interesting and instructive when we can follow assidu- 
 ously the changes in their more minute particulars, using 
 larger instruments. The snowy regions are then seen 
 to be successively notched at their edges; black holes 
 and huge fissures are formed in their interiors; great iso- 
 lated pieces many miles in extent stand out from the prin- 
 cipal mass and, dissolving, disappear a little later. In short, 
 the same divisions and movements of these icy fields pre- 
 sent themselves to us at a glance that occur during the 
 summer of our own arctic regions, according to the de- 
 scriptions of explorers. 
 
 The southern snow, however, presents this peculiarity: 
 The centre of its irregularly rounded figure does not coin- 
 cide exactly with the" pole, but is situated at another point, 
 which is nearly always the same, and is distant from the 
 pole about 300 kilometres (180 miles) in the direction of 
 the Mare Erythraeum. From this we conclude that when 
 the area of the snow is reduced to its smallest extent the 
 south pole of Mars is uncovered, and therefore, perhaps, 
 the problem of reaching it upon this planet is easier than 
 
THE PLANET MARS 145 
 
 upon the earth. The southern snow is in the midst of a 
 huge dark spot, which with its branches occupies nearly one 
 third of the whole surface of Mars, and is supposed to 
 represent its principal ocean. Hence the analogy with our 
 arctic and antarctic snows may be said to be complete, and 
 especially so with the antarctic one. 
 
 The mass of the northern snow cap of Mars is, on the 
 other hand, centred almost exactly upon its pole. It is 
 located in a region of yellow colour, which we are accus- 
 tomed to consider as representing the continent of the 
 planet. From this arises a singular phenomenon which has 
 no analogy upon the earth. At the melting of the snows 
 accumulated at that pole during the long night of ten 
 months and more the liquid mass produced in that opera- 
 tion is diffused around the circumference of the snowy 
 region, converting a large zone of surrounding land into a 
 temporary sea and filling all the lower regions. This pro- 
 duces a gigantic inundation, which has led some observers 
 to suppose the existence of another ocean in those parts, 
 but which does not really exist in that place, at least as a 
 permanent sea. We see then (the last opportunity was 
 in 1884) the white spot of the snow surrounded by a dark 
 zone, which follows its perimeter in its progressive diminu- 
 tion, upon a circumference ever more and more narrow. 
 The outer part of this zone branches out into dark 
 lines, which occupy all the surrounding region, and seem 
 to be distributary canals by which the liquid mass may 
 return to its natural position. This produces in these re- 
 gions very extensive lakes, such as that designated upon 
 the map by the name of Lacus Hyperboreus; the neigh- 
 bouring interior sea called Mare Acidalium becomes more 
 black and more conspicuous. And it is to be remembered 
 as a very probable thing that the flowing of this melted 
 snow is the cause which determines principally the hydro- 
 graphic state of the planet and the variations that are peri- 
 odically observed in its aspect. Something similar would 
 be seen upon the earth if one of our poles came to be 
 located suddenly in the centre of Asia or of Africa. As 
 
 10 
 
I4 6 SCHIAPARELLI 
 
 things stand at present, we may find a miniature image of 
 these conditions in the flooding that is observed in our 
 streams at the melting of the Alpine snows. 
 
 Travellers in the arctic regions have frequent occasion 
 to observe how the state of the polar ice at the beginning 
 of the summer, and even at the beginning of July, is always 
 very unfavourable to their progress. The best season for 
 exploration is in the month of August, and September is 
 the month in which the trouble from the ice is the least. 
 Thus in September our Alps are usually more practicable 
 than at any other season. And the reason for it is clear 
 the melting of the snow requires time; a high temperature 
 is not sufficient; it is necessary that it should continue, 
 and its effect will be so much the greater, as it is the more 
 prolonged. Thus, if we could slow down the course of our 
 season so that each month should last sixty days instead 
 of thirty, in the summer, in such a lengthened condition, 
 the melting of the ice would progress much further, and 
 perhaps it would not be an exaggeration to say that the 
 polar cap at the end of the warm season would be entirely 
 destroyed. But one can not doubt, in any case, that the 
 fixed portion of such a cap would be reduced to much 
 smaller size than we see it to-day. Now, this is exactly 
 what happens on Mars. The long year, nearly double our 
 own, permits the ice to accumulate during the polar night 
 of ten or twelve months, so as to descend in the form of 
 a continuous layer as far as parallel 70, or even farther. 
 But in the day which follows, of twelve or ten months, the 
 sun has time to melt all, or nearly all, of the snow of recent 
 formation, reducing it to such a small area that it seems to 
 us no more than a very white point. And perhaps this 
 snow is entirely destroyed; but of this there is at present 
 no satisfactory observation. 
 
 Other white spots of a transitory character and of a 
 less regular arrangement are formed in the southern hemi- 
 sphere upon the islands near the pole, and also in the oppo- 
 site hemisphere whitish regions appear at times surround- 
 ing the north pole and reaching to 50 and 55 of latitude. 
 
THE PLANET MARS 147 
 
 They are, perhaps, transitory snows, similar to those which 
 are observed in our latitudes. But also in the torrid zone of 
 Mars are seen some very small white spots more or less per- 
 sistent; among others one was seen by me in three con- 
 secutive oppositions (1877-1882) at the point indicated 
 upon our chart by longitude 268 and latitude 16 north. 
 Perhaps we may be permitted to imagine in this place the 
 existence of a mountain capable of supporting extensive ice 
 fields. The existence of such a mountain has also been 
 suggested by some recent observers upon other grounds. 
 
 As has been stated, the polar snows of Mars prove in 
 an incontrovertible manner that this planet, like the earth, 
 is surrounded by an atmosphere capable of transporting 
 vapour from one place to another. These snows are, in 
 fact, precipitations of vapour, condensed by the cold and 
 carried with it successively. How carried with it if not by 
 atmospheric movement? The existence of an atmosphere 
 charged with vapour has been confirmed also by spectro- 
 scopic observations, principally those of Vogel, according 
 to which this atmosphere must be of a composition differing 
 little from our own, and, above all, very rich in aqueous va- 
 pour. This is a fact of the highest importance, because from 
 it we can rightly affirm with much probability that to water 
 and to no other liquid is due the seas of Mars and its polar 
 snows. When this conclusion is assured beyond all doubt, 
 another one may be derived from it of not less importance 
 that the temperature of the Arean climate, notwithstand- 
 ing the greater distance of that planet from the sun, is of 
 the same order as the temperature of the terrestrial one. 
 Because, if it were true, as has been supposed by some 
 investigators, that the temperature of Mars was on the 
 average very low (from 50 to 60 below zero), it would 
 not be possible for water vapour to be an important ele- 
 ment in the atmosphere of that planet, nor could water 
 be an important factor in its physical changes, but would 
 give place to carbonic acid, or to some other liquid whose 
 freezing point was much lower. 
 
 The elements of the meteorology of Mars seem, then, 
 
148 SCHIAPARELLI 
 
 to have a close analogy to those of the earth. But there 
 are not lacking, as might be expected, causes of dissimi- 
 larity. From circumstances of the smallest moment Nature 
 brings forth an infinite variety in its operations. Of the 
 greatest influence must be different arrangement of the seas 
 and the continents upon Mars and upon the earth, regard- 
 ing which a glance at the map will say more than would 
 be possible in many words. We have already emphasized 
 the fact of the extraordinary periodical flood, which at 
 every revolution of Mars inundates the northern polar re- 
 gion at the melting of the snow. Let us now add that 
 this inundation is spread out to a great distance by means 
 of a network of canals, perhaps constituting the principal 
 mechanism (if not the only one) by which water (and with 
 it organic life) may be diffused over the arid surface of the 
 planet. Because on Mars it rains very rarely, or perhaps 
 even it does not rain at all. And this is the proof. 
 
 Let us carry ourselves in imagination into celestial 
 space, to a point so distant from the earth that we may 
 embrace it all at a single glance. He would be greatly in 
 error who had expected to see reproduced there upon a 
 great scale the image of our continents with their gulfs and 
 islands and with the seas that surround them which are 
 seen upon our artificial globes. Then without doubt the 
 known forms or part of them would be seen to appear 
 under a vaporous veil, but a great part (perhaps one half) 
 of the surface would be rendered invisible by the immense 
 fields of cloud, continually varying in density, in form, and 
 in extent. Such a hindrance, most frequent and continu- 
 ous in the polar regions, would still impede nearly half the 
 time the view of the temperate zones, distributing itself in 
 capricious and ever-varying configurations. The seas of 
 the torrid zone would be seen to be arranged in long 
 parallel layers, corresponding to the zone of equatorial and 
 tropical calms. For an observer placed upon the moon the 
 study of our geography would not be so simple an under- 
 taking as one might at first imagine. 
 
 There is nothing of this sort in Mars. In every climate 
 
THE PLANET MARS 149 
 
 and under every zone its atmosphere is nearly perpetually 
 clear and sufficiently transparent to permit one to recog- 
 nise at any moment whatever the contours of the seas and 
 continents and, more than that, even the minor configura- 
 tions. Not, indeed, that vapours of a certain degree of 
 opacity are lacking, but they offer very little impediment 
 to the study of the topography of the planet. Here and 
 there we see appear from time to time a few whitish spots, 
 changing their position and their form, rarely extending 
 over a very wide area. They frequent by preference a few 
 regions, such as the islands of the Mare Australe, and on 
 the continents the regions designated on the map with the 
 names of Elysium and Tempe. Their brilliancy generally 
 diminishes and disappears at the meridian hour of the place, 
 and is re-enforced in the morning and evening with very 
 marked variations. It is possible that they may be layers 
 of cloud, because the upper portions of terrestrial clouds 
 where they are illuminated by the sun appear white. But 
 various observations lead us to think that we are dealing 
 rather with a thin veil of fog instead of a true nimbus cloud, 
 carrying storms and rain. Indeed, it may be merely a 
 temporary condensation of vapour under the form of dew 
 or hoar frost. 
 
 Accordingly, as far as we may be permitted to argue from 
 the observed facts, the climate of Mars must resemble that 
 of a clear day upon a high mountain. By day a very strong 
 solar radiation, hardly mitigated at all by mist or vapour; 
 by night a copious radiation from the soil toward celestial 
 space, and because of that a very marked refrigeration. 
 Hence a climate of extremes, and great changes of tem- 
 perature from day to night, and from one season to an- 
 other. And as on the earth at altitudes of 5,000 and 6,000 
 metres (17,000 to 20,000 feet) the vapour of the atmos- 
 phere is condensed only into the solid form, producing 
 those whitish masses of suspended crystals which we call 
 cirrus clouds, so in the atmosphere of Mars it would be 
 rarely possible (or would even be impossible) to find collec- 
 tions of cloud capable of producing rain of any conse- 
 
150 SCHIAPARELLI 
 
 quence. The variation of the temperature from one season 
 to another would be notably increased by their long dura- 
 tion, and thus we can understand the great freezing and 
 melting of the snow, which is renewed in turn at the poles 
 at each complete revolution of the planet around the sun. 
 
 As our chart demonstrates, in its general topography 
 Mars does not present any analogy with the earth. A third 
 of its surface is occupied by the great Mare Australe, which 
 is strewn with many islands, and the continents are cut 
 up by gulfs and ramifications of various forms. To the gen- 
 eral water system belongs an entire series of small internal 
 seas, of which the Hadriacum and the Tyrrhenum com- 
 municate with it by wide mouths, while the Cimmerium, 
 the Sirenum, and the Solis Lacus are connected with it 
 only by means of narrow canals. We shall notice in the 
 first four a parallel arrangement, which certainly is not 
 accidental, as also not without reason is the corresponding 
 position of the peninsulas of Ausonia, Hesperia, and Atlan- 
 tis. The colour of the seas of Mars is generally brown, 
 mixed with gray, but not always of equal intensity in all 
 places, nor is it the same in the same place at all times. 
 From an absolute black it may descend to a light gray or 
 to an ash colour. Such a diversity of colours may have 
 its origin in various causes, and is not without analogy 
 also upon the earth, where it is noted that the seas of the 
 warm zone are usually much darker than those nearer the 
 pole. The water of the Baltic, for example, has a light, 
 muddy colour that is not observed in the Mediterranean. 
 And thus in the seas of Mars we see the colour become 
 darker when the sun approaches their zenith, and summer 
 begins to rule in that region. 
 
 All of the remainder of the planet, as far as the north 
 pole, is occupied by the mass of the continents, in which, 
 save in a few areas of relatively small extent, an orange 
 colour predominates, which sometimes reaches a dark-red 
 tint, and in others descends to yellow and white. The 
 variety in this colouring is in part of meteorological origin, 
 in part it may depend on the diverse nature of the soil, but 
 
THE PLANET MARS 151 
 
 upon its real cause it is not as yet possible to frame any 
 very well grounded hypothesis. Nevertheless, the cause of 
 this predominance of the red and yellow tints upon the 
 surface of ancient Pyrois is well known. 2 Some have 
 thought to attribute this colouring to the atmosphere of 
 Mars, through which the surface of the planet might be 
 seen coloured, as any terrestrial object becomes red when 
 seen through red glass. But many facts are opposed to 
 this idea, among others that the polar snows appear always 
 of the purest white, although the rays of light derived from 
 them traverse twice the atmosphere of Mars under great 
 obliquity. We must then conclude that the Arean conti- 
 nents appear red and yellow because they are so in fact. 
 Besides these dark and light regions, which we have 
 described as seas and continents, and of whose nature there 
 is at present scarcely left any room for doubt, some others 
 exist, truly of small extent, of an amphibious nature, which 
 sometimes appear yellowish like the continents, and are 
 sometimes clothed in brown (even black in certain cases), 
 and assume the appearance of seas, while in other cases 
 their colour is intermediate in tint, and leaves us in doubt 
 to which class of regions they may belong. Thus all the 
 islands scattered through the Mare Australe and the Mare 
 Erythraeum belong to this category; so too the long pen- 
 insula called Deucalionis Regio and Pyrrhse Regio, and in 
 the vicinity of the Mare Acidalium the regions designated 
 by the names of Baltia and Nerigos. The most natural 
 idea, and the one to which we should be led by analogy, 
 is to suppose these regions to represent huge swamps, in 
 which the variation in depth of the water produces the 
 diversity of colours. Yellow would predominate in those 
 parts where the depth of the liquid layer was reduced to 
 little or nothing, and brown, more or less dark, in those 
 places where the water was sufficiently deep to absorb more 
 light and to render the bottom more or less invisible. That 
 the water of the sea, or any other deep and transparent 
 
 J Pyrois I take to be some terrestrial region, although I have not 
 been able to find any translation of the name. TRANSLATOR. 
 
I5 2 SCHIAPARELLI 
 
 water, seen from above, appears more dark the greater 
 the depth of the liquid stratum, and that the land in com- 
 parison with it appears bright under the solar illumination, 
 is known and confirmed by certain physical reasons. The 
 traveller in the Alps often has occasion to convince himself 
 of it, seeing from the summits the deep lakes with which 
 the region is strewn extending under his feet as black as 
 ink, while in contrast with them even the blackest rocks 
 illumined by the sunlight appeared brilliant. 3 
 
 Not without reason, then, have we hitherto attributed 
 to the dark spots of Mars the part of seas, and that of con- 
 tinents to the reddish areas which occupy nearly two thirds 
 of all the planet, and we shall find later other reasons which 
 confirm this method of reasoning. The continents form in 
 the northern hemisphere a nearly continuous mass, the only 
 important exception being the great lake called the Mare 
 Acidalium, of which the extent may vary according to the 
 time, and which is connected in some way with the inun- 
 dations which we have said were produced by the melting 
 of the snow surrounding the north pole. To the system of 
 the Mare Acidalium undoubtedly belong the temporary 
 lake called Lacus Hyperboreus and the Lacus Niliacus. 
 This last is ordinarily separated from the Mare Acidalium 
 by means of an isthmus or regular dam, of which the con- 
 tinuity was only seen to be broken once for a short time 
 in 1888. Other smaller dark spots are found here and there 
 in the continental area which we may designate as lakes, 
 but they are certainly not permanent lakes like ours, but 
 are variable in appearance and size according to the sea- 
 sons, to the point of wholly disappearing under certain cir- 
 cumstances. Ismenius Lacus, Lunse Lacus, Trivium Cha- 
 rontis, and Propontis are the most conspicuous and durable 
 ones. There are also smaller ones, such as Lacus Moeris 
 
 'This observation of the dark colour which deep water exhibits 
 when seen from above is found already noted by the first author of 
 antique memory, for in the " Iliad " (verses 770, 771 of book v) it is de- 
 scribed how " the sentinel from the high sentry box extends his glance 
 over the wine-coloured sea, otvova irAvrov" In the version of Monti the 
 adjective indicating the colour is lost. 
 
THE PLANET MARS 153 
 
 and Fons Juventse, which at their maximum size do not 
 exceed 100 to 150 kilometres (60 to 90 miles) in diameter, 
 and are among the most difficult objects upon the planet. 
 
 All the vast extent of the continents is furrowed upon 
 every side by a network of numerous lines or fine stripes 
 of a more or less pronounced dark colour, whose aspect is 
 very variable. These traverse the planet for long distances 
 in regular lines that do not at all resemble the winding 
 courses of our streams. Some of the shorter ones do not 
 reach 500 kilometres (300 miles), others, on the other hand, 
 extend for many thousands, occupying a quarter or some- 
 times even a third of a circumference of the planet. Some 
 of these are very easy to see, especially that one which is 
 near the extreme left-hand limit of our map, and is desig- 
 nated by the name of Nilosyrtis. Others in turn are ex- 
 tremely difficult, and resemble the finest thread of spider's 
 web drawn across the disk. They are subject also to great 
 variations in their breadth, which may reach 200 or even 
 300 kilometres (120 to 180 miles) for the Nilosyrtis, while 
 some are scarcely 30 kilometres (18 miles) broad. 
 
 These lines or stripes are the famous canals of Mars, of 
 which so much has been said. As far as we have been 
 able to observe them hitherto, they are certainly fixed con- 
 figurations upon the planet. The Nilosyrtis has been seen 
 in that place for nearly one hundred years, and some of 
 the others for at least thirty years. Their length and ar- 
 rangement are constant, or vary only between very narrow 
 limits. Each of them always begins and ends between the 
 same regions. But their appearance and their degree of 
 visibility vary greatly, for all of them, from one opposition 
 to another, and even from one week to another, and these 
 variations do not take place simultaneously and according 
 to the same laws for all, but in most cases happen apparently 
 capriciously, or at least according to laws not sufficiently 
 simple for us to be able to unravel. Often one or more be- 
 come indistinct, or even wholly invisible, while others in 
 their vicinity increase to the point of becoming conspicuous 
 even in telescopes of moderate power. The first of our 
 
154 SCHIAPARELLI 
 
 maps shows all those that have been seen in a long series of 
 observations. This does not at all correspond to the appear- 
 ance of Mars at any given period, because generally only 
 a few are visible at once. 4 
 
 Every canal (for now we shall so call them) opens at 
 its ends either into a sea, or into a lake, or into another 
 canal, or else into the intersection of several other canals. 
 None of them have yet been seen cut off in the middle of 
 the continent, remaining without beginning or without end. 
 This fact is of the highest importance. The canals may 
 intersect among themselves at all possible angles, but by 
 preference they converge toward' the small spots to which 
 we have given the name of lakes. For example, seven are 
 seen to converge in Lacus Phoenicis, eight in Trivium 
 Charontis, six in Lunae Lacus, and six in Ismenius Lacus. 
 
 The normal appearance of a canal is that of a nearly 
 uniform stripe, black, or at least of a dark colour, similar to 
 that of the seas, in which the regularity of its general course 
 does not exclude small variations in its breadth and small 
 sinuosities in its two sides. Often it happens that such a 
 dark line opening out upon the sea is enlarged into the 
 form of a trumpet, forming a huge bay, similar to the 
 estuaries of certain terrestrial streams. The Margaritifer 
 Sinus, the Aonius Sinus, the Auroras Sinus, and the two 
 horns of the Sabaeus Sinus are thus formed, at the mouths 
 
 *In a footnote the author refers to a drawing of Mars made by 
 himself, September 15, 1892, and says: "... At the top of the disk 
 the Mare Erythrseum and the Mare Australe appear divided by a great 
 curved peninsula, shaped like a sickle, producing an unusual appear- 
 ance in the area called Deucalionis Regio, which was prolonged that 
 year so as to reach the islands of Noachis and Argyre. This region 
 forms with them a continuous whole, but with faint traces of separa- 
 tion occurring here and there in a length of nearly six thousand kilo- 
 metres (four thousand miles). Its colour, much less brilliant than that 
 of the continents, was a mixture of their yellow with the brownish 
 gray of the neighbouring seas." The interesting feature of this note 
 is the remark that it. was an unusual appearance, the region referred 
 to being that in which the central branch of the fork of the Y ap- 
 peared. Since no such branch was conspicuously visible this year, it 
 would therefore seem from the above that it was the opposition of 1892 
 that was peculiar, and not the present one. TRANSLATOR. 
 
THE PLANET MARS 155 
 
 of one or more canals, opening into the Mare Erythraeum 
 or into the Mare Australe. The largest example of such a 
 gulf is the Syrtis Major, formed by the vast mouth of the 
 Nilosyrtis, so called. This gulf is not less that 1,800 kilo- 
 metres (1,100 miles) in breadth, and attains nearly the same 
 depth in a longitudinal direction. Its surface is little less 
 than that of the Bay of Bengal. In this case we see clearly 
 the dark surface of the sea continued without apparent in- 
 terruption into that of the canal. Inasmuch as the surfaces 
 called seas are truly a liquid expanse, we can not doubt that 
 the canals are a simple prolongation of them, crossing the 
 yellow areas or continents. 
 
 Of the remainder, that the lines called canals are truly 
 great furrows or depressions in the surface of the planet, 
 destined for the passage of the liquid mass and constituting 
 for it a true hydrographic system, is demonstrated by the 
 phenomena which are observed during the melting of the 
 northern snows. We have already remarked that at the 
 time of melting they appeared surrounded by a dark zone, 
 forming a species of temporary sea. At that time the 
 canals of the surrounding region become blacker and 
 wider, increasing to the point of converting at a certain 
 time all of the yellow region comprised between the edge 
 of the snow and the parallel of 60 north latitude into 
 numerous islands of small extent. Such a state of things 
 does not cease until the snow, reduced to its minimum area, 
 ceases to melt. Then the breadth of the canals diminishes, 
 the temporary sea disappears, and the yellow region again 
 returns to its former area. The different phases of these 
 vast phenomena are renewed at each return of the seasons, 
 and we were able to observe them in all their particulars 
 very easily during the oppositions of 1882, 1884, and 1886, 
 when the planet presented its northern pole to terrestrial 
 spectators. The most natural and the most simple inter- 
 pretation is that to which we have referred, of a great 
 inundation produced by the melting of the snows; it is 
 entirely logical and is sustained by evident analogy with 
 terrestrial phenomena. We conclude, therefore, that the 
 
1 56 SCHIAPARELLI 
 
 canals are such in fact and not only in name. The network 
 formed by these was probably determined in its origin in 
 the geological state of the planet, and has come to be slowly 
 elaborated in the course of centuries. It is not necessary 
 to suppose them the work of intelligent beings, and, not- 
 withstanding the almost geometrical appearance of all of 
 their system, we are now inclined to believe them to be 
 produced by the evolution of the planet, just as on the 
 earth we have the English Channel and the channel of 
 Mozambique. 
 
 It would be a problem not less curious than compli- 
 cated and difficult to study the system of this immense 
 stream of water, upon which perhaps depends principally the 
 organic life upon the planet, if organic life is found there. 
 The variations of their appearance demonstrated that this 
 system is not constant. When they become displaced, or 
 their outlines become doubtful and ill defined, it is fair to 
 suppose that the water is getting low or is even entirely 
 dried up. Then, in place of the canals there remains either 
 nothing or at most stripes of yellowish colour differing 
 little from the surrounding background. Sometimes they 
 take on a nebulous appearance, for which at present it is 
 not possible to assign a reason. At other times true en- 
 largements are produced, expanding to 100, 200, or more 
 kilometres (60 to 120 miles) in breadth, and this sometimes 
 happens for canals very far from the north pole, according 
 to. laws which are unknown. This occurred in Hydaspes 
 in 1864, in Simois in 1879, in Ackeron in 1884, and in 
 Triton in 1888. The diligent and minute study of the 
 transformations of each canal may lead later to a knowl- 
 edge of the causes of these effects. 
 
 But the most surprising phenomenon pertaining to the 
 canals of Mars is their gemination, which seems to occur 
 principally in the months which precede and in those which 
 follow the great northern inundation at about the times 
 of the equinoxes. In consequence of a rapid process, which 
 certainly lasts at most a few days, or even perhaps only a 
 few hours, and of which it has not yet been possible to 
 
THE PLANET MARS 157 
 
 determine the particulars with certainty, a given canal 
 changes its appearance and is found transformed through 
 all its length into two lines or uniform stripes more or less 
 parallel to one another, and which run straight and equal 
 with the exact geometrical precision of the two rails of a 
 railroad. But this exact course is the only point of resem- 
 blance with the rails, because in dimensions there is no com- 
 parison possible, as it is easy to imagine. The two lines 
 follow very nearly the direction of the original canal and 
 end in the place where it ended. One of these is often 
 superposed as exactly/ as possible upon the former line, 
 the other being drawn anew; but in this case the original 
 line loses all the small irregularities and curvature that it 
 may have originally possessed. But it also happens that 
 both the lines may occupy opposite sides of the former 
 canal and be located upon entirely new ground. The dis- 
 tance between the two lines differs in different gemmations 
 and varies from 600 kilometres (360 miles) and more down 
 to the smallest limit at which two lines may appear sepa- 
 rated in large visual telescopes less than an interval of 
 50 kilometres (30 miles). The breadth of the stripes them- 
 selves may range from the limit of visibility, which we may 
 suppose to be 30 kilometres (18 miles), up to more than 
 loo kilometres (60 miles). The colour of the two lines 
 varies from black to a light red, which can hardly be dis- 
 tinguished from the general yellow background of the con- 
 tinental surface. The space between is for the most part 
 yellow, but in many cases appears whitish. The gemination 
 is not necessarily confined only to the canals, but tends to 
 be produced also in the lakes. Often one of these is seen 
 transformed into two short, broad dark lines parallel to one 
 another and traversed by a yellow line. In these cases the 
 gemination is naturally short and does not exceed the limits 
 of the original lake. 
 
 The gemination is not shown by all at the same time, 
 but when the season is at hand it begins to be produced 
 here and there, in an isolated, irregular manner, or at least 
 without any easily recognisable order. In many canals 
 
!58 SCHIAPARELLI 
 
 (such as the Nilosyrtis, for example) the gemination is lack- 
 ing entirely, or is scarcely visible. After having lasted for 
 some months, the markings fade out gradually and disap- 
 pear until another season equally favourable for their forma- 
 tion. Thus it happens that in certain other seasons (espe- 
 cially near the southern solstice of the planet) few are seen, 
 or even none at all. In different oppositions the gemina- 
 tion of the same canal may present different appearances as 
 to width, intensity, and arrangement of the two stripes*; 
 also in some cases the direction of the lines may vary, 
 although by the smallest quantity, but still deviating by 
 a small amount from the canal with which they are directly 
 associated. From this important fact it is immediately 
 understood that the gemination can not be a fixed forma- 
 tion upon the surface of Mars and of a geographical char- 
 acter like the canals. The second of our maps will give an 
 approximate idea of the appearance which these singular 
 formations present. It contains all the geminations ob- 
 served since 1882 up to the present time. In examining it 
 it is necessary to bear in mind that not all of these appear- 
 ances were simultaneous, and consequently that the map 
 does not represent the condition of Mars at any given 
 period; it is only a sort of topographical register of the ob- 
 servations made of this phenomenon at different times. 5 
 
 The observation of the geminations is one of the great- 
 est difficulty, and can only be made by an eye well prac- 
 tised in such work, added to a telescope of accurate con- 
 struction and of great power. This explains why it is that 
 it was not seen before 1882. In the ten years that have 
 transpired since that time it has been seen and described 
 at eight or ten observatories. Nevertheless, some still deny 
 that these phenomena are real, and tax with illusion (or 
 even imposture) those who declare that they have ob- 
 served it. 
 
 Their singular aspect, and their being drawn with abso- 
 lute geometrical precision, as if they were the work of 
 
 5 This map may be found in " La Planete Mars," by Flammarion, 
 page 440. TRANSLATOR. 
 
THE PLANET MARS 159 
 
 rule or compass, have led some to see in them the work 
 of intelligent beings, inhabitants of the planet. I am very 
 careful not to combat this supposition, which includes 
 nothing impossible. (lo mi guardero bene dal combattere 
 questa supposizione, la quale nulla include d' impossibile.) 
 But it will be noticed that in any case the gemination can 
 not be a work of permanent character, it being certain that 
 in a given instance it may change its appearance and di- 
 mensions from one season to another. If we should assume 
 such a work, a certain variability would not be excluded 
 from it; for example, extensive agricultural labour and irri- 
 gation upon a large scale. Let us add, further, that the 
 intervention of intelligent beings might explain the geo- 
 metrical appearance of the gemination, but it is not at all 
 necessary for such a purpose. The geometry of Nature is 
 manifested in many other facts from which are excluded 
 the idea of any artificial labour whatever. The perfect 
 spheroids of the heavenly bodies and the ring of Saturn 
 were not constructed in a turning lathe, and not with com- 
 passes has Iris described within the clouds her beautiful 
 and regular arch. And what shall we say of the infinite 
 variety of those exquisite and regular polyhedrons in which 
 the world of crystals is so rich? In the organic world, also, 
 is not that geometry most wonderful which presides over 
 the distribution of the foliage upon certain plants, which 
 orders the nearly symmetrical, starlike figures of the flowers 
 of the field, as well as of the animals of the sea, and which 
 produces in the shell such an exquisite conical spiral that 
 excels the most beautiful masterpieces of Gothic archi- 
 tecture? In all these objects the geometrical form is the 
 simple and necessary consequence of the principles and 
 laws which govern the physical and physiological world. 
 That these principles and these laws are but an indication 
 of a higher intelligent Power we may admit, but this has 
 nothing to do with the present argument. 
 
 Having regard, then, for the principle that in the ex- 
 planation of natural phenomena it is universally agreed to 
 begin with the simplest suppositions, the first hypotheses 
 
160 SCHIAPARELLI 
 
 of the nature and cause of the geminations have for the 
 most part put in operation only the laws of inorganic Na- 
 ture. Thus, the gemination is supposed to be due either 
 to the effects of light in the atmosphere of Mars, or to 
 optical illusions produced by vapours in various manners, 
 or to glacial phenomena of a perpetual winter, to which it 
 is known all the planets will be condemned, or to double 
 cracks in its surface, or to single cracks of which the images 
 are doubled by the effect of smoke issuing in long lines 
 and blown laterally by the wind. The examination of these 
 ingenious suppositions leads us to conclude that none of 
 them seem to correspond entirely with the observed facts, 
 either in whole or in part. Some of these hypotheses 
 would not have been proposed had their authors been able 
 to examine the geminations with their own eyes. Since 
 some of these may ask me directly, " Can you suggest any- 
 thing better? " I must reply candidly, " No." 
 
 It would be far more easy if we were willing to intro- 
 duce the forces pertaining to organic Nature. Here the 
 field of plausible supposition is immense, being capable 
 of making an infinite number of combinations capable of 
 satisfying the appearances even with the smallest and sim- 
 plest means. Changes of vegetation over a vast area, and 
 the production of animals, also very small, but in enormous 
 multitudes, may well be rendered visible at such a dis- 
 tance. An observer placed in the moon would be able to 
 see such an appearance at the times in which agricultural 
 operations are carried out upon one vast plain the seed- 
 time and the gathering of the harvest. In such a manner 
 also would the flowers of the plants of the great steppes of 
 Europe and Asia be rendered visible at the distance of 
 Mars by a variety of colouring. A similar system of 
 operations produced in that planet may thus certainly be 
 rendered visible to us. But how difficult for the Lunarians 
 and the Areans to be able to imagine the true causes of 
 such changes of appearance without having first at least 
 some superficial knowledge of terrestrial nature! So also 
 for us, who know so little of the physical state of Mars, and 
 
THE PLANET MARS l6l 
 
 nothing of its organic world, the great liberty of possible 
 supposition renders arbitrary all explanations of this sort 
 and constitutes the gravest obstacle to the acquisition of 
 well-founded notions. All that we may hope is that with 
 time the uncertainty of the problem will gradually diminish, 
 demonstrating if not what the geminations are, at least 
 what they can not be. We may also confide a little in what 
 Galileo called " the courtesy of Nature," thanks to which 
 a ray of light from an unexpected source will sometimes 
 illuminate an investigation at first believed inaccessible to 
 our speculations, and of which we have a beautiful example 
 in celestial chemistry. Let us therefore hope and study. 
 
 ii 
 
METEORITES 
 AND STELLAR SYSTEMS 
 
 BY 
 
 GEORGE HOWARD DARWIN 
 
METEORITES AND STELLAR SYSTEMS 1 
 
 IT is only within the last few years that photographic 
 processes have been so far perfected as to make it pos- 
 sible to photograph a faintly luminous celestial object. 
 The success attained has already been so great that we are 
 made aware of the existence of a multitude of stars which 
 would never have been otherwise perceived, even with the 
 finest telescope and under the purest air. The sensitized 
 plate sums up the effects of light, so that under prolonged 
 exposure even a very faint light at length produces its 
 mark. In this respect the advantage is all on the side of 
 the photograph as compared with the eye, for prolonged 
 gazing is actually detrimental to the acuteness of vision. 
 
 The exposure necessary for an ordinary photograph in 
 the broad daylight may be only a fraction of a second, but 
 with the feeble light of the stars three or four hours are 
 found to be necessary or advantageous. Fortunately for 
 the astronomer the heavens move uniformly, and the in- 
 strument can be made to follow the object by clockwork. 
 But as clocks are imperfect, the motion of the photographic 
 telescope has to be constantly regulated by hand, so as to 
 keep exact pace with a star, which is viewed through a 
 second telescope attached to the first one. It may easily 
 be conceived that it has required an enormous amount of 
 skill and patience to attain to the present high degree of 
 perfection. But the details of celestial photography are 
 outside the scope of this paper, and I am only concerned 
 with some of the conclusions which have been drawn from 
 the photographic method. 
 
 1 From " The Century Magazine," October, 1890. By special per- 
 mission of the Century Company. 
 
 165 
 
1 66 DARWIN 
 
 Mr. Isaac Roberts, of Liverpool, has recently photo- 
 graphed a portion of the heavens, embracing about four 
 square degrees in the constellation of Cygnus, and he esti- 
 mates that his plate shows about sixteen thousand stars, 
 none of which are, I believe, visible to the naked eye. A 
 good idea may be formed of this picture by imagining a 
 sheet of dark paper thoroughly splashed with whitewash. 
 The recent advance of celestial photography is well illus- 
 trated by the fact that this same portion of the heavens, 
 when photographed in 1885, appeared to contain only 
 about five thousand stars. Thus four years has tripled the 
 number. 
 
 Four square degrees comprise only about a ten-thou- 
 sandth of the whole heavens, and if space were everywhere 
 as thickly peopled as the constellation of the Swan, the 
 whole number of stars photographically visible would reach 
 the stupendous total of one hundred and sixty-seven mil- 
 lions. But the Milky Way runs through Cygnus, and this 
 is a crowded portion of the heavens. Yet there is little 
 doubt that a hundred millions of stars would already be 
 perceptible if the whole heavens were surveyed with equal 
 thoroughness. 
 
 These celestial photographs bring vividly before us the 
 utter .insignificance of this world and of ourselves; for our 
 planet is of almost contemptible smallness, and our sun is 
 certainly a star of no great magnitude. 
 
 And yet it is nearly twice as far from the sun's center 
 to his surface as from here to the moon, and the planet 
 Neptune is distant nearly three thousand million miles from 
 the sun. The mind fails to grasp such a number of stars 
 as a hundred million, and a limit to the perfection of celes- 
 tial photography has certainly not yet been reached. 
 
 Each of these millions of stars has its history, and there 
 are among them representatives of every stage of evolution. 
 If they were not, even with the telescope, mere specks of 
 light, we might see the whole process before us, and might 
 study them like the objects in a museum. 
 
 Among the stars there are, however, small luminous 
 
METEORITES AND STELLAR SYSTEMS 167 
 
 clouds called nebulae, which are not immeasurably small. 
 They have, of course, been examined with all the finest 
 telescopes for many years, and many strange vagaries in 
 their structure have been noted. 
 
 It is true that we know the stars and nebulae to be made 
 of materials found on the earth, and we can estimate ap- 
 proximately how hot they are, and which are old in their 
 history and which are young. All this has been discovered 
 by means of that wonderful instrument, the spectroscope, 
 but it can not show us their shapes and structures. Within 
 the last few months, however, there is reason to hope that 
 the telescopic photograph may really bring before us in an 
 intelligible shape many objects from the celestial museum. 
 
 Notwithstanding the paucity of definite knowledge, 
 many theories have been propounded as to the sequence 
 of changes through which the solar system has passed. The 
 most celebrated of these is that associated with the names 
 of the great mathematician Laplace and of the philosopher 
 Kant. It is remarkable that substantially the same theory 
 should have been independently formulated by two men 
 whose intellects were so different. 
 
 They both suggested that the matter which now forms 
 the sun and the planets existed in primitive times as a 
 globular nebula of highly rarefied gas in slow rotation, and 
 their theory is accordingly generally known as the nebular 
 hypothesis. 
 
 Every portion of this nebula of course attracted every 
 other portion, and therefore there must have been a con- 
 densation at the centre, at which point a dense nucleus 
 must ultimately have formed. 
 
 The rotation made the nebula fly out like a trundled 
 mop, but the outward tendency was counteracted by attrac- 
 tion. This battle between the attraction due to gravitation 
 and the repulsion due to rotation caused a flattening of the 
 globe, so that it became orange-shaped. 
 
 The gas of which the nebula was composed possessed 
 heat; the central part being probably very hot and the 
 external part very cold, as estimated by terrestrial stand- 
 
DARWIN 
 
 ards. As the energy of heat was gradually lost by radiation 
 into space the globe shrank, and at the same time the cen- 
 tral portion became still hotter. 
 
 In consequence of the shrinkage, the rate of rotation 
 was increased. This mechanical effect may easily be illus- 
 trated thus: If I whirl a stone attached to a string and let 
 the string wind itself up on my finger, the stone will whirl 
 faster and faster as the string shortens. 
 
 Lastly, with increased rate of rotation the increased re- 
 pulsion due to centrifugal force augmented the flattening 
 of the globe. At length a time arrived when the globe was 
 flattened until it became more like a disk than a globe, and 
 gravitation was then no longer capable of holding it to- 
 gether in a single shape. 
 
 Everywhere in the nebula the gas was being pressed by 
 the surrounding gas, attracted toward the centre of the 
 nebula, and repelled by centrifugal force away from the axis 
 of rotation. The attraction diminishes and the repulsion 
 increases the farther we go from the centre. If at a place 
 near the edge of the disklike globe the attraction and 
 repulsion are just equal to one another, pressure is not 
 called into play in keeping the gas in its place. At this 
 distance from the centre, then, the gas which is outside 
 does not press at all on that which is inside, and the 
 inner gas may part company with the outer gas without 
 disturbance to it. 
 
 In fact, according to the nebular hypothesis, when the 
 flattening had reached a certain degree a ring separated 
 itself from the equatorial regions. The central portion, 
 thus relieved, regained a more globular shape, continued to 
 contract and to spin quicker, until a second crisis super- 
 vened, when another ring was shed. A succession of rings 
 was thus formed, and after the detachment of the last the 
 central portion, continuing to contract, at length formed 
 the sun. 
 
 Each ring, as soon as it was free, began to aggregate 
 round some denser portion in its periphery. Subordinate 
 nebulae were thus formed, and they in their turn contracted 
 
METEORITES AND STELLAR SYSTEMS 169 
 
 and shed rings. The nucleus of the secondary nebulae 
 formed the planets, and their rings condensed into satellites. 
 
 This is an outline of the celebrated nebular hypothesis. 
 I shall now show what an interesting confirmation this 
 theory receives from a recent photograph. 
 
 There is in the constellation of Andromeda a nebula so 
 remarkable that its nebulous character was recognised even 
 long before the invention of the telescope. 
 
 This nebula was first photographed with conspicuous 
 success, in October, 1888, by Mr. Roberts, and again on 
 the 29th of the following December, 1888. 
 
 The result is of the greatest interest, for in it we actually 
 see what Laplace pictured with his mind's eye. There is 
 a bright central condensation surrounded by ring after ring, 
 gradually dying away into faintness. 
 
 In one of the rings there is a region of greater bright- 
 ness, which may fairly be interpreted as the centre of aggre- 
 gation for a planet. At another place which is clearly more 
 remote from the centre, although brought nearer by fore- 
 shortening, we have a brilliant round luminous ball surely 
 a planetary nebula already formed. At a much greater dis- 
 tance there is an elongated nebulosity, which we may con- 
 jecture to be a planetary nebula seen edgewise, but in a 
 further state of advance than the other. It is worthy of 
 notice that the remote planets Neptune and Uranus rotate 
 about axes nearly in the plane of their orbits, and from the 
 direction of elongation of this subordinate nebula it seems 
 as though the like must be true here. 
 
 In 1848 Bond measured the positions of these two 
 bright small nebulae relatively to the large one, and they 
 seem to have changed their positions since that date. This 
 confirms the theory that they are planets, but it must be 
 admitted that measurements with reference to an ill-defined 
 object like a nebula are hard to make with precision. 
 
 I should suppose this to be the greatest triumph yet 
 achieved by celestial photography, and I owe my sincere 
 thanks to Mr. Roberts for allowing me to reproduce it. 
 
 But these pictures, while confirming the substantial 
 
DARWIN 
 
 truth of the nebular hypothesis, fail to clear up many of 
 the obscurities which surround the evolution of a planetary 
 system. There is one difficulty indeed so tundamental that 
 it has led some astronomers virtually to throw over the 
 whole theory, and it forms the special object of this essay 
 to discuss it. 
 
 It is the very essence of the nebular hypothesis that the 
 nebula should be formed of continuous gas, one part of 
 which exercises a pressure on another part; for we have 
 seen how gaseous pressure is instrumental in imparting the 
 globular form to the whole, and how when the globe loses 
 heat and shrinks it is just along that line where the pres- 
 sure vanishes that the ring splits off. 
 
 Now, there is no perceptible trace in the solar system 
 of that all-pervading gas from which the whole is supposed 
 to have been evolved; for the planets do not suffer any 
 sensible retardation in their motion round the sun, as would 
 be the case if they were moving through even a highly 
 rarefied gas. 
 
 On the other hand, there is evidence of abundance of 
 solid bodies flying through space. When these bodies meet 
 our atmosphere they glow up white-hot with friction, and 
 are called falling stars or meteorites. Though they are gen- 
 erally dissipated into dust in their passage through the air, 
 yet once in a while one of them owes its preservation to its 
 greater size, and falls on the earth. We thus know them to 
 be strange-looking stones, largely composed of iron. 
 
 The ring which surrounds the planet Saturn was obvi- 
 ously suggestive of the nebular hypothesis to the minds of 
 Laplace and Kant. But it has been conclusively proved 
 by the researches of Roche and Clerk Maxwell to consist 
 of a swarm of loose stones a shower of brickbats, as Max- 
 well was fond of calling it. And now within the last three 
 years spectroscopic research has led Mr. Lockyer to sug- 
 gest that the luminous gas, which undoubtedly forms the 
 visible portion of the nebulae, is simply gas volatilized from 
 the solid state and rendered incandescent by the violent 
 impact of meteoric stones. 
 
METEORITES AND STELLAR SYSTEMS 171 
 
 These gases, he tells us, cool quickly, cease to be lumi- 
 nous, and condense again into the solid state, but the colli- 
 sions being incessant, the whole nebula shines with a steady 
 light. Mr. Lockyer supports his view by an elaborate 
 comparison of the spectra of stars and nebulae with those 
 of actual meteorites, fused by the electric spark in the 
 laboratory. I have not the knowledge of spectroscopy 
 which would be necessary to examine his theory, but his 
 general conclusions seem to be of the highest importance 
 in the study of stellar systems. 
 
 All these lines of observation conspire to indicate that 
 the immediate antecedent of the sun and planets was not a 
 continuous gas, but a swarm of loose stones. And yet the 
 nebular hypothesis seems as good as proved by this photo- 
 graph of Mr. Roberts. Here, then, we find ourselves in 
 a dilemma; on the one hand we have the meteoric theory 
 denying the continuity of the matter which forms the 
 nebulae, while on the other hand the nebular hypothesis 
 demands such continuity. I wish to emphasize this point: 
 either a nebula is made of a cooling gas, such as hydrogen, 
 nitrogen, oxygen, and the vapours of metals, or it is not 
 so. The nebular hypothesis apparently says it must con- 
 sist of gas, while this is denied by the strong evidence that 
 it consisted of an enormous number of stones. It seems at 
 first that either the nebular hypothesis or the meteoric 
 theory must be untrue. 
 
 I believe, nevertheless, that there is a way in which 
 these conflicting ideas may be brought into harmony and 
 made to re-enforce one another, and the special object of 
 this paper is to effect such a reconciliation. But before 
 coming to that we must leave for a time the world of stars, 
 and must consider the ultimate structure of a gas, such 
 as the air we breathe. A gas is now known to consist of 
 ultra-microscopic molecules, all exactly alike in weight, 
 shape, and structure. Although they are invisible, they 
 can be counted and timed; there are found to be millions 
 in a cubic inch of air, moving indiscriminately in all direc- 
 tions, with great velocity. For example, in the air, at a 
 
1 72 DARWIN 
 
 temperature of 60 Fahr., their average speed is 1,570 feet 
 a second half as fast again as the velocity of sound. The 
 temperature of a gas simply depends on the rate at which 
 the molecules are moving. Millions of times in each sec- 
 ond each one of these molecules happens by chance to 
 strike one of its neighbours, and the two which have struck 
 rebound from each other as though they were of India 
 rubber, or at any rate after such an encounter they behave 
 as though they were perfectly elastic. If we could watch 
 the crowd we should see the individuals darting about in 
 a zigzag course, being deflected into a new direction at 
 each collision. I have often been reminded of this so-called 
 kinetic theory of gases when watching the dance of a little 
 swarm of house-flies as they zigzag about and sharply 
 change their paths, when, for a second, two of them get 
 entangled together. Perhaps this familiar example may 
 help the reader to realize the dance of the molecules in 
 a gas. 
 
 The incessant agitation of molecules is quite independ- 
 ent of winds and draughts, and when as many molecules are 
 going in any one direction as in any other we consider 
 the air to be calm. What we call a wind is when more 
 molecules are going in the direction of the wind than in 
 any other. 
 
 The molecules of a gas are not aimed at one another, 
 and as a collision is all a matter of chance, it is clear that 
 a molecule is sometimes nearly stopped, sometimes im- 
 pelled faster, and sometimes merely deflected. Thus they 
 are moving with all possible speeds, but the great majority 
 are moving with about the average speed of the whole 
 crowd. 
 
 According to this theory, the pressure of a gas is merely 
 the cannonade of millions of molecules against the side of 
 the vessel containing the gas; as the number of impacts 
 per square inch and per second is enormous, the effect is 
 indistinguishable from that of continuous pressure. 
 
 We are accustomed to make statistical inquiries into 
 any question affecting groups of men, and the same method 
 
METEORITES AND STELLAR SYSTEMS 173 
 
 has to be applied to the collisions of the molecules of a 
 gas. These very complex statistics have been profoundly 
 studied by Maxwell, Clausius, and others, and they have 
 shown how to compute from the temperature and density 
 of a gas the average velocity of its molecules, the average 
 frequency of collision, and the average distance travelled 
 between successive collisions. 
 
 It will not be possible to go into these difficult questions 
 at present, and it must be accepted that a gas is actually 
 composed of constantly colliding particles or molecules. 
 But when we look at a gas from this point of view we 
 must take care not to confuse the single molecule with the 
 gas of which it forms part. Gas is merely our word for 
 a crowd of molecules, much as nation is a word for a crowd 
 of men. National history is no more to be learned from 
 the doings and character of a single man than the prop- 
 erties of a gas are to be learned from the doings and char- 
 acter of a single molecule. In " gas " and in " nation " 
 the relationship of all to all is involved. 
 
 Now that the internal structure of common air has been 
 explained, let us examine a little more closely its relation to 
 ourselves. 
 
 If we were to shrink to a ten-millionth of our actual 
 size, how different would air seem! It would then seem to 
 consist of cannon-balls flying about in all directions, at rare 
 intervals, and at a prodigious rate. And yet the supposed 
 change in us would not have affected the nature of air, and 
 it would still be a continuous gas to our former senses. 
 Thus the description we should give of a gas is all a mat- 
 ter of the relative scales of largeness of ourselves and of 
 the gas. 
 
 Now, this theory of a gas affords the idea by which I 
 seek to reconcile the conflicting theories of the evolution 
 of stellar systems. My suggestion is that celestial nebulse 
 are drawn on so large a scale that meteorites may be treated 
 as molecules, and that the collisions of meteorites are so 
 frequent that the whole swarm will behave as though it 
 were a gas. The relationship of us men to this coarse- 
 
174 
 
 DARWIN 
 
 grained meteoric medium is exactly that of the ideal pyg- 
 mies to common air. 
 
 But it is not enough to make such a suggestion as this; 
 the details of the idea must be examined. We must con- 
 sider what may be supposed to happen when two stones 
 clash together, and must see whether they can come into 
 collision often enough to make the swarm into a kind 
 of gas. 
 
 In comparing the behaviour of meteorites to the mole- 
 cules of a gas it will naturally occur to inquire whether they 
 can be supposed to possess that high degree of elasticity 
 which is necessary for a kinetic theory. I believe that this 
 question may be answered in the affirmative. Meteoric 
 stones move with speeds which are very great according to 
 our terrestrial notions; and even without Mr. Lockyer's 
 direct spectroscopic evidence we could not doubt that 
 enough heat is generated in a collision to volatilize part of 
 the solid matter of each stone, and to make the gas incan- 
 descent. Now, a sudden generation of gas at the point of 
 contact of the two stones would be exactly like the explo- 
 sion of a charge of gunpowder between them, and they 
 would be blown apart with great violence. As far as re- 
 gards their velocities after collision the result would be 
 much the same as though they were highly elastic, although 
 this virtual elasticity is of quite a different character from 
 that tendency of a strained solid to recover its shape, which 
 constitutes ordinary elasticity. I may call to mind, as an 
 example of an abnormal elasticity of somewhat the same 
 sort, how a leaden bullet bounds from the surface of the 
 sea, although lead is a very inelastic solid, and water is not 
 solid at all. 
 
 It is not claimed that these considerations prove abso- 
 lutely that two meteorites would bound from each other as 
 if they were very elastic, but it seems highly probable that 
 they would do so, and the matter is not susceptible of strict 
 proof. But granting the elasticity, there is another point 
 to consider. 
 > If two stones meet, the chance of their fracture is 
 
METEORITES AND STELLAR SYSTEMS 175 
 
 greater if they are great than if they are small, and the 
 breakage may go on only until a certain size, dependent on 
 the average velocity of the meteorites, is reached, after 
 which it may become unimportant. 
 
 When the gases generated on collision cool they will 
 condense into a metallic rain, and this may fuse with old 
 meteorites. Some actual meteorites show signs of the 
 fusion of many distinct nuclei. Thus there are both ab- 
 stract reason and direct evidence in support of occasional 
 fusions. The mean size of meteorites in a swarm probably 
 depends on the balance between the opposing forces of 
 breakage and fusion. 
 
 No doubt when two stones meet directly each is shat- 
 tered to fragments. But glancing collisions must be indefi- 
 nitely more frequent, and in these we may suppose that 
 fracture is comparatively rare, and virtual elasticity great. 
 
 The possible frequency of fracture undoubtedly does 
 present a difficulty in the theory, for it would seem as 
 though the whole swarm of stones must gradually degrade 
 into dust. There must be some way out of this difficulty, 
 for meteorites of considerable size fall upon the earth, and 
 unless Mr. Lockyer has misinterpreted the spectroscopic 
 evidence, the nebulae do now consist of meteorites. 
 
 I hold that these considerations justify us in maintain- 
 ing a rough similarity between meteoric stones and the 
 molecules of a gas, as far as regards the actual collisions. 
 If this is so, what is called the temperature of a gas must 
 be translated as meaning the average energy of motion of 
 the meteorites. 
 
 We must now go on to consider how often meteorites 
 collide, and try to discover whether a swarm of meteorites 
 possesses a fine enough texture to permit the applicability 
 of the theory. For this part of the discussion numerical 
 calculations are necessary. Calculations require numerical 
 data, and these can only be derived from a known system, 
 and of course the only one known with any precision is the 
 solar system. The fineness of grain is obviously independ- 
 ent of the amount of flattening of the nebula which arises 
 
DARWIN 
 
 from rotation. In order, therefore, to simplify the matter, 
 and to consider one thing at a time, the nebula is supposed 
 to be one in which there is no rotation. 
 
 The weight of the sun in pounds is four with thirty 
 zeros after it, and I suppose the sun to be broken up into 
 that number of meteorites, each weighing one pound. If 
 the meteorites are supposed to be of iron their exact size 
 is known, because the dimensions of a pound of iron are 
 known. This supposition as to the weight and size of the 
 meteorites is merely adopted as a type, but it suffices for 
 our present purpose. These one-pound iron stones are to 
 be distributed in a swarm extending beyond the present 
 orbit of Neptune. To give numerical precision, let us 
 suppose that the swarm extends half as far again as Nep- 
 tune's orbit that is to say, let it extend to forty-five times 
 the distance of the earth from the sun. 
 
 In this condition the nebula is of extreme tenuity, and 
 if the stones are not then too sparse to make the swarm 
 behave like a gas, it will, a fortiori, behave like a gas when 
 the nebula has shrunk and the stones are more closely 
 packed. The supposition made as to the extension of 
 the solar swarm, therefore, puts the theory to a severe 
 test. 
 
 In the case of a town, density of population means the 
 number of people to the square mile, and for meteorites 
 what we may still call the density of population is the 
 number to the cubic mile. The swarm is not supposed to 
 be rotating, and is therefore a perfect globe, and the layers 
 of equal density of population are also spheres. 
 
 The stones will not be evenly distributed in space, but 
 the density of population will be much greater toward the 
 middle than toward the outside. The reason of this is that 
 every stone is attracted toward the middle, and is only 
 prevented from yielding to the tendency by the blows it 
 receives from its neighbours. Think of a crowd struggling 
 for tickets at a railway station, and you have a picture of 
 what happens. The men squeeze and push and sway about, 
 but the crowd remains of about the same density at each 
 
METEORITES AND STELLAR SYSTEMS 177 
 
 distance from its middle. So in the swarm the dance of the 
 meteorites is incessant, but it arranges itself automatically 
 into a steady condition, in which the density of population 
 has no tendency to shift. 
 
 It is natural to ask why the stones should be moving 
 at all, and how they acquired their great speed. This is 
 a question that imperatively demands an answer, and we 
 are able to answer it with certainty. They derive their 
 speed from gravitation; they have fallen in from a great 
 distance toward a centre of aggregation. A description of 
 the way in which they may have come together will make 
 it clear why they are moving, and will also give the reader 
 some idea of how the actual velocity may be calculated in a 
 swarm of given mass and size. 
 
 Imagine that somewhere in space there is an aggregation 
 of meteorites no matter how it got there and conceive 
 a stone released from a state of rest at a very great distance. 
 Under the attraction of gravitation the stone falls toward 
 the centre of aggregation, and on reaching the confines of 
 the swarm it will have acquired a certain velocity. It then 
 penetrates the swarm for some uncertain distance, until it 
 happens to strike another meteorite. Henceforth its path 
 is zigzag, as it happens to strike, and we need not suppose 
 ourselves to watch it any longer, since it has become one of 
 the swarm. It is, however, important to remark that the 
 supposed visitant from outside space has imported energy 
 of motion into the system, which energy it gradually com- 
 municates to its neighbours by collisions; it has also in- 
 creased the mass of the swarm. When another stone is 
 allowed to fall in, since it is attracted by a slightly greater 
 mass, it arrives at the swarm with slightly greater speed 
 than the first. So if we imagine the swarm to be increased 
 by the addition of stone after stone, we see that in the 
 course of accretion the energy of agitation of its constitu- 
 ent meteorites gradually increases. Also the volume of the 
 globe throughout which (if anywhere) the swarm possesses 
 the mechanical properties of a gas is at the same time 
 gradually increased. We must suppose that at length all 
 
 12 
 
178 DARWIN 
 
 the stones in that part of space are exhausted, the materials 
 of the nebula are collected, and it only remains for them 
 to work out their future fate. 
 
 By this sort of reasoning we find out how fast the stones 
 are moving, but it is proper to add that an important cor- 
 rection has to be applied to allow for the fact that at each 
 collision some speed is lost. In the process of settling into 
 the steady condition each stone retains only seven tenths of 
 the velocity it would have had if it were a fresh arrival from 
 space. 
 
 It will make no material difference in these results by 
 what process the stones were brought together, and this 
 account which I have just given of the formation of a 
 swarm is not intended as a contribution to its history, but 
 is only meant to render intelligible the mechanical prin- 
 ciples involved, and to show in a general way how the 
 matter may be subjected to calculation. 
 
 By such a line of argument as this I found that, when 
 the solar swarm extended half as far again as the planet 
 Neptune, in the central region the stones were moving at 
 an average rate of three miles a second two hundred times 
 as fast as a fast train but that in the outer portion of the 
 swarm the velocity was less. 
 
 We have now to find out how often the stones came 
 into collision, how far they travelled beween collisions, and 
 whether the collisions were frequent enough to allow us to 
 consider the whole nebula as a kind of gas, as is demanded 
 by the nebular hypothesis. 
 
 To how small a pygmy would air still be air? The an- 
 swer is that the pygmy must be just large enough to be 
 struck so often that he loses the sensation of the individual 
 blows, and is only aware of their average effect. It will 
 insure this if he is struck hundreds or thousands of times 
 in the average interval between two collisions of a molecule 
 of air; or we may say that his bulk must be great enough 
 to contain thousands of molecules, or his length thousands 
 of times as great as the average path traversed by a mole- 
 cule between two successive collisions. These conditions 
 
METEORITES AND STELLAR SYSTEMS 179 
 
 are amply satisfied in the relationship of the smallest micro- 
 scopic animalcule to our air. 
 
 It must, however, be a giant who would not feel the 
 individual blows of meteorites, but only realize their aver- 
 age effects. If we might consider the nebula, as a whole, 
 to be a living being, we might say that if she is to behave 
 like a gas she must realize herself as a gas, and so she must 
 be the giant to whose perceptions the meteoric nebula is 
 to be a gas; hence the giant must not be larger than the 
 nebula itself. 
 
 It would not be easy to explain the exact reasoning by 
 which a comparison is made between the dimensions of the 
 giant and the texture of the nebula at every part of itself. 
 It must suffice to say that the comparison is best clothed in 
 a form which may appear something quite different, but 
 which is really substantially the same. 
 
 Except at the moment of a collision, a meteorite is like 
 a very small planet, and accordingly moves in a curved orbit, 
 but at each collision it starts in a new orbit. 
 
 I say, then, that the nebula will behave sufficiently like 
 a gas to allow the nebular hypothesis to be true, if the 
 average path of a meteorite between two collisions is so 
 short that the bit of orbit described departs very little from 
 a straight line. 
 
 We now at length come to the numerical values to 
 which we have been tending, and shall see how often the 
 stones of the solar nebula came into collision with one an- 
 other when the nebula extended in a swarm of one-pound 
 iron meteorites half as far again from its centre as the 
 present distance of Neptune from the sun. 
 
 In this case I find that at the middle of the swarm a 
 meteorite would on the average come into collision every 
 thirteen hours, and would travel 140,000 miles between 
 collisions; at the distance of the small planets, called the 
 asteroids, it would collide every seventeen hours and would 
 travel 190,000 miles between; at the distance of Uranus 
 the collisions would be at intervals of twenty-five days, and 
 the path 6,000,000 miles; and lastly, at the distance of Nep- 
 
l8o DARWIN 
 
 tune, the interval would be one hundred and ninety days 
 and the path 28,000,000 miles. 
 
 I have said that the criterion we have to apply depends 
 on the amount of curvature of the average path of a stone 
 between two successive collisions. Now, it may be shown 
 that the amount of departure from straightness is greater 
 the farther we go from the middle of the swarm; and I 
 find that even at the distance of Neptune the collisions are, 
 speaking relatively, so frequent that gravity only suffices 
 to draw the meteorite aside from the straight path by one 
 sixty-sixth of the path it has traversed. The fraction one 
 sixty-sixth is then the value of the criterion which was to 
 be applied. Now one sixty-sixth is so small a fraction that 
 it may be concluded that the meteoric swarm passes the 
 proposed test, notwithstanding that the great extension 
 which has been attributed to the nebula strains the hy- 
 pothesis severely. 
 
 It follows, therefore, that if meteorites possess virtual 
 elasticity, and if breakages are counterbalanced by fusions, 
 then a swarm of meteorites provides a gaslike medium of 
 a fine enough structure to satisfy the demands of the nebu- 
 lar hypothesis. 
 
 Some such numerical examination as the foregoing is 
 necessary in order to assure us that the quality of a gas 
 can have been imparted to a nebula in the suggested way, 
 and so to lift the hypothesis out of the realms of mere 
 conjecture. 
 
 We may conclude from this discussion that it is possible 
 to justify the contention that the meteoric theory is recon- 
 cilable with the nebular hypothesis, and that we may ac- 
 cordingly hold the truth of both of them at the same time. 
 
 If space permitted I might go on to consider some of 
 the conclusions fairly deducible from this view of a nebula, 
 but it must suffice to say that this theory seems likely to 
 prove fruitful in the further elucidation of this complex and 
 necessarily speculative subject. 
 
 Up to this time we have been occupied with proving, 
 or rendering probable, a modification in Laplace's theory. 
 
METEORITES AND STELLAR SYSTEMS 181 
 
 But it would hardly be satisfactory to leave the matter at 
 this point. I wish it were possible to gain an insight into 
 the origin and previous history of these stones, but on 
 these mysteries I have no suggestion to make. It is, how- 
 ever, possible to see pretty clearly what happened after the 
 nebular stage, and how all this bears on the state of things 
 of which we are witnesses to-day. 
 
 At the various centres of condensation which we now 
 call sun, planets, and satellites the swarm of meteorites be- 
 came denser and denser. The collisions were too frequent 
 to let the gases cool and condense again,, and thus by de- 
 grees the meteorites were entirely volatilized. Thus round 
 these centres we should have at length a mass of glowing 
 gas, and toward the middle fluids and solids. All this must 
 have occurred comparatively early in the case of the sun, 
 later at the planets, and last at the satellites. 
 
 Outside of these condensations there were numbers of 
 free meteorites, but the majority of the stones which formed 
 the swarm in primitive times were already absorbed, and 
 the absorption still went on gradually. 
 
 The collisions among the free meteorites became rarer, 
 because they were scattered more sparsely; and less violent, 
 because at each successive collision some relative motion 
 was lost. Finally, the collisions were nearly annulled. The 
 residue of the meteoric swarm then consisted of sparse 
 flights of meteorites, moving in streams. Such streams 
 give us no evidence of their existence, except under special 
 circumstances. 
 
 The zodiacal light is a lens-shaped luminosity, seen in 
 the east or west shortly before sunrise or after sunset not 
 commonly in the latitude of England, but frequently in 
 the south. It is probably due to the reflection of sunlight 
 from millions of meteorites which have not yet been swal- 
 lowed by the sun. 
 
 Again, if a stream of meteorites moves in a very elliptic 
 orbit, at one part of its course it passes near the sun. In 
 this part of its orbit the flight is packed into a smaller space 
 than before, so that collisions are largely multiplied. More- 
 
182 DARWIN 
 
 over, the flight dashes through a region thickly peopled 
 with the meteorites which make the zodiacal light. It has 
 been proved that there is an intimate relationship between 
 comets and flights of meteorites, and Mr. Lockyer sug- 
 gests that the luminosity of comets is caused jointly by the 
 collisions internal to the flight of stones, and by those 
 which occur as the flight ploughs its way through the zodi- 
 acal light. 
 
 But meteorites are still frequent far outside of the zodi- 
 acal light, although there may not be enough to reflect 
 sunlight to a visible degree. Of this we have familiar evi- 
 dence in the shooting star. 
 
 The orbits of several streams of meteorites are known, 
 and each year, as on certain days the earth crosses those 
 orbits, their existence is proved by volleys of falling stars, 
 which emanate from known radiant points in the heavens. 
 
 But these are the dregs and sawdust of the solar system, 
 and merely serve to give us a memento of the myriads 
 which existed in early days, before the sun and the planets 
 and their satellites were born. 
 
 In this paper I have attempted to touch on only a few 
 points in a large subject. The attempt to reconstruct the 
 history of stars and planets involves ideas grand in them- 
 selves; but the events to be recorded in that history relate 
 to a past so remote that our conclusions can not but be 
 speculative. Thus the value of the investigation of which 
 I have given an account will appear very different to dif- 
 ferent minds. To some men of science it will stand con- 
 demned as altogether too speculative; others will think that 
 it is better to risk error in the chance of winning truth. To 
 me, at least, it seems that the line of thought flows in a 
 true channel; that it may help to give a meaning to the 
 observations of the astronomer and of the spectroscopist; 
 and that many interesting problems may perhaps be solved 
 with sufficient completeness to throw further light on the 
 evolution of nebulae and of planetary systems. 
 
MAGNITUDE OF THE SOLAR 
 SYSTEM 
 
 BY 
 
 WILLIAM HARKNESS 
 
MAGNITUDE OF THE SOLAR SYSTEM 1 
 
 NATURE may be studied in two widely different 
 ways. On the one hand we may employ a powerful 
 microscope which will render visible the minutest 
 forms, and limit our field of view to an infinitesimal frac- 
 tion of an inch situated within a foot of our own noses; or, 
 on the other hand, we may occupy some commanding posi- 
 tion, and from thence, aided perhaps by a telescope, we 
 may obtain a comprehensive view of an extensive region. 
 The first method is that of the specialist, the second is that 
 of the philosopher, but both are necessary for an adequate 
 understanding of Nature. The one has brought us knowl- 
 edge wherewith to defend ourselves against bacteria and 
 microbes, which are among the most deadly enemies of 
 mankind, and the other has made us acquainted with the 
 great laws of matter and force upon which rests the whole 
 fabric of science. All Nature is one, but for convenience 
 of classification we have divided our knowledge into a 
 number of sciences which we usually regard as quite dis- 
 tinct from each other. Along certain lines, or, more prop- 
 erly, in certain regions, these sciences necessarily abut 
 on each other, and just there lies the weakness of the spe- 
 cialist. He is like a wayfarer who always finds obstacles in 
 crossing the boundaries between two countries, while to 
 the traveller who gazes over them from a commanding 
 eminence the case is quite different. If the boundary is 
 
 1 Presidential address delivered before the American Association for 
 the Advancement of Science, at its Brooklyn meeting, August 16, 1894. 
 Printed in " Astronomy and Astro-Physics," vol. xiii, No. 8; also in 
 " American Journal of Science," vol. xlviii, September, 1894; and the 
 " Smithsonian Report for 1894." 
 
 185 
 
HARKNESS 
 
 an ocean shore, there is no mistaking it; if a broad river or 
 a chain of mountains, it is still distinct; but if only a line of 
 posts traced over hill and dale, then it becomes lost in the 
 natural features of the landscape, and the essential unity 
 of the whole region is apparent. In that case the bor- 
 der-land is wholly a human conception of which Nature 
 takes no cognizance, and so it is with the scientific bor- 
 der-land to which I propose to invite your attention this 
 evening. 
 
 To the popular mind there are no two sciences further 
 apart than astronomy and geology. The one treats of the 
 structure and mineral constitution of our earth, the causes 
 of its physical features and its history, while the other treats 
 of the celestial bodies, their magnitudes, motions, distances, 
 periods of revolution, eclipses, order, and of the causes of 
 their various phenomena. And yet many, perhaps I may 
 even say most, of the apparent motions of the heavenly 
 bodies are merely reflections of the motions of the earth, 
 and in studying them we are really studying it. Further- 
 more, precession, nutation, and the phenomena of the tides 
 depend largely upon the internal structure of the earth, and 
 there astronomy and geology merge into each other. 
 Nevertheless, the methods of the two sciences are widely 
 different, most astronomical problems being discussed 
 quantitatively by means of rigid mathematical formulae, 
 while in the vast majority of cases the geological ones are 
 discussed only qualitatively, each author contenting him- 
 self with a mere statement of what he thinks. With pre- 
 cise data the methods of astronomy lead to very exact re- 
 sults, for mathematics is a mill which grinds exceeding fine ; 
 but, after all, what comes out of a mill depends wholly upon 
 what is put into it, and if the data are uncertain, as is the 
 case in most cosmological problems, there is little to choose 
 between the mathematics of the astronomer and the guesses 
 of the geologist. 
 
 If we examine the addresses delivered by former presi- 
 dents of this association, and of the sister perhaps it would 
 be nearer the truth to say the parent association on the 
 
MAGNITUDE OF THE SOLAR SYSTEM 187 
 
 other side of the Atlantic, we shall find that they have gen- 
 erally dealt either with the recent advances in some broad 
 field of science, or else with the development of some spe- 
 cial subject. This evening I propose to adopt the latter 
 course, and I shall invite your attention to the present con- 
 dition of our knowledge respecting the magnitude of the 
 solar system; but in so doing it will be necessary to intro- 
 duce some considerations derived from laboratory experi- 
 ments upon the luminiferous ether, others derived from 
 experiments upon ponderable matter, and still others relat- 
 ing both to the surface phenomena and to the internal 
 structure of the earth, and thus we shall deal largely with 
 the border-land where astronomy, physics, and geology 
 merge into each other. 
 
 The relative distances of the various bodies which com- 
 pose the solar system can be determined to a considerable 
 degree of approximation with very crude instruments as 
 soon as the true plan of the system becomes known, and 
 that plan was taught by Pythagoras more than five hun- 
 dred years before Christ. It must have been known to the 
 Egyptians and Chaldeans still earlier, if Pythagoras really 
 acquired his knowledge of astronomy from them, as is 
 affirmed by some of the ancient writers, but on that point 
 there is no certainty. In public Pythagoras seemingly ac- 
 cepted the current belief of his time, which made the earth 
 the centre of the universe, but to his own chosen disciples 
 he communicated the true doctrine that the sun occupies 
 the centre of the solar system and that the earth is only one 
 of the planets revolving around it. Like all the world's 
 greatest sages, he seems to have taught only orally. A 
 century elapsed before his doctrines were reduced to writ- 
 ing by Philolaus of Crotona, and it was still later before they 
 were taught in public for the first time by Hicetas, or as he 
 is sometimes called Nicetas, of Syracuse. Then the familiar 
 cry of impiety was raised, and the Pythagorean system was 
 eventually suppressed by that now called the Ptolemaic, 
 which held the field until it was overthrown by Copernicus 
 almost two thousand years later. Pliny tells us that Pythag- 
 
1 88 HARKNESS 
 
 oras believed the distances to the sun and moon to be, 
 respectively, 252,000 and 12,600 stadia, or, taking the 
 stadium at 625 feet, 29,837 and 1,492 English miles; but 
 there is no record of the method by which these numbers 
 were ascertained. 
 
 After the relative distances of the various planets are 
 known, it only remains to determine the scale of the system, 
 for which purpose the distance between any two planets 
 suffices. We know little about the early history of the sub- 
 ject, but it is clear that the primitive astronomers must have 
 found the quantities to be measured too small for detection 
 with their instruments, and even in modern times the prob- 
 lem has proved to be an extremely difficult one. Aris- 
 tarchus, of Samos, who flourished about 270 B. c., seems 
 to have been the first to attack it in a scientific manner. 
 Stated in modern language, his reasoning was that when 
 the moon is exactly half full the earth and sun, as seen 
 from its centre, must make a right angle with each other, 
 and by measuring the angle between the sun and moon, 
 as seen from the earth at that instant, all the angles of the 
 triangle joining the earth, sun, and moon would become 
 known, and thus the ratio of the distance of the sun to the 
 distance of the moon would be determined. Although per- 
 fectly correct in theory, the difficulty of deciding visually 
 upon the exact instant when the moon is half full is so 
 great that it can not be accurately done, even with the most 
 powerful telescopes. Of course, Aristarchus had no tele- 
 scope, and he does not explain how he effected the observa- 
 tion, but his conclusion was that at the instant in question 
 the distance between the centres of the sun and moon as 
 seen from the earth is less than a right angle by one thirtieth 
 part of the same. We should now express this by saying 
 that the angle is 87, but Aristarchus knew nothing of trigo- 
 nometry, and in order to solve his triangle he had recourse 
 to an ingenious but long and cumbersome geometrical 
 process, which has come down to us, and affords conclusive 
 proof of the condition of Greek mathematics at that time. 
 His conclusion was that the sun is nineteen times farther 
 
AS COPE 
 Photogravure from a painting. 
 
e sun and moon to be, 
 
 stadia, or, taking the 
 
 .492 English miles; but 
 
 . by which these numbers 
 
 f the various planets are 
 e the scale of the system, 
 >n any two planets 
 iy history of the sub- 
 orners must have 
 iall for detection 
 
 one. 
 
 each other, 
 
 and sun and moon, 
 
 .nt, all the angles of the 
 
 would become 
 
 n to the 
 
 he most 
 
 'io tele- 
 
 bserva- 
 
 in question 
 
 mi and moon as 
 
 ,gle by one thirtieth 
 
 express this by saying 
 
 tha r knew nothing of trigo- 
 
 non. ; iangle he had recourse 
 
 to an ii 1 cumbersome geomet: 
 
 5, and affords conclu 
 thematics at tli 
 eteen times far: 
 
MAGNITUDE OF THE SOLAR SYSTEM 189 
 
 from the earth than the moon, and if we combine that re- 
 sult with the modern value of the moon's parallax viz., 
 3,422.38", we obtain for the solar parallax 180", which is 
 more than twenty times too great. 
 
 The only other method of determining the solar parallax 
 known to the ancients was that devised by Hipparchus 
 about 150 B. c. It was based on measuring the rate of de- 
 crease of the diameter of the earth's shadow cone by noting 
 the duration of lunar eclipses, and as the result deduced 
 from it happened to be nearly the same as that found by 
 Aristarchus, substantially his value of the parallax remained 
 in vogue for nearly two thousand years, and the discovery 
 of the telescope was required to reveal its erroneous char- 
 acter. Doubtless this persistency was due to the extreme 
 minuteness of the true parallax, which we now know is far 
 too small to have been visible upon the ancient instruments, 
 and thus the supposed measures of it were really nothing 
 but measures of their inaccuracy. 
 
 The telescope was first pointed to the heavens by 
 Galileo in 1609, but it needed a micrometer to convert it 
 into an accurate measuring instrument, and that did not 
 come into being until 1639, when it was invented by Wil- 
 liam Gascoigne. After his death, in 1644, his original in- 
 strument passed to Richard Townley, who attached it to a 
 fourteen-foot telescope at his residence in Townley, Lanca- 
 shire, England, where it was used by Flamsteed in observ- 
 ing the diurnal parallax of Mars during its opposition in 
 1672. A description of Gascoigne's micrometer was pub- 
 lished in the " Philosophical Transactions " in 1667, and a 
 little before that a similar instrument had been invented by 
 Auzout, in France, but observatories were fewer then than 
 now, and, so far as I know, J. D. Cassini was the only per- 
 son besides Flamsteed who attempted to determine the 
 solar parallax from that opposition of Mars. Foreseeing the 
 importance of the opportunity, he had Richer despatched to 
 Cayenne some months previously, and when the opposition 
 came he effected two determinations of the parallax: one 
 being by the diurnal method, from his own observations 
 
190 HARKNESS 
 
 in Paris, and the other by the meridian method, from ob- 
 servations in France by himself, Romer, and Picard, com- 
 bined with those of Richer at Cayenne. This was the tran- 
 sition from the ancient instruments with open sights to tele- 
 scopes armed with micrometers, and the result must have 
 been little short of stunning to the seventeenth-century 
 astronomers, for it caused the hoary and gigantic parallax 
 of about 1 80" to shrink incontinently to 10", and thus ex- 
 panded their conception of the solar system to something 
 like its true dimensions. More than fifty years previously 
 Kepler had argued from his ideas of the celestial harmonies 
 that the solar parallax could not exceed 60 ", and a little 
 later Horrocks had shown on more scientific grounds that 
 it was probably as small as 14"; but the final death blow 
 to the ancient values ranging as high as 2! or 3' came 
 from these observations of Mars by Flamsteed, Cassini, 
 and Richer. 
 
 Of course, the results obtained in 1672 produced a keen 
 desire on the part of astronomers for further evidence re- 
 specting the true value of the parallax, and as Mars comes 
 into a favourable position for such investigations only at 
 intervals of about sixteen years, they had recourse to ob- 
 servations of Mercury and Venus. In 1677 Halley ob- 
 served the diurnal parallax of Mercury, and also a transit 
 of that planet across the sun's disk, at St. Helena, and in 
 1 68 1 J. D. Cassini and Picard observed Venus when she 
 was on the same parallel with the sun, but although the 
 observations of Venus gave better results than those of 
 Mercury, neither of them was conclusive, and we now know 
 that such methods are inaccurate even with the powerful in- 
 struments of the present day. Nevertheless, Halley's at- 
 tempt by means of the transit of Mercury ultimately bore 
 fruit in the shape of his celebrated paper of 1716, wherein he 
 showed the peculiar advantages of transits of Venus for 
 determining the solar parallax. The idea of utilizing such 
 transits for this purpose seems to have been vaguely con- 
 ceived by James Gregory, or perhaps even by Horrocks, 
 but Halley was the first to work it out completely, and 
 
MAGNITUDE OF THE SOLAR SYSTEM 191 
 
 long after his death his paper was mainly instrumental in 
 inducing the governments of Europe to undertake the 
 observations of the transits of Venus in 1761 and 1769, 
 from which our first accurate knowledge of the sun's dis- 
 tance was obtained. 
 
 Those who are not familiar with practical astronomy 
 may wonder why the solar parallax can be got from Mars 
 and Venus, but not from Mercury or the sun itself. The 
 explanation depends upon two facts: Firstly, the nearest 
 approach of these bodies to the earth is for Mars 33,874,000 
 miles, for Venus 23,654,000 miles, for Mercury 47,935,000 
 miles, and for the sun 91,239,000 miles. Consequently, for 
 us Mars and Venus have very much larger parallaxes than 
 Mercury or the sun, and of course the larger the parallax 
 the easier it is to measure. Secondly, even the largest of 
 these parallaxes must be determined within far less than 
 one tenth of a second of the truth, and while that degree of 
 accuracy is possible in measuring short arcs, it is quite un- 
 attainable in long ones. Hence, one of the most essential 
 conditions for the successful measurement of parallaxes is 
 that we shall be able to compare the place of the near body 
 with that of a more distant one situated in the same region 
 of the sky. In the case of Mars that can always be done 
 by making use of a neighbouring star, but when Venus is 
 near the earth she is also so close to the sun that stars are 
 not available, and consequently her parallax can be satis- 
 factorily measured only when her position can be accu- 
 rately referred to that of the sun, or, in other words, only 
 during her transits across the sun's disk. But even when 
 the two bodies to be compared are sufficiently near each 
 other, we are still embarrassed by the fact that it is more 
 difficult to measure the distance between the limb of a 
 planet and a star or the limb of the sun than it is to 
 measure the distance between two stars, and since the dis- 
 covery of so many asteroids that circumstance has led to 
 their use for the determination of the solar parallax. Some' 
 of these bodies approach within 75,230,000 miles of the 
 earth's orbit, and as they look precisely like stars, the in- 
 
I9 2 HARKNESS 
 
 creased accuracy of pointing on them fully makes up for 
 their greater distance as compared with Mars or Venus. 
 
 After the Copernican system of the world and the New- 
 tonian theory of gravitation were accepted it soon became 
 evident that trigonometrical measurements of the solar 
 parallax might be supplemented by determinations based 
 on the theory of gravitation, and the first attempts in that 
 direction were made by Machin in 1729 and T. Mayer in 
 1753. The measurement of the velocity of light between 
 points on the earth's surface, first effected by Fizeau in 
 1849, opened up still other possibilities, and thus for deter- 
 mining the solar parallax we have at our command no less 
 than three entirely distinct classes of methods, which are 
 known respectively as the trigonometrical, the gravita- 
 tional, and the photo-tachymetrical. We have already 
 given a summary sketch of the trigonometrical methods as 
 applied by the ancient astronomers to the dichotomy and 
 shadow cone of the moon, and by the moderns to Venus, 
 Mars, and the asteroids, and we shall next glance briefly 
 at the gravitational and photo-tachymetrical methods. 
 
 The gravitational results which enter directly or indi- 
 rectly into the solar parallax are six in number, to wit: 
 First, the relation of the moon's mass to the tides; second, 
 the relation of the moon's mass and parallax to the force 
 of gravity at the earth's surface; third, the relation of the 
 solar parallax to the masses of the earth and moon; fourth, 
 the relation of the solar and lunar parallaxes to the moon's 
 mass and parallactic inequality; fifth, the relation of the 
 solar and lunar parallaxes to the moon's mass and the 
 earth's lunar inequality; sixth, the relation of the constants 
 of nutation and precession to the moon's parallax. 
 
 Respecting the first of these relations it is to be re- 
 marked that the tide-producing forces are the attractions 
 of the sun and moon upon the waters of the ocean, and 
 from the ratio of these attractions the moon's mass can 
 readily be determined. But unfortunately the ratio of the 
 solar tides to the lunar tides is affected both by the depth 
 of the sea and by the character of the channels through 
 
MAGNITUDE OF THE SOLAR SYSTEM 193 
 
 which the water flows, and for that reason the observed 
 ratio of these tides requires multiplication by a correcting 
 factor in order to convert it into the ratio of the forces. 
 The matter is further complicated by this correcting factor 
 varying from port to port, and in order to get satisfactory 
 results long series of observations are necessary. The 
 labour of deriving the moon's mass in this way was for- 
 merly so great that for more than half a century Laplace's 
 determination from the tides at Brest remained unique; but 
 the recent application of harmonic analysis to the data sup- 
 plied by self-registering tide gauges is likely to yield abun- 
 dant results in the near future. 
 
 Our second gravitational relation viz., that connecting 
 the moon's mass and parallax with the force of gravity at 
 the earth's surface affords an indirect method of deter- 
 mining the moon's parallax with very great accuracy if the 
 computation is carefully made, and with a fair approxima- 
 tion to the truth even when the data are exceedingly crude. 
 To illustrate this, let us see what could be done with a rail- 
 road transit such as is commonly used by surveyors, a steel 
 tape, and a fairly good watch. Neglecting small correc- 
 tions due to the flattening of the earth, the centrifugal force 
 at its surface, the eccentricity of its orbit and the mass of 
 the moon, the law of gravitation shows that if we multiply 
 together the length of the seconds-pendulum, the square of 
 the radius of the earth, and the square of the length of the 
 sidereal month, divide the product by four, and take the 
 cube root of the quotient, the result will be the distance 
 from the earth to the moon. To find the length of the sec- 
 onds-pendulum we would rate the watch by means of the 
 railroad transit, and then making a pendulum out of a 
 spherical leaden bullet suspended by a fine thread, we 
 would adjust the length of the thread until the pendulum 
 made exactly three hundred vibrations in five minutes by 
 the watch. Then, supposing the experiment to be made 
 here or in New York city, we would find that the distance 
 from the point of suspension of the thread to the centre of 
 the bullet was about 39^ inches, and dividing that by the 
 13 
 
HARKNESS 
 
 number of inches in a mile viz., 63,360 we would have 
 for the length of the seconds-pendulum -^^ of a mile. 
 The next step would be to ascertain the radius of the earth, 
 and the quickest way of doing so would probably be, first, 
 to determine the latitude of some point in New York city 
 by means of the railroad transit ; next to run a traverse sur- 
 vey along the old post-road from New York to Albany, and 
 finally to determine the latitude of some point in Albany. 
 The traverse survey should surely be correct to one part in 
 three hundred, and as the distance between the two cities 
 is about 2, the difference of latitude might be determined 
 to about the same percentage of accuracy. In that way 
 we would find the length of 2 of latitude to be about 138 
 miles, whence the earth's radius would be 3,953 miles. It 
 would then only remain to observe the time occupied by 
 the moon in making a sidereal revolution around the earth, 
 or, in other words, the time which she occupies in moving 
 from any given star back to the same star again. By noting 
 that to within one quarter of her own diameter we would 
 soon find that the time of revolution is about 27.32 days, 
 and multiplying that by the number of seconds in a 
 day viz., 86,400 we would have for the length of the 
 sidereal month 2,360,000 seconds. With these data the 
 computation would stand as follows: The radius of the 
 earth, 3,953 miles, multiplied by the length of a sidereal 
 month, 2,360,000 seconds, and the product squared gives 
 87,060,000,000,000,000,000. Multiplying that by one 
 fourth of the length of the seconds-pendulum viz., 6 ^ s 
 of a mile and extracting the cube root of the product, we 
 would get 237,700 miles for the distance from the earth 
 to the moon, which is only about 850 miles less than the 
 truth, and certainly a remarkable result considering the 
 crudeness of the instruments by which it might be obtained. 
 Nevertheless, when all the conditions are rigorously taken 
 into account these data are to be regarded as determining 
 the relation between the moon's mass and parallax, rather 
 than the parallax itself. 
 
 Our third gravitational relation to wit, that existing 
 
MAGNITUDE OF THE SOLAR SYSTEM 195 
 
 between the solar parallax, the solar attractive force, and 
 the masses of the earth and moon is analogous to the rela- 
 tion existing between the moon's mass and parallax and the 
 force of gravity at the earth's surface, but it can not be 
 applied in exactly the same way on account of our inability 
 to swing a pendulum on the sun. We are therefore com- 
 pelled to adopt some other method of determining the 
 sun's attractive force, and the most available is that which 
 consists in observing the perturbative action of the earth 
 and moon upon our nearest planetary neighbours, Venus 
 and Mars. From this action the law of gravitation enables 
 us to determine the ratio of the sun's mass to the combined 
 masses of the earth and moon, and then the relation in 
 question furnishes a means of comparing the masses so 
 found with trigonometrical determinations of the solar 
 parallax. Thus it appears that notwithstanding necessary 
 differences in the methods of procedure, the analogy be- 
 tween the second and third gravitational relations holds not 
 only with respect to their theoretical basis, but also in 
 their practical application, the one being used to determine 
 the relation between the mass of the moon and its distance 
 from the earth, and the other to determine the relation be- 
 tween the combined masses of the earth and moon and 
 their distance from the sun. 
 
 Our fourth gravitational relation deals with the connec- 
 tion between the solar parallax, the lunar parallax, the 
 moon's mass, and the moon's parallactic inequality. The 
 important quantities are here the solar parallax and the 
 moon's parallactic inequality, and although the derivation 
 of the complete expression for the connection between them 
 is a little complicated, there is no difficulty in getting a 
 general notion of the forces involved. As the moon moves 
 around the earth she is alternately without and within the 
 earth's orbit. When she is without, the sun's attraction on 
 her acts with that of the earth ; when she is within, the two 
 attractions act in opposite directions. Thus in effect the 
 centripetal force holding the moon to the earth is alternately 
 increased and diminished, with the result of elongating the 
 
196 HARKNESS 
 
 moon's orbit toward the suri and compressing it on the 
 opposite side. As the variation of the centripetal force is 
 not great, the change of form of the orbit is small; never- 
 theless, the summation of the minute alternations thereby 
 produced in the moon's orbital velocity suffices to put her 
 sometimes ahead and sometimes behind her mean place to 
 an extent which oscillates from a maximum to a minimum, 
 as the earth passes from perihelion to aphelion, and aver- 
 ages about 125" of arc. This perturbation of the moon is 
 known as the parallactic inequality, because it depends on 
 the earth's distance from the sun, and can therefore be ex- 
 pressed in terms of the solar parallax. Conversely, the 
 solar parallax can be deduced from the observed value of 
 the parallactic inequality, but unfortunately there are great 
 practical difficulties in making the requisite observations 
 with a sufficient degree of accuracy. Notwithstanding the 
 ever-recurring talk about the advantages to be obtained 
 by observing a small, well-defined crater instead of the 
 moon's limb, astronomers have hitherto found it imprac- 
 ticable to use anything but the limb, and the disadvantage 
 of doing so, as compared with observing a star, is still fur- 
 ther increased by the circumstance that in general only one 
 limb can be seen at a time, the other being shrouded in 
 darkness. If both limbs could always be observed, we 
 should then have a uniform system of data for determining 
 the place of the centre, but under existing circumstances 
 we are compelled to make our observations half upon one 
 limb and half upon the other, and thus they involve all the 
 systematic errors which may arise from the conditions 
 under which these limbs are observed, and all the uncer- 
 tainty which attaches to irradiation, personal equation, and 
 our defective knowledge of the moon's semidiameter. 
 
 Our fifth gravitational relation is that which exists be- 
 tween the solar parallax, the lunar parallax, the moon's 
 mass, and the earth's lunar inequality. Strictly speaking, 
 the moon does not revolve around the earth's centre, but 
 both bodies revolve around the common centre of gravity 
 of the two. In consequence of that, an irregularity arises 
 
MAGNITUDE OF THE SOLAR SYSTEM 197 
 
 in the earth's orbital velocity around the sun, the common 
 centre of gravity moving in accordance with the laws of 
 elliptic motion, while the earth, on account of its revolution 
 around that centre, undergoes an alternate acceleration and 
 retardation which has for its period a lunar month, and is 
 called the lunar inequality of the earth's motion. We per- 
 ceive this inequality as an oscillation superposed on the 
 elliptic motion of the sun, and its semiamplitude is the 
 measure of the angle subtended at the sun by the interval 
 between the centre of the earth and the common centre of 
 gravity of the earth and moon. Just as an astronomer on 
 the moon might use the radius of her orbit around the earth 
 as a base for measuring her distance from the sun, so we 
 may use this interval for the same purpose. We find its 
 length in miles from the equatorial semidiameter of the 
 earth, the moon's parallax, and the moon's mass, and thus 
 we have all the data for determining the solar parallax from 
 the inequality in question. In view of the great difficulty 
 which has been experienced in measuring the solar parallax 
 itself, it may be asked, Why should we attempt to deal with 
 the parallactic inequality, which is about twenty-six per 
 cent smaller? The answer is, Because the latter is derived 
 from differences of the sun's right ascension, which are fur- 
 nished by the principal observatories in vast numbers, and 
 should give very accurate results on account of their be- 
 ing made by methods which insure freedom from constant 
 errors. Nevertheless, the sun is not so well adapted for 
 precise observations as the stars, and Dr. Gill has recently 
 found that heliometer measurements upon asteroids which 
 approach very near to the earth yield values of the paral- 
 lactic inequality superior to those obtained from right 
 ascensions of the sun. 
 
 Our sixth gravitational relation is that which exists be- 
 tween the moon's parallax and the constants of precession 
 and nutation. Every particle of the earth is attracted both 
 by the sun and by the moon, but in consequence of the 
 polar flattening the resultant of these attractions passes a 
 little to one side of the earth's centre of gravity. Thus a 
 
198 
 
 HARKNESS 
 
 couple is set up, which, by its action upon the rotating 
 earth, causes the axis thereof to describe a surface which 
 may be called a fluted cone, with its apex at the earth's 
 centre. A top spinning with its axis inclined describes a 
 similar cone, except that the flutings are absent and the 
 apex is at the point upon which the spinning occurs. For 
 convenience of computation we resolve this action into two 
 components, and we name that which produces the cone 
 the luni-solar precession, and that which produces the flut- 
 ings the nutation. In this phenomenon the part played by 
 the sun is comparatively small, and by eliminating it we 
 obtain a relation between the luni-solar precession, the 
 nutation, and the moon's parallax which can be used to 
 verify and correct the observed values of these quantities. 
 
 In the preceding paragraph we have seen that the rela- 
 tion between the quantities there considered depends 
 largely upon the flattening of the earth, and thus we are 
 led to inquire how and with what degree of accuracy that 
 is determined. There are five methods viz., one geodetic, 
 one gravitational, and three astronomical. The geodetic 
 method depends upon measurements of the length of a 
 degree on various parts of the earth's surface; and with the 
 data hitherto accumulated it has proved quite unsatisfac- 
 tory. The gravitational method consists in determining 
 the length of the seconds-pendulum over as great a range 
 of latitude as possible, and deducing therefrom the ratio 
 of the earth's polar and equatorial semidiameters by means 
 of Clairaut's theorem. The pendulum experiments show 
 that the earth's crust is less dense on mountain plateaus 
 than at the seacoast, and thus for the first time we are 
 brought into contact with geological considerations. The 
 first astronomical method consists in observing the moon's 
 parallax from various points on the earth's surface; and 
 as these parallaxes are nothing else than the angular semi- 
 diameter of the earth at the respective points, as seen from 
 the moon, they afford a direct measure of the flattening. 
 The second and third astronomical methods are based upon 
 certain perturbations of the moon which depend upon the 
 
MAGNITUDE OF THE SOLAR SYSTEM 199 
 
 figure of the earth, and should give extremely accurate 
 results; but unfortunately very great difficulties oppose 
 themselves to the exact measurement of the perturbations. 
 There is also an astronomico-geological method which 
 can not yet be regarded as conclusive on account of our 
 lack of knowledge respecting the law of density which pre- 
 vails in the interior of the earth. It is based upon the 
 fact that a certain function of the earth's moments of inertia 
 can be determined from the observed values of the coeffi- 
 cients of precession and nutation, and could also be deter- 
 mined from the figure and dimensions of the earth if we 
 knew the exact distribution of matter in its interior. Our 
 present knowledge on that subject is limited to a super- 
 ficial layer not more than ten miles thick, but it is usual 
 to assume that the deeper matter is distributed, according 
 to Lagrange's law, and then by writing the function in 
 question in a form which leaves the flattening indetermi- 
 nate, and equating the expression so found to the value 
 given by the precession and nutation, we readily obtain the 
 flattening. As yet these methods do not give consistent 
 results, and so long as serious discrepancies remain be- 
 tween them there can be no security that we have arrived 
 at the truth. 
 
 It should be remarked that in order to compute the 
 function of the earth's moments of inertia which we have 
 just been considering, we require not only the figure and 
 dimensions of the earth and the law of distribution of den- 
 sity in its interior, but also its mean and surface densities. 
 The experiments for determining the mean density have 
 consisted in comparing the earth's attraction with the 
 attraction either of a mountain or of a known thickness of 
 the earth's crust or of a known mass of metal. In the case 
 of mountains the comparisons have been made with plumb 
 lines and pendulums; in the case of known layers of the 
 earth's crust they have been made by swinging pendu- 
 lums at the surface and down in mines; and in the case 
 of known masses of metal they have been made with tor- 
 sion balances, fine chemical balances, and pendulums. The 
 
200 HARKNESS 
 
 surface density results from a study of the materials com- 
 posing the earth's crust, but notwithstanding the appar- 
 ent simplicity of that process it is doubtful if we have yet 
 attained as accurate a result as in the case of the mean 
 density. 
 
 Before quitting this part of our subject it is important 
 to point out that the luni-solar precession can not be directly 
 observed, but must be derived from the general precession. 
 The former of these qualities depends only upon the action 
 of the sun and moon, while the latter is affected in addition 
 by the action of all the planets, and to ascertain what that 
 is we must determine their masses. The methods of doing 
 so fall into two great classes, according as the planets dealt 
 with have or have not satellites. The most favourable case 
 is that in which one or more satellites are present, because 
 the mass of the primary follows immediately from their 
 distances and revolution times; but even then there is a 
 difficulty in the way of obtaining very exact results. By 
 extending the observations over sufficiently long periods 
 the revolution times may be ascertained with any desired 
 degree of accuracy; but all measurements of the distance of 
 a satellite from its primary are affected by personal equa- 
 tion, which we can not be sure of completely eliminating, 
 and thus a considerable margin of uncertainty is brought 
 into the masses. In the cases of Mercury and Venus, 
 which have no satellites, and to a certain extent in the case 
 of the earth also, the only available way of ascertaining the 
 masses is from the perturbations produced by the action of 
 the various planets on each other. These perturbations are 
 of two kinds, periodic and secular. When sufficient data 
 have been accumulated for the exact determination of the 
 secular perturbations they will give the best results, but as 
 yet it remains advantageous to employ the periodic pertur- 
 bations also. 
 
 Passing now to the photo-tachymetrical methods, we 
 have first to glance briefly at the mechanical appliances by 
 which the tremendous velocity of light has been success- 
 fully measured. They are of the simplest possible char- 
 
MAGNITUDE OF THE SOLAR SYSTEM 20 1 
 
 acter, and are based either upon a toothed wheel or upon 
 a revolving mirror. 
 
 The toothed-wheel method was first used by Fizeau, in 
 1849. To understand its operation, imagine a gun-barrel 
 with a toothed wheel revolving at right angles to its muzzle 
 in such a way that the barrel is alternately closed and opened 
 as the teeth and the spaces between them pass before it. 
 Then, with the wheel in rapid motion, at the instant when 
 a space is opposite the muzzle let a ball be fired. It will 
 pass out freely, and after traversing a certain distance let 
 it strike an elastic cushion and be reflected back upon its 
 own path. When it reaches the wheel, if it hits a space it 
 will return into the gun-barrel, but if it hits a tooth it will 
 be stopped. Examining the matter a little more closely, 
 we see that, as the ball requires a certain time to go and 
 return, if during that time the wheel moves through an odd 
 multiple of the angle between a space and a tooth the ball 
 will be stopped, while if it moves through an even multiple 
 of that angle the ball will return into the barrel. Now, 
 imagine the gun-barrel, the ball, and the elastic cushion to 
 be replaced, respectively, by a telescope, a light-wave, and 
 a mirror. Then if the wheel moved at such a speed that 
 the returning light-wave struck against the tooth following 
 the space through which it issued, to an eye looking into 
 the telescope all would be darkness. If the wheel moved a 
 little faster and the returning light-wave passed through the 
 space succeeding that through which it issued, the eye at 
 the telescope would perceive a flash of light, and if the 
 speed was continuously increased a continual succession of 
 eclipses and illuminations would follow each other accord- 
 ing as the returning light was stopped against a tooth or 
 passed through a space farther and farther behind that 
 through which it issued. Under these conditions the time 
 occupied by the light in traversing the space from the wheel 
 to the mirror and back again would evidently be the same 
 as the time required by the wheel to revolve through the 
 angle between the space through which the light issued 
 and that through which it returned, and thus the velocity 
 
202 HARKNESS 
 
 of light would become known from the distance between 
 the telescope and the mirror, together with the speed of 
 the wheel. Of course the longer the distance traversed and 
 the greater the velocity of the wheel the more accurate 
 would be the result. 
 
 The revolving-mirror method was first used by Fou- 
 cault in 1862. Conceive the toothed wheel of Fizeau's ap- 
 paratus to be replaced by a mirror attached to a vertical axis 
 and capable of being put into rapid rotation. Then it will 
 be possible so to arrange the apparatus that light issuing 
 from the telescope shall strike the movable mirror and be 
 reflected to the distant mirror, whence it will be returned to 
 the movable mirror again, and being thrown back into the 
 telescope will appear as a star in the centre of the field of 
 view. That adjustment being made, if the mirror were 
 caused to revolve at a speed of some hundred turns per 
 second it would move through an appreciable angle while 
 the light was passing from it to the distant mirror and back 
 again, and, in accordance with the laws of reflection, the star 
 in the field of the telescope would move from the centre by 
 twice the angle through which the mirror had turned. 
 Thus the deviation of the star from the centre of the field 
 would measure the angle through which the mirror turned 
 during the time occupied by light in passing twice over the 
 interval between the fixed and revolving mirrors, and from 
 the magnitude of that angle, together with the known 
 speed of the mirror, the velocity of the light could be 
 calculated. 
 
 In applying either of these methods the resulting ve- 
 locity is that of light when traversing the earth's atmos- 
 phere, but what we want is its velocity in space, which we 
 suppose to be destitute of ponderable material, and in order 
 to obtain that the velocity in the atmosphere must be mul- 
 tiplied by the refractive index of air. The correct velocity 
 so obtained can then be used to find the solar parallax, 
 either from the time required by light to traverse the semi- 
 diameter of the earth's orbit, or from the ratio of the ve- 
 locity of light to the orbital velocity of the earth. 
 
MAGNITUDE OF THE SOLAR SYSTEM 
 
 203 
 
 Any periodic correction which occurs in computing the 
 place of a heavenly body or the time of a celestial phe- 
 nomenon is called by astronomers an equation, and as the 
 time required by light to traverse the semidiameter of the 
 earth's orbit first presented itself in the guise of a correction 
 to the computed times of the eclipses of Jupiter's satellites, 
 it has received the name of the light equation. The earth's 
 orbit being interior to that of Jupiter, and both having the 
 sun for their centre, it is evident that the distances between 
 the two planets must vary from the sum to the difference of 
 the radii of their respective orbits, and the time required 
 by light to travel from one planet to the other must vary 
 proportionately. Consequently, if the observed times of 
 the eclipses of Jupiter's satellites are compared with the 
 times computed upon the assumption that the two planets 
 are always separated by their mean distance, it will be found 
 that the eclipses occur too early when the earth is at less 
 than its mean distance from Jupiter, and too late when it 
 is farther off, and from large numbers of such observations 
 the value of the light equation has been deduced. 
 
 The combination of the motion of light through our 
 atmosphere with the orbital motion of the earth gives rise 
 to the annual aberration, all the phases of which are com- 
 puted from its maximum value, commonly called the con- 
 stant of aberration. There is also a diurnal aberration due 
 to the rotation of the earth on its axis, but that is quite 
 small and does not concern us this evening. When aberra- 
 tion was discovered the corpuscular theory of light was in 
 vogue, and it offered a charmingly simple explanation of 
 the whole phenomenon. The hypothetical light corpuscles 
 impinging upon the earth were thought to behave pre- 
 cisely like the drops in a shower of rain, and you all know 
 that their apparent direction is affected by any motion on 
 the part of the observer. In a calm day, when the drops 
 are falling perpendicularly, a man standing still holds his 
 umbrella directly over his head, but as soon as he begins 
 to move forward he inclines his umbrella in the same direc- 
 tion, and the more rapidly he moves the greater must be 
 
204 
 
 HARKNESS 
 
 its inclination in order to meet the descending shower. 
 Similarly, the apparent direction of oncoming light cor- 
 puscles would be affected by the orbital motion of the earth, 
 so that in effect it would always be the resultant arising 
 from combining the motion of the light with a motion equal 
 and opposite to that of the earth. But since the falsity of 
 the corpuscular theory has been proved that explanation 
 is no longer tenable, and as yet we have not been able to 
 replace it with anything equally satisfactory based on the 
 now universally accepted undulatory theory. In accord- 
 ance with the latter theory we must conceive the earth as 
 ploughing its way through the ether, and the point which 
 has hitherto baffled us is whether or not in so doing it 
 produces any disturbance of the ether which affects the 
 aberration. In our present ignorance on that point we 
 can only say that the aberration constant is certainly very 
 nearly equal to the ratio of the earth's orbital velocity to 
 the velocity of light, but we can not affirm that it is rigor- 
 ously so. 
 
 The luminiferous ether was invented to account for the 
 phenomena of light, and for two hundred years it was not 
 suspected of having any other function. The emission 
 theory postulated only the corpuscles which constitute light 
 itself, but the undulatory theory fills all space with an im- 
 ponderable substance possessing properties even more re- 
 markable than those of ordinary matter, and to some of the 
 acutest intellects the magnitude of this idea has proved an 
 almost insuperable objection against the whole theory. So 
 late as 1862 Sir David Brewster, who had gained a world- 
 wide reputation by his optical researches, expressed himself 
 as staggered by the notion of filling all space with some 
 substance merely to enable a little twinkling star to send 
 its light to us; but not long after Clerk Maxwell removed 
 that difficulty by a discovery coextensive with the undu- 
 latory theory itself. Since 1845, when Faraday first per- 
 formed his celebrated experiment of magnetizing a ray of 
 light, the idea that electricity is a phenomenon of the ether 
 had been steadily growing, until at last Maxwell perceived 
 
MAGNITUDE OF THE SOLAR SYSTEM 205 
 
 that if such were the fact the rate of propagation of an 
 electro-magnetic wave must be the same as the velocity of 
 light. At that time no one knew how to generate such 
 waves, but Maxwell's theory showed him that their velocity 
 must be equal to the number of electric units of quantity 
 in the electro-magnet unit, and careful experiments soon 
 proved that that is the velocity of light. Thus it was put 
 almost beyond the possibility of doubt that the ether gives 
 rise to the phenomena of electricity and magnetism, as well 
 as to those of light, and perhaps it may even be concerned 
 in the production of gravitation itself. What could be ap- 
 parently more remote than these electric quantities and the 
 solar parallax? And yet we have here a relation between 
 them, but we make no use of it because as yet the same 
 relation can be far more accurately determined from ex- 
 periments upon the velocity of light. 
 
 Now, let us recall the quantities and methods of ob- 
 servation which we have found to be involved, either di- 
 rectly or indirectly, with the solar parallax. They are, the 
 solar parallax, obtained from transits of Venus, oppositions 
 of Mars, and oppositions of certain asteroids; the lunar 
 parallax, found both directly and from measurements of 
 the force of gravity at the earth's surface; the con- 
 stants of precession, nutation, and aberration, obtained 
 from observations of the stars; the parallactic inequality 
 of the moon; the lunar inequality of the earth, usually 
 obtained from observations of the sun, but recently 
 found from heliometer observations of certain asteroids; 
 the mass of the earth, found from the solar parallax 
 and also from the periodic and secular perturbations of 
 Venus and Mars; the mass of the moon, found from the 
 lunar inequality of the earth and also from the ratio of the 
 solar and lunar components of the ocean tides; the masses 
 of all the planets obtained from observations of their satel- 
 lites whenever possible, and when no satellites exist, then 
 from observations of their mutual perturbations, both peri- 
 odic and secular; the velocity of light, obtained from ex- 
 periments with revolving mirrors and toothed wheels, 
 
206 HARKNESS 
 
 together with laboratory determinations of the index of 
 refraction of atmospheric air; the light equation, obtained 
 from observations of the eclipses of Jupiter's satellites; the 
 figure of the earth, obtained from geodetic triangulations, 
 measurements of the length of the seconds-pendulum in 
 various latitudes, and observations of certain perturbations 
 of the moon; the mean density of the earth, obtained from 
 measurements of the attractions of mountains, from pen- 
 dulum experiments in mines, and from experiments on the 
 attraction of known masses of matter made either with 
 torsion balances or with the most delicate chemical bal- 
 ances; the surface density of the earth, obtained from geo- 
 logical examinations of the surface strata; and, lastly, the 
 law of distribution of density in the interior of the earth, 
 which in the present state of geological knowledge we can 
 do little more than guess at. 
 
 Here, then, we have a large group of astronomical, geo- 
 detic, geological, and physical quantities which must all be 
 considered in finding the solar parallax, and which are all 
 so entangled with each other that no one of them can be 
 varied without affecting all the rest. It is therefore im- 
 possible to make an accurate determination of any one of 
 them apart from the remainder of the group, and thus we 
 are driven to the conclusion that they must all be deter- 
 mined simultaneously. Such has not been the practice of 
 astronomers in the past, but it is the method to which 
 they must inevitably resort in the future. A cursory glance 
 at an analogous problem occurring in geodesy may be in- 
 structive. When a country is covered with a net of tri- 
 angles it is always found that the observed angles are sub- 
 ject to a certain amount of error, and a century ago it was 
 the habit to correct the angles in each triangle without 
 much regard to the effect upon adjacent triangles. Conse- 
 quently the adjustment of the errors was imperfect, and in 
 computing the interval between any two distant points the 
 result would vary somewhat with the triangles used in 
 the computation that is, if one computation was made 
 through a chain of triangles running around on the right- 
 
MAGNITUDE OF THE SOLAR SYSTEM 207 
 
 hand side, another through a chain of triangles running 
 straight between the two points, and a third through a 
 chain of triangles running around on the left-hand side, the 
 results were usually all different. At that time things were 
 less highly specialized than now, and all geodetic opera- 
 tions were yet in the hands of first-rate astronomers, who 
 soon devised processes for overcoming the difficulty. They 
 imagined every observed angle to be subject to a small cor- 
 rection, and as these corrections were all entangled with 
 each other through the geometrical conditions of the net, 
 by a most ingenious application of the method of least 
 squares they determined them all simultaneously in such 
 a way as to satisfy the whole of the geometrical conditions. 
 Thus the best possible adjustment was obtained, and no 
 matter what triangles were used in passing from one point 
 to another, the result was always the same. That method 
 is now applied to every important triangulation, and its 
 omission would be regarded as proof of incompetency on 
 the part of those in charge of the work. 
 
 Now, let us compare the conditions existing respectively 
 in a triangulation net and in the group of quantities for the 
 determination of the solar parallax. In the net every angle 
 is subject to a small correction, and the whole system of 
 corrections must be so determined as to make the sum of 
 their weighted squares a minimum and at the same time 
 satisfy all the geometrical conditions of the net. Like the 
 triangles, the quantities composing the group from which 
 the solar parallax must be determined are all subject to 
 error, and therefore we must regard each of them as requir- 
 ing a small correction, and all these corrections must be 
 so determined as to make the sum of their weighted squares 
 a minimum, and at the same time satisfy every one of the 
 equations expressing the relations between the various 
 components of the group. 
 
 Thus it appears that the method required for adjusting 
 the solar parallax and its related constants is in all respects 
 the same as that which has so long been used for adjusting 
 systems of triangulation; and as the latter method was in- 
 
208 HARKNESS 
 
 vented by astronomers, it is natural to inquire, Why have 
 they not applied it to the fundamental problem of their 
 own science? The reasons are various, but they may all be 
 classed under two heads: First, an inveterate habit of over- 
 estimating the accuracy of our own work as compared with 
 that of others; and, second, the unfortunate effect of too 
 much specialization. 
 
 The prevailing opinion certainly is that great advances 
 have recently been made in astronomy, and so they have in 
 the fields of spectral analysis and in the measurement of 
 minute quantities of radiant heat; but the solution of the 
 vast majority of astronomical problems depends upon the 
 exact measurement of angles, and in that little or no 
 progress has been made. Bradley, with his zenith sector 
 a hundred and fifty years ago, and Bessel and Struve, with 
 their circles and transit instruments seventy years ago, 
 made observations not sensibly inferior to those of the 
 present day, and indeed it would have been surprising if 
 they had not done so. The essentials for accurately deter- 
 mining star-places are a skilled observer, a clock, and a 
 transit circle, the latter consisting of a telescope, a divided 
 circle, and four micrometer microscopes. Surely no one 
 will claim that we have to-day any more skilful observers 
 than were Bessel, Bradley, and Struve, and the only way 
 in which we have improved upon the telescopes made by 
 Dollond one hundred and thirty years ago is by increasing 
 their aperture and relatively diminishing their focal dis- 
 tance. The most famous dividing engine now in existence 
 was made by the elder Repsold seventy-five years ago; but 
 as the errors of divided circles and their micrometer micro- 
 scopes are always carefully determined, the accuracy of the 
 measured angles is quite independent of any small improve- 
 ment in the accuracy of the divisions or of the micrometer 
 screws. Only in the matter of clocks has there been some 
 advance, and even that is not very great. On the whole, 
 the star-places of to-day are little better than those of 
 seventy-five years ago, but even yet there is great room 
 for improvement. One of the commonest applications of 
 
MAGNITUDE OF THE SOLAR SYSTEM 
 
 209 
 
 these star-places is to the determination of latitude, but it 
 is very doubtful if there is any point on the face of the 
 earth whose latitude is known certainly within one tenth 
 of a second. 
 
 Looking at the question from another point of view, it 
 is notorious that the contact observations of the transits of 
 Venus in 1761 and 1769 were so discordant that from the 
 same observations Encke and E. J. Stone got respectively 
 for the solar parallax 8.59" and 8.9 1". In 1870 no one 
 thought it possible that there could be any difficulty with 
 the contact observations of the then approaching transits 
 of 1874 and 1882, but we have found from sad experience 
 that our vaunted modern instruments gave very little bet- 
 ter results for the last pair of transits than our prede- 
 cessors obtained with much cruder appliances in 1761 
 and 1769. 
 
 The theory of probability and uniform experience alike 
 show that the limit of accuracy attainable with any instru- 
 ment is soon reached; and yet we all know the fascination 
 which continually lures us on in our efforts to get better 
 results out of the familiar telescopes and circles which have 
 constituted the standard equipment of observatories for 
 nearly a century. Possibly these instruments may be capa- 
 ble of indicating somewhat smaller quantities than we have 
 hitherto succeeded in measuring with them, but their limit 
 can not be far off, because they already show the disturbing 
 effects of slight inequalities of temperature and other un- 
 controllable causes. So far as these effects are accidental 
 they eliminate themselves from every long series of obser- 
 vations, but there always remains a residuum of constant 
 error, perhaps quite unsuspected, which gives us no end of 
 trouble. Encke's value of the solar parallax affords a fine 
 illustration of this. From the transits of Venus in 1761 
 and 1769 he found 8.58" in 1824, which he subsequently 
 corrected to 8.57", and for thirty years that value was uni- 
 versally accepted. The first objection to it came from Han- 
 sen in 1854, a second followed from Leverrier in 1858, both 
 based upon facts connected with the lunar theory, and 
 14 
 
2io HARKNESS 
 
 eventually it became evident that Encke's parallax was 
 about one fourth of a second too small. 
 
 Now please observe that Encke's value was obtained 
 trigonometrically, and its inaccuracy was never suspected 
 until it was revealed by gravitational methods, which were 
 themselves in error about one tenth of a second, and re- 
 quired subsequent correction in other ways. Here, then, 
 was a lesson to astronomers, who are all more or less 
 specialists, but it merely enforced the perfectly well-known 
 principle that the constant errors of any one method are 
 accidental errors with respect to all other methods, and 
 therefore the readiest way of eliminating them is by com- 
 bining the results from as many different methods as pos- 
 sible. However, the abler the specialist the more certain 
 he is to be blind to all methods but his own, and astrono- 
 mers have profited so little by the Encke-Hansen-Leverrier 
 incident of thirty-five years ago that to-day they are mostly 
 divided into two great parties, one of whom holds that 
 the parallax can be best determined from a combination of 
 the constant of aberration with the velocity of light, and 
 the other believes only in the results of heliometer meas- 
 urements upon asteroids. By all means continue the heli- 
 ometer measurements and do everything possible to clear 
 up the mystery which now surrounds the constant of aber- 
 ration; but why ignore the work of predecessors who were 
 quite as able as ourselves? If it were desired to determine 
 some one angle of a triangulation net with special exact- 
 ness, what would be thought of a man who attempted to 
 do so by repeated measurements of the angle in question 
 while he persistently neglected to adjust the net? And yet 
 until very recently astronomers have been doing precisely 
 that kind of thing with the solar parallax. I do not think 
 there is any exaggeration in saying that the trustworthy ob- 
 servations now on record for the determination of the nu- 
 merous quantities which are functions of the parallax could 
 not be duplicated by the most industrious astronomer work- 
 ing continuously for a thousand years. How, then, can we 
 suppose that the result properly deducible from them can 
 
MAGNITUDE OF THE SOLAR SYSTEM 211 
 
 be materially affected by anything that any of us can do 
 in a lifetime unless we are fortunate enough to invent 
 methods of measurement vastly superior to any hitherto 
 imagined? Probably the existing observations for the de- 
 termination of most of these quantities are as exact as any 
 that can ever be made with our present instruments, and 
 if they were freed from constant errors they would certainly 
 give results very near the truth. To that end we have only 
 to form a system of simultaneous equations between all the 
 observed quantities and then deduce the most probable 
 values of these quantities by the method of least squares. 
 Perhaps some of you may think that the value so obtained 
 for the solar parallax would depend largely upon the rela- 
 tive weights assigned to the various quantities, but such is 
 not the case. With almost any possible system of weights 
 the solar parallax will come out very nearly 8.809" 
 0.0057", whence we have for the mean distance between 
 the earth and the sun 92,797,000 miles, with a probable 
 error of only 59,700 miles; and for the diameter of the 
 solar system, measured to its outermost member, the planet 
 Neptune, 5,578,400,000 miles. 
 
THE STABILITY OF THE 
 SOLAR SYSTEM 
 
 BY 
 
 ORMSBY MCKNIGHT MITCHEL 
 
 (Born 1810; died 1862) 
 
THE 
 STABILITY OF THE SOLAR SYSTEM 1 
 
 WHEN, by the application of a single great law, the 
 mind had succeeded in resolving the difficult 
 problems presented by the motions of the earth 
 and its satellite, the moon, it rose to the examination of 
 the higher and more complicated questions of the stability 
 of the entire sytem of planets, satellites, and comets, which 
 are found to pursue their courses round the sun. The 
 number of bodies involved in this investigation, their mag- 
 nitudes and vast periods of revolution, their great dis- 
 tances from the observer, and the exceeding delicacy of the 
 required observations, combined with the high interest 
 which attaches itself to the final results, have united to 
 render this investigation the most wonderful which has ever 
 employed the energies of the human mind. 
 
 To comprehend the dignity and importance of this 
 great subject, let us rapidly survey the system, and moving 
 outward to its known boundaries, mark the number and 
 variety of worlds involved in the investigation. Beginning, 
 then, at the great centre, the grand controlling orb, the sun, 
 we find its magnitude such as greatly to exceed the com- 
 bined masses of all its attendant planets. Indeed, if these 
 could all be arranged in a straight line on the same side 
 of the sun, so that their joint effect might be exerted on 
 that body, the centre of gravity of the entire system, thus 
 located, would scarcely fall beyond the limits of the sun's 
 surface. At a mean distance of 36,000,000 miles from the 
 sun we meet the nearest planet, Mercury, revolving in an 
 
 1 From " Planetary and Stellar Worlds." 
 215 
 
2l6 MITCHEL 
 
 orbit of considerable eccentricity, and completing its cir- 
 cuit around the sun in a period of about 88 of our days. 
 This world has a diameter of only 3,140 miles, and is the 
 smallest of the old planets. Pursuing our journey, at a dis- 
 tance of 68,000,000 miles from the sun, we cross the orbit 
 of the planet Venus. Her magnitude is nearly equal to that 
 of the earth. Her diameter is 7,700 miles, and the length of 
 her year is nearly 225 of our days. The next planet we 
 meet is the earth, whose mean distance from the sun is 
 95,000,000 miles. The peculiarities which mark its move- 
 ments and those of its satellite have been already dis- 
 cussed. Leaving the earth, and continuing our journey 
 outward, we cross the orbit of Mars, at a mean distance 
 from the sun of 142,000,000 miles. This planet is 4,100 
 miles in diameter, and performs its revolution around the 
 sun in about 687 days, in an orbit but little inclined to the 
 plane of the ecliptic. Its features, as we shall see hereafter, 
 are more nearly like those of the earth than any other 
 planet. Beyond the orbit of Mars, and at a mean distance 
 from the sun of about 250,000,000 miles, we encounter a 
 group of small planets, eight in number, presenting an 
 anomaly in the system, and entirely different from any- 
 thing elsewhere to be found. These little planets are called 
 asteroids. Their orbits are, in general, more eccentric, and 
 more inclined to be ecliptic, than those of the other planets; 
 but the most remarkable fact is this, that their orbits are 
 so nearly equal in size that, when projected on a common 
 plane, they are not inclosed the one within the other, but 
 actually cross each other. 
 
 We shall return to an examination of these wonderful 
 objects hereafter. At a mean distance of 485,000,000 miles 
 from the sun we cross the orbit of Jupiter, the largest and 
 most magnificent of all the planets. His diameter is nearly 
 90,000 miles. He is attended by four moons, and performs 
 his revolution round the sun in a period of nearly twelve 
 years. Leaving this vast world, and continuing our jour- 
 ney to a distance of 890,000,000 miles from the sun, we 
 cross the orbit of Saturn, the most wonderful of all the 
 
THE STABILITY OF THE SOLAR SYSTEM 217 
 
 planets. His diameter is 76,068 miles, and he sweeps round 
 the sun in a period of nearly 29^ years. He is sur- 
 rounded by several broad, concentric rings, and is accom- 
 panied by no fewer than seven satellites or moons. The 
 interplanetary spaces, we perceive, are rapidly increasing. 
 The orbit of Uranus is crossed at a mean distance from the 
 sun of 1,800,000,000 miles. His diameter is 35,000 miles 
 and his period of revolution amounts to rather more than 
 84 of our years. He is attended by six moons, and pur- 
 sues his journey at a slower rate than any of the interior 
 planets. Leaving this planet, we reach the boundary of 
 the planetary system at a distance of about 3,000,000,000 
 miles from the sun. Here revolves the last discovered 
 planet, Neptune, attended by one, probably by two moons, 
 and completing his vast circuit about the sun in a period 
 of 164 of our years. His diameter is eight times greater 
 than the earth's, and he contains an amount of matter 
 sufficient to form one hundred and twenty-five worlds such 
 as ours. 
 
 Here we reach the known limit of the planetary worlds, 
 and standing at this remote point and looking back toward 
 the sun, the keenest vision of man could not descry more 
 than one solitary planet along the line we have traversed. 
 The distance is so great that even Saturn and Jupiter are 
 utterly invisible, and the sun itself has shrunk to be scarcely 
 greater than a fixed star. 
 
 There are certain great characteristics which distinguish 
 this entire scheme of worlds. They are all nearly globular, 
 they all revolve on axes, their orbits are all nearly cir- 
 cular, they all revolve in the same direction around the sun, 
 the planes of their orbits are but slightly inclined to each 
 other, and their moons follow the same general laws. With 
 a knowledge of these general facts, it is proposed to trace 
 the reciprocal influences of all these revolving worlds, and 
 to learn, if it be possible, whether this vast scheme has been 
 so constructed as to endure while time shall last, or whether 
 the elements of its final dissolution are not contained within 
 itself, either causing the planets, one by one, to drop into 
 
2i8 MITCHEL 
 
 the sun, or to recede from this great centre, released from 
 its influence, to pursue their lawless orbits through un- 
 known regions of space. 
 
 Before proceeding to the investigation of the great 
 problem of the stability of the universe, let us examine how 
 far the law of gravitation extends its influence over the 
 bodies which are united in the solar system. A broad and 
 distinct line must be drawn between those phenomena for 
 which gravitation must render a satisfactory account, and 
 those other phenomena for which it is in no wise respon- 
 sible. In the solar system we find, for example, that all 
 the planets revolve in the same direction around the sun, 
 in orbits slightly elliptical, and in planes but little inclined 
 to each other. Neither of these three peculiarities is in any 
 way traceable to the law of gravitation. 
 
 Start a planet in its career, and, no matter what be the 
 eccentricity of its orbit, the direction of its movement, or 
 the inclination of the plane in which it pursues its journey, 
 once projected, it falls under the empire of gravitation, and 
 ever afterward this law is accountable for all its movements. 
 We are not, therefore, to regard the remarkable constitu- 
 tion of the solar system as a result of any of the known laws 
 of Nature. 
 
 If the sun were created, and the planetary worlds formed 
 and placed at the disposal of a being possessed of less than 
 infinite wisdom, and he were required so to locate them in 
 space, and to project them in orbits, such that their revo- 
 lutions should be eternal, even with the assistance of the 
 known laws of motion and gravitation, this finite being 
 would fail to construct his required system. 
 
 Let it be remembered that each and every one of these 
 bodies exerts an influence upon all the others. There is no 
 isolated object in the system. Planet always planet, and 
 satellite bends the orbit of satellite, until the primitive 
 curves described lose the simplicity of their character, and 
 perturbations arise, which may end in absolute destruction. 
 There is no chance work in the construction of our mighty 
 system. Every planet has been weighed and poised, and 
 
THE STABILITY OF THE SOLAR SYSTEM 219 
 
 placed precisely where it should be. If it were possible to 
 drag Jupiter from its orbit, and cause him to change places 
 with the planet Venus, this interchange of orbits would be 
 fatal to the stability of the entire system. In contemplat- 
 ing the delicacy and complexity of the adjustment of the 
 planetary worlds, the mind can not fail to recognise the 
 fact that, in all this intricate balancing, there is a higher ob- 
 ject to be gained than the mere perpetuity of the system. 
 
 If stability had been the sole object it might have been 
 gained by a far simpler arrangement. If God had so con- 
 stituted matter that the sun might have attracted the plan- 
 ets, while these should exert no influence over each other 
 that the planets might have attracted their satellites, while 
 these were free from their reciprocal influences then, in- 
 deed, a system would have been formed whose movements 
 would have been eternal and whose stability would have 
 been independent of the relative positions of the worlds 
 and the character of their orbits. Give to them but space 
 enough in which to perform their revolutions around the 
 sun, so that no collisions might occur, freed from this only 
 danger, and every planet and every satellite will pursue the 
 same undeviating track throughout the ceaseless ages of 
 eternity. 
 
 If this statement be true, it may be demanded why such 
 a system was not adopted. It is impossible for us to assign 
 all the reasons which led to the adoption of the present 
 complicated system. Of one thing, however, we are cer- 
 tain: If God designed that in the heavens his glory and 
 his wisdom should be declared, and that in the study of his 
 mighty works his intelligent creatures should rise higher 
 and higher toward his eternal throne, then, indeed, has the 
 present system been admirably constituted for the accom- 
 plishment of this grand design. To have acquired a knowl- 
 edge of a system constituted of independent planets, free 
 from all mutual perturbations, would have required scarcely 
 no effort to the mind, when compared with that put forth 
 in the investigation of the present complex construction 
 of the planetary system. The mind would have lost the op- 
 
220 MITCHEL 
 
 portunity of achieving its greatest triumphs, while the evi- 
 dence of infinite wisdom displayed in the arrangement and 
 counterpoising of the present system would have been lost 
 forever. There is one other thought which here suggests 
 itself with so much force that I can not turn away from it. 
 We speak of gravitation as some inherent quality or prop- 
 erty of matter, as though matter could not exist in case 
 it were deprived of this quality. This is, however, a false 
 idea. Matter might have existed independent of any qual- 
 ity which should cause distant globes to influence each 
 other. The force called gravitation, even admitting that it 
 must have an existence, no special law of its action could 
 have forced itself on matter to the exclusion of all other 
 laws. Why does this force diminish as the square of the 
 distance at which it operates increases? There are almost 
 an infinite number of laws according to which an attraction 
 might have exerted itself, but there is no one which would 
 have rendered the planets fit abodes for sentient beings, 
 such as now dwell on them, and which would at the same 
 time have guaranteed the perpetuity of the system. Ad- 
 mitting, then, that matter can not be matter without ex- 
 erting some influence on all other matter (which I am un- 
 willing to admit), in the selection of the law of the inverse 
 square of the distance there is the strongest evidence of 
 design. 
 
 If we rise above the law of gravitation to the Great 
 Author of Nature, and regard the laws of motion and of 
 gravitation nothing more than the uniform expressions of 
 his will, we perceive at once the impossibility of construct- 
 ing the universe in such manner that the sun should attract 
 the planets, without these attracting each other; or that 
 the planets should attract their satellites without, in turn, 
 being reciprocally influenced by their satellites; for this 
 would be equivalent to saying that the will of the same 
 Almighty Being should exert itself, and not exert, at the 
 same moment, which is impossible. As there is but one 
 God, so there is but one kind of matter, governed by one 
 law, applied by infinite wisdom to the formation of suns 
 
THE STABILITY OF THE SOLAR SYSTEM 221 
 
 and systems without number, crowding the illimitable re- 
 gions of space, all moving harmoniously, fulfilling their 
 high destiny, and all sustained by the single arm of divine 
 Omnipotence. 
 
 We now proceed to an examination of the great ques- 
 tion, Is the system of worlds by which we are surrounded, 
 and of which our earth and its moon form a part, so con- 
 structed that, under the operation of the known laws of 
 Nature, it shall forever endure, without ever passing cer- 
 tain narrow limits of change, which do not in any way 
 involve its stability? 
 
 It is well known that the planets revolve in elliptical 
 orbits of small eccentricity that under the action of the 
 primitive impulse by which they were projected in their 
 orbits they would have moved off in a straight line, with 
 a velocity proportioned to the intensity of the impulse, and 
 which would have endured forever; but being seized by the 
 central attraction of the sun, at the moment of starting in 
 their career, the joint action of these two forces bends the 
 planet from its straight direction and causes it to commence 
 a curvilinear path, which carries it round the sun. 
 
 The question which first presents itself is this: If the 
 central force lodged in the sun has the power to cause a 
 planet to diverge from the straight line in which, but for this, 
 it would have moved, if it draw it into a curved path, will 
 not this central force, which is ever active, finally overcome 
 entirely the impulsive force originally given to the planet, 
 draw it closer and closer to the sun in each successive revo- 
 lution, in a spiral orbit, until, finally, the planet shall fall 
 into the sun and be destroyed forever? This question 
 arises independent of the extraneous influence which the 
 planets exert over each other. It refers to a solitary globe 
 revolving around the sun, under the influence of a central 
 force which varies its action as does the law of gravitation. 
 The problem has been submitted to the most rigorous 
 mathematical examination, and a result has been obtained 
 which settles the question in the most absolute manner. 
 The amount by which the central force, in a moment of 
 
222 MITCHEL 
 
 time, overcomes the effect produced by the primitive im- 
 pulse, is a quantity infinitely small, and of the second order. 
 If it were found to be infinitely small in each moment of 
 time, then might it accumulate so that, at the end of a vast 
 period, it might become finite and appreciable. But be- 
 cause it is of the second order of infinitely small quantities, 
 before it can become an infinitely small quantity of the first 
 order a period equal to infinite ages must roll by, and to 
 make a finite appreciable quantity out of this, an infinite 
 cycle of years must roll round an infinite number of times. 
 
 Such is the answer given by analysis to this wonderful 
 question. " Is there no change? " demands the astronomer. 
 " Yes," answers the all-seeing analysis. " When will it be- 
 come appreciable? " asks the astronomer. " At the end of 
 a period infinitely long, repeated an infinite number of 
 times/' is the reply. 
 
 Having settled this important question, it remains now 
 to examine whether the mutual attractions of the planets 
 on each other may not, in the end, change permanently the 
 form of their orbits, and lead ultimately to the destruction 
 of the system. To comprehend more readily the nature of 
 the examination, let us review the points involved in the 
 permanency of our orbit. 
 
 Take, for example, our own planet, the earth. It now 
 revolves in an elliptic orbit, whose magnitude is deter- 
 mined by the length of its longer axis and by its eccen- 
 tricity. These elements are readily deduced from observa- 
 tion. If it were possible to construct this orbit of some 
 material like wire, which would permit us to take it up 
 and locate it in space at will, to enable us to give it the 
 position now occupied by the actual orbit of the earth, we 
 must first carry its focus to the sun's centre; we must then 
 turn its longer axis around this centre as a fixed point until 
 the nearest vertex of the wire orbit shall fall upon that point 
 of the earth's orbit which is at this time nearest to the sun. 
 Having accomplished this, the axes will coincide in their 
 entire length, and to make the orbits coincide we must 
 revolve the artificial one around the now common axis 
 
THE STABILITY OF THE SOLAR SYSTEM 223 
 
 until its plane shall fall upon the actual orbit of the 
 earth. 
 
 If, now, change should ever come in the absolute coin- 
 cidence of these two orbits, regarding the iron one as fixed 
 and permanent, the orbit of Nature may vary from it in any 
 one or all of the following ways: i. The natural orbit, all 
 other things remaining the same, may leave the fixed orbit 
 by a variation of eccentricity that is, it may become more 
 or less nearly circular. 2. The planes of the orbits remain- 
 ing coincident, the curves may separate from each other, in 
 consequence of an angular movement of the longer axis of 
 the natural orbit, by means of which the vertex of the 
 natural curve shall be carried to the right or to the left of 
 the vertex of the fixed one. 3. While these causes are 
 operating to produce change, an increase of deviation may 
 be occasioned by the fact that the two planes may become 
 inclined to each other, thus causing the natural orbit to 
 lie partly above and partly below the fixed one. These, 
 then, are the several ways in which the orbits of the planets 
 may change; and to settle the question of stability, we 
 must ascertain whether these changes actually exist, and 
 whether any of them, in case they do exist, and are pro- 
 gressing constantly in the same direction, will ever prove 
 fatal to the permanency of the system, finally accomplish- 
 ing its absolute destruction, or rendering it unfit for the 
 sustentation of that life which now exists upon the planet. 
 
 By a close examination of this great subject, both the- 
 oretically and practically, it is found that the system is so 
 constituted that not a single planet or satellite revolves in 
 an orbit absolutely invariable. Theory demonstrates that 
 such changes must exist, and observation confirms this 
 great truth by showing that they actually do exist. 
 
 Draw, in imagination, a straight line from the sun's 
 centre through the perihelion, or nearest point to the sun 
 of the earth's orbit, and let it be extended to the outermost 
 limits of the entire system. On this locate the perihelion 
 points of the orbits of all the planets, and in these points 
 fix the planets themselves. They are now all on the same 
 
224 MITCHEL 
 
 side of the sun, the longer axes of their orbits are in the 
 same direction, and they are located at their nearest dis- 
 tance from the sun, or in perihelion. The planes of the 
 orbits are inclined to each other under their proper angles, 
 and they all intersect in a common line of nodes passing 
 through the sun's centre. Now give the entire group of 
 planets their primitive impulse, and at the same instant they 
 start in their respective orbits round the sun. Now, in case 
 no perturbations existed, the perihelion-points, the inclina- 
 tions, and the lines of nodes would remain fixed forever, 
 and although millions of years might pass away before the 
 planets would again resume their primitive position with 
 reference to each other, yet the time would come when a 
 final restoration would be effected. 
 
 At the end of 164 years Neptune will have completed 
 its revolution round the sun, and will return to its starting 
 point. All the other planets will have performed several 
 revolutions, but each, on reaching the point of departure, 
 will find the perihelion of its orbit changed in position, the 
 inclination altered, and the line of nodes shifted. These 
 changes continue until the longer axes of the orbits, which 
 once coincided, radiate from the sun in all directions. The 
 lines of nodes, once common, now diverge under all an- 
 gles, the inclinations increasing or decreasing, and even the 
 figures of the orbits undergoing constant mutation; and 
 the grand question arises, whether these changes, no matter 
 how slow, are ever to continue progressing in the same 
 direction until all the original features of the system shall 
 be effaced, and the possibility of return to the primitive con- 
 dition destroyed forever. 
 
 Such a problem would seem to be far too deep and 
 complicated ever to be grasped by the human intellect. It 
 is true that no single mind was able to accomplish its com- 
 plete solution, but the advance made by one has been 
 steadily increased by another, until finally not a question 
 remains unanswered. The solution is complete, yielding 
 results of the most wonderful character. 
 
 We shall examine this great problem in detail, and 
 
THE STABILITY OF THE SOLAR SYSTEM 225 
 
 begin with the figure of the orbit of any planet our earth, 
 for example. 
 
 The amount of heat received from the sun by the earth 
 depends, other things being the same, on the minor axis 
 of its elliptic orbit. Any change in the eccentricity operates 
 directly to increase or decrease the shorter axis, and con- 
 sequently to increase or decrease the mean annual amount 
 of heat received from the sun. Now, we know that animal 
 and vegetable life is adjusted in such a way that it re- 
 quires almost exact uniformity in the mean annual amount 
 of heat which it shall enjoy. An increase or decrease of two 
 or three degrees in temperature would make an entire revo- 
 lution in the animals and plants belonging to the region 
 experiencing such a change. If, then, it be true that the 
 eccentricity of the earth's orbit is actually changing under 
 the combined action of the other planets, may this change 
 continue so far as to subvert the order of Nature on its 
 surface? This question has been answered in the most 
 satisfactory manner. 
 
 It is found that the greatest axes of the planetary orbits 
 are subjected to slight and temporary variations, returning 
 in comparatively short periods to their primitive values. 
 This important fact guarantees the permanency of the 
 periodic times, so that it becomes possible to deduce, with 
 the utmost precision, the periodic times of the planets, from 
 the mean of a large number of revolutions. That of the 
 earth is now so accurately known, and so absolutely in- 
 variable, that we know what it will be a million of years 
 hence, should the system remain as it now is, as perfectly 
 as at the present moment. But neither of these elements 
 secures the stability of the eccentricity or of the minor axis. 
 Lagrange, however, demonstrated a relation between the 
 masses of the planets, their major axes and eccentricities 
 such that, while the masses remain constant and the axes 
 invariable, the eccentricity can only vary its value through 
 extremely narrow limits. These limits have been assigned 
 beyond which the change can never pass, and within these 
 narrow bounds we find that the orbits of all the planets 
 15 
 
22 6 MITCHEL 
 
 are slowly vibrating backward and forward in periods which 
 actually stun the imagination. 
 
 This remarkable law for the preservation of the system 
 would not hold in any other organization. It demands 
 orbits nearly circular, with planes nearly coincident with 
 the periodic times related as are those of the planets, and 
 the planets themselves located as they actually are. No 
 interchange of orbits is admissible; but, constituted as the 
 system now is, the perpetuity is absolutely certain, so far 
 as the change of eccentricity is concerned. 
 
 Let us now examine the changes which affect the posi- 
 tion of the major axis in its own plane. The perihelion of 
 every orbit is found to be slowly advancing. Nor is this 
 distance ever to be changed into a retrograde motion. The 
 movement is ever progressive in the same direction, and 
 the perihelion-points of all the orbits are slowly sweeping 
 round the sun. That of the earth's orbit accomplishes its 
 revolution in 111,000 years! How wonderful the fact that 
 such discoveries should be made by man, whose entire life 
 is but a minute fraction of these vast periods of time! 
 
 Owing to a retrograde motion in the vernal equinox, 
 carrying it around in the opposite direction in 25,868 years, 
 the perihelion and equinox pass each other once in 20,984 
 years. Knowing their relative position at this moment, 
 and their rates of motion, it is easy to compute the time of 
 their coincidence. Their last coincidence took place 4,089 
 years before the Christian era, or about the epoch usually 
 assigned for the creation of man. The effect of the coinci- 
 dence of the perihelion with the vernal equinox is to cause 
 an exact equality in the length of spring and summer, com- 
 pared with autumn and winter. In other language, the 
 sun will occupy exactly half a year in passing from the 
 vernal to the autumnal equinox, and the other half in 
 moving from the autumnal to the vernal equinox. 
 
 At present the line of equinoxes divides the earth's 
 elliptic orbit into two unequal portions. The smaller part 
 is passed over in the fall and winter, causing the earth to 
 be nearer the sun at this season than in summer, and mak- 
 
THE STABILITY OF THE SOLAR SYSTEM 227 
 
 ing a difference in the length of the two principal seasons, 
 summer and winter, of some seventeen and a half days. 
 This inequality, which is now in favour of summer, will 
 eventually be destroyed, and the time will come when the 
 earth will be farthest from the sun during the winter and 
 nearest in the summer. But at the end of a great cycle of 
 more than 20,000 years all the changes will have been 
 gone through, and in this respect a complete compensa- 
 tion and restoration will have been effected. 
 
 This epoch of subordinate restoration will find the peri- 
 helion of the earth's orbit located in space far distant from 
 the point primitively occupied. Five of these grand revo- 
 lutions of 20,984 years must roll round before the slow 
 movement of the perihelion shall bring it back to its start- 
 ing point. One hundred and ten thousand years will then 
 restore the axis of the earth's orbit, and the equinoctial line, 
 nearly to their relative positions to each other, and to the 
 same region of absolute space occupied at the beginning 
 of this grand cycle. If, now, we direct our attention to the 
 other planets, we find their perihelion-points all slowly ad- 
 vancing in the same direction. That of the orbit of Jupiter 
 performs its revolution round the sun in 186,207 years, 
 while the perihelion of Mercury's orbit occupies more than 
 200,000 years in completing its circuit round the sun. To 
 effect a complete restoration of the planetary orbits to their 
 original position with reference to their perihelion-points 
 will require a grand compound cycle amounting to millions 
 of years. Yet the time will come when all the orbits will 
 come again to their primitive positions, to start once more 
 on their ceaseless journeys. 
 
 In the changes of the eccentricities, it will be remem- 
 bered, the stability of the system was involved. Should 
 these changes be ever progressive, no matter how slowly, 
 a time would finally come when the original figure of the 
 orbit would be destroyed, the planet either falling into the 
 sun, or sweeping away into unknown regions of space. But 
 a limit is assigned, beyond which the change can never pass. 
 Some of the planetary orbits are becoming more circular, 
 
228 MITCHEL 
 
 others growing more elliptical; but all have their limits 
 fixed. The earth's orbit, for example, should the present 
 rate of decrease of eccentricity continue, in about half a 
 million years will become an exact circle. There the pro- 
 gressive motion of the changes stops, and it slowly com- 
 mences to recover its ellipticity. This is not the case with 
 the motion of the perihelions. Their positions are in no 
 way involved in the well-being of a planet or in its capacity 
 to sustain the life which exists on its surface; and since the 
 stability of the system is not endangered by progressive 
 change, it ever continues in the same direction, until the 
 final restoration is effected, by an entire revolution about 
 the sun. 
 
 Let us now examine the inclinations of the planetary 
 orbits. Here it is found that there is no guarantee for the 
 stability of the system, provided the angles under which the 
 orbits of the planets are inclined to each other do not re- 
 main nearly the same forever. If changes are found to ex- 
 ist, by which the inclinations are made to increase, without 
 stopping and returning to their primitive condition, then 
 is the perpetuity of the system rendered impossible. Its 
 fair proportions must slowly wear away, the harmony which 
 now prevails be destroyed, and chaos must come again. 
 
 Commencing again with the earth, we find that from 
 the earliest ages the inclination of the earth's equator to 
 the ecliptic has been decreasing. Since the measure of 
 Eratosthenes, 2,078 years ago, the decrease has amounted 
 to about 23' 44", or about half a second every year. 
 Should the decrease continue, in about 85,000 years the 
 equator and ecliptic would coincide, and the order of Na- 
 ture would be entirely changed; perpetual spring would 
 reign throughout the year, and the seasons would be lost 
 forever. Of this, however, there is no danger. The diminu- 
 tion will reach its limit in a comparatively short time, when 
 the decrease of inclination will change into an increase, and 
 thus slowly rocking backward and forward in thousands of 
 years, the seasons shall ever preserve their appointed places, 
 and seedtime and harvest shall never fail. These changes 
 
THE STABILITY OF THE SOLAR SYSTEM 229 
 
 of inclination are principally due to the perturbations of 
 Venus, and, arising from configurations, will be ultimately 
 entirely compensated. 
 
 The angles under which the planetary orbits are in- 
 clined to each other are in a constant state of mutation. 
 The orbit of Jupiter at this time forms an angle with the 
 ecliptic of 4,731 seconds, and this angle is decreasing at 
 such a rate that in about 20,000 years the planes would 
 actually coincide. This would not affect the well-being of 
 the planets or the stability of the system; but should the 
 same change now continue, the angle between the orbits 
 might finally come to fix them even at right angles to each 
 other, and a subversion of the present system would result. 
 
 A profound investigation of the problem of the plan- 
 etary inclinations, accomplished by Lagrange, resulted in 
 the demonstration of a relation between the masses of the 
 planets, the principal axes of their orbits, and the inclina- 
 tions, such that, although the angles of inclination may 
 vary, the limits are narrow, and they are all found slowly 
 to oscillate about their mean positions, never passing the 
 prescribed limits, and securing in this particular the per- 
 petuity of the system. 
 
 Here, again, we are presented with the remarkable fact 
 that whenever mutation involves stability, this mutation is 
 of a compensatory character, always returning upon itself, 
 and in the long run correcting its own effects. If all this 
 mighty system was organized by chance, how happens it 
 that the angular motions of the perihelia of the planetary 
 orbits are ever progressive, while the angular motions of 
 the planes of the orbits are vibrating? Design, positive and 
 conspicuous, is written all over the system in characters 
 from which there is no escape. 
 
 We now proceed to an examination of the lines in 
 which the planes of the planetary orbits cut each other, or 
 the lines in which they intersect a fixed plane. These are 
 called the lines of nodes. They all pass through the sun's 
 centre, and, in case they ever were coincident, they now 
 radiate from a common point in all directions. 
 
230 
 
 MITCHEL 
 
 Here is an element in no degree involving in its value 
 the stability of the system, and from analogy we already 
 begin to anticipate that its changes, whatever they may be, 
 will probably progress always in the same direction. This 
 is actually the case. The nodes of the planetary orbits are 
 all slowly retrograding on a fixed plane; and in vast periods, 
 amounting to thousands of years, accomplish revolutions 
 which in the end return them to their primitive positions. 
 
 Thus are we led to the following results: Of the two 
 elements which fix the magnitude of the planetary orbits, 
 the principal axes, and the eccentricity, the axes remain 
 invariable, while the eccentricity oscillates between narrow 
 and fixed points. In the Jong run, therefore, the magni- 
 tudes of the orbits are preserved. 
 
 Of the three elements which give position to the plan- 
 etary orbits viz., the place of the perihelion, the lines of 
 nodes, and the inclinations the first two ever vary in the 
 same direction, and accomplish their restoration at the 
 end of vast periods of revolution, while the inclinations 
 vibrate between narrow and prescribed limits. 
 
 One more point, and we close this wonderful investiga- 
 tion. The last question which presents itself is this: May 
 not the periodic times of the planets be so adjusted to each 
 that the results of certain configurations may ever be 
 repeated without any compensation, and thus, by perpetual 
 accumulation, finally effect a destruction of the system? 
 
 If the periodic times of two neighbourng planets were 
 exact multiples of the same quantity, or if the one was 
 double the other, or in any exact ratio, then the contin- 
 gency above alluded to would arise, and there would be 
 perturbations which would remain uncompensated. A near 
 approach to this condition of things actually exists in the 
 system, and gave great trouble to geometers. It was found, 
 in comparing observations, that the mean periods of Jupiter 
 and Saturn were not constant that one was on the de- 
 crease while the other was on fhe increase. This discovery 
 seemed to disprove the great demonstration which had 
 fixed as invariable the major axes of the planetary orbits, 
 
THE STABILITY OF THE SOLAR SYSTEM 231 
 
 and guaranteed the stability of the mean motions. It was 
 not until after Laplace had instituted a long and laborious 
 research that the phenomenon was traced to its true origin, 
 and was found to arise from the near commensurability of 
 the periodic times of Jupiter and Saturn five of Jupiter's 
 periods being nearly equal to two of Saturn's. In case 
 the equality were exact, it is plain that if the two planets 
 set out from the same straight line drawn from the sun, 
 at the end of a cycle of five of Jupiter's periods, or two of 
 Saturn's, they would be again found in the same relative 
 positions, and whatever effect the one planet had exerted 
 over the other would again i>e repeated under the same 
 precise circumstances. Hence would arise derangements 
 which would progress in the same direction, and eventually 
 lead to the permanent derangement of the system. 
 
 But it happens that five of Jupiter's periods are not ex- 
 actly equal to two of Saturn's, and in this want of equality 
 safety is found. The difference is such that the point of 
 conjunction of the planets does not fall at the same points 
 of their orbits, but at the end of each cycle is in advance 
 by a few degrees. Thus the conjunction slowly works 
 round the orbits of the planets, and in the end the effect 
 produced on one side of the orbit is compensated for on 
 the other, and a mean period of revolution comes out for 
 both planets, which is invariable. In the case of Jupiter 
 and Saturn, the entire compensation is not effected until 
 after a period of nearly a thousand years. 
 
 A similar inequality is found to exist between the earth 
 and Venus, with a period much shorter, and producing re- 
 sults much less easily observed. In no instance do we find 
 the periods of any two planets in an exact ratio. They are 
 all incommensurable with each other, and in this peculiar 
 arrangement we find the stability of the entire system is 
 secured. 
 
 So far, then, as the organization of the great planetary 
 system is concerned, we do not find within itself the ele- 
 ments of its own destruction. Mutation and change are 
 everywhere found all is in motion, orbits expanding or 
 
232 MITCHEL 
 
 contracting, their planes rocking up and down, their peri- 
 helia and nodes sweeping in opposite directions round the 
 sun but the limits of all these changes are fixed; these 
 limits can never be passed, and at the end of a vast period, 
 amounting to many millions of years, the entire range of 
 fluctuation will have been accomplished, the entire system 
 planets, orbits, inclinations, eccentricities, perihelia, and 
 nodes will have regained their original values and places, 
 and the great bell of eternity will have then sounded " One." 
 
 Having reached the grand conclusion of the stability of 
 the system of planets in their reciprocal influences, and 
 that no element of destruction is found in the organization, 
 we propose next to inquire whether the same features are 
 stamped on the subordinate groups composing the plan- 
 etary system. As our limits will not permit us to enter into 
 a full examination of all the subordinate groups, we shall 
 confine our remarks to our own earth and its satellite, 
 Jupiter and his satellites, and to Saturn, his rings and 
 moons. We shall, in this examination, find it practicable 
 to answer, to some extent, the inquiry as to whether either 
 of these systems has received any shock from external 
 causes. We know nothing as to the future, and can, in this 
 particular, only form our conjectures as to what is to be 
 from what has been. 
 
 We commence our inquiry by an examination of two 
 questions viz., Is the velocity of rotation of the earth on 
 its axis absolutely invariable? Has the relation between 
 the earth and moon ever been disturbed by any external 
 cause? There is nothing so important to the well-being 
 of our planet and its inhabitants as absolute invariability in 
 the period of its axial rotation. The sidereal day is the 
 great unit of measure for time, and is of the highest conse- 
 quence in all astronomical investigations. If causes are 
 operating, either to increase or decrease the velocity of 
 rotation, a time will come when the earth will cease to 
 rotate, or else acquire so great a velocity as to destroy its 
 figure, and, in the end, scatter its particles in space. 
 
 It is difficult to ascertain from theory a perfectly satis- 
 
THE STABILITY OF THE SOLAR SYSTEM 233 
 
 factory answer to the question of the invariable velocity of 
 rotation of the earth, but Laplace has demonstrated that 
 the length of the day has not varied by the hundredth of 
 one second during the last 2,000 years that is, the length 
 of the day is neither greater nor less than it was 2,000 years 
 ago by the hundredth of a second. The reasoning leading 
 to this remarkable result is simple, and may be readily com- 
 prehended by all. Two thousand years ago the duration 
 of the moon's period of revolution around the earth was 
 accurately determined and was expressed in days and parts 
 of a day. The measure of the same period has been ac- 
 complished in our own time, and is expressed in days and 
 parts of a day. Now, all the causes operating to change the 
 moon's period of revolution are known, and may be applied. 
 When this is done, it is found that the moon's period now 
 and 2,000 years ago agree precisely, being accomplished 
 in the same number of days and parts of a day which 
 would be impossible if the unit of a measure, the day, had 
 varied ever so slightly. 
 
 The extraordinary relation existing between the moon's 
 period in her orbit and the time occupied in her axial rota- 
 tion gives us the opportunity of ascertaining whether our 
 system has received any external shock. These two periods 
 are so accurately adjusted that in all respects an exact 
 equality exists. The moon ever turns the same hemisphere 
 to the earth, and ever will, unless some external cause 
 should arise to disturb the perfect harmony which now 
 reigns. It is not my purpose to explain why it is that this 
 phenomenon exists. I merely desire to state that this deli- 
 cate balancing of periods furnishes an admirable evidence 
 that, for several thousand years, at least no shock has been 
 received by the earth and its satellite. Steadily have they 
 moved in their orbits, subject only to the influence of causes 
 originating in the constitution of the mighty system of 
 which they constitute a part. 
 
 Moving out to a more complex system, we find in the 
 remarkable arrangement of the satellites of Jupiter a delicate 
 test for the action of sudden and extraneous causes. Here 
 
234 
 
 MITCHEL 
 
 we find the periodic times of the satellites so related that 
 1,000 periods of the first added to 2,000 periods of the third 
 will be precisely equal to 3,000 periods of the second. This 
 delicate balancing of periods would be destroyed by the ac- 
 tion of any external shock, such as might be experienced 
 from the collision of a comet sweeping through the system. 
 Thus far we know that no disturbance has entered, and a 
 knowledge of facts will now pass down to posterity, which 
 will give the means of ascertaining exactly the influence of 
 all disturbing causes which do not form a part of the great 
 system. 
 
 The last subordinate group, and the most extraordinary 
 one to which I will at this time direct your attention, is 
 that of Saturn and his rings. Here we find a delicacy of ad- 
 justment and equilibrium far exceeding anything yet ex- 
 hibited in our examinations. This great planet is sur- 
 rounded certainly by two, probably by three, immense 
 rings, which are formed of solid matter, in all respects like 
 that constituting the central body. These wonderful ap- 
 pendages are nowhere else to be found throughout the 
 entire solar system, at least with certainty. Their existence 
 has elsewhere been suspected, but around Saturn they are 
 seen with a perfection and distinctness which defies all 
 scepticism as to their actual existence. The diameter of 
 the outer ring is no less than 176,000 miles. Its breadth is 
 21,000 miles, while its thickness does not exceed 100 miles. 
 The inner ring is separated from the outer one by a space 
 of about 1, 800 miles, its breadth 34,000 miles, its inner edge 
 being about 20,000 miles from the surface of the planet. 
 Its thickness is the same as that of the outer ring. These 
 extraordinary objects are rotating in the same direction 
 as the planet, and with a velocity so great that objects on 
 the extreme edge of the outer ring are carried through 
 space with the amazing velocity of nearly 50,000 miles an 
 hour, or nearly fifty times more swiftly than the objects on 
 the earth's equator. 
 
 What power of adjustment can secure the stability of 
 these stupendous rings? No solid bond fastens them to 
 
THE STABILITY OF THE SOLAR SYSTEM 235 
 
 the planet; isolated in space, they hold their places, and 
 revolving with incredible velocity around an imaginary 
 axis, they accompany their planet in its mighty orbit round 
 the sun. Such is the exceeding delicacy with which this 
 system is adjusted, that, the slightest external cause once 
 deranging the equilibrium, no readjustment would be 
 effected. The rings would be thrown on the body of the 
 planet, and the system would be destroyed. 
 
 To understand the extraordinary character of this sys- 
 tem we will explain a little more fully the three different 
 kinds of equilibrium. The first is called an equilibrium of 
 instability, and is exemplified in the effort to balance a rod 
 on the tip of the finger. The slightest deviation from the 
 exact vertical increases itself constantly, until the equilib- 
 rium is destroyed. In case the same rod be balanced on 
 its centre on the finger, it presents an example of an equi- 
 librium of indifference that is, if it be swayed slightly to 
 the one side or the other, there is no tendency to restore 
 itself, or to increase its deviation. It remains indifferent to 
 any change. Take the same rod, and suspend it like a pen- 
 dulum. Now cause it to deviate from the vertical to the 
 right or left, and it returns of itself to the condition of 
 equilibrium. This is an equilibrium of stability. We have 
 already seen that this is the kind of equilibrium which exists 
 in the planetary system. There are constant deviations, 
 but a perpetual effort is making to restore the object to its 
 primitive condition. 
 
 Now, in case the rings of Saturn are homogeneous, 
 equally thick, and exactly concentric with the planet, their 
 equilibrium is one of instability. The smallest derange- 
 ment would find no restorative power, and would even per- 
 petuate and increase itself, until the system is destroyed. 
 For a long time it was believed that the rings were equally 
 thick, and concentric with the planet, but when it was dis- 
 covered that such features would produce an equilibrium of 
 instability, and that there existed no guarantee for the per- 
 manency of this exquisite system, an analytic examination 
 was made, which led to this singular result viz., to change 
 
236 , * MITCHEL 
 
 the equilibrium of instability into one of stability, all that 
 is necessary is to make the ring thicker or denser in some 
 parts than in others, and to cause its centre of position to 
 be without the centre of the planet, and to perform around 
 that centre a revolution in a minute orbit. Finding these 
 conditions analytically, it now became a matter of deep 
 interest to ascertain whether these conditions actually ex- 
 isted in Nature. The occasional disappearance of the ring, 
 in consequence of its edge being presented to the eye of 
 the observer, gave a capital opportunity of determining 
 whether it was of uniform thickness. On these rare occa- 
 sions, in the most powerful telescopes, the ring remains 
 visible edgewise, and looks like a slender fibre of silver light 
 drawn across the diameter of the planet. In the gradual 
 wasting away of the two extremities of the ring it has been 
 remarked that the one remains visible longer than the other. 
 As the ring is swiftly revolving, neither extremity can, in 
 any sense, be regarded as fixed, and hence sometimes the 
 one, sometimes the other, fades first from the sight. An 
 exactly uniform thickness in the ring would render such a 
 phenomenon impossible, and hence we conclude that the 
 first condition of stability is fulfilled the rings are not 
 equally thick throughout. 
 
 The micrometer was now applied to detect an eccen- 
 tricity in the central point of the ring. Recent examina- 
 tions by Struve and Bessel have settled this question in 
 the most satisfactory manner. The centre of the ring does 
 not coincide with that of the planet, and it is actually per- 
 forming a revolution around the centre of the planet in 
 a minute orbit, thus forming the second delicate condition 
 of equilibrium. The analogy of the great system is un- 
 broken in the subordinate one. For more than two hundred 
 years have these wonderful circles of light whirled in their 
 rapid career under the eye of man, and freed from all ex- 
 ternal action they are so poised that millions of years shall 
 in nowise affect their beautiful organization. Their graceful 
 figures and beautiful light shall greet the eye of the student 
 when ten thousand years shall have rolled away. 
 
THE STABILITY OF THE SOLAR SYSTEM 237 
 
 Thus do we find that God has built the heavens in wis- 
 dom, to declare his glory, and to show forth his handi- 
 work. There are no iron tracks, with bars and bolts, to 
 hold the planets in their orbits. Freely in space they 
 move, ever changing, but never changed; poised and bal- 
 ancing; swaying and swayed; disturbing and disturbed, 
 onward they fly, fulfilling with unerring certainty their 
 mighty cycles. The entire system forms one grand com- 
 plicated piece of celestial machinery circle within circle, 
 wheel within wheel, cycle within cycle revolution so swift 
 as to be completed in a few hours; movements so slow that 
 their mighty periods are only counted by millions of years. 
 Are we to believe that the Divine Architect constructed 
 this admirably adjusted system to wear out, and to fall 
 in ruins, even before one single revolution of its complex 
 scheme of wheels had been performed? No. I see the 
 mighty orbits of the planets slowly rocking to and fro, their 
 figures expanding and contracting, their axes revolving in 
 their vast periods; but stability is there. Every change 
 shall wear away, and after sweeping through the grand 
 cycle of cycles, the whole system shall return to its primitive 
 condition of perfection and beauty. 
 
THE NEW PLANET, EROS 
 
 BY 
 
 EDMUND LEDGER 
 
THE NEW PLANET, EROS 1 
 
 ON the 1 3th of August, 1898, a little planet was dis- 
 covered at the Urania Observatory of Berlin by 
 Herr G. Witt, to which he has since given the name 
 of Eros. Its discovery has been the great astronomical 
 sensation of the past twelve months, because its orbit passes 
 out of the zone in which all the other small planets move 
 into a very remarkable proximity to the earth a proximity 
 which will cause Eros to be of the highest value in con- 
 nection with some of the most important problems of 
 astronomy. 
 
 In order to appreciate the method of its discovery, and 
 the reasons which make that discovery so important, it will 
 be well briefly to recall what had been previously achieved 
 in the same line of research. Copernicus had proved and 
 Kepler and others had drawn attention to the greatness of 
 the gap between the orbits of Mars and Jupiter. Then, in 
 1781, Sir William Herschel detected the planet Uranus at 
 a distance from the sun agreeing with the next term of a 
 series which Titius and Bode had noticed as almost exactly 
 representing the distances of the other planets, except that 
 for one term, between those which corresponded with Mars 
 and Jupiter, there was no planet known. 
 
 While twenty-four astronomers were arranging a search 
 for such a missing member of the solar system, Piazzi (who 
 was not one of them) unexpectedly detected at Palermo, 
 on the ist of January, 1801, a little planet, afterward named 
 Ceres. Three more were found by two out of the twenty- 
 four astronomers in the course of the next six years. A 
 
 1 From the " Nineteenth Century," April, 1899. 
 16 241 
 
242 LEDGER 
 
 fifth was found in 1845, after an interval of thirty-eight 
 years; then the progress became rapid. Since 1846 no 
 year has passed without such a discovery. In 1868 the 
 total reached 100; in 1879, 200; in 1890, 300; in 1895, 
 400; and now it is nearly 450. After a few of the brighter 
 ones had been detected the search for others became a 
 wearisome process. New star charts had to be constructed, 
 with great labour, so as to include the fainter stars. If a 
 small star was noticed with the telescope which could not 
 be found in them, it was carefully watched, and if it ex- 
 hibited an orbital movement it was entered on the list of 
 planets, and the various elements of its orbit were calculated 
 and recorded. 
 
 But when many astronomers were inclined to look upon 
 all this work as well-nigh profitless, and too wearisome to 
 be continued, photography with startling suddenness did 
 away with all need of star charts and of any comparison of 
 observations with them. Upon a photographic plate suit- 
 ably exposed, Herr Max Wolf, of Heidelberg, found that 
 a little planet had recorded its place on the 226. of Decem- 
 ber, 1891. There was no need to compare the plate with 
 any chart of stars. The planet had asserted its right to the 
 name of Wanderer by moving on a little way in its orbit 
 among the stars during the exposure of the plate. The stars 
 left their traces in dots (the effect of the rotatory motion 
 of the earth having been duly compensated); the planet 
 left its trail in a little straight line drawn by it in the direc- 
 tion of its motion. 
 
 In the next year Max Wolf found thirteen more, and 
 in the same year Charlois ten, by this new method. And 
 since the early part of 1892, out of one hundred discoveries 
 of such planets, only three or four have been made by the 
 old method of eye observation. Once more, however, the 
 great abundance of these photographic discoveries of planet 
 after planet began to make astronomers despair of the pos- 
 sibility of so keeping count of their orbits and positions as 
 to be able to determine whether the little trails, of which 
 several were sometimes found upon the same photographic 
 
THE NEW PLANET. EROS 243 
 
 plate, indicated the presence of planets previously seen or 
 hitherto unknown. It would, indeed, have been quite im- 
 possible to do so had it not been for the unremitting in- 
 dustry of German computers, as evidenced year by year in 
 the " Berliner Jahrbuch." 
 
 These little bodies were even termed astronomical nui- 
 sances. But one of them the 433d has at last proved 
 to be a great astronomical treasure. It has proved that 
 it would have been most unwise to have neglected any of 
 these minute portions of our solar system. Some e. g., 
 Hilda (No. 153), Thule (No. 279), and one which is still 
 unnamed (No. 361) approach so near to the orbit of 
 Jupiter that they will be of much use in the accurate de- 
 termination of that great planet's mass. Others are of 
 especial interest in the comparison of their very oval orbits 
 with those of certain periodic comets. But by far the most 
 important are those whose orbits lie nearest to that of the 
 earth. Only three or four, however, such as JEthra (No. 
 132), Brucia (No. 323), and Ingeborg (No. 391), have 
 hitherto been found which approach the earth, even to a 
 very moderate extent, within the distance of that part of 
 the somewhat oval orbit of Mars in which he is at his far- 
 thest from the sun; and they do so only in a small portion 
 of their orbits. 
 
 But in the case of Eros we meet with something utterly 
 different and unexpected. A new planet has been dis- 
 covered whose average distance from the sun is less than 
 that of Mars; a planet which at times comes within a dis- 
 tance from the earth not much more than one third of the 
 nearest distance within which Mars ever approaches it. 
 
 On the 1 3th of August, 1898, Herr Witt exposed a 
 photographic plate with the hope of obtaining upon it the 
 trail of another previously known minor planet. He suc- 
 ceeded, but upon the plate there was also a second, fainter 
 trail faint and of unusual length because of the rapidity 
 with which the planet had moved. This indicated an un- 
 usual orbit. Further observations were at once made. 
 From them Herr Berberich calculated what proved to be 
 
244 LEDGER 
 
 a most surprising orbit. The path of Mars up to this time 
 had practically formed the boundary beyond which minor 
 planets had hardly transgressed. This new planet came 
 45,000,000 miles within the mean distance of Mars. With 
 the exception of the moon it is by far the nearest celestial 
 neighbour of the earth, the nearest approach even of Venus 
 to the earth being not much less than twice as great. 
 
 But let us now ask, Why should the near approach of 
 Eros to the earth attach an extraordinary value to our ac- 
 quaintance with it? Is it because we may hope to see the 
 details of its surface, or to set up some communication be- 
 tween it and the earth? By no means. If we may judge 
 by the amount of light which it reflects to us, we may con- 
 clude that its diameter is probably less than twenty miles. 
 The largest telescope, therefore, will barely reveal in it 
 any disk of measurable breadth. On the contrary, the great 
 value of this little Eros depends upon its enabling us to 
 measure the scale upon which the whole universe around 
 us is constructed with an accuracy much surpassing any that 
 has been previously attained. 
 
 Our estimate, for instance, of the distance of any star, 
 or of the size of the orbits of any pair of double stars, in 
 fact, all our measurements in the celestial spaces, depend 
 upon our knowledge of the distance of the earth from the 
 sun. To determine that distance a direct trigonometrical 
 method, such as is used in surveying, and such as may be 
 applied to find the distance of the moon from the earth, 
 can not be used, as no instruments can be constructed of 
 the necessary delicacy. But there is a remarkable propor- 
 tion connected with the movements of the planets in their 
 orbits, discovered by Kepler and more fully investigated by 
 the genius of Newton, which enables us at once to deter- 
 mine the distance of the sun, if only we can measure the 
 distance of any one of the other planets from the earth. 
 
 It was at one time hoped that this might be accurately 
 determined in the case of Venus by observations made on 
 those rare occasions when it passes in transit across the 
 sun's disk. But the glare of the sun's light, the ill-defined 
 
FIRST OBSERVATION OF THE TRANSIT 
 OF VENUS. 
 
 Photogravure from an old etching. 
 
a most surprisii: , a th of Mars up to this time 
 
 had practically i adary beyond which minor 
 
 P !anc This new planet came 
 
 45,000,0* m distance of Mars. With 
 
 it is by far the nearest celestial 
 
 Approach even of Venus 
 
 han twice as great. 
 
 Id the near approach of 
 
 ary value to our ac- 
 
 fiope to see the 
 
 imunication be- 
 
 may judge 
 
 may con- 
 
 ie^rhole universe around 
 that 
 
 nee of any star, 
 s, in 
 uretnent^ 
 
 >f the d ,,i the 
 
 listance a < umetrical 
 
 i the earth, 
 
 istructed of 
 
 narkable propor- 
 
 planets in their 
 
 investigated by 
 
 us at once to deter- 
 
 we can measure the 
 
 clistr> ets from the earth. 
 
 iiis might be accunr 
 
 ^rmined ^ by observations made 
 
 ;e rare ? >en it passes in transit across 
 
 isk. But the glare of the sun's light, the ill-' 
 
THE NEW PLANET, EROS 245 
 
 edge of the sun's disk, and the atmosphere of Venus itself, 
 combine to deprive such observations of the necessary ac- 
 curacy. Apart from some other methods, involving long 
 periods of time and highly complicated theoretical investi- 
 gations in their use, attention was therefore next given to 
 an attempt to obtain the distance of the planet Mars when 
 it makes its nearest approaches to the earth. It was, how- 
 ever, found to be difficult to measure the exact position of 
 the centre of its disk. Whereupon it was suggested that 
 some of the nearer minor planets, although they would be 
 farther from the earth, and their distance from it propor- 
 tionately more difficult to determine, might more than com- 
 pensate for this disadvantage by the great accuracy with 
 which the positions of their starlike telescopic images might 
 be observed. And this was found to be the case. The 
 most accurate value of the sun's distance known at the 
 present time is believed to be that which has been skilfully 
 deduced in this way from observations of certain of the 
 nearer minor planets by Dr. Gill, H. M. Astronomer at the 
 Cape of Good Hope. 
 
 The new planet, Eros, is of the utmost value for such 
 observations, because the accuracy of the result which they 
 afford is proportionate to the nearness to the earth of the 
 planet that is observed. The method of calculation em- 
 ployed depends upon the ratio of the planet's distance to 
 the distance between two observers simultaneously look- 
 ing at it from two widely separated points upon the 
 earth's surface, or to the distance through which an ob- 
 server may himself be moved, by the rotation of the earth 
 upon its axis, between two observations made, the one 
 soon after sunset, and the other shortly before sunrise. The 
 movements of the earth and the planet in their orbits in the 
 interval (as also if in the first case the two observations 
 made are not exactly simultaneous) can be allowed for, and 
 will not affect the final result. An observer may be moved 
 between such an evening and early-morning observation 
 when this latter (termed the diurnal) method is employed, 
 provided he be near to the equator, to a position which may 
 
246 LEDGER 
 
 be separated by about 7,000 miles, supposed to be measured 
 in a straight line drawn through the earth, from his pre- 
 vious place. The effect would be the same in altering the 
 apparent direction in which the planet would be seen as 
 if he were looking at it one moment from Jamaica, and 
 then were suddenly transported to see it from Aden. 
 
 The difference in the directions in which the planet is 
 seen from two such standpoints, as compared with the posi- 
 tions of the stars around it, which are so distant that no 
 change is produced in their apparent places, is in such cases 
 large enough to be capable of very accurate measurement, 
 and will be so much the larger and more easily measurable 
 the nearer any planet employed is to the earth. 
 
 It was a few years ago supposed that the diurnal method 
 would prove to be the most satisfactory possible, because in 
 it the same observer and the same instrument can be em- 
 ployed for all the observations. We are inclined to think 
 that this may ultimately prove to be the case if an observa- 
 tory suitably equipped, and situated near to the equator, 
 can be employed. Dr. Gill has, however, introduced such 
 improvements into the other method that it has been 
 chiefly used under his superintendence for the last pub- 
 lished 2 and most accurate result that has yet been ob- 
 tained viz., from observations of the minor planets Vic- 
 toria, Sappho, and Iris, made in 1888 and 1889 at the Cape 
 of Good Hope, and at Yale, Leipsic, Gottingen, Bamberg, 
 and Oxford (Radcliffe) Observatories. But it is very in- 
 teresting that it is also found that observations (nearly 3,500 
 in number) made at the same time by the diurnal method 
 upon the planet Victoria at the Cape, although that ob- 
 servatory is unfavourably situated for the use of this 
 method, in a latitude thirty-four degrees south of the 
 equator, gave almost precisely the same result a value of 
 very nearly 92,875,000 miles as the distance of the sun from 
 the earth. 
 
 This is a very trustworthy value, but it nevertheless in- 
 volves an uncertainty of about 50,000 miles, or possibly 
 2 " Annals of the Cape Observatory," vol. vi, published in 1897. 
 
THE NEW PLANET, EROS 247 
 
 somewhat more. It was obtained by the observation of 
 planets selected for their suitability of position and because 
 their orbits were very accurately known, which did not, 
 however, come within a distance equal to six times that of 
 the nearest approach which Eros may make to the earth. 
 There is every reason, therefore, to hope that our future 
 observations of Eros may give us this all-important unit 
 for all our celestial measurings the distance of the earth 
 from the sun with an accuracy six times greater than any 
 which has hitherto been secured. 
 
 It is very fortunate that Eros, when at its nearest ap- 
 proach to the sun, is also almost in the plane of the earth's 
 orbit. If the earth is at the same time in the corresponding 
 part of its own orbit, the planet's approximation to the 
 earth is consequently in nowise hindered by its being ele- 
 vated above, or depressed below, the earth's orbit. But 
 the two are only simultaneously in these positions once 
 in about every thirty years, and it is very unfortunate that 
 an exceedingly favourable concurrence of such positions 
 occurred in January, 1894, so that the next occasion will 
 not be until January, 1924. In the latter part, however, 
 of 1900, and in the beginning of 1901, the earth and Eros 
 will come within about 31,000,00x3 miles, and this, their 
 nearest approach to one another before the year 1924, 
 will enable observations of much importance to be made, 
 which ought to suffice for a decided improvement in 
 the accuracy of our present estimate of the distance of 
 the sun. 
 
 A few further statements with reference to this very re- 
 markable planet may be of interest. Its mean distance from 
 the sun is 5,500,000 miles less than that of Mars. That 
 distance ranks, therefore, not with those of all the other 
 little planets as between that of Mars and that of Jupiter, 
 but as between the earth's and that of Mars. Owing, how- 
 ever, to the ovalness of its orbit, it passes in one part of 
 its circuit about 11,000,000 miles beyond the maximum dis- 
 tance of Mars, which is, nevertheless, a comparatively slight 
 excess. Its period of revolution round the sun is 643 days, 
 
248 LEDGER 
 
 that of Mars being 687. There is no fear of its ever col- 
 liding with Mars, because the two orbits, where they would 
 otherwise intersect, are separated by an interval of about 
 21,000,000 miles, owing to the difference of their tilts, or 
 inclinations to the ecliptic. 
 
 Besides its great usefulness for the purpose already ex- 
 plained, the perturbations of its motion by the earth's at- 
 traction will afford another indirect and theoretically very 
 interesting method of determining the distance of the sun, 
 by its enabling a comparison to be made between the masses 
 of the earth and the sun. The perturbations of the motion 
 of Eros by Mars will, in addition, be very valuable to 
 astronomers. Certain recondite effects of its proximity 
 in reference to the moon may also prove to be important. 
 The great alterations in its distance from the earth at dif- 
 ferent times will afford an excellent test as to whether the 
 light received from it varies exactly as the inverse square 
 of its distance from us, or meets with any hindrance, or 
 absorbing medium, in its passage. The comparison of its 
 light with the phases of its little disk corresponding to its 
 positions relatively to the earth and the sun will also be 
 instructive. 
 
 The discovery of Eros has afforded a most important 
 proof of the value of stellar photographs carefully kept and 
 preserved. For the more accurate determination of the 
 elements of its orbit it was very desirable to obtain, if pos- 
 sible, records of its exact position in previous years. Very 
 careful search was therefore made among the many plates 
 preserved at the Harvard College Observatory, in America, 
 in order to see if its faint trace could be found upon some 
 of those which had been exposed in 1896. It seemed 
 almost impossible to detect it. But at last success rewarded 
 a search which proved most trying to the eyes. Mrs. 
 Fleming, well known for her splendid work in connection 
 with stellar spectra in the Harvard Observatory, detected 
 the trail upon a plate dated the 5th of June, 1896. It was 
 soon found upon other plates of that year, as its probable 
 position could not be more precisely calculated; then on 
 
THE NEW PLANET, EROS 249 
 
 others of 1893 and 1894; upon thirteen plates in all. Its 
 orbit is consequently known at the present time with great 
 accuracy. 
 
 There is no reason to suppose that Eros is a body re- 
 cently drawn, by the attraction of the earth, into its present 
 orbit. Its near approach to the earth is by no means near 
 enough for such an event as that to have occurred. It has 
 doubtless escaped previous observation because its light 
 has only exceeded that of an eighth-magnitude star (or, for 
 the purpose of photography, as it seems to be wanting in 
 violet rays, that of a ninth-magnitude star) for about two 
 months in the last eleven years. On the comparatively rare 
 occasions of its very nearest approach it will be barely 
 within the range of visibility to the naked eye. 
 
 In conclusion, it may be noticed that the proximity of 
 the orbit of this little neighbour to that of the earth may 
 afford one more argument against the hypothesis put for- 
 ward by Olbers of the supposed origin of the minor planets 
 by the explosion of a larger planet, a hypothesis which for 
 a while met with much acceptance. When only four minor 
 planets had been discovered, three at least of their orbits 
 tended to support his hypothesis. It has, however, since 
 been discarded by nearly all the highest authorities in 
 astronomy, to the regret, no doubt, of those whom the idea 
 of any celestial catastrophe seems to fascinate, whether it be 
 the possible collision of two suns, the destruction of the 
 earth by a comet, or the blowing of a planet into pieces 
 by its own inherent forces. Even if such an event could 
 have occurred, and have produced the minor planets, it 
 must have been at an exceedingly remote epoch; other- 
 wise, by the laws of mechanics every fragment would have 
 continued, once in each of its subsequent revolutions round 
 the sun, to pass again and again through the point of ex- 
 plosion. Millions of years would have been required to 
 enable the mutual perturbing attractions of the fragments 
 upon each other so to change their orbits as to have 
 effaced all trace of the point where the catastrophe took 
 place. Apart, however, from this, and apart from the fact 
 
250 LEDGER 
 
 that it is difficult to conceive how the orbits could have 
 been spread over seven eighths of the vast interval which 
 separates those of Mars and Jupiter, apart also from the 
 great extension of the region in which they move which is 
 involved in the newly found orbit of Eros, another argu- 
 ment against the hypothesis seems to be conclusive. 
 
 It appears impossible to conceive of such an amount of 
 explosive energy in any globe as should not only hurl away 
 a number of ejected portions in the directions and with the 
 velocities which would produce such widely differing or- 
 bits, but such as should break up and suitably project the 
 whole mass of the globe, leaving no fragment of any im- 
 portance unprojected. A cannon-ball, as Proctor has re- 
 marked in his " Old and New Astronomy," " might be 
 driven by a certain charge of gunpowder to a distance of 
 two or three miles, but a thousand times that charge would 
 not scatter the fragments of the cannon (if the ball had been 
 tightly driven in) over a similar distance all round the place 
 of explosion. Nothing known about our earth's interior, 
 nothing which we can infer about the interior of any other 
 planet formed by processes such as we recognise in the de- 
 velopment of the solar system as at present understood, 
 suggests the possibility that a millionth part of the force 
 necessary to shatter a planet, as Olbers's theory requires, 
 can ever be generated or accumulated within the planet's 
 interior." 3 
 
 Rather may we see in a planet such as Eros a portion of 
 the primeval solar nebula unused in the formation either of 
 Mars or of the earth. The minor planets are probably no 
 fragments of a larger planet previously existing, but the 
 fragments that might have helped to form a larger planet 
 had it not been for the influence of the mighty globe of 
 Jupiter. We may see in them one more instance of the 
 effect of that process of tidal action which Professor Darwin 
 has of late so wonderfully applied to show how the matter 
 of the moon may, in bygone time, have been disrupted from 
 the then viscous earth, in the form of a succession of lumps 
 8 " Old and New Astronomy," p. 563. 
 
THE NEW PLANET, EROS 251 
 
 broken off by centrifugal effect from the summits of great 
 tidal waves a hypothesis which is found to be of ever- 
 widening application, as, for instance, to the genesis of 
 double stars, and to the temporary outburst of such stars 
 as that which Kepler saw in 1572 in Cassiopeia. 
 
 The attraction of the globe of Jupiter, as the solar 
 nebula contracted within his orbit, may well have produced 
 such tides in its mass as, in place of allowing a greater 
 quantity of matter or a nebulous ring to be more quietly 
 detached at some subsequent epoch, so as to form another 
 large globe, may have caused many and many a smaller 
 portion to have been broken off and left behind. These 
 portions we may now see in the hundreds of minor planets 
 which have so far been discovered. After a while, it may 
 be supppsed that the influence of Jupiter was so far left 
 behind by the continued contraction of the solar nebula that 
 the formation of larger globes, such as those of Mars and 
 the earth, Venus and Mercury, began again. 
 
 However this may be, let us hope that in the succession 
 of celestial photographs now being continuously secured 
 other similar fragments may ere long be revealed whose 
 orbits may be as interesting as that of Eros, whether they 
 may revolve within, or, like it, outside of the orbit of the 
 earth. Let us hope that some of them may approach the 
 earth even more closely than Eros. If so, they will be still 
 more useful rewards of the unwearied industry of observers 
 and computers, and of the skill displayed in astronomical 
 photography. 
 
SIDEREAL ASTRONOMY: 
 OLD AND NEW 
 
 PHOTOGRAPHY THE SERVANT 
 OF ASTRONOMY 
 
 THE BEGINNINGS OF AMERICAN 
 ASTRONOMY 
 
 BY 
 
 EDWARD SINGLETON HOLDEN 
 
SIDEREAL ASTRONOMY: OLD AND NEW 1 
 
 I. THE DATA IT HAS COLLECTED 
 
 WHEN did astronomy have its beginnings on the 
 earth? There have been many learned attempts 
 to answer this question. They all have led to the 
 conclusion that long before the historic period there was 
 a large common stock of knowledge; so large, in fact, that 
 one distinguished writer finds it simplest to ascribe the 
 origin of astronomy to the teaching of an extinct race: " Ce 
 peuple ancien qui nous a tout appris excepte son nom et 
 son existence," his commentator adds. 
 
 Astronomy is older than the first records of any nation. 
 In order that the records might exist, it was first necessary 
 to divide the years and times by astronomical observations. 
 On the other hand, I believe the travellers of to-day have 
 found no tribe so degraded as to be without some knowl- 
 edge of the sort. 
 
 It is extremely doubtful if animals notice special celes- 
 tial bodies. Birds seem to be inspired by the approach of 
 day and not by the actual presence of the sun. It is a ques- 
 tion whether dogs " bay the moon " or only the moon's 
 light. A friend maintains that her King Charles spaniel 
 watched the progress of an occupation of Venus by the 
 crescent moon with the most vivid interest. This is the 
 only case which I have been able to collect in which the 
 attention of animals has been even supposed to have been 
 held by a celestial phenomenon. The actions of the most 
 ignorant savages during a total solar eclipse, compared with 
 
 1 From the " Century Magazine," August and September, 1888. By 
 special permission of the Century Company. 
 
 255 
 
256 HOLDEN 
 
 those of animals, throw much light on the question of 
 whereabouts in the scale of intelligence the attention begins 
 to be directed to extra-terrestrial occurrences. The savages 
 are appalled by the disappearance of the sun itself, while 
 animals seem to be concerned with the advent of darkness 
 simply. 
 
 I am told that the Eskimos of Smith's Sound have 
 names for a score or more of stars, and that their long 
 sledge journeys are safely made by the guidance of these 
 stars alone. I have myself seen a Polynesian islander em- 
 bark in a canoe, without compass or chart, bound for an 
 island three days' sail distant. His course would need to 
 be so accurately laid that at the end of his three days he 
 should find himself within four or five miles of his haven; 
 if he passed the low coral island at a greater distance it 
 could not be seen from his frail craft. There can be little 
 doubt but that he used the sun by day and the stars by 
 night to hold his course direct. 
 
 There must have been centuries during which such 
 knowledge was passed from man to man by word of mouth, 
 woven into tales and learned as a part of the lore of the 
 sailor, the hunter, or the tiller of the soil. No one can say 
 how early this knowledge of the sky was put into the for- 
 mal shape of maps, globes, or catalogues. Eudoxus is said 
 to have constructed a celestial globe B. c. 366. Globes would 
 naturally precede maps, and maps mere lists or catalogues. 
 
 The prototype of all sidereal catalogues is the " Alma- 
 gest " of Ptolemy (A. D. 150), which includes not only the 
 observations of Ptolemy, but those of the great Hippar- 
 chus (B. c. 127). It contains the description of 1,022 stars, 
 their positions, and their brightness. Here we meet for 
 the first time the name magnitude of a star. Ptolemy 
 divides all the stars into magnitudes degrees of bright- 
 ness. Sirius, Capella, are of the first magnitude; the faint- 
 est stars visible to the eyes are of the sixth. But Ptolemy 
 has gone further, and divides each magnitude into three 
 parts. The moderns divide each class into ten parts that 
 is, decimally. 
 
SIDEREAL ASTRONOMY: OLD AND NEW 257 
 
 SCALE OF MAGNITUDES 
 
 In assigning magnitudes in this way, we have uncon- 
 sciously adopted a scale. A star of the third magnitude is 
 brighter than one of the fourth. How much brighter? 
 Sirius and the brightest stars are about one hundred times 
 more brilliant than the very faintest stars which can be seen 
 with the naked eye. In general a star of any magnitude, 
 as fifth, is four tenths as bright as the star of the next 
 brighter magnitude, as fourth. Ten fifth-magnitude stars 
 taken together are as bright as four fourth-magnitude stars, 
 and so on. This relation between the brightness of stars 
 of consecutive magnitudes gives us a means of computing 
 the total amount of light received from stars. For example, 
 there are ten stars in our sky as bright as the brilliant star 
 Vega, or Alpha Lyrse, which we see in our zenith during 
 the summer months. The collective light of these ten first- 
 magnitude stars is 10 times that of Vega. The 37 second- 
 magnitude stars are together 7.4 times as bright as Vega; 
 the 128 third-magnitude stars are 10.2 times as bright; and 
 so on down to the 4,328 sixth-magnitude stars, which, 
 taken together, are 22.1 times as bright. Taking all the 
 stars visible to us without a telescope and adding their 
 brilliancy, we find that all the naked-eye stars give us a 
 light 67.6 times as bright as that from Vega. Now, the 
 stars of the seventh and eighth magnitudes have been 
 counted; there are 13,593 of the seventh, 57,960 of the 
 eighth, and they too send light to us, although they are 
 individually invisible. All the seventh-magnitude stars 
 taken together give us 27.8 times as much light as Vega, 
 and the eighth give us 47.4 as much; so that we have from 
 both of these classes 75.2 times the light of Vega that is, 
 more light comes to us from stars so faint as to be indi- 
 vidually invisible than from the less numerous and brighter 
 stars that we see with the naked eye. We may recollect that 
 more than half of the light of a starlit night comes from the 
 collective lustre of stars, each of which is totally invisible 
 except in the telescope. 
 17 
 
258 HOLDEN 
 
 METHODS OF NAMING THE STARS 
 
 In Ptolemy's " Almagest," and for fifteen centuries 
 later, there were two, and but two, ways of designating a 
 particular star. A few of the brighter stars had special 
 names. 
 
 By far the greater number were described by their situa- 
 tion in their constellation. The brightest star in Taurus was 
 the eye of the Bull, and so for others, as the belt and sword 
 of Orion. This was all very well for the brighter stars, and 
 it did not require that the boundaries of the constellations 
 should be very accurately fixed. There was no mistaking 
 Regulus, Cor Leonis the heart of the lion. But when we 
 come to the small pairs of stars which make the paws of the 
 Great Bear, or to some of the stars in the windings of Ser- 
 pens, then it is evident that Ptolemy must have had ac- 
 curately bounded constellations laid down on charts or 
 globes. Not a single ancient globe or chart has come 
 down to us. The oldest extant are but Arabian copies of 
 the tenth century. 
 
 Where, then, do we derive our figures of the constella- 
 tions? If any one of my readers will ask some astronomical 
 friend to show him a copy of Flamsteed's "Atlas Ccelestis " 
 he will see the beautiful and spirited drawings of the con- 
 stellation figures, and be charmed and delighted with their 
 vigour and character. Who could have drawn these out- 
 lines, instinct with life? Who of the ancients knew the 
 whole character of the timid hare, or who could draw 
 Andromeda, and put a modern resignation in her chained 
 despair? These figures were drawn by a master indeed, for 
 they are from the hand of Albert Diirer himself. If we fol- 
 low the history of how he came to make them for an edi- 
 tion of Ptolemy, and think of him patiently fitting his mar- 
 vellously free outlines to match the stars in the sky and 
 the crabbed descriptions in Ptolemy's book, the pleasure 
 does not dimmish. About 1603 Bayer introduced the prac- 
 tice of designating the brighter stars of each constellation 
 by the letters of the Greek alphabet, so that Cor Leonis or 
 
SIDEREAL ASTRONOMY: OLD AND NEW 259 
 
 Regulus became a Leonis; Aldebaran became a Tauri, and 
 so on. As the number of the well-determined stars has 
 vastly increased, the practice of referring to them by their 
 numbers in some well-known catalogue has come into 
 vogue; so that a Leonis, for example, might be known as 
 Bradley, 1406, from its number in Bradley 's catalogue; 
 or as Lalande, 19,755, an< ^ so on - ^ * s not to be denied 
 that astronomical nomenclature in this direction could be 
 greatly improved. 
 
 URANOMETRIES 
 
 The word uranometry has received a limited technical 
 meaning in astronomy. It is used to denote a description 
 of the fixed stars which are visible to the naked eye only. 
 The description of each star places it in its proper con- 
 stellation, assigns its latitude and longitude, and gives its 
 brightness or magnitude. Variable stars, which change 
 their brightness periodically and there are many such 
 are treated separately. 
 
 Ptolemy's " Almagest " (1,022 stars) was an incomplete 
 uranometry, since there were more than 3,000 stars visible 
 to him. Al-Sufi's revision of it, in the tenth century, added 
 no stars, but simply revised the magnitudes given by Ptol- 
 emy. Bayer (1603) gave 1,200 stars. None of the very 
 important works of Flamsteed (1753), Harris (1725), Wol- 
 laston (1811), Harding (1822), were complete that is, no 
 one gave every star down to a certain brightness. It was 
 reserved for Argelander (1843) to give in the " Uranometria 
 Nova" the position of brightness of every star visible to the 
 naked eye at Bonn. This was a picture of the sky; changes 
 could no longer occur without detection. This work gave 
 the places of 3,256 stars, from first to sixth magnitudes, and 
 very careful eye-estimates of their magnitudes. Argelan- 
 der's work has been repeated by Heis (1872). The southern 
 sky has been treated in the same way by Dr. Gould, in the 
 " Uranometria Argentina" (1879), containing 6,694 south- 
 ern and 991 northern stars, of magnitudes between the first 
 and seventh. Houzeau, during a residence in Jamaica, 
 
260 HOLDEN 
 
 made a uranometry which embraces every star in both 
 hemispheres, and which has a special value owing to the 
 fact that the estimates of magnitude were all made by a 
 single person, and are therefore consistent. 
 
 We have, then, a complete picture of our sky, as seen 
 with the naked eye, based on eye-estimates of the bright- 
 ness of the stars. It should be said that the magnitudes so 
 determined are extremely accurate, approaching closely to 
 the exactness which can be reached with the best pho- 
 tometers, or instruments for measuring the relative bright- 
 ness of stars. 
 
 Up to 1877, when Professor Pickering became director 
 of the Harvard University Observatory, there was no single 
 observatory devoted to photometry as a chief end. The 
 important works of this nature had been done as a part 
 of other duties. Professor Pickering turned the whole 
 strength of the observatory in this direction, and by means 
 of new methods and new instruments he and his assistants 
 have just completed a work of the first importance the 
 " Harvard Photometry." It contains the positions and 
 the measured brightness of 4,260 stars visible at Cam- 
 bridge, together with a comparison with the magnitudes 
 of all other observers. The actual number of single ob- 
 servations is 95,000. Each one of these consists in a direct 
 photometric comparison of the relative brightness of a star 
 with one of the polar stars. The polar stars are always 
 visible; the stars to be measured were taken as they crossed 
 the meridian; and these direct measures, suitably com- 
 bined, give the relative brightness of each of the stars of the 
 list. We have now a sure basis for all future work, and a 
 perfect picture of the sky at this time. 
 
 THE NUMBER OF THE STARS 
 
 The total number of stars one can see will depend very 
 largely upon the clearness of the atmosphere and the keen- 
 ness of the eye. There are in the whole celestial sphere 
 
SIDEREAL ASTRONOMY: OLD AND NEW 261 
 
 about 6,000 stars visible to an ordinarily good eye. Of 
 these, however, we can never see more than a fraction at 
 any one time, because a half of the sphere is always below 
 the horizon. If we could see a star in the horizon as easily 
 as in the zenith, a half of the whole number, or 3,000, would 
 be visible on any clear night. But stars near the horizon 
 are seen through so great a thickness of atmosphere as 
 greatly to obscure their light, and only the brightest ones 
 can there be seen. As a result of this obscuration, it is not 
 likely that more than 2,000 stars can ever be taken in at 
 a single view by any ordinary eye. About 2,000 other stars 
 are so near the south pole that they never rise in our lati- 
 tudes. Hence, out of 6,000 supposed to be visible, only 
 4,000 ever come within the range of our vision, unless we 
 make a journey toward the equator. 
 
 As telescopic power is increased, we still find stars of 
 fainter and fainter light. But the number can not go on 
 increasing forever in the same ratio as with the brighter 
 magnitudes, because, if it did, the whole sky would be a 
 blaze of starlight. If telescopes with powers far exceeding 
 our present ones were made, they would no doubt show 
 new stars of the twentieth and twenty-first, etc., magni- 
 tudes. But it is highly probable that the number of such 
 successive orders of stars would not increase in the same 
 ratio as is observed in the eighth, ninth, and tenth magni- 
 tudes, for example. The enormous labour of estimating 
 the number of stars of such classes will long prevent the 
 accumulation of statistics on this question; but this much 
 is certain, that in special regions of the sky, which have 
 been searchingly examined by various telescopes of suc- 
 cessively increasing apertures, the number of new stars 
 found is by no means in proportion to the increased instru- 
 mental power. If this is found to be true elsewhere, the 
 conclusion may be that, after all, the stellar system can be 
 experimentally shown to be of finite extent and to contain 
 only a finite number of stars. In the whole sky an eye of 
 average power will see about 6,000 stars, as I have just 
 said. With a telescope this number is greatly increased, 
 
262 
 
 HOLDEN 
 
 and the most powerful telescopes of modern times will show 
 more than 60,000,000 stars. Of this number not one out 
 of one hundred has ever been catalogued at all. 
 
 In Argelander's " Durchmusterung " of the stars of the 
 northern heavens, there are recorded as belonging to the 
 northern hemisphere: 
 
 io stars 
 
 37 
 
 128 
 
 310 
 
 1,016 
 
 4,328 
 
 13,593 
 
 237,544 
 
 between the i.o magnitude and 
 2.0 
 3.0 
 4.0 
 5-0 
 6.0 
 7.0 
 8.0 
 9.0 
 
 the 1.9 magnitude. 
 2.9 
 
 3-9 
 4.9 
 
 5-9 
 6.9 
 
 7-9 
 8.9 
 
 9-5 
 
 In all, 314,926 stars, from the first to the 9j magnitudes, 
 are contained in the northern sky, or about 600,000 in both 
 hemispheres. All of these can be seen with a three-inch 
 object-glass. 
 
 THE CHARTS OF THE BERLIN ACADEMY 
 
 In 1824 Bessel wrote to the Academy of Berlin some- 
 what as follows: 
 
 " It is of the highest astronomical interest that every 
 fixed star in the sky should be known, and its position 
 fixed. Completeness in this task is unattainable; but when 
 we once have maps of all the stars down to a certain mag- 
 nitude, then the object will be attained. The limit I set is 
 at those stars which can just be plainly seen in one of 
 Fraunhofer's excellent comet-seekers 2 that is, at about 
 the ninth or tenth magnitude." 
 
 Bessel then gives briefly the reasons why such a com- 
 plete list would be valuable, in addition to its importance 
 as a finished picture of the sky so far as it went; and con- 
 tinues: 
 
 " For all these reasons I have often expressed my hope 
 that we might have such a complete list, if even over only 
 a portion of the sky; and I think the time of an astronomer, 
 * A telescope with about three inches aperture, magnifying ten times. 
 
SIDEREAL ASTRONOMY: OLD AND NEW 263 
 
 and of an observatory, could not be better spent than in 
 aiding a systematic attempt to carry out this plan. I my- 
 self designed the instruments of the Konigsberg Observa- 
 tory for such a purpose, and since 1821 I have observed 
 as many as possible of the stars from 15 north to 15 south 
 of the equator. In all there are 36,000 observations of 
 32,000 stars. If the stars are equally numerous over the 
 whole sky, there are 125,000 such. I am about to carry on 
 these zones up to 45 from the equator." 
 
 With this introduction Bessel unfolds his plan, which 
 was to have 24 astronomers join in an undertaking to 
 make the 24 separate charts required to extend round the 
 whole 24 hours, and in width over the 30 from 15 north 
 to 15 south of the equator. He himself made a small chart 
 as a beginning, " to break the path," and as a model. The 
 Academy welcomed Bessel's plan, and the work began in 
 1825. 
 
 The first two charts were received in 1828, and the 
 work on the others continued slowly. One of these charts 
 has a great history. It had been engraved but not yet dis- 
 tributed, and was lying in the Berlin Observatory for ex- 
 amination. On the evening of September 23, 1846, Le 
 Verrier's letter, giving the place of a new planet, Neptune, 
 was received in Berlin. The planet had never been seen, 
 but its existence had been predicted from the otherwise in- 
 explicable motions of Uranus. The predicted place of the 
 planet fell within the limit of the lately finished chart, which 
 was taken to the telescope. In very truth there was an 
 eighth-magnitude star in the sky which was not on the 
 chart. This star was in motion; it had the planetary light 
 and disk; it was, in fact, Neptune. The proposal of Bessel 
 had borne splendid fruit. Besides this major planet, many 
 of the minor planets (asteroids) were discovered by these 
 maps. Finally, in 1859, thirty-five years after Bessel's let- 
 ter, this series was finished. But before it was finished a 
 greater undertaking was begun, of which we must give a 
 short account. One thing must be continually kept in 
 sight. Every one of the systematic " Durchmusterungen," 
 
264 HOLDEN 
 
 as the Germans say we have no word for them is the 
 direct outcome of Bessel's original proposition. 
 
 ARGELANDER'S " DURCHMUSTERUNG " 
 
 Argelander was Bessel's pupil. In the great zones of 
 Konigsberg, Bessel had pointed the telescope on the stars 
 as they, passed, and Argelander read the verniers which 
 showed their position. Finally, Argelander had an observa- 
 tory of his own at Bonn, and his two young assistants, 
 Drs. Krueger and Schoenfeld, were all to him that he had 
 been to Bessel. The years 1852 to 1862 were spent in the 
 tremendous task of observing every star plainly visible in 
 such a comet-seeker as we have described, over more than 
 half of the whole heavens. The telescope was pointed and 
 fixed in position. The time of the passage of every star 
 over a wire in the field of view was noted; the part of the 
 wire crossed by the star was also noted, and finally the 
 brightness of the star. 
 
 Not counting the time for the computations, the ob- 
 servations alone lasted seven years and one month; 1,797 
 hours were spent in observing the comet-seeker zones on 
 625 nights; and 227 other nights were used in part or 
 wholly in revision zones to correct errors of one nature or 
 another, or to solve doubts. 
 
 In the comet-seeker zones 850,000 single observations 
 were made, or on the average 473 stars per hour, or 8 per 
 minute. In specially rich parts of the Milky Way more 
 than 1 6 stars per minute were often observed, and the rich- 
 est zone had 1,226 stars in the hour, or 20^ per minute 
 one every 3 seconds. Counting all the observations to- 
 gether, there were no less than 1,065,000, and this million 
 of observations gave the positions and the brightness of 
 324,198 stars that is, the position and brightness of every 
 star plainly visible in the telescope used, from the north 
 pole down to 2 south of the equator. 
 
 The very enumeration of the observations makes one 
 fatigued. Only the astronomer can know the multifarious 
 nature of the calculations connected with the observations 
 
SIDEREAL ASTRONOMY: OLD AND NEW 265 
 
 themselves. Millions on millions of figures had to be made, 
 and made correctly; and, finally, every star had to be en- 
 graved on charts, and engraved correctly both as to posi- 
 tion and magnitude. 
 
 How this work could have been finished in ten years 
 one does not see. That Argelander and his two assistants 
 had the courage to persevere in this tremendous task is 
 itself a marvel. But the work is done, is printed, and is in 
 daily use by scores of astronomers. Its value will never be 
 less. It will remain forever as a picture of the sky, avail- 
 able for every purpose. 
 
 Mr. Proctor has done a very useful w r ork in represent- 
 ing the results of Argelander's " Durchmusterung " in a 
 single chart. For every star in Argelander's catalogue 
 Mr. Proctor has laid down a dot, correct as to position and 
 magnitude 324,198 dots in all. The resulting map is 
 photographed down so that the individual dots are, in 
 general, hard to distinguish, but the law of aggregation of 
 the stars is all the better brought out. The map is most 
 interesting, not only in relation to the mere positions and 
 brilliancy of the stars, but as showing, better than any 
 other means can, the apparently capricious manner in 
 which the stars are spread over the surface of the sky. 
 Some evidences of law can be made out, and, in the origi- 
 nal, the great features of the Milky Way come forth in 
 a most striking manner. It must be remembered that this 
 map contains, besides the stars visible to the naked eye, all 
 those visible in an ordinary three-inch telescope. 
 
 SCHOENFELD'S " DURCHMUSTERUNG " 
 Argelander's original plan was to extend his observa- 
 tions to 23 south of the equator. Professor Schoenfeld, 
 his successor at Bonn, and his aid in the original under- 
 taking, in 1885 completed the plan projected by Bessel in 
 1824, and so nobly followed at Bonn from 1852 to 1860. 
 From 1876 to 1884 he has catalogued the stars from 2 
 to 23 south of the equator, and the work is just finished. 
 Soon we shall have this new " Durchmusterung," with its 
 
266 HOLDEN 
 
 charts, showing the position and brightness of 133,658 
 southern stars. 
 
 It is most desirable that this enumeration should be 
 extended over the whole southern sky. So long ago as 
 1866 the work was begun in the southern hemisphere, but 
 apparently it was abandoned, though there is reason to 
 believe that the observatory of the Argentine Republic at 
 Cordoba may begin anew. Professor Stone, at Cincinnati, 
 has partly completed the zone between 23 and 31 (south). 
 
 A recognition of the enormous advantages which pho- 
 tography would have over ordinary visual methods of chart- 
 ing is now leading several observatories to attempt the 
 cataloguing of stars from photographic negatives. 
 
 The difficulties are many, but success seems to be tol- 
 erably certain, and the observatories of Harvard University 
 and of Paris have already produced wonderful results in 
 this direction. The observatory of the Cape of Good Hope, 
 also, has seriously begun a southern " Durchmusterung " 
 by photographic methods. 
 
 SYSTEMATIC OBSERVATORIES OF THE STARS IN ZONES 
 
 These " Durchmusterungen " are most important. 
 They give us an index to the stars of the whole sky. But 
 it is clear that the positions of the separate stars can not 
 be accurate when so many as eight or ten per minute are 
 observed. What the astronomer wants is the accurate posi- 
 tion of a star its latitude and longitude, as it were. We 
 shall see how much pains is necessary to fix the position of 
 a single star with real precision. Scores of observations are 
 needed, and each observation requires at least five minutes 
 to make and an hour to calculate. When we say that many 
 thousand stars have their positions known with this high 
 precision, we shall be giving a feeble idea of the amount 
 of labour devoted to this question. 
 
 But it is impossible to fix the position of every one of 
 the 600,000 stars of the " Durchmusterungen " with this 
 last degree of precision, and yet it is important to know 
 very closely the place of each star. The positions of all faint 
 
SIDEREAL ASTRONOMY: OLD AND NEW 267 
 
 comets, of asteroids, etc., are known by referring them to 
 neighbouring stars. We must know the positions of these 
 stars. These positions are determined by a special kind 
 of observations zone observations, so called. A telescope 
 is fixed in the meridian so that it can only move north 
 and south. A divided circle is attached to this, the indica- 
 tions of which give the altitude of the stars seen in the field. 
 One observer at the telescope moves it slowly up and down 
 until some star enters the field. The motion is stopped. 
 The transit of the star is observed over spider lines stretched 
 in the field, while a second observer reads the altitude of 
 this star from the divided circle. In this way it is possible 
 to obtain very accurate positions, and by confining the 
 work to a narrow zone the observations are increased as 
 to number, and the subsequent computations are much 
 simplified. 
 
 Before the days of the Berlin charts, or of the " Durch- 
 musterungen," Lalande, in Paris (1870), had fixed the 
 places of more than 50,000 stars in this way, and the Abbe 
 Lacaille(i75i)had made a special expedition to the Cape of 
 Good Hope to determine the places of 9,766 southern stars. 
 Bessel took up the same research in the years 1821-^33, and 
 his results are given in two magnificent catalogues, which 
 include 62,000 of the most important stars from 15 south 
 to 45 north of the equator. He made 75,011 single ob- 
 servations, employing 868 hours in observing alone that 
 is, about 84 stars per hour were observed. Argelander 
 read the altitudes of the stars from the circle while Bessel 
 observed their transits. One of Argelander's first works, 
 when he took charge of the observatory at Bonn, was to 
 continue this series of zones from 45 up to 80 north of 
 the equator that is, to within 10 of the pole. In this 
 region he made 26,424 observations of 22,000 stars, or 83 
 stars per hour. Not content with this extension of Bessel's 
 zones to the north, Argelander next began a series of 
 southern zones from 15 to 31 south of the equator. This 
 task he also completed, with 23,250 observations of 17,600 
 stars, or 83 stars per hour. 
 
268 HOLDEN 
 
 Bessel and Argelander alone had pushed their zones 
 from 31 south to 80 north of the equator, making nearly 
 125,000 separate observations and fixing the positions of 
 101,600 stars. We have no space to speak of the 38,000 
 observations made at the Naval Observatory in Washing- 
 ton in the years 1846-^49, or of the zones observed by 
 Lieutenant Gilliss, of our navy, in Chile (1850), which cov- 
 ered the region for 25 round the south pole (27,000 stars). 
 It is most unfortunate for the credit of American astron- 
 omers, as well as for the good of the science, that these col- 
 lections are not yet suitably published. 
 
 One would think that the 100,000 stars of Bessel and 
 Argelander would have been sufficient for the needs of 
 astronomy. But the German Astronomical Society, at its 
 meeting in Bonn, in 1867, deliberately resolved upon the 
 task of accurately determining the position of every star as 
 bright as the ninth magnitude contained in Argelander's 
 " Durchmusterung." 
 
 The veteran Argelander presided at this meeting, and it 
 is interesting to note how serious the undertaking appeared 
 to be to him. No one knew better how gigantic a task it 
 was. The plan was well laid. A set of 539 very-well-deter- 
 mined stars was assumed as fundamental, and the society 
 resolved that the position of the stars to be determined 
 should be referred to these. The sky was cut up into zones 
 five degrees wide, and various observatories undertook to 
 finish one or more of these zones. The Polar Zone (90 
 to 80 north of the equator) had lately been completed by 
 Carrington, in England, and did not need revision. 
 
 The observatories of Kazan (8o-75), Dorpat (75- 
 70), Christiania (7o-65), Helsingfors (65-55), Har- 
 vard University (55-5O), Bonn (5o-4O), Lund (40- 
 35), Leyden (35-3O), Cambridge, England (3O-25), 
 Berlin (25-i5), Leipsic (i5-5), Albany (5-!), Niko- 
 laief (i-2 south), joined in the work, and to-day it is 
 nearly completed. 
 
 But this is only a beginning. Schoenfeld's " Durch- 
 musterung " to 23 south will soon be printed, and it is 
 
SIDEREAL ASTRONOMY: OLD AND NEW 269 
 
 the intention of the German Astronomical Society to push 
 the zones to this point, to join on to the great series of 
 southern zones printed by our countryman, Dr. B. A. 
 Gould, at the National Observatory of the Argentine Re- 
 public. Dr. Gould is himself a pupil of Argelander, and 
 his magnificent work may be fairly called an outcome of 
 the spirit of Bessel, the master; 105,000 observations of 
 some 73,000 stars, from 23 south to 65 south of the 
 equator, have been printed by Dr. Gould as part of the re- 
 sults of fourteen years' labour in a foreign country. Thus 
 from the north to the south poles the labours of Carrington, 
 Argelander, Bessel, Gould, and Gilliss 3 have given us an 
 almost complete catalogue of accurate positions of nearly all 
 the principal stars. Besides this we shall shortly have the 
 region from 80 north to 2 south completely reobserved, 
 and by 1900 the region to 23 south will be done also. 
 
 SPECIAL CATALOGUES OF STARS 
 
 Besides these gigantic undertakings there have been 
 scores of separate catalogues pretending to greater pre- 
 cision even, the very names of which we can not mention. 
 The observatories of Greenwich, Oxford, Edinburgh, Paris, 
 Pultowa, Dorpat, Bonn, Berlin, Palermo, Washington, 
 Harvard University, Melbourne, Cape of Good Hope, and 
 many others have issued such accurate collections. 
 
 It is also necessary to say that a certain small number 
 of stars several thousands have had their positions and 
 motions determined with extreme precision; and of these 
 again, a few hundreds of the brightest stars have been 
 observed for so long, and so many times, that their result- 
 ing positions are now almost as accurate as they can be 
 made, and their motions so well known as to admit of 
 very little improvement by the work of the next genera- 
 tion. These are our fundamental stars, so called. 
 
 Such, then, are our data: a few hundred stars deter- 
 mined with the last degree of precision, a few thousand 
 nearly as well, 200,000 with considerable accuracy, and 
 "Two Germans, one Englishman, two Americans. 
 
2/0 HOLDEN 
 
 nearly half a million separate stars known by the approxi- 
 mate positions of the " Durchmusterungen," or additional 
 to these from the southern zones. We can add to these 
 too the 200,000 or more stars laid down in the ecliptic 
 charts of Paris, Vienna, and Clinton (New York), which 
 serve as nets to catch the minor planets just now, but 
 which have an incalculable value as accurate pictures of 
 the sky at a given instant. 
 
 The brightness of some 10,000 stars is very accurately 
 known, and that of nearly half a million has been very 
 approximately fixed. Lastly, the distances of some fifteen 
 of the brighter stars from the earth are known with tolerable 
 certainty, and that of a few more with a good degree of 
 approximation. 
 
 These are the materials available mighty monuments 
 to human ingenuity, skill, patience, devotion. But what 
 further problems will they solve for us? What far-reach- 
 ing conclusions can be drawn? In a succeeding article I 
 shall try to show to what results a combination of the data 
 so painfully accumulated may lead, and what conclusions 
 may safely be drawn even now. 
 
 The science of the positions and the motions of the 
 stars is not so young as that other science so well described 
 by Professor Langley in his admirable articles on " The 
 New Astronomy " (" The Century " for September, Octo- 
 ber, December, 1884, and March, 1885), but it nas its 
 modern period as well as the historical one which has been 
 here set forth. The old astronomy has set itself to solve 
 such problems as these: What is the rate at which the 
 whole solar system is moving on through space? What 
 are the distances and what are the masses of the stars? 
 What is the shape of the stellar cluster to which our sun 
 belongs? Are the stars in general broken up into subordi- 
 nate universes? or do they, as a whole, form one mighty 
 system, with one common motion? 
 
 Some of these and other such questions are answered; 
 some seem almost unanswerable; some are still in the way 
 of solution. 
 
SIDEREAL ASTRONOMY: OLD AND NEW 
 
 II. THE RESULTS THAT IT HAS ATTAINED 
 
 271 
 
 In the preceding chapter we collected the data which 
 the ancient and the modern astronomy has placed at our 
 disposition. We saw that a few hundred of the stars have 
 their positions fixed with the last degree of precision; a 
 few thousand are known nearly as well; half a million have 
 their places approximately known, and half of these last are 
 tolerably well determined. The brightness of some ten 
 thousand stars is well known, while the brightness of nearly 
 half a million is known with fair approximation. The dis- 
 tances of a few stars (about fifteen) are known with pre- 
 cision; the distances of a few more are approximately 
 known. 
 
 These are the data which have been amassed by the ob- 
 serving astronomers of the modern period, beginning with 
 Bradley (1750). In the present paper we are to see some 
 of the general conclusions which may be drawn from these 
 data. What are the distances, what are the dimensions, of 
 the stars? What is the orbit in which our sun, with its 
 group of planets, is travelling? What stars are our nearest 
 neighbours and travelling with us? Are stars in general 
 aggregated into systems of comparatively small size, or are 
 the stars as a whole collected into one vast system, bound 
 together by a common bond, and endowed with a common 
 motion? 
 
 The stellar universe, as we see it at any moment, is 
 quite complete. Change does not seem to belong to the 
 region of fixed stars. Yet every one of the millions of 
 observations has been made to fix a position so accurately 
 that the slow changes which must be going on may not 
 escape us; so that the laws of these changes can be for- 
 mulated. If we know that a star retains its position in- 
 variably, if we know positively that its brightness and 
 colour remain the same, it becomes for these very reasons 
 a most useful standard of reference, but it does not, as yet, 
 help us to solve the problem of the stellar universe. We 
 must seek a clew elsewhere, among the stars where changes 
 
272 
 
 HOLDEN 
 
 are manifest, so that the unknown laws of these changes 
 may be unfolded. 
 
 PROPER MOTIONS OF STARS 
 
 As we said, nothing appears to be more invariable or 
 unalterable than the region of the fixed stars, and, in a 
 general sense, nothing is more so. But when we come to 
 a closer view all is change there as well as elsewhere. 
 
 Since Rome was built the apparent situation of Sirius 
 has changed more than a diameter of the moon, Arcturus 
 has moved more than three such angular diameters, and 
 so with other stars. 
 
 If gravitation is truly universal, if all the stars are 
 bound together in one system by this law, as we believe, 
 then no star can move without affecting every other. As 
 one moves all must move. The real motion of any star 
 is along some line or curve; we see this real motion pro- 
 jected on the ground of the heavens as an apparent change 
 of its latitude and longitude. Knowing the latitude and 
 longitude of the star now, by observation, we may com- 
 pare these with the positions of twenty, fifty, or a hundred 
 years ago. It is possible to allow by calculation for every 
 one of the complex changes produced in the apparent posi- 
 tion of a star by every cause not in the star itself. Each 
 one of the several observations, when reduced to a com- 
 mon epoch, should give the same position, except for the 
 small and unavoidable errors of observation and the proper 
 motion of the stars. 
 
 For example, here are the observations made by Dr. 
 Gould in the last twelve years on a southern star, all re- 
 duced to what they would have been if all had been made 
 on January i, 1875: 
 
 YEAR OF OBSERVATION. 
 
 Right ascension. 
 
 South declination. 
 
 1877 . , 
 
 23^ 58 m O 8 Q2 
 
 a*7 eg' TI* o 
 
 1876 
 
 2IO 
 
 jl *-5 -V 
 
 1881 
 
 46^ 
 
 >IA T 
 
 1885... 
 
 6 60 
 
 4.2 o 
 
 
 
 
SIDEREAL ASTRONOMY: OLD AND NEW 273 
 
 These do not agree. They ought not to differ by more 
 than o?.2O or 3" 4 if the star were at rest. If we assume 
 that the star is moving in right ascension by o s -482 and 
 in declination by 2". 45 yearly, and apply these numbers, 
 the positions will harmonize. 
 
 1873 is two years before 1875, and we add twice o s -482 
 and twice 2". 45; and subtract for the other intervals. The 
 observations thus corrected give 
 
 For 1873 23 h 58 m i.88 37* 58' i8".8 
 
 1876 1.71 18.4 
 
 1881 1.74 19.4 
 
 1885 1.78 17.5 
 
 and are harmonious within the errors of observation. If 
 we assume that this star is as near to the earth as the very 
 nearest of all the stars, it is certainly moving no less than 
 600,000,000 miles per year. Yet it will require more than 
 3,000 years for it to move from its present place by so 
 much as one diameter of the moon. 
 
 The calculation that has been outlined here for one 
 star has been performed for several thousands of the better- 
 known stars, especially for the 3,222 stars which were most 
 carefully determined by Bradley in 1750. For each one of 
 these the proper motion has been determined with the 
 greatest nicety. The results at first sight are interesting 
 only in a very special way. No. i, for example, may be 
 moving 21" in a century along a path inclined by 10 to 
 the equator. No. 2 moves 44" in a century along another 
 path inclined by another angle, and so on to No. 3,222. 
 Here seem to be three thousand isolated facts, each one 
 useful in its narrow relations, but each having no connec- 
 tion with any other. 
 
 Let us suppose for a moment that the sun, with the 
 solar system, and the earth, our point of view, are moving 
 onward in space, and imagine how such a motion would 
 affect the appearance of a universe of stars scattered all 
 about us. If the sun alone has a motion, all the stars to- 
 * Errors of observation of this magnitude may exist 
 18 
 
274 HOLDEN 
 
 ward which we are moving will appear to be retreating en 
 masse from the point in the sky toward which our course 
 is directed. The nearer stars will move most rapidly; those 
 more distant, less so. 
 
 In the same way the stars from which we are retreat- 
 ing will appear to crowd together and approach each other. 
 It is as if one were riding on the rear of a railroad train 
 and watching the rails over which one had just passed. As 
 one recedes from any point the rails at that point seem to 
 come nearer and nearer together. If we were passing 
 through a forest we should see the trunks of the trees from 
 which we were going apparently moving nearer and nearer 
 to each other, while those at the sides would retain their 
 distance apart and those in front would be moving wider 
 and wider apart. 
 
 Here is a case in which we are sensible of our own 
 motion and observe the effects of that motion in the posi- 
 tions of the fixed objects about us. We may turn the ques- 
 tion about, and inquire whether the observed motions of 
 the stars indicate any real motion of our own. 
 
 The outline of the problem is here much as it pre- 
 sented itself to Sir William Herschel in 1782. The de- 
 tails are extremely complicated. It is certain that we 
 are not passing along through space among a vast number 
 of fixed stars. Each star has a motion peculiar to itself. It 
 also is moving along a vast orbit, and this real motion of 
 the star is evident to our instruments. Combined with the 
 veritable motion of the star itself is the parallactic motion 
 produced by the shifting of our own point of view as the 
 earth sweeps forward through space. 
 
 It is for analysis to separate the effects of these two 
 motions and to determine what is the real direction and the 
 real amount of the solar motion. The processes of the 
 analysis can not be given here, but fortunately it is easy 
 to exhibit both the data and the results graphically. This 
 has been well done by M. Flammarion in his edition of 
 Dien's Star-Atlas. 
 
 The circle marked " Northern Hemisphere " gives the 
 
SIDEREAL ASTRONOMY: OLD AND NEW 275 
 
 positions of those northern stars which are known to have 
 a proper motion. The size of the dot representing each 
 star gives the magnitude (i. e., brilliancy) of the star. The 
 arrows attached to the star represent the directions in which 
 the stars move on the surface of the sky by their proper 
 motions. The lengths of the arrows represent the velocities 
 with which the stars move. At the time of making the 
 map the stars are in the positions marked by the dots. At 
 the end of 50,000 years they will be at the ends of their 
 respective arrows. 
 
 Thus the data are all presented graphically. Notice 
 what variety there is. Notice, too, the striking fact that 
 some of the largest proper motions belong to some of the 
 smallest stars. One would think that the brighter stars 
 would be the nearer, and therefore that on the average they 
 would have the larger proper motions. For evidence on 
 this point I have compiled the paragraph which follows 
 from Argelander's list of the 250 stars with the best-known 
 proper motions. I have chosen the fainter magnitude 
 classes in order to get a sufficient number of stars: 
 
 " Seventy-seven stars between sixth and seventh mag- 
 nitudes have a proper motion of o".54 yearly; 80 stars be- 
 tween seventh and eighth magnitudes have a proper motion 
 of o".$6 yearly; 58 stars between eighth and ninth magni- 
 tudes have a proper motion of o".?! yearly." 
 
 That is, the proper motions do not seem to diminish 
 as the numerical magnitude diminishes. 
 
 But to return to the plate. In the constellation Her- 
 cules, not far from the bright star Vega, which is near our 
 zenith in the summer sky, is the point toward which the 
 sun is moving. In the corresponding position on the map 
 of the southern hemisphere is a similar point; it is the 
 point from which we come. All over the map are arrows 
 not attached to any stars. These show the direction and 
 the velocity of that part of the proper motion due to the 
 motion of the solar system alone. In general the arrows 
 belonging to the stars should agree in length and in direc- 
 tion with these unattached arrows and in general they 
 
276 HOLDEN 
 
 do, for the latter were derived from computations based 
 on the former. But there are many exceptional cases; 
 and, at first glance, it is the exceptions which seem to be 
 the rule. 
 
 There is no space to refer to special cases except in 
 passing; but we should note a pair of stars marked 21,258 
 (of Lalande's catalogue) and 1,830 (of Groombridge's 
 catalogue). They were about 15 apart in 1880. In 
 50,000 years they will be more than 200 diameters of the 
 moon apart, while now they are not more than 30 such an- 
 gular diameters. Proper motion alone will in time change 
 the whole aspect of the sky. 
 
 So much for the map. Mathematical analysis gives 
 the same results in numbers. It declares that the apex of 
 solar motion is in right ascension 260 and in declination 
 36 north, which defines the point in Flammarion's map 
 marked by the figure like the sun; and analysis further de- 
 clares that the amount of the solar motion in 100 years, if 
 viewed from a point at the average distance of the 3,222 
 Bradley stars, would be 5.O5. 
 
 If we know this average distance in miles, we can assign 
 our own velocity in miles. With our best present knowl- 
 edge, it follows that the sun, the earth, and the whole solar 
 system are moving through space at the rate of 
 
 586,000,000 miles per year. 
 1,600,000 " " day. 
 67,000 " " hour. 
 i8i " 4< second. 
 
 The earth moves about the sun in its own orbit at about 
 the same rate of 19 miles per second, while sun, earth, and 
 orbit move along in space another 19 miles. 
 
 We can now go back to the stars themselves, and sub- 
 tract from the observed proper motion of each star that 
 portion (motus parallacticus) which is due to the motion of 
 the solar system, and leave that portion which is due to the 
 star's own motion (motus peculiaris). 
 
SIDEREAL ASTRONOMY: OLD AND NEW 
 
 277 
 
 Is there anything common to the truly proper motions 
 of the stars? In the first place, it may be said that, so far 
 as we know up to this time, these motions are, in general, 
 not curved. They are practically straight lines. They have 
 no common centre. There is no great central body around 
 which revolve the suns of all other systems. If there be 
 such a body it will be many centuries before we shall know 
 it; and we may certainly say that, so far as our knowledge 
 goes, there is none. 
 
 SYSTEMATIC MOTIONS OF THE FIXED STARS PARALLEL TO THE 
 
 MILKY WAY 
 
 But if we are obliged to consider the motions of all the 
 stars to be practically in right lines, and not in closed orbits, 
 there is no reason why we should not examine the ques- 
 tion of whether the stars as a whole do not have some 
 systematic motion whether there is not among this variety 
 some unity. The most natural hypothesis to start with is 
 that the stars have a vast rotation in planes parallel to the 
 Milky Way. We already have good data for examining 
 this, and in a few years, when the zones of the " Astrono- 
 mische Gesellschaft " are complete, much material will be 
 added. Without some assumption of the sort, that the stars 
 rotate in planes parallel to the Milky Way, it is hardly pos- 
 sible to explain the existence of the Milky Way itself. It 
 would necessarily disintegrate more and more with the 
 lapse of time, and it would be a pure accident that we hap- 
 pen to live at an epoch when this disintegration has not 
 been accomplished. The investigation of this possible rota- 
 tion has been carried out by two pupils of Professor Gylden 
 and of Professor Schoenfeld respectively. While the result 
 in one case is fairly against the hypothesis of such a rota- 
 tion, in the other it is somewhat in its favour. The doubt in 
 the matter arises solely from the deficiency of the data, and 
 this will soon be supplied. In the mean time it should be an 
 answer to those objectors who ask what is the use of another 
 new catalogue of stars, that this catalogue, and every 
 other catalogue, goes a certain way toward providing the 
 
278 HOLDEN 
 
 means for solving the very greatest problem that can be 
 presented to the human mind by natural objects. 
 
 Look at the Milky Way stretching across the summer 
 sky with the bright star Vega burning near it. Think that 
 the few proper motions laboriously determined by Halley 
 and Maskelyne enabled Herschel to announce that the sun, 
 the earth, and every planet is moving toward a spot near 
 Vega which he could point out. Think, too, that the 
 smallest efforts of every faithful observer, the world over, 
 go to the solution of the question, How do all these thou- 
 sands of stars that I see move in space? Are they bound 
 up with that Milky Way in one fate? Or is that permanent 
 shining track, which seems unchanged since Job and the 
 patriarchs looked upon it is that doomed to destruction? 
 The finger of analysis can point out the fate of those myri- 
 ads of shining stars, and man becomes fit to live under 
 their influence when his mind adds the beauty of law to 
 the wayward beauty of their shining. 
 
 SPECTROSCOPIC PROPER MOTIONS MOTIONS IN THE LINE OF 
 
 SIGHT 
 
 The observation of a star's position is really nothing but 
 the determination of the place where the line joining eye 
 and star pierces the celestial sphere. The determination of 
 its proper motion is nothing but the determination of the 
 rate at which its apparent position changes. If a star is 
 moving directly toward us, or directly away from us, its 
 apparent place in the sky will remain unchanged. But we 
 have in the spectroscope a means of measuring the motion 
 of a star in the line of sight. The principle of the method 
 is simple. The application of it is most difficult. Every 
 one has noticed, in travelling upon an express train, the 
 sudden clang of the bell of a train passing in the contrary 
 direction; and how the note, the pitch, of the sound of 
 this bell rapidly changes from high back to low again. 
 Nothing is more certain than that the bell has but one 
 essential pitch. Why, then, does it change? The engineer 
 of the passing train hears his own bell giving always the 
 
SIDEREAL ASTRONOMY: OLD AND NEW 279 
 
 same note, and this note is determined by the length of the 
 sound waves that reach his ear. Suppose them to come at 
 the rate of about 500 per second to him. He is always 
 moving at the same rate as his bell. But to us in the other 
 train the case is different. When the bell is just opposite 
 us, 500 waves come to us per second; when we are ap- 
 proaching the passing train more than 500 come to us (not 
 only the 500 sent out by the Bell, but those others which 
 we meet by our velocity); as we leave the passing train 
 less than 500 waves overtake us per second. Hence the 
 pitch (the number of waves per second) varies. The same 
 thing happens in the case of light. In the spectrum of a 
 star there are certain dark lines the presence of which is 
 due to hydrogen in the star's atmosphere. If the star is 
 at rest with respect to us, these lines are not displaced in 
 its spectrum; a definite number of waves per second (say 
 A) come to us from the spectrum on both sides of these 
 lines. If the star is approaching us, more waves than A 
 reach us; if the star is receding, fewer waves reach us. The 
 pitch of the line, so to say, is altered; and the spectroscope 
 can measure this change of pitch. 
 
 When this is done with respect to the principal stars 
 the most interesting results follow. 
 
 Vega (a Lyrse) is found to be approaching us at the rate 
 of 10 miles per second. Castor is approaching us at 18 
 miles, Arcturus at 5 miles, etc.; while Pollux is receding 
 from us i mile per second, Aldebaran is receding 30 miles, 
 and so on. After years the aspect of our sky will change. 
 We shall have new glories in the galaxy, and after thou- 
 sands of years these again will leave us. There is ceaseless 
 change here as everywhere. 5 
 
 PARALLAXES OF THE STARS 
 
 The ancients placed all the fixed stars on the inner sur- 
 face of a vast sphere which turned about the earth's centre 
 
 8 Later observations, particularly those at Potsdam and at the Lick 
 Observatory, have given more accurate velocities than those originally 
 printed, and the later values have been inserted here. 
 
280 HOLDEN 
 
 once each day. They had absolutely no way of even guess- 
 ing how far off this sphere might be. In 1618 Kepler's 
 guess was 4,000,000 times as far as the sun; in 1698 Huy- 
 gens placed Sirius 28,000 times as far as the sun; in 1841 
 Picard showed that the errors of observation with the in- 
 struments of his time were as great as the parallaxes of 
 the stars themselves, and that therefore the problem was 
 indeterminate to him; in 1806 Delambre concluded that 
 the same thing remained true, notwithstanding the im- 
 provements of the instruments in the meanwhile. It was 
 not till 1836 that W. Struve and Bessel really determined 
 the parallax, and hence the distance, of two different stars 
 a Lyrae and 61 Cygni. 
 
 It is familiar to all that the distances of even the near- 
 est stars are not to be conceived when they are expressed 
 in miles or familiar units. No star is so near to us as 
 200,000 times 93,000,000 miles. We have to express 
 these distances in terms of the time required for light to 
 pass from star to earth. For 61 Cygni that time is 2,377 
 days, or 6J years. It was the elder Herschel who put these 
 immense distances before us in the true light, by showing 
 that if to-day the star were blotted out of existence its mild 
 light would shine on for years, until the last ray that left 
 it had finally ended its long journey and reached the earth, 
 more than six years afterward. 
 
 But all stars are not equally distant. The light from 
 one star may be 10, from another 100, from another 1,000 
 years old when it reaches us. We must no longer regard 
 the study of the stars as a study of their contemporaneous 
 existence. It is rather the ancient history of the universe 
 which is exhibited to us by the vault of heaven. Assiduous 
 observers have determined the parallaxes of about a score 
 of stars. The first stars to be examined were either the 
 brightest (as in the case of Vega), or those of large proper 
 motion (as 61 Cygni). In general, the brightest stars should 
 be the nearest, one would think, and yet the very largest 
 parallaxes belong to the fainter stars. Similarly the star 
 with the greatest proper motion has a very small parallax. 
 
SIDEREAL ASTRONOMY: OLD AND NEW 28 1 
 
 By treating all the certain data in various ways, Pro- 
 fessor Gylden has come to the conclusion that the average 
 parallax of a star of the first magnitude is about o".o84, or 
 that the average distance of our brightest star is 160,000,- 
 000,000,000,000 miles. But to make further steps in the 
 problem of the " construction of the heavens," we must 
 know more than the average parallax of the brightest stars. 
 We must be able to assign the average parallax of stars of 
 each order of magnitude, and this in both hemispheres. 
 
 This task is now undertaken for stars down to the 
 fourth magnitude by two observers who have already dis- 
 tinguished themselves in this field Dr. Gill, Royal Astron- 
 omer at the Cape of Good Hope, and Dr. Elkin, now at 
 Yale University Observatory. These gentlemen have de- 
 voted their energies to this one problem, which will require 
 perhaps ten years for its solution in the form that they have 
 chosen for it. Dr. Ball, Royal Astronomer for Ireland, is 
 systematically searching for stars of large parallax and in- 
 cidentally proving many stars to have small parallax a 
 fact which it is just as important to know as its converse. 
 
 The next dozen years will show immense strides in our 
 knowledge of the stellar distances of individual stars, and 
 it may well be that some general relation between distance, 
 brightness, and proper motion of situation in the sky will 
 result from the great increase of data. 
 
 DISTANCES OF STARS OF EACH MAGNITUDE 
 
 The golden time for astronomers will come when the 
 parallaxes of enough stars have been determined for them 
 to be able to say that the distance of an average third, 
 fourth, sixth, or tenth magnitude star is so many, or so 
 many, times the sun's distance. That time has not yet 
 come, nor will it have come even when the great work 
 begun by Messrs. Gill and Elkin has been ended. There 
 is no certain way of assigning the stellar distances but by 
 measurements such as they are making. But it is a fair 
 procedure to make certain assumptions as to stellar dis- 
 tances, to work out the logical consequences of these as- 
 
282 HOLDEN 
 
 sumptions, and to compare these consequences with known 
 facts. An agreement with the facts will, in some degree, 
 support the assumptions. If we assume the stars to be of 
 equal brilliancy one with another, we have one basis of 
 computation. If we suppose them, further, to be equally 
 distributed in space on the average, we have another basis. 
 These conditions lead at once to the following table: 
 
 Magnitudes. Relative distances. 
 
 I ... 1. 00 
 
 2 1.54 
 
 3 2.36 
 
 4 3-64 
 
 5 5-59 
 
 6 8.61 
 
 7 13-23 
 
 8 20.35 
 
 We can test these assumptions to some extent. If they 
 are true, then the ratio of the actual number of stars of 
 any brightness to the actual number of stars of the next 
 lower grade of brightness, raised to the two-thirds power, 
 should be 0.400. Using the stars of the sixth and seventh 
 magnitudes, this number results 0.426; of the seventh and 
 eighth, it results 0.4003, etc. The two hypotheses are in 
 the main not far from correct, and therefore the relative dis- 
 tances above given are not very far wrong for stars down 
 to the eighth magnitude. There is strong reason to believe 
 that the fainter stars, from eleventh to fifteenth magnitudes, 
 do not follow the same law. We have seen that the average 
 distance of a first-magnitude star is 160,000,000,000,000,- 
 ooo miles. Multiply this by 20.35 an ^ you have the best 
 estimate now available of the distance of an eighth-magni- 
 tude star. It is inconceivable, but no more so than the first 
 number. Light would require 600 years and more to reach 
 us from such stars. 
 
 DISTRIBUTION OF THE STARS OVER THE SURFACE OF THE 
 CELESTIAL SPHERE 
 
 The real question to be solved is, How are the stars dis- 
 tributed throughout solid space itself? To solve this ques- 
 tion completely the distance of every star from the earth 
 
SIDEREAL ASTRONOMY: OLD AND NEW 283 
 
 must be measured (which is a simple impossibility), or else 
 we must find some law which connects the brightness, or the 
 proper motion, or the position of a star with its distance. 
 Suppose that ten stars of each magnitude from the bright- 
 est down to the faintest are selected say 150 or 160 in all 
 and that the parallax of each individual star is determined. 
 This would be a tremendous labour in itself, and would 
 require the work of several observers for a score of years. 
 But suppose this work done. Suppose that the average 
 distances of the ten stars of each group resulting from 
 
 the measures were I, II, III, IV, V XIII, 
 
 XIV, XV, XVI. Would any general relation exist be- 
 tween the magnitudes I 16 and the correspond- 
 ing distances I XVI ? From the measures that 
 
 we already possess this is by no means sure. In fact, 
 the evidence seems to be directly opposed to this conclu- 
 sion. The average measured parallax of five first-magni- 
 tude stars is about o".27; of three fourth-magnitude stars 
 about o".i3; of three fifth-magnitude stars about o".3i; 
 of seven sixth-magnitude stars about o".2i. That is, 
 the parallax does not seem materially to decrease as the 
 brilliancy diminishes from the first to the sixth magnitude. 
 If, instead of comparing the magnitudes with the distances, 
 we compare the proper motions, there seems to be no evi- 
 dent agreement. The stars with the largest proper motions 
 do not in general have the largest parallaxes (and hence 
 the smallest distances). We have not enough determina- 
 tions of parallax to decide whether the region of the sky 
 in which a star is situated has any relation to its distance; 
 so that for the present we are not sure that a series of 
 measures even so extensive as the one we have imagined 
 would solve the question of the relation between magni- 
 tude, or proper motion, and parallax. Such a series would 
 go a great way toward deciding whether the question was 
 solvable or not. It would add enormously to the very 
 small number of certain facts bearing on the subject of the 
 constitution of the stellar system. And it is to the great 
 credit of this generation of astronomers that such a series 
 
284 HOLDEN 
 
 has actually been begun (for stars of from first to fourth 
 magnitudes) by Messrs. Gill and Elkin at the Cape of Good 
 Hope and New Haven respectively, as has been mentioned 
 already. 
 
 In the absence of definite knowledge with regard to the 
 distribution of the stars in space, much labour has been ex- 
 pended on the study of what we may call stellar statistics 
 the statistics of the distribution of the stars on the surface 
 of the celestial vault. This distribution of the stars is known 
 when once we have a map of their positions, which it is 
 comparatively easy to make. A more rapid method of 
 studying this distribution may be employed that of star 
 gauging, so called by Herschel, its inventor. This con- 
 sists essentially in counting the number of stars visible in 
 the field of the telescope as it is directed to various known 
 portions of the sky. The mere number of stars visible at 
 each pointing may be laid down on a map, like the sound- 
 ings on a hydrographic chart. The data are easily gath- 
 ered. How are they to be interpreted? We may briefly 
 indicate one obvious method. Suppose that we have made 
 such star gauges with telescopes of five different powers 
 over the same areas in the sky. The largest telescope will 
 show all the stars, say, down to and including the fifteenth 
 magnitude; the next smaller those to the fourteenth; the 
 next to the thirteenth, the twelfth, the eleventh (the actual 
 distribution of the individual stars from first to tenth mag- 
 nitudes is known by the " Durchmusterungen "). In any 
 area the difference between all the " Durchmusterung " 
 stars (from one to tenth magnitude) and the number seen 
 in telescope I (the smallest of the five supposed) will give 
 the number of the eleventh-magnitude stars in that region. 
 
 The difference between the counts by telescope I 
 (which shows all stars down to and including the eleventh 
 magnitude) and telescope II (which shows all to twelfth 
 magnitude) will give the actual number of twelfth-magni- 
 tude stars. Combining the results of the telescopes II and 
 III we should have the number of thirteenth-magnitude 
 stars for this region, and so on for the fourteenth and 
 
SIDEREAL ASTRONOMY: OLD AND NEW 285 
 
 fifteenth magnitudes. Thus the actual number of the stars 
 of each magnitude in this area (and similarly for other 
 areas) will be known. We may interpret these figures 
 somewhat in this way: Take a map which shall have 
 spaces on it for the whole sky, and devote this map to ex- 
 hibiting the results of our gauges for the fifteenth-magni- 
 tude stars. Wherever there are 100 of these to the square 
 degree lay on one tint of colour; wherever there are 200, 
 two tints; 300, three tints, and so on. The final map will 
 exhibit to the eye the results of our gauges for the fif- 
 teenth-magnitude stars. Where the tint is deep, there are 
 more stars; where it is light, fewer. Another such map 
 must be made for the fourteenth-magnitude stars; another 
 for the thirteenth, and so on. Now place these fifteen maps 
 side by side before you, and it will be possible to obtain 
 at once a number of definite conclusions. Here the stars 
 that we call fifteenth and those that we call fourteenth are 
 really connected together in space. Why? Because this 
 long ray of many fifteenth-magnitude stars on one map 
 is matched by this other long ray of just the same position 
 and shape of the fourteenth-magnitude stars. The thir- 
 teenth, too, we will say, is similar. But the ninth, tenth, 
 eleventh, and twelfth do not in their distribution at all re- 
 semble the fainter stars in this region, but they do resemble 
 each other. In this way, passing from region to region, 
 the general peculiarities of each region may be made out, 
 and much light may be thrown on the vital question, How 
 many magnitudes of stars exist at the same distance from 
 us? Are the stars of the so-called ninth, tenth, and eleventh 
 magnitudes all really at the same distance from us, and are 
 their differences in brightness simply due to differences in 
 size, or are they really at different distances? 
 
 A large amount of evidence upon these fundamental 
 points already exists, and more is being accumulated, and 
 it appears possible that a skilful use of it may throw much 
 light on the real question. The new photographic pro- 
 cesses will be of immense importance for this investigation. 
 We have not the space to go further into this method of 
 
286 HOLDEN 
 
 research, but we may just refer in passing to one interesting 
 form of it. We have already elaborate maps of certain 
 portions of the sky showing the position and magnitude of 
 every star down to the thirteenth. These are the maps 
 used for the discovery of asteroids. From each of these 
 maps we can make thirteen others, each one of which 
 shall show the stars of one magnitude only. Now compare 
 these thirteen derived maps, and see what the evidence 
 is that the stars of any two magnitudes are connected or 
 independent. This method is capable of bringing out most 
 interesting conclusions when it is thoroughly carried out, 
 as it has not yet been to any large degree. The local ar- 
 rangements of stars can be adequately studied in this way, 
 and it is not too much to expect that the typical forms 
 of stellar systems distorted by perspective, of course 
 may be exhibited here. 
 
 MASSES OF BINARY AND OTHER STARS 
 
 The binary systems are those composed of two stars 
 which are connected with each other by a mutual gravita- 
 tion. They revolve about a common centre of gravity in 
 orbits which can be calculated. In some few cases the 
 parallax of these stars is known; and in every such case the 
 sum of the masses of the two stars becomes known in 
 terms of the mass of our own sun. It is especially note- 
 worthy that in every known case the mass of the binary 
 system is not very different from the mass of our own sun. 
 That is to say, all the stars whose masses are known at all 
 are such bodies as our sun is: they shine with light like 
 his; they are of the same order of magnitude mass. 
 
 The term " hypothetical parallax " is applied to a paral- 
 lax computed for a binary star on the supposition that the 
 mass of the binary, although unknown, may be hypotheti- 
 cally assumed to be the same as the sun's mass. So far as 
 we can judge, these hypothetical parallaxes must be provi- 
 sionally accepted as essentially correct. 
 
 If we can assume that the intrinsic brilliancy of the 
 fixed stars is the same for each star, which does not seem 
 
SIDEREAL ASTRONOMY: OLD AND NEW 287 
 
 to be a very violent supposition, several interesting con- 
 clusions follow which can only be stated here. 
 
 If it be true that for the stars, taken one with another, 
 a square mile of surface shines with an equal light for each 
 star, then among stars of known distances some must be 
 at least 270 times as great in diameter as others. This is 
 about the proportion of the sun to Mercury. Also it fol- 
 lows that binary stars whose colours are alike must be com- 
 posed of stars of like size; and also, that on the average 
 the brightest star of any cluster is about four times as large 
 as the smallest star of the cluster. No star is more than 
 200,000 times farther than the nearest fixed star. Other 
 assumptions which might serve as a basis for computation 
 will give other results; but for the present we have to 
 content ourselves with some such assumption, and in the 
 infinite variety of circumstances choose that one as general 
 which seems to be the most likely a priori, and which leads 
 to results which agree with actual observation. 
 
 THE CLUSTER OF STARS TO WHICH OUR SUN BELONGS 
 
 The " Uranometria Nova " of Argelander gave the 
 positions of the lucid stars of the northern sky, and it has 
 been supplemented by the " Uranometria Argentina " of 
 Dr. Gould, which covers the southern sky. With the stellar 
 statistics of the whole sky before him Dr. Gould was in 
 a position to draw some extremely interesting conclusions 
 with respect to the arrangement of the brighter stars in 
 space, and to the situation of our solar system in relation 
 to them. The outline of his reasoning can be given here, 
 but the numerical evidence upon which his conclusions are 
 founded must be omitted. In the first place, it is assumed 
 that in general the stars that are visible to the naked eye 
 (the lucid stars) are distributed at approximately equal 
 distances one from another, and that on the average they 
 are of approximately equal brilliancy. If we make a table 
 of the number of stars of each separate magnitude in the 
 whole sky we shall find that there are proportionately many 
 more of the brighter ones (from first to fourth magnitudes) 
 
288 HOLDEN 
 
 than of the fainter (from fourth to seventh magnitudes) 
 that is, there is an " unfailing and systematic excess of the 
 observed number of the brighter stars." We can not sup- 
 pose, taking one star with another, that the difference be- 
 tween their apparent brightness arises simply from real dif- 
 ference in size, but we must conclude that the stars from 
 the first to fourth magnitudes (some 500) are really nearer 
 to us than the fainter stars. It therefore follows that these 
 brighter stars form a system whose separation from that 
 of those of the fainter stars is marked by the change of 
 relative numerical frequency. 
 
 What, then, is the shape of this system? and have we 
 any independent proof of its existence? Sir John Herschel 
 and Dr. Gould have pointed out that there is in the sky a 
 belt of brighter stars which is very nearly a great circle 
 of the sphere. This belt is plainly marked, and it is inclined 
 about 80 to the Milky Way, which it crosses near Cas- 
 siopea and the Southern Cross. Taking all the stars down 
 to 4.0 magnitude, Dr. Gould shows that they are more 
 symmetrically arranged with reference to this belt than 
 they are with reference to the Milky Way. In fact, the 
 belt has 264 stars on one side of it and 263 on the other, 
 while the corresponding numbers for the Milky Way are 
 245 and 282. From this and other reasons it is concluded 
 that this belt contains brighter stars because it contains 
 the nearest stars, and that this set of nearer and brighter 
 stars is distinctively the cluster to which our sun belongs. 
 Leaving out the brighter stars which may be accidentally 
 projected among the true stars belonging to this cluster, 
 Dr. Gould concludes that our sun belongs to a cluster of 
 about 400 stars; that it lies in the principal plane of the 
 cluster (since the belt of bright stars is a great, not a 
 small circle); and that this solar cluster is independent of 
 the vast congeries of stars which we call the Milky Way. 
 
 We know that the sun is moving in space. It becomes 
 a question whether this motion is one common to the solar 
 cluster and to the sun, or only the motion of the sun in 
 the solar cluster. The motion has been determined on the 
 
SIDEREAL ASTRONOMY: OLD AND NEW 289 
 
 supposition that the sun is moving and that its motion is 
 not systematically shared by the stars which Dr. Gould 
 assigns to the solar cluster. But a very important research 
 will be to investigate the solar motion without employing 
 these 400 stars as data. 
 
 In what has gone before I have tried to exhibit some 
 of the main questions in purely sidereal astronomy; to show 
 some of the more important results already reached, and 
 especially to indicate the directions along which present 
 researches are tending. It is impossible to give a complete 
 view in this or in any other single branch of astronomy, for 
 they are all indissolubly bound together. 
 
 The methods of the new astronomy have taught us that 
 in the condition of the variable stars, where the intense 
 glow has cooled to a red heat, we can see the future of 
 our own sun as well as its past in the brilliant white and 
 violet of the brightest and youngest stars. It requires the 
 profound mathematical analysis of Gylden to interpret his 
 equations so as to explain to the new astronomy exactly 
 how the phenomena of the rotation of variable stars pro- 
 duce the effects which are observed by its methods. 
 
 Professor Langley measures the light and heat of the 
 moon by the new methods; Professor Darwin interprets 
 the mathematical theory of the tides so as to trace back 
 the origin of that heat to the remote time when the earth 
 and moon formed one mass, and rotated in less than an 
 eighth part of our present day. All the parts of the com- 
 plex science are intimately connected, and no one can be 
 separately treated without losing sight of many lines of 
 research of the greatest promise and importance. 
 
 But I hope that enough has been said to show that the 
 old astronomy is not idle; that it has its new side; and that 
 its energies are addressed to the solution of problems of the 
 highest significance. In broad terms, it is the noble aim of 
 the new astronomy to trace the life-history of an individual 
 star, and of the old to show how all these single stars are 
 bound together to make a universe. There is no antago- 
 nism in their objects. Each is incomplete without the other. 
 
PHOTOGRAPHY THE SERVANT OF 
 ASTRONOMY J 
 
 IN order to appreciate the present state of astronomy, its 
 new methods, its novel instruments, its recondite prob- 
 lems, it is necessary to glance at its condition a half cen- 
 tury ago. The great astronomers, Bessel and W. Struve, 
 were then contending in friendly rivalry to found the sci- 
 ence on a sure basis. They had a perfectly definite object, 
 and that object has been attained through their efforts, and 
 through the efforts of the school of young men whom they 
 trained either directly or indirectly Argelander, Schoen- 
 feld, Krueger, Auwers, Winnecke, Wagner, Schiaparelli in 
 Europe, Walker, Coffin, Hubbard, Gould in America. 
 
 The attention of astronomers was then almost exclu- 
 sively directed to the question of the motions of the 
 heavenly bodies, as determined by the law of universal 
 gravitation. The vast catalogues of stars which have been 
 made in the past half century, as well as the accurate dis- 
 cussion and rediscussion of the older observations of 
 Bradley (1750), at Greenwich, were all undertaken for this 
 sole object. The school of mathematical astronomers 
 founded by Euler, Laplace, Lagrange, Gauss, utilized these 
 observations to the utmost. The examination of the sur- 
 faces of the planets was an entirely secondary question, 
 and was largely left to amateur astronomers. The surface 
 of the sun was studied only in the crudest manner, simply 
 for the enumeration of the solar spots. 
 
 The fact that these spots were periodic was only estab- 
 lished in 1851. Sir John Herschel was almost the only 
 
 1 From the " Overland Monthly," November, 1886. 
 290 
 
PHOTOGRAPHY THE SERVANT OF ASTRONOMY 
 
 astronomer by profession who devoted himself to observa- 
 tions not " of precision." 
 
 In this fifty years, an entirely new science has arisen 
 astrophysics which is, indeed, the daughter of astronomy, 
 but the cousin-german of chemistry, technics, physics. 
 
 This new science always had its cultivators, even before 
 it had a name. The elder Herschel set himself the problem 
 " to find out the construction of the heavens/' and this is 
 the problem of astrophysics, in contradistinction to the 
 problem of exact astronomy " to find out how the heav- 
 enly bodies move." The modern form of Herschel's phrase 
 is, " to determine the present constitution and the evolu- 
 tion history of the stars, the comets, the sun, the planets." 
 
 We must regard Sir William Herschel as the founder of 
 the science. He has had great followers: Schroeter, Sir 
 John Herschel, Beer, Maedler, Fraunhofer, Kirchhoff, 
 Bunsen, Lassell, Bond, De la Rue, Rutherfurd, Draper, 
 Schiaparelli, Vogel, Janssen, Lockyer, Young, Langley, 
 Pickering, not to speak of a host of other familiar names. 
 
 To-day there are several observatories devoted exclu- 
 sively to the new science and their number is growing. 
 This should be so. There are too many astronomical ob- 
 servatories idle. If the charm of the new fields is enough 
 to make them efficient in forwarding the science as a whole, 
 we must welcome the new impetus. But there is a note 
 of warning to which we must give attention. We must 
 keep strictly before us the means by which the older astron- 
 omy has arrived at its proud position as the chief of the 
 physical sciences. For hundreds, yes, thousands of years, 
 one principle has run through all of astronomy. Assidu- 
 ous observations must be made according to well-consid- 
 ered plans, matured after deep reflection. The results of 
 these observations must be compared with a theory ex- 
 pressed rigorously in the terms of mathematics. The dif- 
 ferences between observation and theory must be treated 
 by a profound analysis, so to derive corrections to the pro- 
 visional theory. This provisional theory will in its turn 
 become the basis of comparison with Nature, and so on, 
 
292 HOLDEN 
 
 until the ideal is reached by successive approximations. 
 This ideal is simple, and in many researches it has been 
 attained already. It is to push the successive approxima- 
 tions until we can predict the position or the motion of a 
 heavenly body as accurately as we can observe it. When 
 this stage is reached we may leave the special problem in 
 hand, until the methods of observation are themselves im- 
 proved. 
 
 If astrophysics will accept this ideal and strive for it, 
 there is no future so brilliant that we may not claim it for 
 her portion. If this straight and narrow way is departed 
 from, although the new science is followed never so assidu- 
 ously, no essential progress can be expected, and real harm 
 is sure to follow. 
 
 Astrophysics has three well-marked lines of research 
 namely: spectrum analysis (now a quarter of a century old), 
 celestial photometry (half a century), celestial photography 
 (dating back exactly forty-six years). Schiaparelli's theory 
 of meteor-streams and their connection with comets be- 
 longs to this science in so far as it throws light upon the 
 material out of which comets are built; and every part of 
 physics which treats of the action of one body upon an- 
 other body at a distance, whether through gravitation, 
 heat, magnetism, electricity, has close relations to it. But 
 the three main paths are spectroscopy, celestial photometry, 
 and celestial photography. It is of the latter path that I 
 wish to speak in this paper. We shall follow it assiduously 
 at the Lick Observatory, and we shall have unrivalled op- 
 portunities to do so. 
 
 Spectroscopy in certain of its lines we shall also follow, 
 and our opportunities in this branch also are unique. 
 Photometry is so thoroughly done at the Harvard College 
 Observatory that it would be a waste of energy for another 
 American observatory to devote any great part of its time 
 to such researches. 
 
 I assume that some slight explanation of the differences 
 between a photographic telescope and an ordinary one will 
 not be superfluous. The object-glass of an ordinary tele- 
 
PHOTOGRAPHY THE SERVANT OF ASTRONOMY 293 
 
 scope brings the rays by which we see (those having a wave- 
 length of about 6,000 ten-millionths of a millimetre) to 
 an accurate focus. These can not be photographed except 
 by special plates and with special difficulty. The rays which 
 affect the photographic salts of silver have a wave-length 
 of about 4,000 ten-millionths of a millimetre, and to bring 
 these to a focus the two lenses of the ordinary achromatic 
 object-glass must be supplemented by a third lens. This 
 third lens is so arranged that it can be placed in front of 
 (and close against) the ordinary objective, and it turns the 
 telescope from a seeing instrument into a camera. It is 
 also necessary to say that if the telescope remains fixed, 
 while a bright star is passing across its field of view, the 
 image of the star will pass across the sensitive plate, and 
 will leave a " trail " which is the visible representative of 
 the direction of the star's diurnal motion. Equatorial stars 
 as faint as the eighth or ninth magnitude will give trails. 
 
 If, on the contrary, we attach an accurate driving clock 
 to the telescope, and cause it to follow the star in its 
 motion from east to west, rising to setting, we shall have 
 instead of a trail a bright point, the photographic image. 
 If we wish to make a picture of the sky, we must register 
 the stars by such points as these. The trails have, however, 
 various advantages, one of which is that they can not be 
 mistaken for dust or for pin holes on the plate itself. The 
 position of the dots in latitude and longitude can be very 
 accurately measured. The latitude of the star can be even 
 better determined from its trail, but its longitude must then 
 be determined by special devices, which I need not de- 
 scribe. In the ordinary methods of observing, the astron- 
 omer views the visual images of the heavenly bodies, and 
 either examines their surfaces, or determines their position 
 with reference to adjacent bodies (as, for example, the posi- 
 tions of satellites relative to their planet), by means of ex- 
 tremely accurate and refined micrometers, forming a part 
 of the eyepiece of his telescope. 
 
 To utilize photographic plates fully, and especially to 
 make them a substitute for micrometric measures, it is 
 
2 9 4 
 
 HOLDEN 
 
 necessary to contrive elaborate measuring engines to take 
 the place of the costly micrometers, ordinarily used with 
 telescopes. These engines measure the positions of the 
 dots or trails on the plates, after these have been removed 
 from the telescope. 
 
 Mr. Rutherfurd first made a satisfactory engine of this 
 kind; it was then improved upon in the design of Pro- 
 fessor Harkness adopted by the United States Transit of 
 Venus Commission, in 1874, and the Lick Observatory 
 owns the finest specimen of this class, which was made for 
 it under the supervision of Professor Harkness. 
 
 The very first essay in astronomical photography was 
 that of Professor John William Draper, of New York, who, 
 in the year 1840, took a satisfactory daguerreotype of the 
 moon. The experiments of Dr. Draper were repeated by 
 George Bond, Director of the Harvard College Observa- 
 tory, in 1850, and a lunar daguerreotype made by him was 
 exhibited at London in 1851, at the World's Fair, where 
 it attracted much attention. 
 
 During the years 1853 to 1857, Mr. De la Rue, of 
 London, made lunar daguerreotypes and photographs, 
 some of great excellence. In 1864 Dr. Lewis Rutherfurd, 
 of New York, made an eleven-and-a-half-inch objective, 
 which was corrected only for the photographic rays, and by 
 means of this he obtained the finest photographs of the 
 moon which had yet been made. Dr. Henry Draper, 
 about the same time, made a fifteen-inch reflecting tele- 
 scope with which he also took excellent lunar photographs. 
 These latter have been enlarged to three and even to four 
 feet in diameter, from the original picture of about two 
 inches and a half. A long-focus telescope is of great ad- 
 vantage in these researches. The pictures in the principal 
 focus of the Melbourne reflector are some six inches in 
 diameter, and I have seen a few of these of great excellence. 
 Such pictures can be enlarged, in printing, from six to 
 twelve times. 
 
 The photographs of the moon in the focus of the Lick 
 equatorial will be six inches in diameter, and will probably 
 
PHOTOGRAPHY THE SERVANT OF ASTRONOMY 
 
 295 
 
 stand an enlargement of twelve times, so as to be six feet 
 finally. 
 
 Lunar photographs have not advanced our knowledge 
 in any important degree up to this time, however. 
 
 Solar daguerreotypes were first taken by Foucault and 
 Fizeau, in 1845, at Paris, on the advice of Arago. In 1857 
 Mr. De la Rue contrived the photoheliograph for the Kew 
 Observatory, by which solar photographs have been taken 
 since that time daily at Kew and Greenwich. 
 
 M. Janssen, of Mendon, near Paris, about 1878, succeed- 
 ed in making his exquisite photographs of the sun on glass, 
 which show an astonishing amount of detail. I understand 
 that these are chiefly made by means of a six-inch refractor, 
 and I have never been able to comprehend how so much de- 
 tail can be shown w r ith an objective of such a small separat- 
 ing power, nor to rid myself of an impression that some, at 
 least, of these details are due to atmospheric disturbances. 
 
 If the exposures are made extremely short (^-jrVir to 
 Tinnr f a second), very successful results can be obtained 
 in solar photography. There is, undoubtedly, an important 
 field of research still open here, especially with large ob- 
 jectives of great separating power. 
 
 The first photographs of a solar eclipse were made by 
 Busch, at Konigsberg, in 1851, and by Bartlett at West 
 Point, in 1854. The eclipse photographs of Secchi and 
 De la Rue, in 1860, were of high scientific importance, 
 since they established beyond doubt the fact that the solar 
 protuberances were really appendages of the sun, and not 
 of the moon. 
 
 I believe the first photograph of the spectrum of the 
 sun at a solar eclipse was taken at the Egyptian eclipse of 
 1882, by Professor Schuster, and also by the party under 
 Mr. Lockyer. Very perfect photographs of the solar spec- 
 trum were taken at the total eclipse of 1883 in the Pacific 
 Ocean, by the English parties and by the French parties, 
 and the subject does not now present any great difficulties. 
 
 Photography served a very useful purpose in its appli- 
 cation to the transits of Venus of 1874 and 1882. 
 
296 
 
 HOLDEN 
 
 According to Professor Pickering 1 , the first daguerreo- 
 type of a star was taken at Harvard College Observatory, 
 on July 17, 1850, under the direction of the elder Bond. 
 The star Vega was satisfactorily daguerreotyped, and later 
 the double star Castor gave an elongated image, which was 
 plainly due to its two components. The sensitiveness of 
 the daguerreotype plates then in use was so small that even 
 such bright stars as these gave faint images, and no im- 
 pression whatever was obtained from the Pole Star, no mat- 
 ter how long the exposure. These experiments were re- 
 peated with various stars and clusters, but finally the work 
 was abandoned on account of photographic difficulties. In 
 1857 the younger Bond resumed the research. At this 
 time the collodion process had greatly reduced the time of 
 exposure, and the plates were of much greater sensitive- 
 ness. An impression of the double star Zeta Ursse Majoris 
 was obtained in eight seconds. A trail was obtained from 
 the image of the bright star Vega. The faintest star photo- 
 graphed was the companion of Epsilon Lyrae, which is of 
 the sixth magnitude that is, just visible to the naked eye. 
 
 A series of measures was made of the relative positions 
 and distances of the various double stars photographed, in 
 order to see whether measures made upon a photographic 
 plate could be used to replace those made in the ordinary 
 manner at the telescope. It was found that a single measure 
 made upon the plate was about of the same value as a 
 single measure made by an astronomer with the ordinary 
 micrometer. Professor Bond pointed out very clearly how 
 photographic images might be used to determine accu- 
 rately the relative brightness of stars, and also what the ad- 
 vantages of photography were for the permanent registra- 
 tion of star positions. Mr. De la Rue and Dr. Rutherfurd 
 soon after repeated these experiments of Professor Bond, 
 and a very extended investigation was undertaken in 1864 
 by Dr. Rutherfurd, and continued by him for many years. 
 Most of the principal clusters in the northern heavens were 
 photographed, as well as most of the brighter double stars. 
 These researches have never been fully utilized for the 
 
PHOTOGRAPHY THE SERVANT OF ASTRONOMY 297 
 
 following reason: the photographs were measured in the 
 most careful manner on a measuring engine, in which the 
 distances of one star from another were determined by 
 means of a very accurate screw. After the series of meas- 
 ures had been continued for several years, it was discovered 
 that the screw itself had worn considerably, so that the 
 value of its revolutions was not the same as it had for- 
 merly been. It was impossible to discover at what time 
 this wear commenced, or how it progressed, and therefore 
 these excellent photographs have remained undiscussed up 
 to the present time. 2 The distances, which must be accu- 
 rately measured, are about so } 00 of an inch. The faintest 
 stars shown in Dr. Rutherfurd's eleven-inch telescope are 
 about of the ninth magnitude. The plates used by Dr. 
 Rutherfurd were, I believe, exclusively wet plates. 
 
 Dr. Henry Draper attacked the same problem in 1880, 
 using, however, the most sensitive dry plates then available. 
 In 1 88 1, with an eleven-inch refractor constructed by the 
 Clarks, he obtained a photograph of the nebula in Orion, 
 in which one of the stars is shown whose magnitude is 
 not more than 14^. This star is barely visible with a tele- 
 scope of the same aperture as that with which the photo- 
 graph was taken. The photographic plate now had be- 
 come as efficient an instrument of research as the eye itself. 
 M. Janssen also photographed the nebula in Orion in 
 1 88 1, but the best of all such photographs has been made 
 by Mr. Common, of England, with his three-foot silver-on- 
 glass reflector. 
 
 Dr. B. A. Gould, in his expedition to the southern 
 hemisphere (i87O-'84), carried with him a photographic 
 lens of eleven inches aperture, and during his entire stay 
 of more than ten years employed all the available time 
 at his command in accumulating negatives of the principal 
 southern double stars and clusters. These photographs 
 have not yet been discussed, and Dr. Gould has discovered 
 that there are signs that the films on the negatives are now 
 
 1 They are now (1900) being remeasured and rediscussed at Colum- 
 bia University. 
 
298 HOLDEN 
 
 beginning to deteriorate. Probably this extensive and im- 
 portant series will soon receive discussion. 3 
 
 During the years i882-'86 many observatories have 
 undertaken some researches in stellar photography. The 
 Royal Astronomer at the Cape of Good Hope, Dr. Gill, 
 has undertaken to make a map of the whole southern 
 heavens by photographic means only. The Rev. T. E. 
 Espin, of Liverpool, has published a catalogue of the mag- 
 nitudes of 500 stars, determined by means of photography 
 alone. The most extensive investigation is that of the 
 brothers Paul and Prosper Henry, of the Observatory of 
 Paris. Important investigations have also been made at 
 the Astrophysical Observatory of Potsdam and at two 
 Physical observatories in Hungary. 
 
 In 1863 Dr. Huggins, of London, obtained a photo- 
 graphic image of the spectrum of Sirius, but no lines were 
 visible in this spectrum. The first successful photograph of 
 the spectrum of a star was obtained by Dr. Henry Draper in 
 1872. Both of these astronomers succeeded in 1876 in 
 obtaining valuable spectrum photographs of the brightest 
 stars. In 1882 they both obtained a photograph of the 
 spectrum of the nebula in Orion. Since 1882 many astron- 
 omers and observatories have devoted themselves to pho- 
 tographic researches, but little has been published, except 
 by the Observatory of Harvard College. Here the years 
 1882-^85 were spent in very elaborate experiments, prelimi- 
 nary to undertaking larger and more important researches. 
 A very large number of photographs have been taken of 
 the regions lying about the north celestial pole. The pho- 
 tographic telescope employed is eight inches in aperture. 
 The chief results up to now have been the establishing the 
 relative brightness of 117 stars within one degree of the 
 pole. A second research in progress is the determination 
 of the relative brilliancy of all the brighter stars. Other 
 experiments are in hand, but, as they all relate to pho- 
 tometry or to spectroscopy, they may be passed over here, 
 
 8 This work has lately been published under the direction of Dr. 
 S. C. Chandler. 
 
PHOTOGRAPHY THE SERVANT OF ASTRONOMY 299 
 
 after merely calling attention to them as the most impor- 
 tant researches of the kind. 
 
 In 1882 Dr. Gill, at the Cape of Good Hope, succeeded 
 in photographing the great comet of that year, and in 
 doing this he proved the practicable possibility of making 
 star rnaps, which should contain all the stars down to the 
 tenth magnitude. In 1885 the Royal Society granted 300 
 to the Cape of Good Hope Observatory for photographic 
 purposes. Dr. Gill has set himself to the solution of two 
 problems: First, that of securing as soon as possible a 
 complete photographic map of the southern heavens, con- 
 taining every star visible down to the tenth magnitude, 
 so as to continue the " Durchmusterung " of Argelander; 
 secondly, to test the possibility of photographing the solar 
 corona daily without an eclipse, by the method first sug- 
 gested by Dr. Huggins. For the first purpose Dr. Gill 
 makes use of one of Dallmeyer's rapid rectilinear combina- 
 tions, composed of two concavo-convex achromatic com- 
 binations of six inches aperture. This camera is mounted 
 on an equatorial stand, and is pointed by means of a tele- 
 scope of forty-five inches focal length and three and a 
 half inches aperture. The exposures are an hour long 
 when the sky is clear. Each plate is six inches square, and 
 covers an area of about thirty-six degrees. Every such 
 area is photographed twice, so as to render it impossible to 
 confound the images of faint stars with minute dust specks. 
 In this way a great portion of the sky has already been 
 photographed in duplicate. 
 
 The same observatory has recently obtained a much 
 more powerful optical apparatus through the generosity of 
 Mr. James Nasmyth, who has purchased a specially cor- 
 rected photographic objective of nine inches aperture and 
 nine feet focal length, made by Mr. Grubb, of Dublin. The 
 field of this Nasmyth lens will be much more limited than 
 that of the Dallmeyer apparatus, but it is expected to ob- 
 tain from it a photograph of all stars to the twelfth or thir- 
 teenth magnitude inclusive within a circle of a radius of one 
 or one and a half degree. 
 
HOLDEN 
 
 Mr. Roberts, in England, has erected a reflector of 
 twenty inches aperture and of one hundred inches focus 
 for stellar photography alone, and has made considerable 
 progress in the work of charting the northern heavens. 
 The size of the field of Mr. Roberts's telescope is two de- 
 grees in declination and one and a half degree in right 
 ascension. The time of exposure is fifteen minutes in a 
 clear sky. The companion to the Pole Star is just visible 
 in four seconds under the best circumstances. Mr. Roberts 
 refers to an important difficulty, which is, that in most 
 photographic plates there are small specks in the film, 
 many of which look like stars, and which are extremely 
 difficult to distinguish from stars even when they are 
 viewed through a microscope. Dr. Gill, at the Cape of 
 Good Hope, avoids this difficulty by taking two photo- 
 graphs of the same field successively, giving to each an 
 exposure of one hour. At Paris three exposures of an 
 hour each are made, on the same plate. 
 
 Mr. Common's experiments commenced in 1879. At 
 this time, using dry plates with his three-foot reflector, he 
 took successful pictures of the Pleiades, with one and a 
 half minute exposure, showing all the stars to the eighth 
 and ninth magnitude. In 1882 he devoted his time to pho- 
 tographing the nebula in Orion, and has obtained won- 
 derful results. 
 
 After making such a splendid success with his three- 
 foot reflector, Mr. Common is now making one of five feet 
 in aperture. There is no doubt that a mirror of this aper- 
 ture can be accurately figured by the optician. The dif- 
 ficulties in using it come from unequal flexure of its various 
 parts and from their differing temperatures. Difficulties 
 of this nature have never yet been successfully overcome 
 for reflectors of more than thirty-six inches of aperture, but 
 Mr. Common's great mechanical skill and knowledge and 
 experience lead us to hope that he may succeed in this 
 important undertaking. 
 
 In September, 1884, Dr. Lohse used the eleven-inch 
 refractor of the Potsdam Observatory to photograph the 
 
PHOTOGRAPHY THE SERVANT OF ASTRONOMY 301 
 
 star cluster in Perseus. An exposure of forty-five minutes 
 was given, and stars as faint as the tenth and eleventh mag- 
 nitude were registered. 
 
 A number of other star clusters have also been photo- 
 graphed by Dr. Lohse. The Savilian Observatory at Ox- 
 ford (England) has undertaken to study two constellations 
 (Lyra and Cassiopeia) by photography on plates one de- 
 gree square. 
 
 The early experiments at the Paris Observatory, 1884, 
 were made with a telescope with an aperture of -^^ of a 
 metre (6.3 inches), and they were so successful that it was 
 decided to make a larger instrument specially for photog- 
 raphy, and soon an objective of -fl^ of a metre aperture 
 (13.4 inches) and 3 -^metres focal length (134 inches) was 
 made. Parallel to this photographic telescope, one of about 
 the same focus and of -ffo of a metre (9.5 inches) aperture 
 is placed as a directing telescope. In May, 1885, the new 
 photographic telescope was first brought into use, and a 
 few of the important results that have been reached by it 
 are mentioned below. Stars down to the fifteenth mag- 
 nitude are photographed with an exposure of one hour, 
 the plates being something more than two degrees square. 
 From one to two thousand stars are shown to each square 
 degree with this exposure, using dry plates. On these 
 plates three separate exposures of an hour each are given, 
 the instrument being moved between each exposure, so 
 as to change the position of the image on the plate about 
 five seconds of arc each time. The three images of the 
 same star thus form a little triangle. By means of this tele- 
 scope a new and very faint nebula has been discovered in 
 the Pleiades, which would never have been discovered if 
 we depended on the eye alone. Admirable photographs 
 of Saturn have been taken by direct enlargement of the 
 primary image, through a non-achromatic eyepiece, which 
 gives a magnifying power of eleven times. Hyperion, the 
 faintest satellite of Saturn, a difficult object in the twenty- 
 six-inch telescope at Washington, has been photographed 
 with an exposure of thirty minutes, and the satellite of 
 
302 
 
 HOLDEN 
 
 Neptune can be taken in any part of its orbit. With an 
 exposure of one hour the eleventh- and fifteenth-magni- 
 tude stars have an actual diameter of about 16 * 00 of an 
 inch that is, in arc about one and a half second. Stars 
 of the fifth or sixth magnitude are about one minute in 
 diameter with long exposures. With a properly limited 
 exposure, these also are of extremely minute dimen- 
 sions. 
 
 The proper exposure for a first-magnitude star, like 
 Sirius or Vega, is not more than y^ of a second. For 
 a star just visible to the naked eye, half a second is suf- 
 ficient. For stars of the tenth magnitude, twenty sec- 
 onds; of the twelfth, two minutes; of the thirteenth, five 
 minutes; of the fourteenth, thirteen minutes; and for the 
 faintest visible, an hour and twenty-three minutes. These 
 results are, of course, a minimum, and but approximate. 
 
 So far as is known, the growth of the image of a star 
 upon the photographic plate is equal, and concentric with 
 the point of the plate, on which the image of the star falls. 
 The faintest stars on these Paris plates are, as was said, 
 arranged in little groups of three. As the brighter stars on 
 these plates are examined, it is found that the size of each 
 of the images increases until they overlap; this continues 
 until the complete images of any one bright star are very 
 much larger than the original small triangle. The appear- 
 ance is as if three circles nearly as large as the resulting 
 image had been struck from the centre of what would have 
 been (in the case of a smaller star) the three separate stars 
 of a group. It is the intention of the director of the Ob- 
 servatory of Paris to use this photographic telescope to 
 continue the construction of ecliptic charts. And it is 
 suggested by the director that by means of the co-opera- 
 tion of six or eight observatories the whole heavens should 
 be charted in a similar way. There are 41,000 square de- 
 grees in the whole heavens, and if six square degrees can 
 be registered on a plate (with one hour's exposure), 7,000 
 such plates must be made, requiring at least 7,000 hours. 
 To avoid mistakes, at least two exposures must be given 
 
PHOTOGRAPHY THE SERVANT OF ASTRONOMY 303 
 
 for each region, or 14,000 plates and 14,000 hours are 
 necessary. 
 
 ,i. $ If we allow loo clear nights in a year (which is a fair 
 allowance for all observatories except the Lick Observa- 
 tory, where we can count on at least 200), it would require 
 140 years at any one observatory to do this work, or 14 
 at ten observatories. I personally doubt whether the strict 
 adherence to a plan, which is indispensable to success, 
 could be maintained at so many establishments for so long 
 a period. The whole subject is yet in too unsettled a state 
 to warrant an international undertaking of such magnitude 
 at present. A number of years must be spent in tentative 
 researches before the right paths are struck out. I give 
 some of the most obvious directions for these trials in what 
 follows. 
 
 The two hundred and sixty or more small planets 
 (asteroids) which lie between Mars and Jupiter have all 
 been discovered by the slow process of comparing a star 
 map, night after night, with the heavens. A star not on 
 the map is either an omitted star to be inserted, or a minor 
 planet, known or unknown. A photographic objective of 
 twelve inches aperture will show a trail for a star of the 
 magnitude of the brighter asteroids with an exposure of 
 half an hour. An hour's exposure will probably show the 
 trail of the faintest asteroids (twelve to thirteen magnitude). 
 One of the immediate results of the application of photog- 
 raphy will undoubtedly be to greatly increase the number 
 of known asteroids. 
 
 There are reasons to believe in the existence of a major 
 planet exterior to Neptune. If such a planet exists it is 
 not likely to be brighter than the tenth magnitude, and its 
 motion will be very slow. Hence it is unlikely, at least, 
 that such a planet can be discovered by its trail on the plate. 
 The method of three exposures on the same plate employed 
 at Paris probably would not disclose the existence of a 
 trans-Neptunian planet, though it would suffice for the de- 
 tection of Neptune itself in most parts of its orbit. Prob- 
 ably the surest way to detect such a body, if it exists, 
 
304 
 
 HOLDEN 
 
 would be to take photographs of the same region on suc- 
 cessive days. Such plates would then have to be laboriously 
 compared, star by star. Doubtful cases would require a 
 third night's work to be done in order to decide. 
 
 A blue-print of two such plates will enable all the 
 brighter stars to be quickly compared and disposed of. 
 The real labour will then be confined to the stars less 
 bright than the faintest which can be blue-printed. The 
 problem of the constitution of the stellar universe must be 
 studied, it seems, by some kind of celestial statistics de- 
 rived from counts or gauges of the stars. Nearly all the 
 conclusions we have so far reached are based on the counts 
 made by Sir William Herschel. I have myself spent much 
 time in continuing these. All such work is now useless. 
 Photographic maps will give us all the requisite data, and 
 will throw much light, too, on another closely connected 
 problem the extinction of light in space provided only 
 that all negatives taken for this object are made strictly 
 comparable in every respect. This proviso is of the utmost 
 importance, and very difficult to be lived up to in any 
 work done by co-operating observatories. It is just pos- 
 sible that photometric measures of the photographs of a 
 very eccentric asteroid can now be made with sufficient 
 delicacy to settle the question whether light is, or is not, 
 extinguished in space. 
 
 The precision of the photographic images of stars is so 
 great that there is no doubt that measures of the negatives 
 of double stars, of star clusters and groups, will, at least in 
 most instances, take the place of the painful and laborious 
 micrometric measures which are now employed by ob- 
 servers. The photographs have their own errors, and 
 many of them; but these are all susceptible of investi- 
 gation. 
 
 The shrinkage of the gelatine films of the negatives is 
 likely to prove a grave difficulty in the application of pho- 
 tography to exact astronomy, but this can always be de- 
 tected by photographing a network of lines on glass. Very 
 serious difficulties of this kind have lately been met with 
 
PHOTOGRAPHY THE SERVANT OF ASTRONOMY 305 
 
 by Professor Pritchard, of Oxford, in his researches on the 
 (photographic) parallax of 61 Cygni. 
 
 But photographic plates have also many capital advan- 
 tages. For example, the photographic impress of a star 
 gives really its mean or average position, freed from those 
 accidental and transitory variations of place which are due 
 to variations of atmospheric refraction a constant source 
 of error. The saving of time is also important. 
 
 An exposure of an hour has given (at the Paris Observa- 
 tory) a map of 5,000 stars in four square degrees in the 
 constellation Cygnus. The best maps we now have give 
 170 of the brightest stars only in this place. To map 5,000 
 stars by the eye alone would require several years. The 
 writer spent all the time he could spare from routine ob- 
 servations during four years with the twenty-six-inch equa- 
 torial, at Washington, in a study of the nebula in Orion. 
 Every important result reached by that study, and very 
 many not comprised in it, was attained by Mr. Common's 
 photograph (subsequently taken), which required an ex- 
 posure of forty minutes only. 
 
 Another important advantage of the new methods is 
 that they do not require highly skilled observers. It re- 
 quired a Bessel or a Struve to determine the parallax of 61 
 Cygni or of Vega. But photographic exposures can be 
 made, and glass negatives successfully measured by well- 
 trained assistants, after the plan of observation has once 
 been thoroughly thought out. This is no slight benefit. 
 The skill of the astronomer is reserved for real difficulties, 
 and the merely laborious work can be done in duplicate, 
 if necessary, by younger men. 
 
 Again, the chemical plate is sensitive to a whole series 
 of rays, which produce no effect on the human eye. Only 
 half of the faintest stars of any photographic map are visible 
 to the eye in the same telescope. Photographic methods 
 thus increase the range of our vision immensely; they also 
 increase its sharpness. The photographic plate will regis- 
 ter the sum of all the impressions it receives. It does not 
 tire, as the eye does, and refuse to pay attention for more 
 
 20 
 
306 HOLDEN 
 
 than a small fraction of a second, but it will faithfully 
 record every ray of light that falls upon it, even for hours, 
 and finally it will produce its automatic register, so that 
 the eye can see it, and so that this can be measured, if 
 necessary, again and again. The permanence of the records 
 is of the greatest importance, and, so far as we know, it 
 is complete when the best modern plates are employed. 
 
 We can hand down to our successors a picture of the 
 sky locked in a box. What would we not give for such 
 a record bequeathed to us by Hipparchus or by Galileo! 
 
 It will be of interest to briefly state here how far the 
 equipment of the Lick Observatory will fit it to engage in 
 this important branch of research. It is known that the 
 situation of the observatory is the finest in the world, both 
 as to the number of clear days and as to the quality of steady 
 atmosphere. The observatory will be completely equipped 
 for all micrometric work, and also for all spectroscopic 
 researches. 
 
 Mr. Lick's bequest for the observatory was $700,000, 
 of which nearly $200,000 will remain after the observatory 
 is completed The income from this sum must support 
 the observatory for the present. Although the whole plan 
 of the observatory has been made with direct reference to 
 keeping its running expenses low, it is clear that the com- 
 pany of astronomers will have to be kept small. The work 
 of these observers must be concentrated on the large equa- 
 torial, and even then their energies will not be sufficient to 
 utilize every moment. It is not our intention to jealously 
 guard the immense scientific opportunity for ourselves, for 
 California, or even for the United States. The real gift 
 of Mr. Lick was to the world. We mean to put the large 
 telescope at the disposition of the world, by inviting its 
 most distinguished astronomers to visit us one at a time, 
 and to give them the use of the instrument during certain 
 specific hours of the twenty-four. Each day there will be 
 certain hours set apart when the observatory staff will re- 
 linquish the use of the equatorial to distinguished special- 
 ists who will come upon our invitation from the United 
 
THE LICK OBSERVATORY, 
 Photogravure from a photograph. 
 

 a small fra< 
 
 ray < 
 and * 
 the 
 
 d, but it will faithfully 
 
 t falls upon it, even for hours, 
 
 its automatic register, so that 
 
 that this can be measured, if 
 
 The permanence of the records 
 
 :e, and, so far as we know, it 
 
 ii plates are employed. 
 
 :cessors a picture of the 
 
 .ve not give for such 
 
 .r> or by Galileo! 
 
 ;iere how far the 
 
 it it to engage in 
 
 . that the 
 
 both 
 
 -ervatory was $700,000, 
 
 n after the observatory 
 
 sum must support 
 
 whole plan 
 
 ork 
 
 large equa- 
 
 be sufficient to 
 
 T intention to jealously 
 
 ry for ourselves, for 
 
 The real gift 
 
 > mean to put the large 
 
 by inviting its 
 
 t us one at a time, 
 
 and to give .ment during cer 
 
 Each day there 
 
 rs set apart when the observatory staff 
 the use of the equatorial to distinguished 
 >me upon our invitation from 
 
PHOTOGRAPHY THE SERVANT OF ASTRONOMY 307 
 
 States and from Europe to solve or to attack some one of 
 the many unsolved problems of astronomy. In this way 
 we hope to make the gift of Mr. Lick one which is truly 
 a gift to science, and not merely a gift to California and 
 to its university. 
 
 Even under such circumstances it will be impossible 
 to utilize the instrumental outfit to the full. It was clearly 
 the duty of the Lick trustees to make this observatory 
 perfect in every respect, and to provide it with all the in- 
 struments necessary to a complete equipment. This they 
 have done economically and wisely. So far as I can judge, 
 there is nothing that should be altered. The instruments 
 are all necessary, and they are mounted in the most perfect 
 manner. 
 
 Each one is directly subordinate to the large equatorial 
 and accessory to it. Nothing has been purchased, and no 
 work has been done, which does not directly tend to make 
 the observations made by the large equatorial either more 
 complete or more immediately useful. The cost of the 
 whole observatory may fairly be said to be the cost of 
 the great telescope in place and entirely ready for work. 
 The objective itself has cost $52,000. The photographic 
 lens will add $13,000 to this. The mounting which is to 
 carry the tube of nearly 60 feet in length is to be made 
 and delivered for $42,000. The dome, of 70 feet interior 
 diameter, will be built in San Francisco, and I have no 
 doubt that it will be materially better than any now made. 
 The chief novelty will be the adoption of Mr. Grubb's in- 
 genious plan for placing the observer in a proper position 
 with reference to his telescope. We have to recollect that 
 the eyepiece of the telescope may be about 5 feet from 
 the floor of the dome when the telescope is pointed to the 
 zenith, or it may be 35 feet when the telescope is in the 
 horizontal position. The ordinary observing chair, which 
 is convenient enough when it is not more than 16 feet high, 
 becomes a cumbrous and inconvenient affair when it is 
 extended to 35 feet. Mr. Grubb proposed to remedy this 
 by raising the whole floor of the dome, like an elevator, 
 
308 HOLDEN 
 
 to the proper height. The whole floor will be raised ver- 
 tically a distance of i6J feet by four hydraulic jacks. The 
 ascent is made in eight minutes with a perfectly parallel 
 motion. The water supply for this purpose comes from the 
 watershed of the dome itself. The last mechanical diffi- 
 culty is now overcome, and it is expected that the steel 
 dome will be mounted during the present year, or at least 
 in the spring of 1887. The contract price for the dome 
 delivered and erected is $56,800, and for the moving floor 
 $14,250. The sum of these items is $178,000, and if this 
 is increased by others not named here it will raise the cost 
 of the large instrument in place to $200,000. The prepara- 
 tion of the top of the mountain to receive the buildings, the 
 erection of the buildings themselves and the observers* 
 houses, and above all the provision of an adequate water 
 supply, have been covered by the remaining $300,000. 
 
 With faithfulness on the part of the company of astron- 
 omers to which this magnificent equipment is confided, 
 and with the generous support of the friends of science in 
 California, much may be expected to follow from this 
 splendid gift to America and to the world. 
 
THE BEGINNINGS OF AMERICAN 
 ASTRONOMY 1 . 
 
 IT is impossible, even in the briefest sketch, not to em- 
 phasize the debt of American science and learning to 
 the intelligent interest and patronage of our early Presi- 
 dents Washington, John Adams, Jefferson, Madison, 
 Monroe, John Quincy Adams. The powerful impetus 
 given by them and through them has shaped the liberal 
 policy of our Governments, national and State, toward edu- 
 cation and toward science. Sir Lyon Playfair, in his ad- 
 dress to the British Association for the Advancement of 
 Science (1885), has recognised this influence in the truest 
 and most graceful way. He said: " In the United King- 
 dom we are just beginning to understand the wisdom of 
 Washington's ' Farewell Address to his Countrymen ' 
 (1796) when he said: 'Promote, as an object of primary 
 importance, institutions for the increase and diffusion of 
 knowledge; in proportion as the structure of a govern- 
 ment gives force to public opinion, it is essential that public 
 opinion should be enlightened/ ' 
 
 Until the Revolution (1776) American science was but 
 English science transplanted, and it looked to the Royal 
 Society of London as its censor and patron. Winthrop, 
 Franklin, and Rittenhouse were, more or less, English 
 astronomers. Franklin was the sturdiest American of the 
 three. As early as 1743 he suggested the formation of the 
 American Philosophical Society of Philadelphia. John 
 Adams founded the American Academy of Arts and Sci- 
 ences in Boston in 1780. These two societies, together 
 
 1 From " Science," June 18, 1897. 
 309 
 
310 HOLDEN 
 
 with Harvard College (founded in 1636), Yale College 
 (1701), the University of Virginia (founded by Jefferson in 
 1825), and the United States Military Academy at West 
 Point (1801), were the chief foci from which the light of 
 learning spread. Other colleges were formed or forming 
 all over the Eastern and Middle States during the early 
 years of the century. 
 
 The leading school of pure science was the Military 
 Academy at West Point, and it continued to hold this place 
 until the civil war of 1861. From its corps of professors 
 and students it gave two chiefs to the United States Coast 
 Survey; and the army, particularly the corps of engineers, 
 provided many observers to that scientific establishment, 
 besides furnishing a large number of professors and teachers 
 of science to the colleges of the country. The observatory 
 of the academy was founded by Bartlett in 1841, and much 
 work was done there, only a small part of which is pub- 
 lished. The Coast Survey was a school of practice for 
 army officers, and their experience was utilized in numer- 
 ous boundary surveys during the period 1830 '50. Colonel 
 J. D. Graham, for example, v/as astronomer of the survey 
 of the boundary between Texas and the United States in 
 1839-^40; commissioner of the Northeast boundary survey, 
 1840-^43; astronomer of the Northwest boundary survey, 
 1843-^47; of the boundary between the United States and 
 Canada, 1848-^50; of the survey of the boundary between 
 Pennsylvania and Virginia, 1849-^50; of the boundary 
 survey between Mexico and the United States, i85O-'5i. 
 The names of Bonneville, Talcott, Cram, Emory, and other 
 army officers are familiar in this connection, and their 
 work was generally of a high order. It was in such service 
 that Talcott invented or re-invented the Zenith Telescope, 
 now universally employed for all delicate determinations of 
 latitude. The mechanical tact of Americans has served 
 astronomy well. The sextant was invented by Thomas 
 Godfrey, of Philadelphia, in 1730, a year before Hadley 
 brought forward his proposal for such an instrument. 2 The 
 * In 1700 Sir Isaac Newton sent drawings and descriptions of a 
 
THE BEGINNINGS OF AMERICAN ASTRONOMY 311 
 
 chronograph of the Bonds, the Zenith Telescope of Talcott, 
 and the break-circuit chronometer of Winlock are univer- 
 sally used to-day The diffraction gratings of Rutherfurd 
 were the best to be had in the world till they were replaced 
 by those of Rowland. The use of a telescope as a collimator 
 was first proposed by Rittenhouse. The pioneer opticians 
 of the United States were Holcomb (1826), Fitz (1846 or 
 earlier), Clark (1845), Spencer (1851). Only the Clarks 
 have a world-wide reputation. Wurdemann, instrument 
 maker to the United States Coast Survey (1834), had a 
 decided influence on observers and instrument makers 
 throughout the United States, as he introduced extreme 
 German methods and models among us where extreme 
 English methods had previously prevailed. The system of 
 rectangular land surveys which proved to be so convenient 
 for the public lands east of the Rocky Mountains was de- 
 vised and executed by Mansfield, a graduate of the Mili- 
 tary Academy. 
 
 The list of army officers who became distinguished in 
 civil life as professors in the colleges of the country is a 
 very long one. Courtenay (class of 1821 at West Point) 
 was professor of mathematics at the University of Penn- 
 sylvania, 1 834-^36, at the University of Virginia, 1842-^43, 
 and was the author of admirable text-books. Norton (class 
 of 1831) became professor at New Haven, and wrote -a 
 very useful text-book of astronomy in 1839; and the list 
 could be much extended. The excellent training in mathe- 
 matics at West Point (chiefly in French methods) early 
 made itself felt throughout the whole country. The mathe- 
 matical text-books of Peirce, of Harvard, and of Chauvenet, 
 of the Naval Academy, brought the latest learning of Eu- 
 rope to American students. Mitchel (class of 1829 at West 
 Point) was the only graduate who became a professional 
 
 reflecting sextant to Halley for his advice. At Halley's death these 
 were found among his papers. Hadley's device (1731) was undoubtedly 
 derived from Newton's manuscripts. The Royal Society of London 
 granted two hundred pounds to Godfrey for his invention, which his 
 brother, Captain Godfrey, had previously put into practical use in the 
 West Indies. 
 
3 i2 HOLDEN 
 
 astronomer (i842-'6i). His direct service to practical ob- 
 serving astronomy is small, but his lectures (i842-'48), the 
 conduct of the Cincinnati Observatory (i845~'59), and his 
 publication of the "Sidereal Messenger" (i846-'48), to- 
 gether with his popular books, excited an intense and wide- 
 spread public interest in the science, and indirectly led to 
 the foundation of many observatories. He was early con- 
 cerned in the matter of using the electric current for longi- 
 tude determinations, and his apparatus was only displaced 
 because of the superior excellence of the chronograph de- 
 vised by the Bonds. His work was done under immense 
 disadvantages, in a new community (Ohio), but the endow- 
 ment of astronomical research in America owes a large 
 debt to his energy and efforts. 
 
 The navy and the United States Naval Academy 
 (founded by Bancroft in 1845, at the suggestion of Chau- 
 venet) were very active in astronomical work. Chauvenet 
 (Yale College, 1840) published a text-book of trigonom- 
 etry, in 1850, which had an important share in directing 
 attention to rigid, elegant, and general methods of research. 
 His "Astronomy" (1863) is a handbook for all students. 
 Walker, Gilliss, Coffin, Hubbard, Ferguson, Keith, Yarnall, 
 Winlock, Maury, Wilkes, were all connected with the 
 navy more or less intimately. Walker's career was espe- 
 cially brilliant; he graduated at Harvard College in 1825, 
 and established the observatory of the Philadelphia High 
 School in 1840. He was the leading spirit in the United 
 States Naval Observatory at Washington (1845-^47), 
 and introduced modern methods into its practice at the 
 beginning. From the observatory he went to the Coast 
 Survey to take charge of its longitude operations, and he 
 continued to direct and expand this department until his 
 death, in 1853. To him, more than to any single person, 
 is due the idea of the telegraphic method (" the American 
 method ") of determining differences of longitude. His 
 assistant in this work was Gould, who succeeded to the 
 charge of it in 1853. His researches extended to the field 
 of mathematical astronomy also, and his theory of the 
 
THE BEGINNINGS OF AMERICAN ASTRONOMY 
 
 313 
 
 planet Neptune (then newly discovered) marks an impor- 
 tant step forward. His investigations and those of Peirce 
 were conducted in concert, and attracted general and de- 
 served attention. 
 
 The exploring expedition of Wilkes required corre- 
 sponding observations to be made in America, and during 
 the period 1838-^42 William Bond, at Dorchester, and 
 Lieutenant Gilliss, at Washington, maintained such a series 
 with infinite assiduity and with success. The results of Gil- 
 liss's astronomical expedition to the southern hemisphere 
 (Chile, 1 849-^52) were most creditable to him and to the 
 navy, though his immediate object the determination of 
 the solar parallax was not attained. 
 
 The Coast Survey began its work in 1817 under Hass- 
 ler, a professor from West Point, who impressed upon the 
 establishment a thoroughly scientific direction. Bache, his 
 successor (a grandson of Benjamin Franklin), was a gradu- 
 ate of West Point in the class of 1825, and took charge of 
 the Survey in 1843. He is the true father of the institu- 
 tion, and gave it the practical efficiency and high standard 
 which characterized its work. He called around him the 
 flower of the army and navy, and was ably seconded by 
 th^ permanent corps of civilian assistants Walker, Sax- 
 ton, Gould, Dean, Blunt, Pourtales, Boutelle, Hilgard, 
 Schott, Goodfellow, Cutts, Davidson, and others. 
 
 Silliman's (and Dana's) " American Journal of Science " 
 had been founded at New Haven in 1818, and served as a 
 medium of communication among scientific men. A great 
 step forward was made in the establishment of the " Astro- 
 nomical Journal" by Dr. Gould on his return from Europe 
 at the close of i849. 3 Silliman's "Journal" was chiefly con- 
 cerned with the non-mathematical sciences, though it has 
 always contained valuable papers on mathematics, astron- 
 omy, and physics, especially from the observers of Yale 
 College Olmsted, Herrick, Bradley, Twining, Norton, 
 Newton, Lyman, and others. In Mason, who died in 1840 
 
 * The " Astronomische Nachrichten " had been founded in Altona, 
 by Schumacher, in 1821. 
 
314 
 
 HOLDEN 
 
 at the age of twenty-one, the country lost a practical astron- 
 omer of the highest promise. 4 Gould's " Journal " was an 
 organ devoted to a special science. It not only gave a con- 
 venient means of prompt publication, but it immediately 
 quickened research and helped to enforce standards already 
 established and to form new ones. The " Astronomical No- 
 tices " of Briinnow (1858-' 62) might have been an ex- 
 ceedingly useful journal with an editor who was wil- 
 ling to give more attention to details, but, in spite of 
 Briinnow's charming personality and great ability, it 
 had comparatively little influence on the progress of 
 the science. 
 
 The translation of the " Mecanique Celeste " of Laplace 
 by Nathaniel Bowditch, the supercargo of a Boston ship 
 (1815 '17), marks the beginning of an independent mathe- 
 matical school in America. The first volume of the trans- 
 lation appeared in 1829; at that time there were not more 
 than two or three persons in the country who could read 
 it critically. The works of the great mathematicians and 
 astronomers of France and Germany Laplace, Lagrange, 
 Legendre, Olbers, Gauss, W. Struve, Bessel were almost 
 entirely unknown. 
 
 Bowditch's translation of the " Mecanique Celeste," 
 and, still more, his extended commentary, brought this 
 monumental work to the attention of students and within 
 their grasp. His " Practical Navigator " 5 contained the 
 latest and best methods for determining the position of a 
 ship at sea, expressed in simple rules. American navigators 
 had no superiors in the first half of this century. Nan- 
 tucket whalers covered the Pacific, Salem ships swarmed 
 in the Indies, and the clipper ships made passages round 
 the Horn to San Francisco, which are a wonder to-day. 
 Part of their success is due to the bold enterprise of their 
 
 4 See the " International Review," vol. x, p. 585. 
 
 8 First edition, 1802. Sumner's method in navigation (1843) a very 
 original and valuable contribution from a Boston sea-captain and 
 Maury's " Wind and Current Charts," begun in 1844, are two other 
 notable contributions from a young country to an art as old as com- 
 merce. 
 
THE BEGINNINGS OF AMERICAN ASTRONOMY 315 
 
 captains (who were said to carry deck-loads of studding-sail 
 booms to replace those carried away), but an important 
 part depended on their skill as observers with the sextant. 
 One of the sister ships to the one of which Bowditch was 
 supercargo was visited at Genoa by a European astronomer 
 of note (Baron de Zach), who found that the latest methods 
 of working lunar distances to determine the longitude were 
 known to all on board, sailors as well as officers. His be- 
 wilderment reached its climax when the navigator called 
 the negro cook from the galley and bade him expound the 
 methods of determining the longitude to the distinguished 
 visitor. 
 
 On Bowditch's own ship there was " a crew of twelve 
 men, every one of whom could take and work a lunar ob- 
 servation as well, for all practical purposes, as Sir Isaac 
 Newton himself." Such crews were only to be found on 
 American ships in the palmy days of democracy. All were 
 cousins or neighbours, and each had a " venture " in the 
 voyage. But these anecdotes may serve as illustrations of 
 the intellectual awakening which came about as soon as 
 our young country was relieved from the pressure of the 
 two wars of 1776 and 1812. An early visitor, Baron Hyde 
 de Neuville (1805), felt " an unknown something in the 
 air," " a new wind blowing." This new spirit, born of free- 
 dom, entered first into practical life, as was but natural; 
 science next felt its impulse, and, last of all, literature was 
 born. Emerson hailed it (in 1837) " as the sign of an in- 
 destructible instinct." " Perhaps the time is already come," 
 he says, " when the sluggard intellect of this country will 
 look from under its iron lids and fill the postponed expecta- 
 tion of the world with something better than the exertions 
 of mechanical skill. Our day of dependence, our long ap- 
 prenticeship to the learning of other lands, draws to a close. 
 The millions that around us are rushing into life can not 
 always be fed with the sere remains of foreign harvests." 
 
 Benjamin Peirce, a graduate of Harvard in the class of 
 1829, had been concerned with the translation of the 
 " Mecanique Celeste," and was early familiar with the best 
 
316 HOLDEN 
 
 mathematical thought of Europe. He became professor in 
 Harvard College in 1833, and, after the death of Bowditch, 
 in 1838, he was easily the first mathematical astronomer in 
 the country. His instruction was precisely fitted to de- 
 velop superior intelligences, and this was his prime useful- 
 ness. Just such a man was needed at that time. Besides 
 his theoretical researches on the orbits of the planets (espe- 
 cially Uranus and Neptune) and of the moon, his study of 
 the theory of perturbations, and his works on pure math- 
 ematics and mechanics, he concerned himself with ques- 
 tions of practical astronomy, although the observations 
 upon which he depended were the work of others. He was 
 the consulting astronomer of the " American Ephemeris 
 and Nautical Almanac " from its foundation in 1849, an ^ 
 its plans were shaped by him to an important degree. His 
 relative, Lieutenant Davis, United States Navy (the trans- 
 lator of Gauss's " Theoria Motus Corporum Ccelestium ") 
 (1857), was placed in charge of the " Ephemeris," and the 
 members of its staff Runkle, Ferrel, Wright, Newcomb, 
 Winlock, and others most effectively spread its exact 
 methods by example and precept. Professor Peirce under- 
 took the calculations relating to the sun, Mars, and Uranus 
 in the early volumes of the " Ephemeris." As a compli- 
 ment to her sex, Miss Maria Mitchell was charged with 
 those of Venus, Mercury was computed by Winlock, 
 Jupiter by Kendall, Saturn by Downes, Neptune by Sears 
 Walker. 
 
 The Smithsonian Institution was founded in 1846, and 
 Joseph Henry was called from Princeton College to direct 
 it. There never was a wiser choice. His term of service 
 (1846-^78) was so long that his ideals became firmly fixed 
 within the establishment, and were impressed upon his con- 
 temporaries and upon a host of younger men. The inter- 
 ests of astronomy were served by the encouragement of 
 original research through subsidies and otherwise, by the 
 purchase of instruments for scientific expeditions, by the 
 free exchange of scientific books between America and 
 Europe, and by the publication of the results of recondite 
 
THE BEGINNINGS OF AMERICAN ASTRONOMY 317 
 
 investigations. It is by these and like services that the 
 institution is known and valued among the wide commu- 
 nity of scientific men throughout the world. 
 
 But this enumeration of specific benefits does not con- 
 vey an adequate idea of the immense influence exercised 
 by the institution upon the scientific ideals of the coun- 
 try. It was of the first importance that the beginnings of 
 independent investigation among Americans should be 
 directed toward right ends and by high and unselfish aims. 
 In the formation of a scientific and, as it were, a moral 
 standard a few names will ever be remembered among us; 
 and no one will stand higher than that of Henry. His wise, 
 broad, and generous policy, and his high personal ideals, 
 were of immense service to his colleagues and to the 
 country. 
 
 The establishment of a national observatory in Wash- 
 ington was proposed by John Quincy Adams in 1825; but 
 it was not until 1844 tnat tne United States Naval Observa- 
 tory was built by Lieutenant Gilliss, of the navy, from 
 plans which he had prepared. By what seems to have been 
 an injustice Gilliss was not appointed to be its first di- 
 rector. 6 This place fell to Lieutenant M. F. Maury. Gilliss 
 had been on detached service for some years, and a rigid 
 construction of rules required that he should be sent to 
 sea, and not remain to launch the institution which he had 
 built and equipped. 
 
 The first corps of observers at Washington (1845) con- 
 tained men of first-class ability Walker, Hubbard, Coffin. 
 Gilliss's work as astronomer to the Wilkes Exploring Expe- 
 dition (1838-^42), at his little observatory on Capitol Hill, 
 had shown him to be one of the best of observers, as well 
 as one of the most assiduous. His study and experience 
 in planning and building the Naval Observatory had broad- 
 ened his mind. To the men just named, with Peirce, 
 Gould, and Chauvenet, and to their coadjutors and pupils, 
 we owe the introduction of the methods of Gauss, Bessel, 
 and Struve into the United States, and it is for this reason 
 8 He was, however, director during the years 1861-65. 
 
318 HOLDEN 
 
 that American astronomy is the child of German and not 
 of English science. 
 
 The most natural evolution might seem to have been 
 for Americans to follow the English practice of Maskelyne 
 and Pond. But the break caused by the War of Independ- 
 ence, by the War of 1812, and by the years necessary for 
 our youthful governments to consolidate (1776-1836), 
 allowed our young men of science to make a perfectly un- 
 biased choice of masters. The elder Bond (William Cranch 
 Bond, born 1789, director of Harvard College Observatory, 
 i84O-'59) was one of the older school and received his 
 impetus from British sources during a visit to England 
 in 1815. 
 
 In estimating the place of the elder Bond among sci- 
 entific men it is necessary to take into account the circum- 
 stances which surrounded him. He was born in the first 
 year of the French Revolution (1789); he was absolutely 
 self-taught; practically no astronomical work was done in 
 America before 1838. While Admiral Wilkes was seeking 
 for coadjutors to prosecute observations in the United 
 States during the absence of his exploring expedition he 
 was indeed fortunate in finding two such men as Bond and 
 Gilliss. Their assiduity was beyond praise, and it led each 
 of them to important duties. Bond became the founder 
 and director of the Observatory of Harvard College, while 
 Gilliss is the father of the United States Naval Observatory 
 at Washington, as well as of that of Santiago de Chile, the 
 oldest observatory in South America. Cambridge, though 
 the seat of the most ancient university in America, was but 
 a village in 1839. The college could afford no salary to 
 Bond, but only the distinction of a title, " Astronomical 
 Observer to the University," and the occupancy of the 
 Dana house, in which his first observatory was established. 
 His work there, as elsewhere, was well and faithfully done, 
 and it led the college authorities to employ him as the 
 astronomer of the splendid observatory which was opened 
 for work in 1847. At that time the two largest telescopes 
 in the world were those of the Imperial Observatory of 
 
THE BEGINNINGS OF AMERICAN ASTRONOMY 
 
 319 
 
 Russia (Pultowa) and its companion at Cambridge. Each 
 of these instruments has a long and honourable history. 
 Their work has been very different. Who shall say that 
 one has surpassed the other? We owe to Bond and his 
 son the discovery of an eighth satellite to Saturn, of the 
 dusky ring to that planet, the introduction of stellar pho- 
 tography, the invention of the chronograph, by which the 
 electric current is employed in the registry of observations, 
 the conduct of several chronometric expeditions between 
 Liverpool and Boston to determine the transatlantic longi- 
 tude, and a host of minor discoveries and observations. 
 
 Gilliss visited France for study in 1835, before he took 
 up his duties at Washington. The text-books of Bond and 
 Gilliss were the " Astronomies " of Vince (1797-1808) and 
 of Pearson (1824-^29). The younger Bond (George Phil- 
 lips Bond, born 1825; Harvard College, 1844; director of 
 the Harvard College Observatory, 1859-^65) and his con- 
 temporaries, on the other hand, were firmly grounded in 
 the German methods, then, as now, the most philosophical 
 and thorough. 
 
 It was not until 1850, or later, that it was indispensable 
 for an American astronomer to read the German language 
 and to make use of the memoirs of Bessel, Encke, and 
 Struve and the text-books of Sawitsch and Brunnow. 7 
 This general acquaintance with the German language and 
 methods came nearly a generation later in England. The 
 traditions of Piazzi and Oriani came to America with the 
 Jesuit Fathers of Georgetown College (1844), of whom 
 Secchi and Sestini are the best known. 
 
 The dates of the foundation of a few observatories of 
 the United States may be set down here. Those utilized 
 for the observation of the transit of Venus in 1769 were 
 temporary stations merely. The first college observatory 
 was that of Chapel Hill, North Carolina (1831); Williams 
 College followed (1836); Hudson Observatory (Ohio) 
 (1838); the Philadelphia High School (1840); the Dana 
 
 T Dr. Bowditch learned to read German in 1818, at the age of 
 forty-five. 
 
320 HOLDEN 
 
 House Observatory of Harvard College (1840); West 
 Point (1841); the United States Naval Observatory (1844); 
 the Georgetown College Observatory (1844); the Cincin- 
 nati Observatory (1845); tne new observatory of Harvard 
 College (1846); the private observatory of Dr. Lewis M. 
 Rutherfurd in New York city (1848); the observatory at 
 Ann Arbor (1854); the Dudley Observatory at Albany 
 (1856); and that of Hamilton College (1856). 
 
 These dates and the summary history just given will 
 serve to indicate the situation of astronomy in the United 
 States during the first half of the present century. A little 
 attention to the dates will enable the reader to place an indi- 
 vidual or an institution on its proper background. It must 
 constantly be kept in mind that the whole country was very 
 young, and that public interest in astronomical matters 
 was neither educated nor very general. The data here set 
 down will have a distinct value as a contribution to the 
 history of astronomy in America. The developments of 
 later years have been so amazing that we forget that the 
 first working observatories were founded so late as 1845. 
 
 American science is scarcely more than half a century 
 old. The day will soon come it is now here when we 
 shall look back with wonder and gratitude to ask who were 
 the men who laid the wide and deep foundations which 
 already maintain so noble an edifice. 
 
STELLAR PARALLAX 
 
 BY 
 
 SIR JOHN FREDERICK WILLIAM HERSCHEL 
 
 (Born 1792; died 1871) 
 
 21 
 
STELLAR PARALLAX 1 
 
 GENTLEMEN: The report of the council has placed 
 before you so ample a view of the state of the so- 
 ciety, of its labours during the last year, of the ac- 
 cessions of its members, and of the many and severe losses 
 it has had to deplore, that little is left for me to add, except 
 my congratulations on its continued and increasing pros- 
 perity. It would be inexpressibly gratifying to me if I 
 could persuade myself that my own exertions in its chair 
 had contributed, even in a small degree, to that prosperity ; 
 but, alas! I have felt only too sensibly how very feebly and 
 inefficiently, especially during the last year, owing to a 
 variety of causes, but chiefly to residence at a distance from 
 London, I have been able to fill that most honourable 
 office. 
 
 The immediate object of my now addressing you, gen- 
 tlemen, is to declare the award by your council of the gold 
 medal of this society to our eminent associate, M. Bessel, 
 for his researches on the annual parallax of that remarkable 
 double star 61 Cygni researches which it is the opinion 
 of your council have gone so far to establish the existence 
 and to measure the quantity of a periodical fluctuation, 
 annual in its period and identical in its law with parallax, 
 as to leave no reasonable ground for doubt as to the reality 
 of such fluctuation, as something different from mere in- 
 strumental or observational error: an inequality, in short, 
 which, if it is not parallax, is so inseparably mixed up with 
 
 1 Address on presenting the gold medal of the Royal Astronomical 
 Society to Prof. F. W. Bessel, Director of the Observatory of Konigs- 
 berg. (From " Monthly Notices of the Royal Astronomical Society," 
 1841.) 
 
 323 
 
324 HERSCHEL 
 
 that effect as to leave us without any criterion by which to 
 distinguish them. Now, in such a case, parallax stands to 
 us in the nature of a vera causa, and the rules of philoso- 
 phizing will not justify us in referring the observed effect 
 to an unknown and, so far as we can see, an inconceivable 
 cause, when this is at hand ready to account for the whole 
 effect. 
 
 I say in the nature of a vera causa, since each particu- 
 lar star must of necessity be of some parallax. Every real, 
 existing material body must enjoy that indefeasible attri- 
 bute of body viz., definite place. Now, place is defined 
 by direction and distance from a fixed point. Every body, 
 therefore, which does exist, exists at a certain definite dis- 
 tance from us and at no other, either more or less. The dis- 
 tance of every individual body in the universe from us 
 is, therefore, necessarily admitted to be finite. 
 
 But though the distance of each particular star be not 
 in strictness infinite, it is yet a real and immense accession 
 to our knowledge to have measured it in any one case. To 
 accomplish this has been the object of every astronomer's 
 highest aspirations ever since sidereal astronomy acquired 
 any degree of precision. But hitherto it has been an object 
 which, like the fleeting fires that dazzle and mislead the 
 benighted wanderer, has seemed to suffer the semblance 
 of an approach only to elude his seizure when apparently 
 just within his grasp, continually hovering just beyond the 
 limits of his distinct apprehension, and so leading him on 
 in hopeless, endless, and exhausting pursuit. 
 
 The pursuit, however, though eager and laborious, has 
 been far from unproductive, even in those stages where its 
 immediate object has been baffled. 
 
 The fact of a periodical fluctuation of some kind in the 
 apparent places of the stars was recognised by Flamsteed, 
 and erroneously attributed to parallax. The nearer exami- 
 nation of this phenomenon with far more delicate instru- 
 ments, infinitely greater refinement of methods, and clearer 
 views of the geometrical relations of the subject, rewarded 
 Bradley with his grand discoveries of aberration and nuta- 
 
STELLAR PARALLAX 325 
 
 tion, and enabled him to restrict the amount of possible 
 parallax of the stars observed by him within extremely nar- 
 row limits. 
 
 Bradley failed to detect any appreciable parallax, though 
 he considered i" as an amount which would not have 
 escaped his notice. And since his time this quantity has 
 been assumed as a kind of conventional limit, which it 
 might be expected to attain, but hardly to surpass. But 
 this was rather because, in the best observations from 
 Bradley's time forward, i" has been a tolerated error; a 
 quantity for which observation and mechanism, joined to 
 atmospheric fluctuations and uncertainties of reduction, 
 could not be held rigidly accountable even in mean results, 
 than from any reason in the nature of the case, or any 
 distinct perception of its reality. If parallax were to be 
 detected at all by observations of the absolute places of the 
 stars, it could only emerge as a " residual phenomenon," 
 after clearing away all the effects of the uranographical cor- 
 rections as well as of refraction, when it would remain 
 mixed up with whatever uncertainties might remain as to 
 the coefficients of the former, with the casual irregularities 
 of the latter, and with all the forms of instrumental and 
 observational error. Now, these have hitherto proved suf- 
 ficient, even in the observation of zenith stars, quite to 
 overlay and conceal that minute quantity of which astron- 
 omers were in search. 
 
 It is not my intention, gentlemen, to enter minutely 
 into the history of the attempts of various astronomers on 
 this problem, whether by the discussion of observations 
 of one star, or by the combination of those of pairs of stars 
 opposite in right ascension; nor with the occasional gleams 
 of apparent success which, however, have always proved 
 illusory, which have attended these attempts. For such a 
 history, and, indeed, for a complete and admirably drawn- 
 up monograph of the whole subject, I must refer to a paper 
 lately read to this society by Mr. Main, and which is now 
 in process of publication in the forthcoming volume of our 
 " Memoirs." In whatever reference I may have to make 
 
326 HERSCHEL 
 
 to the history of the subject, I must take this opportunity 
 to acknowledge my obligations to the author of this paper, 
 as well as for his exceedingly luminous exposition of the 
 results of those more successful attempts on the problem 
 by Henderson, Struve, and Bessel, which I shall now pro- 
 ceed more especially to consider. 
 
 It would be wrong, however, not to notice that the first 
 indication of some degree of impression beginning to be 
 made on the problem seems to be found in Struve's dis- 
 cussion of the differences of right ascension of circumpolar 
 stars in 1819, 1820, and 1821. The only positive result, 
 indeed, of these observations is that, in the case of twenty- 
 seven stars examined, none has a parallax amounting to 
 half a second. But below this there certainly do seem to 
 be indications in the nature of a real parallax, which might 
 at least suffice to raise the sinking hopes of astronomers 
 and excite them to further efforts. 
 
 But the time arrived when the problem was to be at- 
 tacked from a quarter offering the greater advantages, and 
 exposed to few or none of those unmanageable sources of 
 irregular error to which the determination of absolute 
 places are liable. I mean by the measurement of the dis- 
 tances of such double stars as consist of individuals so dif- 
 ferent in magnitude as to authorize a belief of their being 
 placed at very different distances from the eye; or, as 
 Struve expresses it, optically and not physically double. 
 This, in fact, was the original notion which led to the 
 micrometrical measurements of double stars; but not only 
 was anything like a fair trial of the method precluded by 
 the imperfections of all the micrometers in use until re- 
 cently, but the interesting phenomena of another kind, 
 which began to unfold themselves in the progress of those 
 measurements, led attention off altogether from this their 
 original application, which thus lay dormant and neglected, 
 until the capital modern improvements, both in the optical 
 and mechanical parts of refracting telescopes, and the great 
 precision which it was found practicable, by their aid, to 
 attain in these delicate measurements, revived the idea of 
 
STELLAR PARALLAX 327 
 
 giving this method, what it never before had, a fair trial. 
 The principle on which the determination of parallax by 
 means of micrometrical observations of a double star turns, 
 is extremely simple. If we conceive two stars very nearly 
 in a line with the eye, but of which one is vastly more re- 
 mote than the other, each, by the effect of parallax, will 
 appear to describe annually a small ellipse about the mean 
 place as its centre. These two ellipses, however, though 
 similar in form, will differ in dimension, that described by 
 the more remote star being comparatively much smaller; 
 consequently, the apparent places being similarly situated 
 in each, their apparent distance on the line joining these 
 apparent places will both oscillate in angular position and 
 fluctuate in length, thus giving rise to an annual relative 
 alternate movement between the individuals both in posi- 
 tion and distance, which is greater the greater the differ- 
 ence of the parallaxes. 
 
 Thus it is not the absolute parallax of either, but the 
 differences of their parallaxes, which is effectively measured 
 by this method i. e., by repeating the measurements of 
 their mutual distance at all times of the year. But, on the 
 other hand, aberration, nutation, precession, and refrac- 
 tion act equally on both stars, or so very nearly so as 
 to leave only an exceedingly small fraction of these cor- 
 rections bearing on the results. And when the stars are 
 very unequal in magnitude, there is a presumption that the 
 difference of their parallaxes is very nearly equal to the 
 whole parallax of the nearer one. 
 
 The selection of a star for observation involves many 
 considerations. In that pitched on by M. Bessel (61 
 Cygni) the large star so designated is, in fact, a fine double 
 star; nay, one that has been ascertained to be physically 
 double. It is in every respect a highly remarkable star. 
 The mutual distance of its individuals is great, being about 
 i6y . Now, this being necessarily less than the axis of their 
 mutual orbit, affords in itself a presumption that the star 
 is a near one. And this presumption is increased by the 
 unusually great proper motion of this binary system, which 
 
328 
 
 HERSCHEL 
 
 amounts to nearly 5" per annum, and which has beeri 
 made by Sir James South the subject of particular inquiry, 
 and found to be not participated in by several small sur- 
 rounding stars, which, therefore, are not physically con- 
 nected with it. Moreover, the angular rotation of the two, 
 one about the other, has been well ascertained. 
 
 Now, it fortunately happens that of these small sur- 
 rounding stars there are two very advantageously situated 
 for micrometrical comparison with either of the individuals 
 of the binary star, or with the middle point between them. 
 The one of these (a), at a distance of f 42", is situated 
 nearly at right angles to the direction of the double star; 
 the other (b), at a distance of n' 46", nearly in that direc- 
 tion. Considering a and I as fixed points, then, and 
 measuring at any instant of time their distances from c, 
 the middle point of the double star, the situation of c 
 relative to a and b is ascertained; and if this be done at 
 every instant, the relative locus of c, or the curve described 
 by it on the plane of the heavens with respect to the fixed 
 base line a b, will become known. 
 
 Now, on the hypothesis of parallax, that locus ought 
 to be an ellipse of one certain calculable eccentricity and 
 no other, and its major and minor axes ought to hold with 
 respect to the points, a, b, certain calculable positions and 
 no other. Hence it follows that the distance a c and b c will 
 each of them be subject to annual increase and diminution; 
 and that, first, in a given and calculable ratio the one to the 
 other; and, secondly, so that the maxima and minima of 
 the one distance a c shall be nearly contemporaneous with 
 the mean values of the other distance (b c), and vice versa. 
 
 Thus we have, in the first place, several particulars in- 
 dependent of mere numerical magnitudes; and, in the sec- 
 ond place, several distinct relations a priori determined, to 
 which those numerical values must conform, if it be true 
 that any observed fluctuations in these distances (a c, b c) 
 be really parallactic. So that if they be found in such con- 
 formity, and the above-mentioned maxima and minima do 
 observe that interchangeable law above stated; and if, 
 
STELLAR PARALLAX 329 
 
 moreover, all due care be proved to have been taken to 
 eliminate every instrumental source of annual fluctuation 
 there becomes accumulated a body of probability in favour 
 of the resulting parallax which can not but impress every 
 reasonable mind with a strong degree of belief and con- 
 viction. 
 
 Now, all these circumstances have been found by M. 
 Bessel, in his discussion of the measures taken by him 
 (which have been very carefully and rigorously examined 
 by Mr. Main in the paper alluded to, as have also M. Bes- 
 sel's formulae and calculations, for in such matters nothing 
 must remain unverified), to prevail in a very signal and 
 satisfactory manner. Not one case of discordance, in so 
 many independent particulars, has been found to subsist; 
 and this, of itself, is high ground of probability. But we 
 may go much further. Mr. Main has projected graphically 
 the deviations of the distances a c and b c from their 
 mean quantities (after clearing them of the effects of 
 proper motion and of the minute differences of aberration, 
 etc.). Taking the time for an abscissa, and laying down 
 the deviations in the distances so cleared as ordinates, two 
 curves are obtained, the one for the star a, the other for 
 the star b. Each of these curves ought alternately to lie 
 for half a year above and for half a year below its axis. 
 It does so. Each of them ought to intersect its axis at 
 those dates when the maximum and minimum of the other 
 above and below the axis occurs. With only a slight de- 
 gree of hesitation at one crossing, it does so. The points 
 of intersection with the axis ought to occur at dates in like 
 manner calculated a priori; and so they do within very 
 negligible limits of error. And, lastly, the general forms, 
 magnitudes, and fixtures of the curves ought to be identical 
 with those of curves similarly projected by calculation on 
 an assumed resulting parallactic coefficient. This is the 
 final and severe test; Mr. Main has applied it, and the re- 
 sults have been placed before you: oculis subjecta fidelibus. 
 If all this does not carry conviction along with it, it seems 
 difficult to say what ought to do so. 
 
330 HERSCHEL 
 
 The only thing that can possibly be cavilled at is the 
 shortness of the period embraced by the observations viz., 
 from August, 1837, to the end of March, 1840. But this 
 interval admits of five intersections of each curve with its 
 axis; of two maxima and two minima in its excursions on 
 either side; and of ample room for trying its agreement in 
 general form with the true parallactic curves. Under such 
 circumstances it is quite out of the question to declare the 
 whole phenomenon an accident or an illusion. Something 
 has assuredly been discovered, and if that something be not 
 parallax, we are altogether at fault, and know not what 
 other cause to ascribe it to. 
 
 The instrument with which Bessel made these most re- 
 markable observations is a heliometer of large dimensions, 
 and with an exquisite object-glass by Fraunhofer. I well 
 remember to have seen this object-glass at Munich before 
 it was cut, and to have been not a little amazed at the 
 boldness of the maker who would devote a glass, which at 
 that time would have been considered in England almost 
 invaluable, to so hazardous an operation. Little did I then 
 imagine the noble purpose it was destined to accomplish. 
 By the nature and construction of this instrument, espe- 
 cially when driven by clockwork, almost every conceivable 
 error which can effect a micrometrical measure is destroyed, 
 when properly used; and the precautions taken by M. 
 Bessel in its use have been such as might be expected from 
 his consummate skill. The only possible apparent opening 
 for an annually fluctuating error seems to be in the cor- 
 rection for temperature of its scale. But this correction 
 has been ascertained by M. Bessel by direct observation, 
 in hot and cold seasons, and applied. Nor could this cause 
 destroy the evidence arising from the simultaneous observa- 
 tion of the two companion stars, since a wrong correction 
 for temperature would affect both their distances propor- 
 tionally, leaving the apparent parallactic movement still 
 unaccounted for. 
 
 The resulting parallax is an extremely minute quantity, 
 only -f^f of a second, which would place the star in ques- 
 
STELLAR PARALLAX 331 
 
 tion at a distance from us of nearly 670,000 times that of 
 the sun! 2 Such is the universe in which we exist, and 
 which we have at length found the means to subject to 
 measurement, at least in one of its members, probably 
 nearer to us than the rest. 
 
 It becomes necessary for me now to refer to two series 
 of researches on this important subject, which have been 
 held by your council to merit very high and honourable 
 mention; though neither of them, separately, for reasons 
 which I shall state, would have been considered as carrying 
 that weight of probability in favour of its conclusions, which 
 would justify any immediate decision of the nature which 
 they have come to in the case of M. Bessel's. I allude to 
 M. Struve's inquiries, by the method of micrometric meas- 
 ures, into the parallax of a Lyrae, and to Mr. Henderson's, 
 by that of meridian observations, on the parallax of a 
 Centauri. 
 
 a Lyrse is accompanied by a very minute star, at the 
 distance of about 43". That this star is unconnected with a 
 by any physical relation is clear from the fact, ascertained 
 by Sir James South and myself, that it does not participate 
 in the proper motion of the large star. The mutual angular 
 distance of these stars has been made by M. Struve the sub- 
 ject of a very extensive series of micrometric measures with 
 the celebrated Dorpat achromatic, bearing this object 
 steadily in view, and working it out to a conclusion of the 
 very same kind, and though materially inferior in the degree 
 and nature of its evidence to that of Bessel, yet certainly 
 entitled to high consideration. M. Struve's observations on 
 this star, and for this purpose, extend from November, 
 1835, to August, 1838, and are distributed over sixty 
 nights, averaging twenty per annum; and from their com- 
 bination, according to the principle of probabilities, he con- 
 cludes a parallax of 0.261". Mr. Main has subjected these 
 observations to an analysis and graphical projection pre- 
 
 2 The orbit described by the two stars of 61 Cygni about each other 
 will, therefore, be about fifty times the diameter of the earth's about 
 the sun, or two and a half times that of Uranus. 
 
332 
 
 HERSCHEL 
 
 cisely similar in principle to those I have explained in the 
 case of 61 Cygni. The curves so projected have been sub- 
 jected to your inspection, and that inspection certainly does 
 leave a very strong impression of a real and tolerably well- 
 ascertained parallax having been detected in this star. But 
 at the same time an impression no less decided, owing to 
 irregularities in the march of the curve, when compared 
 with the true parallactic curve, is created : that the errors of 
 observation are far from being eliminated; that, on the con- 
 trary, they bear such a proportion to the parallax itself as 
 to leave room for some degree of hesitation, and to justify 
 an appeal to a longer series of observations, and to con- 
 current evidence from other quarters, before declaring any 
 positive opinion. The evidence of this kind, in short, is not 
 equal to that afforded by the similar projection of Bessel's 
 observations of either of his two comparison stars. And to 
 this it must be added that only one star of comparison ex- 
 isting in the line of a Lyrae, the possible effect of tem- 
 perature and annual instrumental variation is not elimi- 
 nated from the result in the way in which it is from the 
 measures of 61 Cygni; while all that great mutual support 
 which the observations of parallaxes of the two comparison 
 stars afford each other in the latter case is altogether want- 
 ing in the former. These considerations, without any 
 underestimation of the great importance and value of M. 
 Struve's researches, yet formed essential drawbacks on the 
 immediate admission of his results. 
 
 In a word, I conceive the question of discovery as be- 
 tween these illustrious but most generous and amicable 
 rivals may be thus fairly stated. M. Struve's meridian ob- 
 servations in 1819-^2 1 seem to have made the first im- 
 pression on the general problem, but too slight to authorize 
 more than a hope that it would yield at no distant day. 
 His micrometric measures of a Lyrse commenced more 
 than a year earlier, and have extended altogether over a 
 longer period than M. Bessel's of 61 Cygni. From their 
 commencement they afford indications of parallax, and 
 these indications accumulating with time have amounted to 
 
STELLAR PARALLAX 
 
 333 
 
 a high degree of probability, and rendered the supposition 
 of parallax more admissible than that of instrumental or 
 casual errors producing the same influence on the measures. 
 On the other hand, M. Bessel's measures, commencing a 
 year later and continued on the whole through somewhat 
 less time, have exhibited a compact and consistent body of 
 evidence drawn from two distinct systems of measures mu- 
 tually supporting each other, and so steadily bearing on 
 their object as to leave no more reasonable doubt of the 
 truth than in the case of many things which we look upon 
 as, humanly speaking, certain. And this conviction, once 
 obtained, reacts on our belief in the other results, and in- 
 duces us to receive and admit it on the evidence adduced 
 for it; which, without such conviction so obtained, we 
 might hesitate to do until after longer corroboration of the 
 same kind. 
 
 The other series of observations to w r hich I must now 
 call your attention are those of Mr. Henderson, made at 
 the Cape of Good Hope, on the great star a Centauri, the 
 third star in brightness which the heavens offer to our 
 view. It is a magnificent double star consisting of two 
 individuals, the one of a high and somewhat brownish 
 orange, the other of a fine yellow colour, and each of which 
 I consider fairly entitled to be classed in the first magni- 
 tude. 3 Their distance is at present about 15" asunder, but 
 it is rapidly diminishing, and in no great lapse of time they 
 will probably occult one another, their angular motion be- 
 ing comparatively small. Their apparent distance was for- 
 merly much greater how much we can not say for want 
 of observations, but probably the major axis of their mutual 
 orbit is little short of a minute of space. They, therefore, 
 afford strong indications of being very near our system. 
 Add to which their proper motion is very considerable, and 
 participated in by both, which proves their connection as 
 a binary system; and an additional presumption in favour 
 
 'I have seen both their images projected on a screen of three 
 thicknesses of stout paper, the eye being on the opposite side of the 
 screen from that on which the images were depicted. 
 
334 
 
 HERSCHEL 
 
 of their proximity may be drawn from their situation in 
 what, from general aspect, I gather to be the nearest region 
 of the Milky Way, among an immensity of large stars. 
 
 Mr. Henderson observed these stars with great care 
 both in right ascension and declination with the very fine 
 transit, and (in spite of certain grievous defects in the axis) 
 the otherwise really good and finely divided mural circle of 
 the Royal Observatory in that colony. Since his return to 
 England he has reduced these observations with a view to 
 parallax, and the result is the apparent existence of that 
 element to what, after what has been said, we must now call 
 the great and conspicuous amount of a full second. Mr. 
 Main, to whom I am so largely indebted for allowing me 
 to draw so freely on his labours, has also discussed these 
 results, and comes to the conclusion that (as might, per- 
 haps, be expected) the right ascension observations afford 
 a trace, but an equivocal one, of parallax, but that in declina- 
 tion (I use his words) " the law of parallax is followed re- 
 markably well. There is scarcely an exception to the 
 proper change of sign, according to the change of sign of 
 the coefficients of parallax. This is quite as much as can 
 reasonably be expected in a series of individual results ob- 
 tained from any meridional instrument for observing zenith 
 distances. We can not expect to find the periodical func- 
 tion regularly exhibited by the differences. On the whole, 
 therefore, we should say that, in addition to the claims of 
 a Centauri on our attention with relation to its parallax, 
 arising from its forming a binary system, its great proper 
 motion, and its brightness, it derives now much additional 
 importance, in this point of view, from the investigation of 
 Mr. Henderson. This we are at least entitled to assume 
 until some distinct reason, independent of parallax, shall 
 have been assigned for the changes in the declinations. 
 Such I do not consider impossible, having before my eyes 
 the results which Dr. Brinkley derived, in the cases of cer- 
 tain stars, from the Dublin circle. For the present it must 
 be considered that the star well deserves a rigorous exam- 
 ination by all the methods which the author himself has so 
 
STELLAR PARALLAX 335 
 
 well pointed out; and that, in the event of a parallax at all 
 comparable with that assigned by Mr. Henderson being 
 found, he will deserve the merit of its first discovery, and 
 the warmest thanks of astronomers, as an extender of the 
 knowledge which we possess of our connection with the 
 sidereal system." 
 
 With this view of Mr. Henderson's labours I fully agree, 
 and await with highly excited interest the result of Mr. 
 Maclear's larger and complete series of observations on this 
 star, both with the old circle and that more perfect one with 
 which the munificence of Government has recently supplied 
 the observatory. Should a different eye and a different 
 circle continue to give the same results, we must, of course, 
 acquiesce in the conclusion, and the distinct and entire 
 merit of the first discovery of the parallax of a fixed star 
 will rest indisputably with Mr. Henderson. At present, 
 however, we should not be justified in so far anticipating a 
 decision which time alone can stamp with the seal of abso- 
 lute authenticity. 
 
 Gentlemen of the Astronomical Society, I congratulate 
 you and myself that we have lived to see the great and 
 hitherto impassable barrier to our excursions into the side- 
 real universe that barrier against which we have chafed so 
 long and so vainly (aestuantes angusto limite mundi) 
 almost simultaneously overleaped at three different points. 
 It is the greatest and most glorious triumph which practical 
 astronomy has ever witnessed. Perhaps I ought not to 
 speak so strongly; perhaps I should hold some reserve in 
 favour of the bare possibility that it may be all an illusion, 
 and that further researches, as they have repeatedly before, 
 so may now, fail to substantiate this noble result. But I 
 confess myself unequal to such prudence under such excite- 
 ment. Let us rather accept the joyful omens of the time, 
 and trust that, as the barrier has begun to yield, it will 
 speedily be effectually prostrated. Such results are among 
 the fairest flowers of civilization. They justify the vast ex- 
 penditure of time and talent which have led up to them; 
 they justify the language which men of science hold, or 
 
336 HERSCHEL 
 
 ought to hold, when they appeal to the governments of 
 their respective countries for the liberal devotion of the 
 national means in furtherance of the great objects they pro- 
 pose to accomplish. They enable them not only to hold 
 out, but to redeem their promises, when they profess them- 
 selves productive labourers in a higher and richer field than 
 that of mere material and physical advantages. It is then 
 when they become (if I may venture on such a figure with- 
 out irreverence) the messengers from Heaven to earth of 
 such stupendous announcements as must strike every one 
 who hears them with almost awful admiration, that they 
 may claim to be listened to when they repeat in every 
 variety of urgent instance that these are not the last of such 
 announcements which they shall have to communicate; 
 that there are yet behind, to search out and declare, not only 
 secrets of Nature which shall increase the wealth or power 
 of man, but truths which shall ennoble the age and coun- 
 try in which they are divulged, and, by dilating the intel- 
 lect, react on the moral character of mankind. Some truths 
 are things quite as worthy of struggles and sacrifices as 
 many of the objects for which nations contend, and ex- 
 haust their physical and moral energies and resources. 
 They are gems of real and durable glory in the diadems of 
 princes, and conquests which, while they leave no tears 
 behind them, continue forever unalienable. 
 
 It must be needless for me to express a hope that these 
 researches will be followed up. Already we have to con- 
 gratulate astronomy on the resolution taken by one of our 
 great academic institutions to furnish its observatory with 
 a heliometer of the same description as Bessel's; nor can 
 we fear but that the research will speedily be extended to 
 other stars, offering varieties of magnitude and other indi- 
 cations to draw attention to them. 
 
 On the whole, then, the award of our medal, which the 
 council have agreed on, seems to me, under the circum- 
 stances, fully justified. I will now request the foreign sec- 
 retary to convey it to our distinguished associate; and in 
 so doing I will add our hope that, in the painful and dis- 
 
STELLAR PARALLAX 337 
 
 tressing visitation with which it has pleased Providence re- 
 cently to try him, he may find occasion to withdraw his 
 mind awhile from that melancholy contemplation to re- 
 ceive with satisfaction such a tribute to this his last and 
 perhaps his greatest achievement, accompanied as it is by 
 the truest regard for his private worth and the most re- 
 spectful sympathy for his present distress. 
 
 " If we ask to what end magnificent establishments are 
 maintained by states and sovereigns, furnished with master- 
 pieces of art, and placed under the direction of men of 
 first-rate talent and high-minded enthusiasm, sought out 
 for those qualities among the foremost in the ranks of 
 science, if we demand cui bono? for what good a Bradley 
 has toiled, or a Maskelyne or a Piazzi has worn out his 
 venerable age in watching, the answer is, not to settle 
 mere speculative points in the doctrine of the universe; not 
 to cater for the pride of man by refined inquiries into the 
 remoter mysteries of Nature; not to trace the path of our 
 system through space, or its history through past and future 
 eternities. These, indeed, are noble ends, and which I am 
 far from any thought of depreciating; the mind swells in 
 their contemplation, and attains in their pursuit an expan- 
 sion and a hardihood which fit it for the boldest enterprise. 
 But the direct practical utility of such labours is fully wor- 
 thy of their speculative grandeur. The stars are the land- 
 marks of the universe; and, amid the endless and compli- 
 cated fluctuations of our system, seem placed by its Creator 
 as guides and records, not merely to elevate our minds by 
 the contemplation of what is vast, but to teach us to direct 
 our actions by reference to what is immutable in his works. 
 It is, indeed, hardly possible to over-appreciate their value 
 in this point of view. Every well-determined star, from the 
 moment its place is registered, becomes to the astrono- 
 mer, the geographer, the navigator, the surveyor, a point 
 of departure which can never deceive or fail him, the same 
 forever and in all places, of a delicacy so extreme as to be 
 a test for every instrument invented by man, yet equally 
 
 22 
 
338 HERSCHEL 
 
 adapted for the most ordinary purposes; as available for 
 regulating a town clock as for conducting a navy to the 
 Indies; as effective for mapping down the intricacies of a 
 petty barony as for adjusting the boundaries of transatlan- 
 tic empires. When once its place has been thoroughly 
 ascertained and carefully recorded, the brazen circle with 
 which that useful work was done may moulder, the marble 
 pillar may totter on its base, and the astronomer himself 
 survive only in the gratitude of posterity; but the record 
 remains, and transfuses all its own exactness into every de- 
 termination which takes it for a groundwork, giving to 
 inferior instruments nay, even to temporary contrivances, 
 and to the observations of a few weeks or days all the pre- 
 cision attained originally at the cost of so much time, la- 
 bour, and expense." 
 
THE HISTORY OF THE 
 TELESCOPE 
 
 BY 
 
 CHARLES S. HASTINGS 
 
THE HISTORY OF THE TELESCOPE 1 
 
 THERE is no instrument which has done so much to 
 widen the scope of human knowledge, to extend 
 our notions of the universe, and to stimulate intel- 
 lectual activity as has the telescope, unless the microscope 
 be regarded as a successful rival. But even admitting a 
 parity in scientific importance, the former instrument is 
 incomparably more interesting in its history, in the same 
 degree that its history is more simple and more compre- 
 hensible. To trace its development from a curious toy in 
 the hands of its discoverer, for we shall see that this term 
 is more appropriate than inventor, to the middle of this 
 century, is to be brought into contact with most of the 
 great philosophers, from the time of the Renaissance, who 
 have achieved greatness in physical science, Galileo, Tor- 
 ricelli, Huygens, Cassini, Newton, Halley, Kepler, Euler, 
 Calaiault, the Herschels, father and son, Fraunhofer, Gauss 
 from only a portion of the list of great names. Its 
 growth toward perfection has constantly carried with it 
 increased precision in the applied sciences of navigation and 
 of all branches of engineering. It would be easy to show 
 that even pure mathematics would be in a far less forward 
 state had there been no problems of astronomy and physics 
 which were first suggested by the employment of the tele- 
 scope. It is to this history that I venture to invite your 
 attention this evening. I purpose to review succinctly the 
 origin and development of this potent aid in the study of 
 
 1 Address delivered at the dedication of the Goodsell Observatory of 
 Carleton College, Northfield, Minn., June n, 1891. (From the " Side- 
 real Messenger," August, 1891, vol. x, pp. 335-354.) 
 
342 HASTINGS 
 
 Nature, to name some of the more important achievements 
 depending upon it, and to trace its gradual improvement 
 to the magnificent and complicated instrument which con- 
 stitutes the modern equatorial. After this sketch I shall 
 try to give an idea of the imperfections which the consci- 
 entious artisan has to contend with in attaining perfection, 
 and to make clear the methods which have been employed 
 in reducing these imperfections in the noble instrument 
 now erected at this institution, 2 and explain why its pos- 
 sessors are so hopeful of gratifying success. 
 
 Galileo learned, in 1609, while visiting Venice, that a 
 marvellous instrument had been invented the preceding 
 year in Holland, which would enable an observer to see a 
 distant object with the same distinctness as if it were only 
 at a small fraction of its real distance. It required but little 
 time for the greatest physicist of his age to master the prob- 
 lem thus suggested to his mind, and after his return to 
 Padua, where he held the position of professor of mathe- 
 matics in the famous university of that city, he set himself 
 earnestly to work making telescopes. Such was his success 
 that in August of the same year he sent to the Venetian 
 senate a more perfect instrument than they had been able 
 to procure from Holland ; and in January of the next year, 
 by means of a telescope magnifying thirty times, he discov- 
 ered the four satellites of Jupiter. This brilliant discovery 
 was followed by that of the mountains in the moon ; of the 
 variable phases of Venus, which established the Copernican 
 theory of the solar system as incontestable, and of the true 
 nature of the Milky Way, together with many others of less 
 philosophical importance. Though Galileo did not change 
 the character of the telescope as it was known to its dis- 
 coverer in Holland, he made it much more perfect; and 
 above all, made the first and most fertile application of the 
 instrument to increase the bounds of human knowledge, 
 so that it is inevitable that his name should be indissolubly 
 connected with the instrument. Thus the form which he 
 used is to this day known as the Galilean telescope. 
 3 Carleton College. 
 
THE HISTORY OF THE TELESCOPE 343 
 
 Considering the enormous interest excited throughout 
 intellectual Europe by the invention of the telescope, it 
 seems surprising that its early history is so confused. Less 
 than two years after it was first heard of, a discovery, per- 
 haps the greatest of a thousand years in the domain of 
 natural philosophy, had been made by its means. Notwith- 
 standing these facts, the three contemporary, or nearly 
 contemporary, investigators assign the honour to three 
 different persons, and if we should write out the names 
 of all those to whom more modern writers have attributed 
 the invention, the list would be a long one. The surprise 
 will not be boundless, however, if we consider the task 
 before a historian in the next century who undertakes 
 to justly apportion the honour of the invention of the 
 telephone among its numerous claimants. The analogy, 
 though suggested in the obvious fact that the telephone 
 is to hearing just what the telescope is to sight, may be 
 made much closer if we could imagine the future historian 
 deprived of all but verbal description, that contemporary 
 diagrams and models were wholly wanting. Under such 
 conditions it is difficult to believe that the historian would 
 easily escape antedating the discovery of the telephone 
 proper on account of descriptions, generally imperfect, of 
 the acoustic telephone. But this would fairly represent 
 the condition of the material at the command of an inves- 
 tigator of the present day into a question of science of the 
 early part of the seventeenth century. No wonder, then, 
 that the invention has been attributed to Archimedes, to 
 Roger Bacon, to Porta, and to many others who have 
 written on optics; but to find the name of Satan in the 
 list is certainly surprising. Still we read that a very learned 
 man of the seventeenth century, named Arias Montanus, 
 finds in the fourth chapter of Matthew, eighth verse, evi- 
 dence that Satan possessed, and probably invented, a tele- 
 scope; otherwise, how could he have "shown him all the 
 kingdoms of the world and the glory of them "? 3 It seems 
 
 8 The history of the telescope is admirably treated in Poggendorff's 
 " Geschichte der Physik," from which the statements above are taken. 
 
344 
 
 HASTINGS 
 
 to be well established now, however, that Franz Lippershey, 
 or Lippersheim, a spectacle-maker at Middleberg, was the 
 real inventor of the telescope, and that Galileo's first tele- 
 scope, avowedly suggested by news of the Hollander's 
 achievement, was an independent invention. 
 
 That this discovery was really an accident we may be 
 quite sure, for not only was there no developed theory of 
 optics at that time, but even the law of refraction, which 
 lies at the basis of such theory, was quite unknown. So, 
 too, it seems to me quite certain that Galileo's invention 
 must have been empirical and guided by somewhat precise 
 information, such as that the instrument consisted essen- 
 tially of two lenses, of which one was a magnifying and the 
 other a diminishing lens. At least, that Galileo's telescope 
 was like that of the Hollander; that, theoretically consid- 
 ered, it is not so simple as that made of two magnifying 
 lenses, as is evinced by the fact that Kepler, the first phi- 
 losopher to establish an approximate theory of optical 
 instruments, only two years later invented the latter and 
 prevailing form; and finaly, that Galileo published no con- 
 tributions to the theory of optics, seem quite sufficient rea- 
 sons for such a belief. But, in any case, Galileo's merit is 
 in no wise lessened by having failed to do what could not 
 be done at that time, and the value of his discoveries in 
 emancipating men's minds from authority in matters of 
 pure reason is incalculable. 
 
 No other discoveries of great moment were made until 
 over a generation after Galileo proved the existence of 
 spots on the sun in 1611. This cessation of activity was 
 doubtless owing to the difficulty of securing telescopes of 
 greater efficiency than that possessed by Galileo, and which 
 he would hardly have left until its powers of discovery had 
 been fully exhausted in his own hands. By the middle of 
 the seventeenth century, however, several makers of lenses 
 had so far improved the methods of grinding and polishing, 
 that telescopes notably superior in power to that of Galileo 
 were procurable. Of these Torricelli, Divini, and Campani, 
 all Italians; Auzout, who constructed a telescope six hun- 
 
THE HISTORY OF THE TELESCOPE 345 
 
 dred feet in length, though no means was ever found for 
 directing such an enormous instrument toward the heavens; 
 but above all, Huygens have won distinction as telescope 
 makers. The last-named philosopher discovered, by means 
 of a telescope of his construction, the largest satellite of 
 Saturn in 1655, thus adding a fifth member to the list of 
 planetary bodies unknown to the ancients. But his most 
 important astronomical discovery, made also in 1655, was 
 the nature of the rings of Saturn. This object had greatly 
 puzzled Galileo, to whose small telescope the planet ap- 
 peared to consist of a larger sphere flanked on either side 
 by a smaller one; but when in the course of the orbital 
 motion of Saturn the rings entirely disappeared he was 
 wholly unable to suggest an explanation. This planet had 
 thus presented a remarkable problem to all astronomical 
 observers for more than forty years, and the records of the 
 efforts to solve it during that interval afford us a most ex- 
 cellent means of judging the progress in practical optics. 
 Huygens announced these discoveries early in 1656, but 
 that relating to the ring was given in the form of an ana- 
 gram, the solution of which was first published in 1659. 
 This discovery was contested in Italy by Divini, but was 
 finally confirmed by members of the Florentine Academy 
 with one of Divini's own telescopes. 
 
 A few years later the famous astronomer Cassini, having 
 come to Paris from Italy as royal astronomer, commenced 
 a series of brilliant discoveries with telescopes made by 
 Campani, of Rome. With these, varying in length from 
 35 feet to 136 feet, he discovered four satellites to Saturn 
 in addition to the one discovered by Huygens. The 
 whole number was increased by Herschel's discovery of 
 two smaller ones in 1789, a hundred and five years after 
 Cassini's last discovery, and again by Bond's discovery of 
 an eighth in 1848. The Saturnian system, to which the 
 telescope has doubtless been directed more frequently than 
 to anything else, thus serves as a record of the successive 
 improvements of the telescope. Highly significant is the 
 fact that the discoveries of the eighteenth century were 
 
346 HASTINGS 
 
 made with a reflecting telescope, the others all being with 
 refracting instruments. 
 
 Cassini's discovery in 1684 of the two satellites now 
 known as Tethys and Dione, was not accepted as conclu- 
 sive until long afterward, when Pound, 1718, with a tele- 
 scope 123 feet in length, which Huygens had made and 
 presented to the Royal Society, saw all five. This particu- 
 lar instrument is of especial interest, because it is the only 
 one of those of the last half of the seventeenth century 
 which has been carefully compared with modern instru- 
 ments. Moreover, it is without doubt quite equal in merit 
 to any of that period. But we find that, although it had a 
 diameter of six inches, its performance was hardly better 
 than that of a perfect modern telescope of four inches in 
 diameter, and, perhaps, four feet and a half in length, while 
 in regard to convenience in use the modern compact in- 
 strument is incomparably superior. 
 
 Another notable discovery of this period was that of the 
 duplicity of the rings of Saturn by the Ball brothers in 1665, 
 though its independent discovery by Cassini ten years later 
 first attracted the attention of astronomers. The earlier 
 discovery was made by means of a telescope thirty-eight 
 feet long, which seems to have been of English manufac- 
 ture. We must regard Cassini's discovery of the third and 
 fourth satellites of Saturn, however, as marking the very 
 farthest reach of the old form of telescope; a century was 
 to elapse and an entirely new form of telescope was to be 
 developed before another considerable addition to our 
 knowledge of the aspect of the heavenly bodies was to be 
 made. It is true larger telescopes were made, and Huygens 
 invented a means by which -they could be used without 
 tubes, but notwithstanding this improvement they proved 
 so cumbersome as to be impracticable. 
 
 The older opticians had found that if they attempted to 
 increase the diameter of a telescope they were obliged to 
 increase its length in a much more rapid ratio to secure dis- 
 tinct vision. The reason of this was not clearly understood, 
 but it was supposed to be owing to the fact that a wave 
 
THE HISTORY OF THE TELESCOPE 347 
 
 front, changed in curvature by passing through a spherical 
 surface, is no longer strictly spherical. This deviation in 
 shape of the refracted wave from a true sphere is called 
 spherical aberration. When the refracting surfaces are large 
 and of considerable curvature, this soon becomes very seri- 
 ous, but by using small curvature, which, in a telescope, ob- 
 viously corresponds to great length, the effects of the error 
 can be made insensible. Newton's discovery of the com- 
 posite nature of light and of the phenomenon of dispersion 
 enabled him to explain the true cause of indistinctness in 
 short telescopes namely, that the refraction by the objec- 
 tive varies for different colours; consequently, if the ocular 
 is placed for one particular colour, it will not be in the right 
 position for any of the others, whence the image of a star 
 or planet will seem to be surrounded by a fringe of coloured 
 light. Newton found this source of indistinctness in the 
 image, which is now known as chromatic aberration, many 
 hundred times as serious as the spherical aberrations. As 
 he was persuaded by his experiments that this obstacle 
 to further improvement in the refracting telescope was in- 
 superable, he turned his attention to a form of telescope 
 which had been suggested a number of years earlier, in 
 which the image was to be formed by reflection from a 
 concave mirror, and constructed a small one with his own 
 hands which is still in the possession of the Royal Society. 
 This little instrument seems to have been of about the 
 same power as Galileo's instrument with which he discov- 
 ered the satellites of Jupiter, but it was hardly more than 
 six inches in length. 
 
 Since that time the reflecting telescope has had a re- 
 markable history of development in the hands of a number 
 of most skilful mechanicians, who have also for the most 
 part been distinguished by their discoveries in physical 
 astronomy; we may therefore advantageously depart from 
 the chronological treatment and follow the history of this 
 type of instrument. This course is the more natural be- 
 cause we may probably regard the supremacy of the re- 
 flector (undisputed a century ago) as passed away forever. 
 
34 8 HASTINGS 
 
 Even after Newton's invention was made public, little 
 was done toward the improvement of telescopes for half 
 a century, until Hadley presented a reflector of his own 
 construction to the Royal Society in 1723, which was found 
 to be equal to the Huygens refractor of 123 feet in 
 length. From this time we may date the beginning of the 
 superiority of reflectors. A few years later Short com- 
 menced his career as a practical optician, and for thirty 
 years he was unapproached in the excellence of his instru- 
 ments. During this time many telescopes, more powerful 
 than the best of the previous century and infinitely more 
 convenient in use, had been made and scattered throughout 
 Europe, but during this period also there was a singluar 
 dearth of telescopic discovery. Perhaps men thought that 
 the harvest had already been gathered ; or, perhaps, we may 
 find the explanation in that the great cost of telescopes so 
 restricted their use that the impulse to discovery by their 
 means was confined to a very small class. In view of the 
 remarkable manner in which the standstill in this branch 
 of science was finally followed by a brilliant period of dis- 
 covery, rivalled alone by that of Galileo, we might well re- 
 gard the latter cause as the chief one. 
 
 William Herschel was born in 1738, in Hanover. In 
 1755 he left his native country, and going to England, se- 
 cured a position as organist in Octagon Chapel, Bath, 
 where we find him in 1766. Here he became so profoundly 
 interested in the views of the heavens which a borrowed 
 telescope of moderate power yielded, that he tried to pur- 
 chase one in London. The cost of a satisfactory instru- 
 ment proving beyond his command, he determined to con- 
 struct one with his own hands. Thus he entered upon a 
 course which was to reflect honour upon himself, his coun- 
 try, and his age, and which was to add more to physical 
 astronomy than any other one man has added before or 
 since. With almost inconceivable industry and persever- 
 ance he cast, ground, and polished more than four hundred 
 mirrors for telescopes, varying in diameter from six to 
 forty-eight inches. This in itself would imply a busy life 
 
THE HISTORY OF THE TELESCOPE 
 
 349 
 
 in any artisan, but when we remember that all this was 
 merely subsidiary to his main work of astronomical dis- 
 covery, we can not withhold our admiration. 
 
 Fortunately for science as well as for himself, he made 
 early in his career a discovery of the very first importance, 
 which attracted the attention of all Christendom. On the 
 night of March 13, 1781, Herschel was examining small 
 stars in the constellation of Gemini with one of his tele- 
 scopes of a little more than six inches in diameter, when he 
 perceived one that appeared " visibly larger than the rest." 
 This proved to be a new world, now known as Uranus. 
 The discovery led 'in the following year to his appointment 
 as astronomer to the king, George III, with a salary suf- 
 ficient to enable him to devote his whole time to astronomy. 
 
 One of the fruits of this increased leisure was the con- 
 struction of a telescope far more powerful than had been 
 dreamed of by his predecessors namely, a telescope four 
 feet in diameter and forty feet in length. Commenced in 
 1785, Herschel dated its completion as August 28, 1789, 
 when he discovered by its means a sixth satellite of Saturn, 
 and, less* than a month later, a seventh, even closer to the 
 planet and smaller than the sixth. We may regard this 
 achievement as marking the limit of progress in the reflect- 
 ing telescope, for, although at least one as large is now in 
 use, and one even half as large again has been constructed, 
 it is more than doubtful whether they were ever as perfect 
 as Herschel's at its best. 
 
 There has been one improvement, however, in the re- 
 flecting telescope since the time of Herschel which ought 
 not to be left unnoticed here namely, that of replacing 
 the heavy metal mirror by one of glass, made even more 
 highly reflective than the old mirrors by a thin coating of 
 silver deposited by chemical methods upon the polished 
 glass. The great advantage of this modern form of reflector 
 lies not so much in the greater lightness and rigidity of 
 the material as in that the surface when tarnished can be 
 renewed by the simple process of replacing the old silver 
 film by a new one; whereas in the metal reflectors a tar- 
 
350 
 
 HASTINGS 
 
 nished surface required a repetition of the most difficult and 
 critical portion of the whole process of construction. The 
 construction is also so comparatively simple that an efficient 
 reflector is far less expensive than are refracting telescopes 
 of like power, so that this may be regarded as particularly 
 the amateur's telescope. On the other hand, such tele- 
 scopes are, like their predecessors, extremely inconstant, 
 and they require much more careful attention to keep them 
 in working order. It is for these reasons, doubtless, that 
 silver-on-glass reflectors have done so little for the advance- 
 ment of astronomical discovery. In astronomical photog- 
 raphy, however, they promise to do much; and, indeed, 
 at the present date by far the best photographs we have of 
 any nebulae have been made by Mr. Common's magnificent 
 reflector of three feet in diameter, and by the twenty-inch 
 reflector of Mr. Roberts. 
 
 We must go back now to a quarter of a century before 
 Herschel discovered the new planet, to the very year, in- 
 deed, when that great astronomer first set foot on English 
 soil, in order to trace the history of another form of tele- 
 scope which has remained unrivalled for the last half cen- 
 tury in the more difficult fields of astronomical research, 
 and which to-day finds its most perfect development in the 
 instruments at Mount Hamilton, at Pultowa, at Vienna, 
 and at Washington. 
 
 Newton had declared that, as a result from his experi- 
 ments, separation of white light into its constituent colours 
 was an inevitable accompaniment of deviation by refraction, 
 and consequently the shortening of the unwieldy refractors 
 was impracticable. The correctness of the experiments re- 
 mained unquestioned for nearly a century; but a famous 
 German mathematician, Euler, did question his conclusion. 
 His argument was that since the eye does produce colour- 
 less images of white objects it might be possible by the 
 proper selection of curves to so combine lenses of glass and 
 of water as to produce a telescope free from the colour de- 
 fect. Although Euler's premise was an error, since the eye 
 is not free from dispersion, his efforts had the effect of lead- 
 
THE HISTORY OF THE TELESCOPE 351 
 
 ing to much more critical study of the phenomena involved. 
 In this John Dolland, an English optician, met with bril- 
 liant success. Repeating an experiment of Newton's with 
 a prism of water opposed by a prism of glass he found that 
 deviation of light could be produced without accompany- 
 ing dispersion into prismatic colours. More than this, he 
 found that the two varieties of glass, then as now common 
 in England crown or common window glass, and flint 
 glass, which is characterized by the presence of a greater or 
 less quantity of lead oxide possessed very different powers 
 in respect to dispersion; thus, of two prisms of these two 
 varieties of glass which would deflect the light by the same 
 angle, that made of flint glass would form a spectrum nearly 
 twice as long as the other; hence, if a prism of crown glass 
 deflecting a transmitted beam of light, say ten degrees, were 
 combined with one of flint glass which would deflect the 
 beam of light five degrees in the opposite direction, there 
 would remain a deflection of five degrees without division 
 into colour. It also follows that a positive lens of crown 
 combined with a negative lens of flint of half the power 
 would yield a colourless image. Such combinations of two 
 different substances are called achromatic systems. It is 
 a singular fact, worth noting in passing, that more than 
 twenty years before Dolland's success, Mr. Chester More 
 Hall had invented and made achromatic telescopes, but this 
 remained unknown to the world of science until after Dol- 
 land's telescopes became famous. 
 
 For a long time this ingenious invention remained 
 fruitless for astronomical discovery (though they were early 
 applied to meridian instruments), on account of the im- 
 possibility of securing sufficiently large and perfect pieces 
 of glass, more particularly of flint glass. Not until after 
 the beginning of this century was any real advance in this 
 branch of the arts exhibited. Even then success appeared, 
 not in England or France, where most strenuous efforts had 
 been made to improve the quality of optical glass, but in 
 Switzerland. There a humble mechanic, a watchmaker 
 named Guinaud, spent many years in efforts, long unfruit- 
 
352 HASTINGS 
 
 ful, to make large pieces of optical glass. What degree of 
 success he attained there during twenty years of experi- 
 ment we do not know, though from the fact that during 
 that period good achromatic telescopes of more than five 
 inches in diameter were unknown, we must conclude that 
 his success was limited. In 1805 he joined the optical 
 establishment of Fraunhofer and Utzschneiden in Munich. 
 Here he remained nine years, and with the increased means 
 at his disposal, and the aid of Fraunhofer, he perfected his 
 methods so far that the production of large disks of homo- 
 geneous glass became only a matter of time and cost that 
 is to say, all of the large pieces of optical glass which have 
 since been produced, whether in Germany, France, or Eng- 
 land, have been made by direct heirs of the practical secrets 
 of this Swiss watchmaker. 
 
 Fraunhofer was a genius of a high order. Although he 
 died at the early age of thirty-nine, he had not only brought 
 the achromatic telescope to a degree of optical perfection 
 which made it a rival of the most powerful of the reflector 
 type, and so far improved its method of mounting that his 
 system has replaced all others; but he also made some 
 capital discoveries in the domain of physical optics. His 
 great achievement was the construction of an achromatic 
 telescope 9.6 inches in diameter, with which the elder Struve 
 made at Dorpat his remarkable series of discoveries and 
 measurements of double stars. The character of Struve's 
 work demonstrates the excellence of the telescope, and 
 shows us that it is to be ranked as the equal of all but the 
 very best of its predecessors. Indeed, it may fairly be con- 
 cluded that not more than one or two telescopes, and those 
 made and used by Herschel, had ever been of greater power, 
 while in convenience for use the new refractor was vastly 
 superior. 
 
 For a long time Fraunhofer and his successors, Merz 
 and Mahler, from whom the great telescopes of Pultowa 
 and of the Harvard Observatory were procured, remained 
 unrivalled in this field of optics. But they have been fol- 
 lowed by a number of skilful constructors whose products 
 
THE HISTORY OF THE TELESCOPE 353 
 
 have, since the middle of the century, been scattered all 
 over the world. In Germany, Steinheil and Schroder; in 
 France, Canchois, Martin, and the Henry brothers; in Eng- 
 land, Cook and Grubb; and in this country the Clarks and 
 Brashear, have each produced one or more great telescopes 
 which has rendered his name familiar to all readers of 
 astronomical history. Of these the Clarks, father and son, 
 have beyond a doubt won the first place, whether deter- 
 mined by the character of the discoveries made by means 
 of their instruments or by the fact that the two most power- 
 ful telescopes in existence were made by them namely, 
 the new refractor of thirty inches in diameter at Pultowa, 
 and the great refractor of three feet diameter of the Lick 
 Observatory in California. The most notable discoveries 
 made with their telescopes are the satellites of Mars and the 
 companion to Sirius; but besides these there is a long list 
 of double stars of the most difficult character discovered 
 by the makers themselves, by Dawes in England, by Burn- 
 ham in our own country, and by a number of other ob- 
 servers. 
 
 We ought not to terminate our review of the develop- 
 ment of the telescope without a reference to the parallel 
 development of the mounting of great telescopes. Indeed, 
 did this not lead us too far from the immediate aim in view, 
 we might find a great deal of interest and be brought into 
 agreeable contact with some of the cleverest mechanicians 
 and engineers of two centuries by tracing its course. We 
 should meet with Huygens, as the inventor of the aerial 
 telescope, and perhaps consider the claims of his contem- 
 porary, Robert Hook, as a rival inventor, for we may be 
 sure that nothing which brings us to a study of that curious 
 and able philosopher would fail to possess interest. We 
 should find Herschel confronted with the problem as to 
 how he should use his great forty-foot telescope, and the 
 study of his solution would guide us in valuing the results 
 of the subsequent efforts of Lassel and Rosse. The same 
 line of study would bring us to Grubb's clever and inter- 
 esting equatorial mounting of that anachronism, the four- 
 23 
 
354 
 
 HASTINGS 
 
 foot Melbourne reflector. But we should find nothing of 
 very notable interest in the mounting of refractors, after the 
 time of Huygens and Hook, until Fraunhofer invented a 
 type of mounting for the famous Dorpat equatorial, which 
 still remains in its essential features as the type in universal 
 use. With the increase in size of the telescopes to be di- 
 rected toward the heavens, however, the number and com- 
 plexity of the mechanical problems to be solved has been 
 vastly increased, so that they have taxed the best powers 
 of some of the ablest mechanicians. The Repsolds, of Ger- 
 many, and Sir Howard Grubb, of Dublin, have specially 
 distinguished themselves in this field of activity. But it 
 seems to me that none have shown greater fertility of re- 
 sources, greater skill in the solution of every problem af- 
 fecting the comfort and efficacy of the observer, and greater 
 taste, combined with accurate workmanship, than have the 
 celebrated firm which has mounted the telescope at Mount 
 Hamilton and that at Carleton College. 
 
 We come now to a consideration of the present state of 
 the art of lens-making. We ask why such a very large pro- 
 portion of the telescopes in existence are bad; why there 
 was a time, brief it is true, during which the glass-maker 
 was certainly in advance of the demands of telescope- 
 makers; and why, finally, the first of the great modern ob- 
 jectives was in the hands of the most skilful optician in 
 Great Britain for seven years, and even then this maker 
 asserted that it was incomplete. 
 
 These questions can not be answered in a word, but we 
 can, at least, gain much in perspicuity by recognising that 
 the reasons are of two distinct kinds namely, purely tech- 
 nical and theoretical; and by regarding them briefly in suc- 
 cession. 
 
 The art of lens-making can be certainly traced back to 
 the thirteenth century, though the methods at a much later 
 day than that were so rude that, as we have seen, Galileo 
 had the utmost difficulty in making a lens good enough 
 to bear a magnifying power of thirty times. At the present 
 day there is little difficulty in selecting a spectacle glass 
 
THE HISTORY OF THE TELESCOPE 355 
 
 which would rival that most famous of all telescopes. Not 
 until after another generation of effort was there such 
 notable improvement in the technique of lens-making that 
 further astronomical discovery was possible. The reasons 
 for this slow progress are to be found in the extremely 
 critical requirements for a good lens. A departure by a 
 fraction of r^Vinr P art f an mcn horn a correct geo- 
 metrical surface will greatly impair the performance of an 
 objective. But even at this day the limit of accurate meas- 
 urement may be set at about 1 * of an inch, while 
 it is quite probable that ten times that value was vanishingly 
 small to the artisans of a century or more ago. It was 
 necessary, therefore, to devise a method of polishing for 
 it is a comparatively simple matter to grind a surface accu- 
 rately which should keep the surface true within a limit 
 far transcending the range of measurements. Huygens is 
 the first who seems to have done this, by polishing upon a 
 paste which was formed to the glass and then dried, and by 
 using only the central portion of a large lens. In Italy 
 Campani developed a system which he most jealously 
 guarded as a secret until his death, consisting of polishing 
 with a dry powder on paper cemented to the grinding tools. 
 This method still survives in Paris to the exclusion of 
 almost all others, and it is probably the best for work which 
 does not demand the highest scientific precision. 
 
 Newton, however, was the first to introduce a method 
 which has since been developed to a state of surprising 
 delicacy. Casting about for a means which should be suf- 
 ficiently " tender," to use his own expression, for polishing 
 the soft speculum metal, he fixed upon pitch, shaped to 
 the mirror while warm, as a bed to hold the polishing pow- 
 der. But the enormous value of this substance lies not so 
 much in the comparative immunity which it gives from 
 scratching, but in the fact that under slowly changing forces 
 it is a liquid, but under those of short duration it behaves 
 like a hard and brittle solid. Thus it is possible to slowly 
 alter the shape of a lens while polishing, in any desired 
 direction. It was only after the practical recognition of 
 
' 
 
 356 HASTINGS 
 
 this fact that really excellent lenses were much more than 
 a question of good fortune. The perfecting of this method 
 belongs without doubt to the English of the last century 
 and the early part of this. In the " Philosophical Transac- 
 tions " we find many long papers relating to this art, con- 
 tributed by skilful and successful amateurs. We may there- 
 fore regard the technique of the art of lens-making as prac- 
 tically complete at the middle of this century and as com- 
 mon property, so that success no longer depends upon the 
 holding of some special or secret method. 
 
 We are now (after this, I fear, somewhat dry discussion 
 of a necessary point) in a condition to explain the differ- 
 ences between the processes pursued by most telescope- 
 makers and that of the maker of the Carleton College 
 telescope. 
 
 This is the ordinary method: After securing perfect 
 pieces of glass, crown and flint, as like as possible to those 
 generally used, and having fixed upon the general shape 
 of the lenses, a guess is made as to the proper radii of the 
 four surfaces to determine the desired focal length and cor- 
 rections both for colour and spherical aberration. The suc- 
 cess of this guess has much to do with the necessary outlay 
 of labour, and therefore past experience is of great value 
 as a guide. After working the four surfaces to the dimen- 
 sions provisionally adopted so far as to admit of fairly good 
 seeing through the objective, an examination of the errors 
 is made. Should the errors of colour be so small that their 
 final correction will not make the telescope more than 
 from three to ten per cent greater or less than the desired 
 focal length, the crown lens will probably be completed in 
 accordance with the provisional figures. Then the flint 
 lens will be modified in such a direction as will tend to 
 correct the observed errors of colour and figure, until, by a 
 purely tentative process, the colour error is practically neg- 
 ligible and the error of figure is small. Then follows a 
 process when the qualities of skill, conscientiousness, and 
 perseverance have full scope. This process, first introduced, 
 or at least made public, by Foucault, is known as local cor- 
 
THE HISTORY OF THE TELESCOPE 357 
 
 recting. It consists in slowly polishing away portions 
 of the lens surfaces so that errors in the focal image be- 
 come so small, not that they can not be detected, but 
 that one can not determine whether they are on the one 
 side of truth or the other. Local correcting has always 
 seemed to me to be eminently unscientific and unneces- 
 sary. It is a process of making errors which ought not 
 to exist. 
 
 Mr. Brashear's method is essentially different from this. 
 Before the glasses are touched every dimension and con- 
 stant of the finished objective is known with great accuracy. 
 His whole aim is to make the surfaces geometrically per- 
 fect; and by ingenious polishing machinery, which em- 
 bodies twelve years of his thought and experience, he is 
 enabled to do this with truly astonishing exactness. All 
 the surfaces which admit of investigation usually three in 
 his ordinary construction are made rigidly true without 
 regard to the character of the focal image. This leaves only 
 one surface which is known to be very nearly a sphere, but 
 probably deviating slightly within in the direction of a 
 prolate or oblate spheroid. A glance at the character of 
 the focal image will determine this point. Then the polish- 
 ing machine is adapted to bring about a change in the 
 proper direction, and after action during a measured in- 
 terval of time, the image is again examined, and from the 
 observed change in character the necessary time for com- 
 plete correction by the same or contrary action may be de- 
 duced. It will be observed that by this means it is quite 
 possible to correct errors which are much too small to 
 betray their nature, since a step in the wrong direction 
 carries with it no consequences of the slightest moment, 
 since any step may be retraced. 
 
 When we learn that Mr. Brashear's telescope objec- 
 tives have always had a focal length differing only from -^ 
 to y^j- of one per cent of the value prescribed, we have a 
 suggestion of the success of his efforts. But adding to that 
 the fact that he is absolutely untrammelled by purely me- 
 chanical considerations, either as to the shape of his lenses 
 
358 HASTINGS 
 
 or the character of his materials, leaving these questions 
 to be decided alone by the requirements of the astronomer, 
 it seems to me that we may fairly accord to him the merit 
 of the most important improvements introduced into his 
 art for a very long period. 
 
 I shall not venture to demand much of your time in 
 considering the purely theoretical difficulties in telescope 
 construction, not merely because the subject has already 
 taxed our patience, but because it would be of almost too 
 technical a character did we allow ourselves to regard any- 
 thing but the most general features. 
 
 The obvious requirements are that in a good objective 
 the light coming from a point in the object should be con- 
 centrated at a point in the image; but this, combined with 
 a prescribed focal length, may be reduced to three con- 
 ditions: First, a fixed focal length; second, freedom from 
 colour error; third, freedom from spherical aberration for 
 a particular colour or wave-length of light. Now let us 
 catalogue what provisions we have for satisfying these 
 conditions. They are, four surfaces, which must be spheri- 
 cal but may have any radii we please, the two thicknesses 
 of the two lenses, and the distance which separates the 
 lenses that is, seven elements which may be varied to suit 
 our requirements. As a matter of fact, however, on ac- 
 count of the cost of the material and the fact that glass is 
 perfectly transparent, for powerful telescopes we must make 
 the lenses as thin as possible; and we shall find also that 
 separating the lenses introduces errors away from the axis 
 which are, to say the least, undesirable. We have left, 
 therefore, only the four radii as arbitrary constants. These, 
 however, are more than enough to meet the three require- 
 ments. To make the problem determinate we must add 
 another condition. The suggestion of this fourth condi- 
 tion and carrying the problem to its solution is the work 
 of the great mathematicians who have directed their 
 thought to it. Clairault proposed to make the fourth con- 
 dition that the two adjacent surfaces should fit together 
 and the lenses be cemented. This condition would be, 
 
THE HISTORY OF THE TELESCOPE 359 
 
 doubtless, of great value were it possible to cement large 
 lenses without changing their shapes to a degree which 
 would quite spoil their performance. Sir John Herschel 
 published a very important paper in 1821, in which he made 
 the fourth condition that the spherical aberration should 
 vanish, not only for objects at a very great distance, but 
 also for those at a moderate distance. In this paper he 
 computed a table, afterward greatly extended by Prof. 
 Baden-Powell, for the avowed purpose of aiding the prac- 
 tical optician. It was this feature undoubtedly which 
 brought his construction, not at all a good one, as we shall 
 see, into more general use than any other for some time. 
 But, as all Herschel's tables were derived from calculations 
 which wholly disregarded the thickness of the lenses, I am 
 quite unable to see how they could have been of any 
 material aid, and am inclined to suspect that the discredit 
 with which opticians have received the dicta of mathema- 
 ticians concerning their instruments may have been due in 
 part to this very fact. It is a singular fact, for which I have 
 in vain sought the explanation, that Fraunhofer's objec- 
 tives are of just such a form as to comply with the Her- 
 schelian solution, although they must have been made quite 
 independently. 
 
 Gauss made the fourth condition that another colour or 
 wave-length of light should be also free from spherical aber- 
 ration. This seems to have been a tour de force as a mathe- 
 matician, not as a sober suggestion of an improvement in 
 construction, for in a point of fact the construction is very 
 bad. It was generally believed that this condition could 
 not be fulfilled; therefore Gauss, who was particularly fond 
 of doing what all the rest of the world believed impossible, 
 straightway did it. There has been only one effort to carry 
 out this suggestion of Gauss, and that forty years later by 
 Steinheil, but it proved a disappointment. A much larger 
 objective made by Clark a few years ago, of the general 
 form of Gauss's objective, probably does not meet the 
 Gaussian condition; at least, this condition is extremely 
 critical, and I believe it is not asserted that the objective 
 
360 HASTINGS 
 
 was ever thoroughly investigated. It has been the father 
 of no others. 
 
 It is hardly surprising, since none of these forms have 
 any real merit, that the practical optician has, following the 
 line of least resistance, adopted a form which costs him less 
 labour than those heretofore mentioned, and is quite as 
 good. By making the curve equi-convex the trouble and 
 expense of making one pair of tools are saved, although 
 this would hardly appear a satisfactory reason for choice of 
 a particular form to the astronomer, who simply demands 
 the best possible instrument of research. 
 
 The reason for so much futile work on the theory of the 
 telescope objective is not far to seek. It had always been 
 tacitly assumed that the condition of colour correction, one 
 of those which serves to determine the values of the arbi- 
 trary constants, was readily determinable in fact, one of 
 the donne of the problem, whereas it is just this datum 
 which has offered peculiar difficulties. Fraunhofer brought 
 all the resources at the command of his genius to bear upon 
 this point, and frankly failed, although in the effort he 
 made a splendid discovery, which has assured a permanence 
 to his fame no less than that of the history of science itself 
 the discovery of the dark, or Fraunhofer, lines in solar 
 and stellar spectra. Gauss proposed the condition that the 
 best objective is that which produces the most perfect con- 
 centration of light about the place of the geometrical image 
 of a point, just as the best rifle practice is that which pro- 
 duces the maximum concentration of hits about the centre 
 of the target. That this is a false guide appears at once 
 from the consideration that if we take even as much as ten 
 per cent of the light from an object, and diverted from the 
 image so far that it can not be found, the telescope may still 
 be practically perfect; all of Herschel's did much worse 
 than this. But if you take that same ten per cent and con- 
 centrate it very close about the image, the telescope will 
 be absolutely worthless. 
 
 The true difficulty with most of the theories is this: 
 There is no recognition of the relative weight or impor- 
 
THE HISTORY OF THE TELESCOPE 361 
 
 tance of unavoidable errors. The optician is confronted at 
 the very outset by the fact that absolute elimination of 
 colour error is impossible, for certain physical reasons which 
 we have not time for considering further. He can reduce 
 the colour error of the old single-lens type of telescopes 
 hundreds of times, and hence the length of the telescope 
 tens of times. It is this fact which prevents the still further 
 shortening of telescopes, which keeps the ratio of length to 
 diameter not less than fifteen to one in large telescopes. 
 This restriction being recognised, let us revise our limiting 
 conditions. They now become, first, fixed focal length; 
 second, best colour correction; third, freedom from spheri- 
 cal aberration for a particular wave-length of light. We 
 therefore have still one arbitrary constant undetermined. 
 How shall we fix its value, and thus solve the problem 
 completely? Surely there is only one rational guide. Con- 
 sider the residual errors and make the fourth condition such 
 as to reduce these errors as far as possible. Now the only 
 remaining errors are secondary colour error and spherical 
 aberration for colours other than that for which it is elimi- 
 nated, or, more scientifically, chromatic difference of spheri- 
 cal aberration. Which of these is the gravest defect? Our 
 answer must depend upon the use to which the objective is 
 to be put. If it is a high-power microscope objective, it 
 is certainly the second. If it is an objective to be used for 
 photographing at considerable angular distances from the 
 axis, our question loses its physical significance, since we 
 have excluded the consideration of eccentric refraction. 
 But if the objective is to be for a visual telescope, there is 
 no question that the defect of secondary colour error is 
 incomparably the most serious. Our fourth and determin- 
 ing condition must, therefore, be better colour correction. 
 These are therefore the considerations which have 
 served as guides in the construction of the Carleton College 
 objective. First, the selection of the materials which, in 
 the present condition of the art of optical glass-making, 
 possess in the highest degree the desired physical prop- 
 erties; second, a general discussion of every possible com- 
 
362 HASTINGS 
 
 bination of these two pieces of glass and a selection of the 
 forms which yield the best attainable results. This consci- 
 entious strife after scientific perfection, the unexcelled skill 
 with which the results of analysis have been interpreted 
 into the reality of substance, the gratifying identity of pre- 
 dicted and realized values of physical characteristics all 
 of these have led some of those who have watched the 
 growth of this new instrument of research with the most 
 solicitous attention to the belief that although not the niost 
 powerful in existence, it may well be the most perfect great 
 telescope yet made. Let us therefore congratulate the 
 possessors of this noble instrument, wish them Godspeed 
 in their search after knowledge, while we remind them that 
 although no astronomer can ever make another discovery 
 which will rival that made by the insignificant tube first 
 directed toward the heavens by the Paduan philosopher, yet 
 no mind can weigh the importance of any truth, however 
 trivial in appearance, which may be added to that store 
 which we call " science." 
 
RESULTS 
 
 OF SPECTRUM ANALYSIS 
 APPLIED TO HEAVENLY BODIES 
 
 CELESTIAL SPECTROSCOPY 
 THE NEW ASTRONOMY 
 
 BY 
 
 WILLIAM HUGGINS 
 
THE RESULTS OF SPECTRUM ANALYSIS 
 APPLIED TO THE HEAVENLY BODIES 1 
 
 AJ important invention or discovery seldom, if ever, 
 remains sterile and alone. It gives birth to other 
 discoveries. The telescope and the microscope have 
 led to remarkable discoveries in astronomy and in minute 
 anatomy and physiology, which would not have been pos- 
 sible without those instruments. The observation that a 
 magnetic body, free to move, arranges itself nearly north 
 and south, has not only contributed immensely to the ex- 
 tension of commerce and of geographical discovery, but 
 also has founded the important science of terrestrial mag- 
 netism. 
 
 This evening I have to bring before you some additions 
 to our knowledge in the department of astronomy, which 
 have followed from a comparatively recent discovery. The 
 researches of Kirchhoff have placed in the hands of the 
 astronomer a method of analysis which is specially suitable 
 for the examination of the heavenly bodies. So unexpected 
 and important are the results of the application of spectrum 
 analysis to the objects in the heavens that this method of 
 observation may be said to have created a new and distinct 
 branch of astronomical science. 
 
 Physical astronomy, the imperishable and ever-growing 
 monument to the memory of Newton, may be described as 
 the extension of terrestrial dynamics to the heavens. It 
 seeks to explain the movements of the celestial bodies on 
 the supposition of the universality of an attractive force 
 similar to that which exists upon the earth. 
 
 1 Delivered before the British Association for the Advancement of 
 Science, August 23, 1866. 
 
 365 
 
366 MUGGINS 
 
 The new branch of astronomical science which spectrum 
 analysis may be said to have founded has for its object to 
 extend the laws of terrestrial physics to the other phe- 
 nomena of the heavenly bodies, and it rests upon the now 
 established fact that matter of a nature common to that of 
 the earth, and subject to laws similar to those which prevail 
 upon the earth, exists throughout the stellar universe. 
 
 The peculiar importance of KirchhofFs discovery to 
 astronomy becomes obvious if we consider the position in 
 which we stand to the heavenly bodies. Gravitation and 
 the laws of our being do not permit us to leave the earth; 
 it is, therefore, by means of light alone that we can obtain 
 any knowledge of the grand array of worlds which sur- 
 round us in cosmical space. The starlit heavens is the 
 only chart of the universe we have, and in it each twinkling 
 point is the sign of an immensely vast though distant re- 
 gion of activity. 
 
 Hitherto the light from the heavenly bodies, even when 
 collected by the largest telescope, has conveyed to us but 
 very meagre information, and in some cases only of their 
 form, their size, and their colour. The discovery of Kirch- 
 hoff enables us to interpret symbols and indications hidden 
 within the light itself, which furnish trustworthy informa- 
 tion of the chemical, and also to some extent of the physi- 
 cal, condition of the excessively remote bodies from which 
 the light has emanated. 
 
 We are indebted to Newton for the knowledge that the 
 beautiful tints of the rainbow are the common and neces- 
 sary ingredients of ordinary light. He found that when 
 white light is made to pass through a prism of glass it is 
 decomposed into the beautiful colours which are seen in 
 the rainbow. These colours when in this way separated 
 from each other form the spectrum of the light. Let this 
 white plate represent the transverse section of a beam of 
 white light travelling toward you. Let now a prism be 
 interposed in its path. The beam of white light is not 
 turned aside as a whole, but the coloured lights composing 
 it are deflected differently, each in proportion to the rapidity 
 
SPECTRUM ANALYSIS 367 
 
 of its vibrations. An obvious consequence will be that, on 
 emerging from the prism, the coloured lights which formed 
 the white light will separate from each other, and in place 
 of the white light which entered the prism we shall have its 
 spectrum that is, the coloured lights which composed it 
 in a state of separation from each other. Wollaston and 
 Fraunhofer discovered that when the light of the sun is 
 decomposed by a prism, the rainbow colours which form 
 its spectrum are not continuous, but are interrupted by a 
 large number of dark lines. These lines of darkness are the 
 symbols which indicate the chemical constitution of the 
 sun. It was not until recently, in the year 1859, that Kirch- 
 hoff taught us the true nature of these lines. He himself 
 immediately applied his method of interpretation to the 
 dark lines of the solar spectrum, and was rewarded by the 
 discovery that several of the chemical elements which exist 
 upon the earth are present in the solar atmosphere. 
 
 It is my intention to bring before you this evening the 
 results of the extension of this method of analysis to the 
 heavenly bodies other than the sun. These researches have 
 been carried on in my observatory during the last four years. 
 In respect of a large part of these investigations viz., those 
 of the moon, the planets, and fixed stars I have had the 
 great pleasure of working conjointly with the very distin- 
 guished chemist and philosopher, Dr. William A. Miller. 
 Half a century ago Fraunhofer recognised several of the 
 solar lines in the light of the moon, Venus, and Mars, and 
 also in the spectra of several stars. Recently Donati, Jans- 
 sen, Secchi, Rutherfurd, and the Astronomer Royal have 
 observed lines in the spectra of some stars. Before I de- 
 scribe the results of our observations I will state, in a few 
 words, the principles of spectrum analysis upon which our 
 interpretation of the phenomena we have observed has been 
 based, and also the method of observing which we have 
 employed. 
 
 When light which has emanated from different sources 
 is decomposed by a prism, the spectra which are obtained 
 may differ in several important respects from each other. 
 
368 HUGGINS 
 
 All the spectra which may present themselves can be con- 
 veniently arranged in three general groups. A spectrum 
 illustrating each of these three orders is placed upon the 
 diagram : 
 
 1. The special character which distinguishes spectra of 
 the first order consists in that the continuity of the coloured 
 band is unbroken either by dark or bright lines. By means 
 of the electric lamp, Mr. Ladd will throw a spectrum of this 
 order upon the screen. We learn from such a spectrum 
 that the light has been emitted by an opaque body, and 
 almost certainly by matter in the solid or liquid state. A 
 spectrum of this order gives to us no knowledge of the 
 chemical nature of the incandescent body from which light 
 comes. In the present case the light is emitted by the 
 white-hot carbon points of the electric lamp. A spectrum 
 in all respects similar would be formed by the light from 
 incandescent iron, or lime, or magnesia. 
 
 2. Spectra of the second order are very different. These 
 consist of coloured lines of light separated from each other. 
 From such a spectrum we may learn much. It informs us 
 that the luminous matter from which the light has come is 
 in the state of gas. It is only when a luminous body is 
 free from the molecular trammels of solidity and liquidity 
 that it can exhibit its own peculiar power of radiating some 
 coloured rays alone. Hence substances, when in a state of 
 gas, may be distinguished from each other by their spectra. 
 Each element, and every compound body that can become 
 luminous in the gaseous state without suffering decomposi- 
 tion, is distinguished by a group of lines peculiar to itself. 
 These green lines are produced by silver in a state of gas, 
 and only by silver gas. It is obvious that if the groups of 
 lines characterizing the different terrestrial substances be 
 known, a comparison of these as standard spectra with the 
 spectrum of light from an unknown source will show 
 whether any of these terrestrial substances exist in the 
 source of the light. 
 
 3. The third order consists of the spectra of incan- 
 descent solid or liquid bodies, in which the continuity of 
 
SPECTRUM ANALYSIS 369 
 
 the coloured light is broken by dark lines. These dark 
 spaces are not produced by the source of the light. They 
 tell us of vapours through which the light has passed on its 
 way, and which have robbed the light, by absorption, of cer- 
 tain definite colours or rates of vibration; such spectra are 
 formed by the light of the sun and stars. 
 
 Kirchhoff has shown that if vapours of terrestrial sub- 
 stances come between the eye and an incandescent body 
 they cause groups of dark lines, and, further, that the group 
 of dark lines produced by each vapour is identical in the 
 number of the lines and in their position in the spectrum 
 with the group of bright lines of which its light consists 
 when the vapour is luminous. 
 
 Mr. Ladd will throw upon the screen the spectrum of 
 incandescent carbon points which contain sodium. Ob- 
 serve in addition to the continuous spectrum of the incan- 
 descent carbon a bright-yellow band, which indicates the 
 presence of sodium. Now a piece of metallic sodium will 
 be introduced into the lamp. The sodium will be va- 
 pourized by the heat, and will fill the lamp with its vapour. 
 This vapour absorbs, quenches the light that it emits 
 when luminous. There will thus be produced a black 
 line exactly in the place where the bright-yellow line was 
 seen. 
 
 It is evident that Kirchhoff by this discovery has fur- 
 nished us with the means of interpreting the dark lines 
 of the solar spectrum. For this purpose it is necessary to 
 compare the bright lines in the spectra of the light of ter- 
 restrial substances, when in the state of gas, with the dark 
 lines in the solar spectrum. When a group of bright lines 
 coincides with a similar group of dark lines, then we know 
 that the terrestrial substance producing the bright lines is 
 present in the atmosphere of the sun; for it is this sub- 
 stance, and this substance alone, which, by its own pecul- 
 iar power of absorption, can produce that particular group 
 of dark lines. In this way Kirchhoff discovered the pres- 
 ence of several terrestrial elements in the solar atmos- 
 phere. 
 
 24 
 
370 HUGGINS 
 
 METHODS OF OBSERVATION 
 
 I now pass to the special methods of observation by 
 which, in our investigations, we have applied these prin- 
 ciples of spectrum analysis to the light of the heavenly 
 bodies. I may here state that several circumstances unite 
 to make these observations very difficult and very irksome. 
 In our climate, on few only even of those nights in which 
 the stars shine brilliantly to the naked eye, is the air suf- 
 ficiently steady for these extremely delicate observations. 
 Further, the light of the star is feeble. This difficulty has 
 been met, in some measure, by the employment of a large 
 telescope. The light of a star falling upon the surface of 
 an object-glass of eight inches aperture is gathered up and 
 concentrated at the focus into a minute and brilliant point 
 of light. 
 
 Another inconvenience arises from the apparent move- 
 ment of the stars, caused by the rotation of the earth, which 
 carries the astronomer and his instruments with it. This 
 movement was counteracted by a movement given by 
 clockwork to the telescope in the opposite direction. In 
 practice, however, it is not easy to retain the image of a 
 star for any length of time exactly within the jaws of a 
 slit only the ^ of an inch apart. By patient perseverance 
 these difficulties have been overcome, and satisfactory re- 
 sults obtained. We considered that the trustworthiness 
 of our results must rest chiefly upon direct and simultane- 
 ous comparison of terrestrial spectra with those of celestial 
 objects. For this purpose we contrived the following ap- 
 paratus. 
 
 By an outer tube the instrument is attached to the 
 eye end of the telescope, and is carried round with it by 
 the clock motion. Within this outer tube a second tube 
 slides, carrying a cylindrical lens. This lens is for the pur- 
 pose of elongating the round pointlike image of the star 
 into a short line of light, which is made to fall exactly within 
 the jaws of a nearly closed slit. Behind the slit an achro- 
 matic lens (and at the distance of its own focal length) 
 
SPECTRUM ANALYSIS 371 
 
 causes the pencils to emerge parallel. They then pass into 
 two prisms of dense flint glass. The spectrum which results 
 from the decomposition of the light by the prisms is viewed 
 through a small achromatic telescope. This telescope is 
 provided with a micrometer screw, by which the lines of 
 the spectrum may be measured. 
 
 The light of the terrestrial substances which are to be 
 compared with the stellar spectra is admitted into the in- 
 strument in the following manner: 
 
 Over one half of the slit is fixed a small prism, which 
 receives the light reflected into it by the movable mirror 
 placed above the tube. The mirror faces a clamp of ebonite, 
 provided with forceps to contain fragments of the metals 
 employed. These metals are rendered luminous in the state 
 of gas by the intense heat of the sparks from a powerful 
 induction coil. The light from the spark reflected into the 
 instrument by means of the mirror and the little prism 
 passes on to the prisms in company with that from the star. 
 In the small telescope the two spectra are viewed in juxta- 
 position, so that the coincidence and relative positions of 
 the bright lines in the spectrum of the spark with dark lines 
 in the spectrum of the star can be accurately determined. 
 
 MOON AND PLANETS 
 
 I now pass to the results of our observations. 
 
 I refer in a few words only to the moon and planets. 
 These objects, unlike the stars and nebulae, are not original 
 sources of light. Since they shine by reflecting the sun's 
 light, their spectra resemble the solar spectrum, and the 
 only indications in their spectra which may become sources 
 of knowledge to us are confined to any modifications which 
 the solar light may have suffered, either in the atmosphere 
 of the planets or by reflection at their surfaces. 
 
 Moon. On the moon the results of our observations 
 have been negative. The spectra of the various parts of 
 the moon's surface, when examined under different condi- 
 tions of illumination, showed no indication of an atmos- 
 phere about the moon. I also watched the spectrum of a 
 
372 HUGGINS 
 
 star as the dark edge of the moon advanced toward the 
 star and then occulted it. No signs of a lunar atmosphere 
 presented themselves. 
 
 Jupiter. In the spectrum of Jupiter lines are seen which 
 indicate the existence of an absorptive atmosphere about 
 this planet. In this diagram these lines are presented as 
 they appeared when viewed simultaneously with the spec- 
 trum of the sky, which, at the time of observation, reflected 
 the light of the setting sun. One strong band corresponds 
 with some terrestrial atmospheric lines, and probably indi- 
 cates the presence of vapours similar to those which are 
 about the earth. Another band has no counterpart among 
 the lines of absorption of our atmosphere, and tells us of 
 some gas or vapour which does not exist in the earth's 
 atmosphere. 
 
 Saturn. The spectrum of Saturn is feeble, but lines 
 similar to those which distinguish the spectrum of Jupiter 
 were detected. These lines are less strongly marked in the 
 ansae of the rings, and show that the absorptive power of 
 the atmosphere about the rings is less than that of the 
 atmosphere which surrounds the ball. A distinguished for- 
 eigner present at the meeting, Janssen, has quite recently 
 found that several of the atmospheric lines in this part of 
 the spectrum are produced by aqueous vapour. It appears 
 to be very probable that aqueous vapour exists in the 
 atmospheres of Jupiter and Saturn. 
 
 Mars. On one occasion some remarkable groups of 
 lines were seen in the more refrangible part of the spectrum 
 of Mars. These may be connected with the source of the 
 red colour which distinguishes this planet. 
 
 Venus. Though the spectrum of Venus is brilliant, 
 and the lines of Fraunhofer were well seen, no additional 
 lines affording evidence of an atmosphere about Venus 
 were detected. The absence of lines may be due to 
 the circumstance that the light is probably reflected, 
 not from the planetary surface, but from clouds at some 
 elevation above it. The light which reaches us in this 
 way by reflection from clouds would not have been ex- 
 
SPECTRUM ANALYSIS 373 
 
 posed to the absorbent action of the lower and denser 
 strata of the planet's atmosphere. 
 
 THE FIXED STARS 
 
 The fixed stars, though immensely more remote and 
 less conspicuous in brightness than the moon and planets, 
 yet, because they are original sources of light, furnish us 
 with fuller indications of their nature. 
 
 To each succeeding age the stars have been a beauty 
 and a mystery. Not only children, but the most thought- 
 ful of men, often repeat the sentiment expressed in the 
 well-known lines: 
 
 "Twinkle, twinkle, little star; 
 How I wonder what you are! " 
 
 The telescope was appealed to in vain, for in the largest in- 
 struments the stars remain diskless brilliant points merely. 
 
 The stars have indeed been represented as suns, each 
 upholding a dependent family of planets. This opinion 
 rested upon a possible analogy alone. It was not more 
 than a speculation. We possessed no certain knowledge 
 from observation of the true nature of those remote points 
 of light. This long and earnestly coveted information is 
 at last furnished by spectrum analysis. We are now able 
 to read in the light of each star some indications of its 
 nature. Since I have not a magician's power to convert 
 this theatre into an observatory, and so exhibit to you the 
 spectra of the stars themselves, I have provided photo- 
 graphs of careful drawings. These photographs Mr. Ladd 
 will exhibit upon the screen by means of the electric lamp. 
 I will take first the spectra of two bright stars which we 
 have examined with great care. 
 
 The upper one represents the spectrum of Aldebaran, 
 and the other that of Betelgeux, the star marked a in the 
 constellation of Orion. 
 
 The positions of all these dark lines, about eighty in each 
 star, were determined by careful and repeated measures. 
 These measured lines form but a small part of the numer- 
 
374 HUGGINS 
 
 ous fine lines which may be seen in the spectra of these 
 stars. 
 
 Beneath the spectrum of each star are represented the 
 bright lines of the metals which have been compared with 
 it. These terrestrial spectra appeared in the instrument as 
 you now see them upon the screen, in juxtaposition with 
 the spectrum of the star. By such an arrangement it is pos- 
 sible to determine with great accuracy whether or not any 
 of these bright lines actually coincide with any of the dark 
 ones. For example: 
 
 This closely double line is characteristic of sodium. 
 You see that it coincides, line for line, with a dark line simi- 
 larly double in the star. The vapour of sodium is therefore 
 present in the atmosphere of the star, and sodium forms 
 one of the elements of the matter of this brilliant but re- 
 mote star. 
 
 These three lines in the green are produced, so far as 
 we know, by the luminous vapour of magnesium alone. 
 These lines agree in position exactly line for line with three 
 dark stellar lines. The conclusion, therefore, appears well 
 founded that another of the constituents of this star is mag- 
 nesium. 
 
 Again, there are two strong lines peculiar to the ele- 
 ment hydrogen; one line has its place in the red part of the 
 spectrum, the other at the blue limit of the green. Both 
 of these correspond to dark lines of absorption in the 
 spectrum of the star. Hydrogen, therefore, is present in 
 the star. 
 
 In a similar way, other elements, among them bismuth, 
 antimony, telurium, and mercury, have been shown to exist 
 in the star. 
 
 Now, in reference to all those elements, the evidence 
 does not rest upon the coincidence of one line, which would 
 be worth but little, but upon the coincidence of a group 
 of two, three, or four lines, occurring in different parts of 
 the spectrum. Other corresponding lines are probably also 
 present, but the faintness of the star's light limited our 
 comparisons to the stronger lines of each element. 
 
SPECTRUM ANALYSIS 
 
 375 
 
 What elements do the numerous other lines in the star 
 represent? Some of them are probably due to the vapours 
 of other terrestrial elements, which we have not yet com- 
 pared with these stars. But may not some of these lines be 
 the signs of primary forms of matter unknown upon the 
 earth? Elements new to us may here show themselves 
 which form large and important series of compounds, and 
 therefore give a special character to the physical conditions 
 of these remote systems. In a similar manner the spectra 
 of terrestrial substances have been compared with several 
 other stars. The results are given in the diagrams. Five 
 or six elements have been detected in Betelgeux. Ten 
 other elements do not appear to have a place in the consti- 
 tution of this star. 
 
 ft Pegasi contains sodium, magnesium, and perhaps 
 barium. 
 
 Sirius contains sodium, magnesium, iron, and hy- 
 drogen. 
 
 a Lyrae (Vega) contains sodium, magnesium, and iron. 
 
 Pollux contains sodium, magnesium, and iron. 
 
 About sixty other stars have been examined, all of 
 which appear to have some elements in common with the 
 sun and earth, but the selective grouping of the elements 
 in each star is probably peculiar and unique. 
 
 A few stars, however, stand out from the rest, and 
 appear to be characterized by a peculiarity of great sig- 
 nificance. These stars are represented by Betelgeux and 
 ft Pegasi. The general grouping of the lines of absorption 
 in these stars is peculiar, but the remarkable and excep- 
 tional feature of their spectra is the absence of the two 
 lines which indicate hydrogen, one line in the red and the 
 other in the green. These lines correspond to Fraunhofer's 
 C and F. The absence of these lines in some stars shows 
 that the lines C and F are not due to the aqueous vapour 
 of the atmosphere. 
 
 We hardly venture to suggest that the planets which 
 may surround these suns probably resemble them in not 
 possessing the important element hydrogen. To what 
 
3/6 
 
 MUGGINS 
 
 forms of life could such planets be adapted? Worlds with- 
 out water! A power of imagination like that possessed by 
 Dante would be needed to people such planets with living 
 creatures. 
 
 It is worthy of consideration that, with these few ex- 
 ceptions, the terrestrial elements which appear most widely 
 diffused through the host of stars are precisely some of 
 those which are essential to life such as it exists upon the 
 earth namely, hydrogen, sodium, magnesium, and iron. 
 Besides, hydrogen, sodium, and magnesium represent the 
 ocean, which is an essential part of a world constituted like 
 the earth. 
 
 We learn from these observations that in plan of struc- 
 ture the stars, or at least the brightest of them, resemble the 
 sun. Their light, like that of the sun, emanates from in- 
 tensely white-hot matter, and passes through an atmosphere 
 of absorbent vapours. With this unity of general plan of 
 structure there exists a great diversity among the individual 
 stars. Star differs from star in chemical constitution. May 
 we not believe that the individual peculiarities of each star 
 are essentially connected with the special purpose which it 
 subserves, and with the living beings which may inhabit 
 the planetary worlds by which it may possibly be sur- 
 rounded? 
 
 When we had obtained this new information respecting 
 the true nature of the stars, our attention was directed to 
 the phenomena which specially distinguish some of the 
 stars. 
 
 COLOURS OF THE STARS 
 
 When the air is clear, especially in southern climes, the 
 twinkling stars do not all resemble diamonds; here and 
 there may be seen in beauteous contrast richly coloured 
 gems. 
 
 The colour of the light of the stars which are bright to 
 the naked eye is always some tint of red, orange, or yellow. 
 When, however, a telescope is employed, in close compan- 
 ionship with many of these ruddy and orange stars, other 
 
ANALYSIS 377 
 
 fainter stars become visible, the colour of which may be 
 blue, or green, or purple. 
 
 Now it appeared to us to be probable that the origin 
 of these differences of colour among the stars may be indi- 
 cated by their spectra, 
 
 Since we had found that the source of the light of the 
 stars is incandescent solid or liquid matter, it appeared to 
 be very probable that at the time of its emission the light of 
 all the stars is white alike. The colours observed among 
 them must then be caused by some modification suffered 
 by the light after its emission. 
 
 Again, it was obvious that if the dark lines of absorp- 
 tion were more numerous or stronger in some part of the 
 spectrum, then those colours would be subdued in power, 
 relatively to the colour in which few lines only occur. 
 These latter colours, remaining strong, would predominate, 
 and give to the light, originally white, their own tints. 
 These suppositions have been confirmed by observations. 
 
 Mr. Ladd will throw upon the screen the spectrum of 
 Sirius, which may be taken as an illustration of the stars the 
 light of which is white. 
 
 As might be expected, the spectra of these stars are re- 
 markable for their freedom from strong groups of absorp- 
 tion lines. The dark lines, though present in great number, 
 are all, with one exception, very thin and faint, and too 
 feeble to modify the original whiteness of the light. The 
 one exception consists of three very strong single lines: 
 one line corresponding to Fraunhofer's C, one to F, and 
 the other near G. Two of these certainly indicate the pres- 
 ence of hydrogen. This peculiarity, which seems invariably 
 connected with colourless stars, is very suggestive, and in- 
 vites speculation. May it be a sign of a temperature of 
 extreme fierceness? 
 
 Let us now examine the spectrum of an orange star. 
 
 This diagram represents the spectrum of the brighter 
 of the two stars which form the double star a Herculis. In 
 the spectrum of this star the green and blue parts of the 
 light, and also the deep red, are subdued with strong groups 
 
378 HUGGINS 
 
 of lines, while the orange and yellow rays preserve nearly 
 their original intensity, and therefore predominate in the 
 star's light. 
 
 The question yet remained to be answered, Would the 
 faint telescopic stars, which are blue, green, and purple, and 
 which are never found alone in the heavens, but always 
 under the protection of a strong ruddy or orange star, fur- 
 nish spectra in accordance with this theory? 
 
 With some little difficulty, and by means of a special 
 arrangement of the spectrum apparatus, we succeeded in 
 observing the spectra of the components of some double 
 stars. There will now be thrown upon the screen the well- 
 known double star Cygni. In a large telescope the 
 colours of the two stars are beautifully contrasted, as they 
 now appear upon the screen. The spectra of these stars 
 are now shown. The upper spectrum represents the orange 
 star, the lower one that of its beautiful blue but feeble com- 
 panion. In the orange star you observe that the dark lines 
 are strongest and most closely grouped in the blue and 
 violet parts of the spectrum, and the orange rays therefore, 
 which are comparatively free from lines, predominate. 
 
 In the delicate blue companion, the strongest groups of 
 lines are found in the yellow, orange, and in part of the 
 red. In the arrangement of these groups of lines we have 
 a sufficient cause for the predominance of the other por- 
 tions of the spectrum which unite in the eye to give the 
 blue purple colour of the light of this star. 
 
 We have, therefore, shown that the colours of the stars 
 are produced by the vapours existing in their atmosphere. 
 The chemical constitution of a star's atmosphere will de- 
 pend upon the elements existing in the star and upon its 
 temperature. 
 
 VARIABLE STARS 
 
 The brightness of many of the stars is found to be 
 variable. From night to night, from month to month, or 
 from season to season, their light may be observed to be 
 continually changing, at one time increasing, at another 
 
SPECTRUM ANALYSIS 379 
 
 time diminishing. The careful study of these variable stars 
 by numerous observers has shown that their continual 
 changes do not take place in an uncertain or irregular man- 
 ner. The greater part of these remarkable objects wax and 
 wane in accordance with a fixed law of periodic variations 
 which is peculiar to each. 
 
 We have been seeking for some time to throw light 
 upon this strange phenomenon by means of observation of 
 their spectra. If in any case the periodic variation of bright- 
 ness is associated with physical changes occurring in the 
 star, we might obtain some information by means of the 
 prism. Again, if the diminution in brightness of a star 
 should be caused by the interposition of a dark body, then, 
 in that case, if the dark body be surrounded with an atmos- 
 phere, its presence might possibly be revealed to us by the 
 appearance of additional lines of absorption in the spectrum 
 of the star when at its minimum. One such change in the 
 spectrum of a variable star we believe we have already ob- 
 served. 
 
 Betelgeux is v a star of a moderate degree of variability. 
 When this star was at its maximum brilliancy, in February 
 last, we missed a group of lines, the exact position of which 
 we had determined with great accuracy by micrometric 
 measurement some two years before. 
 
 We have observed the spectra of several variable stars at 
 different phases of their periodic variation, but our results 
 are not yet complete. 
 
 It is worthy of notice that the variable stars which have 
 a ruddy or an orange tint possess spectra analogous to that 
 of Betelgeux and ft Pegasi. 
 
 As an example of this group of variable stars, Mr. Ladd 
 will throw upon the screen the spectrum of /* Cephei when 
 at its maximum. 
 
 TEMPORARY STARS 
 
 With the variable stars modern opinion would associate 
 the remarkable phenomena of the so-called new stars which 
 occasionally, but at long intervals, have suddenly appeared 
 
380 HUGGINS 
 
 in the sky. But in no case has a permanently bright star 
 been added to the heavens. The splendour of all these 
 objects was temporary only, though whether they died 
 out or still exist as extremely faint stars is uncertain. In 
 case of the two modern temporary stars, that seen by 
 Mr. Hind in 1845, an< 3 the bright star recently -observed 
 in Corona, though they have lost their ephemeral glory, 
 they still continue as stars of the tenth and eleventh mag- 
 nitudes. 
 
 The old theories respecting these strange objects must 
 be rejected. We can not believe with Tycho Brahe that 
 objects so ephemeral are new creations, nor with Riccioli 
 that they are stars brilliant on one side only, which have 
 been suddenly turned round by the Deity. The theory that 
 they have suddenly darted toward us with a velocity greater 
 than that of light, from a region of remote invisibility, will 
 not now find supporters. 
 
 On the 1 2th of May last a star of the second magnitude 
 suddenly burst forth in the constellation of the Northern 
 Crown. Thanks to the kindness of the discoverer of this 
 phenomenon, Mr. Birmingham, of Tuam, I was enabled, 
 conjointly with Dr. Miller, to examine the spectrum of 
 this star on the i6th of May, when it had not fallen much 
 below the third magnitude. 
 
 I ought to state that Mr. Barker, of London, Canada 
 West, who announced an observation of this star on the 
 1 4th of May in the " Canadian Free Press," now claims 
 to have seen the star on May 4th, and states that it in- 
 creased in brilliancy up to May loth, when it was at its 
 maximum. 
 
 The spectrum of this star consists of two distinct spectra. 
 One of these is formed by these four bright lines. The 
 other spectrum is analogous to the spectra of the sun and 
 stars. 
 
 These two spectra represent two distinct sources of 
 light. Each spectrum is formed by the decomposition of 
 light which is independent of the light which gives birth to 
 the other spectrum. 
 
SPECTRUM ANALYSIS 381 
 
 The continuous spectrum, crowded with groups of dark 
 lines, shows that there exists a photosphere of incandescent 
 solid or liquid matter. Further, that there is an atmos- 
 phere of cooler vapours, which give rise by absorption to 
 the groups of dark lines. 
 
 So far the constitution of this object is analogous to that 
 of the sun and stars; but in addition there is the second 
 spectrum, which consists of bright lines. There is, there- 
 fore, a second and distinct source of light, and this must 
 be, as the character of the spectrum shows, luminous gas. 
 Now the position of the two principal bright lines of 
 this spectrum informs us that one of the luminous gases 
 is hydrogen. The great brightness of these lines shows 
 that the luminous gas is hotter than the photosphere. 
 These facts, taken in connection w r ith the suddenness of the 
 outburst of light in the star, and its immediate very rapid 
 decline in brightness from the second magnitude down to 
 the eighth magnitude in twelve days, suggested to us the 
 startling speculation that the star had become suddenly 
 inwrapped in the flames of burning hydrogen. In conse- 
 quence, it may be, of some great convulsion, enormous 
 quantities of gas were set free. A large part of this gas con- 
 sisted of hydrogen, which was burning about the star in 
 combination with some other element. This flaming gas 
 emitted the light represented by the spectrum of bright 
 lines. The increased brightness of the spectrum of the 
 other part of the star's light may show that this fierce gase- 
 ous conflagration had heated to a more vivid incandescence 
 the solid matter of the photosphere. As the free hydrogen 
 became exhausted the flames gradually abated, the photo- 
 sphere became less vivid, and the star waned down to its 
 former brightness. 
 
 We must not forget that light, though a swift mes- 
 senger, requires time to pass from the star to us. The great 
 physical convulsion which is new to us is already an event 
 of the past with respect to the star itself. For years the star 
 has existed under the new conditions which followed this 
 fiery catastrophe. 
 
382 HUGGINS 
 
 NEBULAE 
 
 I now pass to objects of another order. 
 
 When the eye is aided by a telescope of even moderate 
 power, a large number of faintly luminous patches and 
 spots come forth from the darkness of the sky, which are 
 in strong contrast with the brilliant but pointlike images 
 of the stars. A few of these objects may be easily discerned 
 to consist of very faint stars closely aggregated together. 
 Many of these strange objects remain, even in the largest 
 telescopes, unresolved into stars, and resemble feebly shin- 
 ing clouds, or masses of phosphorescent haze. During the 
 last one hundred and fifty years the intensely important 
 question has been continually before the mind of astron- 
 omers, " What is the true nature of these faint, cometlike 
 masses? " 
 
 The interest connected with an answer to this question 
 has much increased since Sir William Herschel suggested 
 that these objects are portions of the primordial material 
 out of which the existing stars have been fashioned; and 
 further, that in these objects we may study some of the 
 stages through which the suns and planets pass in their 
 development from luminous cloud. 
 
 The telescope has failed to give any certain information 
 of the nature of the nebulae. It is true that each successive 
 increase of aperture has resolved more of these objects into 
 bright points, but at the same time other fainter nebulae 
 have been brought into view, and fantastic wisps and dif- 
 fused patches of light have been seen, which the mind 
 almost refuses to believe can be due to the united glare of 
 innumerable suns still more remote. 
 
 Spectrum analysis, if it could be successfully applied 
 to objects so excessively faint, was obviously a method of 
 investigation specially suitable for determining whether 
 any essential physical distinction separates the nebulae from 
 the stars. 
 
 I selected for the first attempt, in August, 1864, one of 
 the class of small but comparatively bright nebulae. My 
 
SPECTRUM ANALYSIS 383 
 
 surprise was very great on looking into the small telescope 
 of the spectrum apparatus to perceive that there was no 
 appearance of a band of coloured light, such as a star would 
 give, but in place of this there were three isolated bright 
 lines only. 
 
 This observation was sufficient to solve the long-agi- 
 tated inquiry in reference to this object at least, and to show 
 that it was not a group of stars, but a true nebula. 
 
 A spectrum of this character, so far as our knowledge 
 at present extends, can be produced only by light which 
 has emanated from matter in the state of gas. The light 
 of this nebula, therefore, was not emitted from incandescent 
 solid or liquid matter, as is the light of the sun and stars, 
 but from glowing or luminous gas. 
 
 It was of importance to learn, if possible, from the posi- 
 tion of these bright lines, the chemical nature of the gas 
 or gases of which this nebula consists. 
 
 Measures taken by the micrometer of the most brilliant 
 of the bright lines showed that this line occurs in the spec- 
 trum very nearly in the position of the brightest of the 
 lines in the spectrum of nitrogen. The experiment was 
 then made of comparing the spectrum of nitrogen directly 
 with the bright lines of the nebula. I found that the bright- 
 est of the lines of the nebula coincided with the strongest 
 of the group of lines which are peculiar to nitrogen. It 
 may be, therefore, that the occurrence of this one line only 
 indicates a form of matter more elementary than nitrogen, 
 and which our analysis has not yet enabled us to detect. 
 
 In a similar manner the faintest of the lines was found 
 to coincide with the green line of hydrogen. 
 
 The middle line of the three lines which form the spec- 
 trum of the nebula does not coincide with any strong line 
 in the spectra of about thirty of the terrestrial elements. It 
 is not far from the line of barium, but it does not coincide 
 with it. Besides these bright lines, there was also an ex- 
 ceedingly faint continuous spectrum. The spectrum had 
 no apparent breadth, and must, therefore, have been formed 
 by a minute point of light. The position of this faint spec- 
 
384 HUGGINS 
 
 trum, which crossed the bright lines about the middle of 
 their length, showed that the bright point producing it was 
 situated about the centre of the nebula. Now, this nebula 
 possesses a minute but bright nucleus. We learn from this 
 observation that the matter of the nucleus is almost cer- 
 tainly not in a state of gas, as is the material of the sur- 
 rounding nebula. It consists of opaque matter, which may 
 exist in the form of an incandescent fog of solid or liquid 
 particles. 
 
 The new and unexpected results arrived at by the pris- 
 matic examination of this nebula showed the importance of 
 examining as many as possible of these remarkable bodies. 
 Would all the nebulae give similar spectra? Especially it 
 was of importance to ascertain whether those nebulae which 
 the telescope had certainly resolved into a close aggre- 
 gation of bright points would give a spectrum indicating 
 gaseity. 
 
 The observation with the prism of these objects is ex- 
 tremely difficult, on account of their great faintness. Be- 
 sides this, it is only when the sky is very clear and the moon 
 is absent that the prismatic arrangement of their light is 
 even possible. During the last two years I have examined 
 the spectra of more than sixty nebulae and clusters. These 
 may be divided into two great groups. One group con- 
 sists of the nebulae which give a spectrum similar to the one 
 I have already described, or else of one or two only of the 
 three bright lines. Of the sixty objects examined, about 
 one third belong to the class of gaseous bodies. The light 
 from the remaining forty nebulae and clusters becomes 
 spread out by the prism into a spectrum which is appar- 
 ently continuous. 
 
 I will exhibit upon the screen a few of the more remark- 
 able of the nebulae which are gaseous in their constitution. 
 
 This photograph is from a drawing by Lord Rosse of 
 a small nebula in Aquarius. (I. H. IV.) 
 
 We have here a gaseous system which reminds the 
 observer of Saturn and his rings. The ring is seen edge- 
 wise. 
 
SPECTRUM ANALYSIS 385 
 
 The three bright lines represent the spectrum into which 
 the light of this object is resolved by the prism. 
 
 In this other nebula we find probably an analogous 
 general form of structure. In consequence of the nebula 
 lying in a different position to us, its ring is seen, not edge- 
 wise, but open on the flat. The spectrum consists of three 
 bright lines. 
 
 The arrangement of the streams of light in the object 
 now on the screen suggests a spiral structure. This nebula 
 is remarkable as the only one in which, in addition to the 
 three bright lines, a fourth line was also seen. 
 
 The most remarkable, and possibly the nearest to our 
 system, of the nebulae presenting a ring formation, is the 
 well-known annular nebula in Lyra. The spectrum con- 
 sists of one bright line only. When the slit of the instru- 
 ment crosses the nebula, the line consists of two brighter 
 portions, corresponding to the sections of the ring. A 
 much fainter line joins them, which shows that the faint 
 central portion of the nebula has a similar constitution. 
 
 A nebula remarkable for its large extent and peculiar 
 form is that known as the dumb-bell nebula. The spectrum 
 of this nebula consists of one line only. A prismatic exam- 
 ination of the light from different parts of this object shows 
 that it is throughout of a similar constitution. 
 
 The most widely known, perhaps, of all the nebulae is the 
 remarkable cloudlike object in the sword-handle of Orion. 
 
 This object is also gaseous. Its spectrum consists of 
 three bright lines. Lord Rosse informs me that the bluish- 
 green matter of the nebula has not been resolved by his 
 telescope. In some parts, however, he sees a large number 
 of very minute red stars, which, though apparently con- 
 nected with the irresolvable matter of the nebula, are yet 
 doubtless distinct from it. These stars would be too faint 
 to furnish a visible spectrum. 
 
 I now pass to some examples of the other great group 
 of nebulae and clusters. 
 
 All the true clusters, which are resolved by the telescope 
 into distinct bright points, give a spectrum, which does not 
 25 
 
386 
 
 HUGGINS 
 
 consist of separate bright lines, but is apparently continu- 
 ous in its light. There are many nebulae which furnish a 
 similar spectrum. 
 
 I take, as an example of these nebulae, the great nebula 
 in Andromeda, which is visible to the naked eye, and is not 
 seldom mistaken for a comet. The spectrum of this nebula, 
 though apparently continuous, has some suggestive peculi- 
 arities. The whole of the red and part of the orange are 
 wanting. Besides this character, the brighter parts of the 
 spectrum have a very unequal and mottled appearance. 
 
 It is remarkable that the easily resolved cluster in Her- 
 cules has a spectrum precisely similar. The prismatic con- 
 nection of this cluster with the nebula in Andromeda is con- 
 firmed by telescopic observation. Lord Rosse has dis- 
 covered in this cluster dark streaks or lines similar to those 
 which are seen in the nebula in Andromeda. 
 
 In connection with these observations, it was of great 
 interest to ascertain whether the broad classification 
 afforded by the prism of the nebulae and clusters would cor- 
 respond with the indications of resolvability furnished by 
 the telescope. Would it be found that all the unresolved 
 nebulae are gaseous, and that those which gave a continuous 
 spectrum are clusters of stars? 
 
 Lord Oxmantown has examined all the observations of 
 the sixty nebulae and clusters in my list, which have been 
 made with the great reflecting telescope erected by his 
 father, the Earl of Rosse. 
 
 The results are given in this table: 
 
 
 Continuous 
 spectrum. 
 
 Gaseous 
 spectrum. 
 
 Clusters ... . . . . . ....... 
 
 IO 
 
 o 
 
 
 
 o 
 
 
 IO 
 
 6 
 
 Blue or green, no resolvability, no resolvability seen . -I 
 
 
 
 6 
 
 4 
 5 
 
 Not observed by Lord Rosse 
 
 3i 
 
 IO 
 
 15 
 
 4" 
 
 
 
 
 
 41 
 
 iQ 
 
SPECTRUM ANALYSIS 387 
 
 Considering the great difficulty of successful telescopic 
 observation of these objects, the correspondence between 
 the results of prismatic and telescopic observation may be 
 regarded as close and suggestive. 
 
 Half of the nebulae which give a continuous spectrum 
 have been resolved, and about one third more are probably 
 resolvable; while of the gaseous nebulae none have been 
 certainly resolved, according to Lord Rosse. 
 
 The inquiry now presents itself upon us, What super- 
 structure of interpretation have we a right to raise upon the 
 new facts with which the prism has furnished us? 
 
 Is the existence of the gaseous nebulae an evidence of 
 the reality of that primordial nebulous matter required by 
 the theories of Sir William Herschel and Laplace? 
 
 Again, if we do not accept the view that these nebulae 
 are composed of portions of the original elementary mat- 
 ter out of which suns and planets have been elaborated, 
 what is the cosmical rank and relation which we ought to 
 assign to them? 
 
 As aids to a future determination of these great ques- 
 tions I will refer in a few words to some other observations. 
 
 COMETS 
 
 There are objects in the heavens which occasionally, 
 and under some conditions, resemble closely some of the 
 nebulae. In some positions in their orbits some of the 
 comets appear as round vaporous masses, and, except by 
 their motion, can not be distinguished from nebulae. Does 
 this occasional general resemblance indicate a similarity of 
 nature? If such be the case, if the material of the comets 
 is similar to that of the nebulae, then the study of the won- 
 derful changes which comets undergo in the neighbour- 
 hood of the sun may furnish useful information for a more 
 correct interpretation of the structure and condition of the 
 nebulae. In 1864 Donati found that the spectrum of a 
 comet visible in that year consisted of bright lines. 
 
 Last January a small telescopic comet was visible. Its 
 appearance in a large telescope is represented on the screen. 
 
388 HUGGINS 
 
 It was a nearly circular, very faint vaporous mass. Nearly 
 in the centre a small and rather dim nucleus was seen. 
 When this object was viewed in the spectroscope, two 
 spectra were distinguished a very faint continuous spec- 
 trum of the coma showing that it was visible by reflecting 
 solar light. About the middle of this faint spectrum a 
 bright point was seen. This bright point is the spectrum 
 of the nucleus, and shows that its light is different from that 
 of the coma. This short, bright line indicates that the 
 nucleus of this comet was self-luminous, and further, the 
 position of this line of the spectrum suggests that the ma- 
 terial of the comet was similar to the matter of which the 
 gaseous nebulae consist. 
 
 MEASURES OF THE INTRINSIC BRIGHTNESS OF THE NEBULAE 
 
 It appeared to me that some information of the nature 
 of the nebulae might be obtained from observations of an- 
 other order. If physical changes of the magnitude neces- 
 sary for the conversion of the gaseous bodies into suns are 
 now in progress in the nebulse, surely this process of de- 
 velopment would be accompanied by marked changes in 
 the intrinsic brightness of their light, and in their size. 
 
 Now, since the spectroscope shows these bodies to be 
 continuous masses of gas, it is possible to obtain an approxi- 
 mate measure of their real brightness. It is known that as 
 long as a distant object remains of sensible size, its bright- 
 ness remains unaltered. By a new photometric method I 
 found the intrinsic intensity of the light of three of the 
 gaseous nebulas in terms of a sperm candle burning at the 
 rate of 158 grains per hour: 
 
 Nebula No. 4,628, -j-^-g- part of the intensity of the 
 candle. 
 
 Annular nebula, Lyra, -^V^ part of the intensity of the 
 candle. 
 
 Dumb-bell nebula, 3-5-$-^ part of the intensity of the 
 candle. 
 
 These numbers represent not the apparent brightness 
 only, but the true brightness of these luminous masses, ex- 
 
SPECTRUM ANALYSIS 389 
 
 cept so far as it may have been diminished by a possible 
 power of extinction existing in cosmical space, and by 
 the absorption of our atmosphere. It is obvious that simi- 
 lar observations, made at considerable intervals of time, 
 may show whether the light of these objects is undergoing 
 increase or diminution, or is subject to a periodic variation. 
 If the dumb-bell nebula, the feeble light of which is not 
 more than 3-5-0 inr P ar t f tnat f a candle, be in accordance 
 with popular theory a sun-germ, then it is scarcely possible 
 to put in an intelligible form the enormous number of times 
 by which its light must increase before this faint nebula, 
 feebler now in its glimmering than a rushlight, can rival 
 the dazzling splendour of our sun. 
 
 MEASURES OF THE NEBULAE 
 
 Some of the nebulae are sufficiently defined in outline 
 to admit of accurate measurement. By means of a series 
 of micrometric observations, it may be possible to ascertain 
 whether any considerable alteration in size takes place in 
 nebulae. 
 
 METEORS 
 
 Mr. Alexander Herschel has recently succeeded in sub- 
 jecting another order of the heavenly bodies to prismatic 
 analysis. He has obtained the spectrum of a bright meteor, 
 and also the spectra of some of the trains which meteors 
 leave behind them. A remarkable result of his observations 
 appears to be that sodium, in the state of luminous vapour, 
 is present in the trains of most meteors. 
 
 CONCLUSION 
 
 In conclusion, the new knowledge that has been gained 
 from the observations with the prism may be summed up 
 as follows: 
 
 1. All the brighter stars, at least, have a structure 
 analogous to that of the sun. 
 
 2. The stars contain material elements common to the 
 sun and earth. 
 
 3. The colours of the stars have their origin in the 
 
390 MUGGINS 
 
 chemical constitution of the atmospheres which surround 
 them. 
 
 4. The changes in brightness of some of the variable 
 stars are attended with changes in the lines of absorption of 
 their spectra. 
 
 5. The phenomena of the star in Corona appear to 
 show that in this object, at least, great physical changes 
 are in operation. 
 
 6. There exist in the heavens true nebulae. These ob- 
 jects consist of luminous gas. 
 
 7. The material of comets is very similar to the matter 
 of the gaseous nebulae, and may be identical with it. 
 
 8. The bright point of the star clusters may not be in 
 all cases stars of the same order as the separate bright stars. 
 
 It may be asked what cosmical theory of the origin and 
 relations of the heavenly bodies do these new facts suggest? 
 It would be easy to speculate, but it appears to me that it 
 would not be philosophical to dogmatize at present on a 
 subject of which we know so very little. Our views of the 
 universe are undergoing important changes ; let us wait for 
 more facts, with minds unfettered by any dogmatic theory, 
 and therefore free to receive the obvious teaching, what- 
 ever it may be, of new observations. 
 
 Star differs from star in glory, each nebula and each 
 cluster has its own special features; doubtless in wisdom, 
 and for high and important purposes, the Creator has made 
 them all. 
 
CELESTIAL SPECTROSCOPY 1 
 
 IT is now many years since this association has done hon- 
 our to the science of astronomy in the selection of its 
 president. Since Sir George Airy occupied the chair, in 
 1851, and the late Lord Wrottesley nine years later, in 1860, 
 other sciences have been represented by the distinguished 
 men who have presided over your meetings. The very re- 
 markable discoveries in our knowledge of the heavens 
 which have taken place during this period of thirty years 
 one of amazing and ever-increasing activity in all branches 
 of science have not passed unnoticed in the addresses of 
 your successive presidents; still, it seems to me fitting that 
 I should speak to you to-night chiefly of those newer 
 methods of astronomical research which may have led to 
 those discoveries, and which have become possible by the 
 introduction since 1860 into the observatory of the spec- 
 troscope and the modern photographic plate. 
 
 In 1866 I had the honour of bringing before this asso- 
 ciation, at one of the evening lectures, an account of the 
 first fruits of the novel and unexpected advances in our 
 knowledge of the celestial bodies which followed rapidly 
 upon KirchhofFs original work on the solar spectrum and 
 the interpretation of its lines. 
 
 Since that time a great harvest has been gathered in 
 the same field by many reapers. Spectroscopic astronomy 
 has become a distinct and acknowledged branch of the sci- 
 ence, possessing a large literature of its own, and observa- 
 tories specially devoted to it. The more recent discovery 
 of the gelatine dry plate has given a further great impetus 
 
 1 From " Report of the British Association for the Advancement of 
 Science " (1891). 
 
 391 
 
392 HUGGINS 
 
 to this modern side of astronomy, and has opened a path- 
 way into the unknown of which even an enthusiast thirty 
 years ago would scarcely have dared to dream. 
 
 In no science, perhaps, does the sober statement of the 
 results which have been achieved appeal so strongly to the 
 imagination, and make so evident the almost boundless 
 powers of the mind of man. By means of its light alone to 
 analyze the chemical nature of a far-distant body; to be able 
 to reason about its present state in relation to the past and 
 future; to measure within an English mile or less per sec- 
 ond the otherwise invisible motion which it may have to- 
 ward us or from us; to do more, to make even that which is 
 darkness to our eyes light, and from vibrations which our 
 organs of sight are powerless to perceive, to evolve a reve- 
 lation in which we see mirrored some of the stages through 
 which the stars may pass in their slow evolutional progress 
 surely the record of such achievements, however poor the 
 form of words in which they may be described, is worthy 
 to be regarded as the scientific epic of the present century. 
 
 I do not purpose to attempt a survey of the progress of 
 spectroscopic astronomy from its birth at Heidelberg in 
 1859, but to point out what we do know at present, as dis- 
 tinguished from what we do not know, of a few only of its 
 more important problems, giving a prominent place, in ac- 
 cordance with the traditions of this chair, to the work of 
 the last year or two. 
 
 In the spectroscope itself advances have been made by 
 Lord Rayleigh by his discussion of the theory of the in- 
 strument, and by Professor Rowland in the construction of 
 concave gratings. 
 
 Lord Rayleigh has shown that there is not the neces- 
 sary connection, sometimes supposed, between dispersion 
 and resolving power, as, besides the prism or grating, other 
 details of construction and of adjustment of a spectroscope 
 must be taken into account. 
 
 The resolving power of the prismatic spectroscope is 
 proportional to the length of path in the dispersive me- 
 dium. For the heavy flint glass used in Lord Rayleigh's 
 
CELESTIAL SPECTROSCOPY 393 
 
 experiments the thickness necessary to resolve the sodium 
 lines came out 1.02 centimetre. If this be taken as a unit, 
 the resolving power of a prism of similar glass will be in 
 the neighbourhood of the sodium lines equal to the number 
 of centimetres of its thickness. In other parts of the spec- 
 trum the resolving power will vary inversely as the third 
 power of the wave-length, so that it will be eight times as 
 great in the violet as in the red. The resolving power of 
 a spectroscope is therefore proportional to the total thick- 
 ness of the dispersive material in use, irrespective of the 
 number, the angles, or the setting of the separate prisms 
 into which, for the sake of convenience, it may be dis- 
 tributed. 
 
 The resolving power of a grating depends upon the 
 total number of lines on its surface and the order of spec- 
 trum in use, about one thousand lines being necessary to 
 resolve the sodium lines in the first spectrum. 
 
 As it is often of importance in the record of observations 
 to state the efficiency of the spectroscope with which they 
 were made, Professor Schuster has proposed the use of a 
 unit of purity as well as of resolving power, for the full re- 
 solving power of a spectroscope is realized in practice only 
 when a sufficiently narrow slit is used. The unit of purity 
 also is to stand for the separation of two lines differing by 
 10 1 ofl of their own wave-length; about the separation of the 
 sodium pair at D. 
 
 A further limitation may come in from the physiological 
 fact that, as Lord Rayleigh has pointed out, the eye when 
 its full aperture is used is not a perfect instrument. If we 
 wish to realize the full resolving power of a spectroscope, 
 therefore, the emergent beam must not be larger than about 
 one third of the opening of the pupil. 
 
 Up to the present time the standard of reference for 
 nearly all spectroscopic work continues to be Angstrom's 
 map of the solar spectrum, and his scale based upon his 
 original determinations of absolute wave-length. It is well 
 known, as was pointed out by Thalen in his work on the 
 spectrum of iron in 1884, that Angstrom's figures are 
 
394 
 
 HUGGINS 
 
 slightly too small, in consequence of an error existing in 
 a standard metre used by him. The corrections for this 
 have been introduced into the tables of the wave-lengths 
 of terrestrial spectra collected and revised by a commit- 
 tee of this association from 1885 to 1887. Last year the 
 committee added a table of corrections to Rowland's 
 scale. 
 
 The inconvenience caused by a change of standard 
 scale is, for a time at least, considerable; but there is little 
 doubt that in the near future Rowland's photographic map 
 of the solar spectrum, and his scale based on the determina- 
 tions of absolute wave-length by Peirce and Bell, or the 
 Potsdam scale based on original determinations oy Miiller 
 and Kempf, which differs very slightly from it, will come 
 to be exclusively adopted. 
 
 The great accuracy of Rowland's photographic map is 
 due chiefly to the introduction by him of concave gratings, 
 and of a method for their use, by which the problem of the 
 determination of relative wave-lengths is simplified to meas- 
 ures of near coincidences of the lines in different spectra by 
 a micrometer. 
 
 The concave grating and its peculiar mounting, in 
 which no lenses or telescope are needed, and in which all 
 the spectra are in focus together, formed a new departure of 
 great importance in the measurement of spectral lines. The 
 valuable method of photographic sensitizers for different 
 parts of the spectrum has enabled Professor Rowland to 
 include in his map the whole visible solar spectrum, as well 
 as the ultra-violet portion as far as it can get through our 
 atmosphere. Some recent photographs of the solar spec- 
 trum, which include A, by Mr. George Higgs, are of great 
 technical beauty. 
 
 During the past year the results of three independent 
 researches have appeared, in which the special object of 
 the observers has been to distinguish the lines which are 
 due to our atmosphere from those which are truly solar 
 the maps of M. Thollon, which, owing to his lamented 
 death just before their final completion, have assumed the 
 
CELESTIAL SPECTROSCOPY 395 
 
 character of a memorial to him; maps by Mr. Becker; and 
 sets of photographs of a high and a low sun by Mr. Mc- 
 Clean. 
 
 At the meeting of this association in Bath. Mr. Janssen 
 gave an account of his own researches on the terrestrial 
 lines of the solar spectrum, which owe their origin to the 
 oxygen of our atmosphere. He discovered the remarkable 
 fact that while the intensity of one class of bands varies as 
 the density of the gas, other diffuse bands vary as the 
 square of the density. These observations are in accord- 
 ance with the work of Egoroff and of Olszewski, and of 
 Liveing and Dewar on condensed oxygen. In some recent 
 experiments Olszewski, with a layer of liquid oxygen thirty 
 millimetres thick, saw, as well as four other bands, the band 
 coincident with Fraunhofer's A; a remarkable instance of 
 the persistence of absorption through a great range of 
 temperature. The light which passed through the liquid 
 oxygen had a light-blue colour resembling that of the sky. 
 
 Of not less interest are the experiments of Angstrom 
 which show that the carbonic acid and aqueous vapour 
 of the atmosphere reveal their presence by dark bands in 
 the invisible infra-red region, at the positions of bands of 
 emission of these substances. 
 
 It is now some thirty years since the spectroscope gave 
 us for the first time certain knowledge of the nature of the 
 heavenly bodies, and revealed the fundamental fact that ter- 
 restrial matter is not peculiar to the solar system, but that 
 it is common to all the stars which are visible to us. 
 
 In the case of a star, such as Capella, which has a 
 spectrum almost identical with that of the sun, we feel justi- 
 fied in concluding that the matter of which it is built up 
 is similar, and that its temperature is also high, and not very 
 different from the solar temperature. The task of analyzing 
 the stars and nebulae becomes, however, one of very great 
 difficulty when we have to do with spectra differing from 
 the solar type. We are thrown back upon the laboratory 
 for the information necessary to enable us to interpret the 
 indications of the spectroscope as to the chemical nature, 
 
396 HUGGINS 
 
 the density and pressure, and the temperature of the celes- 
 tial masses. 
 
 What the spectroscope immediately reveals to us are 
 the waves which were set up in the ether filling all inter- 
 stellar space, years or hundreds of years ago, by the motion 
 of the molecules of the celestial substances. As a rule it 
 is only when a body is gaseous and sufficiently hot that the 
 motions within its molecules can produce bright lines and 
 a corresponding absorption. The spectra of the heavenly 
 bodies are, indeed, to a great extent absorption spectra, but 
 we have naturally to study them through the corresponding 
 emission spectra of bodies brought into the gaseous form 
 and rendered luminous by means of flames or of electric 
 discharges. In both cases, unfortunately, it has been shown 
 recently by Professors Liveing and Dewar, Wullner, E. 
 Wiedemann, and others, that there appears to be no certain 
 direct relation between the luminous radiation as shown in 
 the spectroscope and the temperature of the flame, or of 
 the gaseous contents of the vacuum tube that is, in the 
 usual sense of the term as applied to the mean motion of 
 all the molecules. In both cases the vibratory motions 
 within the molecules to which their luminosity is due are 
 almost always much greater than would be produced by 
 encounters of molecules having motions of translation no 
 greater than the average motions which characterize the 
 temperature of the gases as a whole. The temperature of 
 a vacuum tube through which an electric discharge is tak- 
 ing place may be low, as shown thermometrically, quite 
 apart from the consideration of the extreme smallness of 
 the mass of gas, but the vibrations of the luminous mole- 
 cules must be violent in whatever way we suppose them to 
 be set up by the discharge; if we take Schuster's view that 
 comparatively few molecules are carrying the discharge, 
 and that it is to the fierce encounters of these alone that 
 the luminosity is due, then if all the molecules had simi- 
 lar motions, the temperature of the gas would be very 
 high. 
 
 So in flames where chemical changes are in progress, 
 
CELESTIAL SPECTROSCOPY 397 
 
 the vibratory motions of the molecules which are luminous 
 may be, in connection with the energy set free in these 
 changes, very different from those corresponding to the 
 mean temperature of the flame. Under the ordinary con- 
 ditions of terrestrial experiments, therefore, the tempera- 
 ture or the mean vis viva of the molecules may have no 
 direct relation to the total radiation, which, on the other 
 hand, is the sum of the radiation due to each luminous 
 molecule. These phenomena have recently been discussed 
 by Ebert from the standpoint of the electro-magnetic theory 
 of light. 
 
 Very great caution is therefore called for when we at- 
 tempt to reason by the aid of laboratory experiments to 
 the temperature of the heavenly bodies from their radia- 
 tion, especially on the reasonable assumption that in them 
 the luminosity is not ordinarily associated with chemical 
 changes or with electrical discharges, but is due to a simple 
 glowing from the ultimate conversion into molecular mo- 
 tion of the gravitational energy of shrinkage. 
 
 In a recent paper Stas maintains that electric spectra are 
 to be regarded as distinct from flame spectra, and, from re- 
 searches of his own, that the pairs of lines of the sodium 
 spectrum other than D are produced only by disruptive 
 electric discharges. As these pairs of lines are found re- 
 versed in the solar spectrum, he concludes that the sun's 
 radiation is due mainly to electric discharges. But Wolf 
 and Diacon, and later Watts, observed the other pairs of 
 lines of the sodium spectrum when the vapour was raised 
 above the ordinary temperature of the Bunsen flame. Re- 
 cently Liveing and Dewar saw easily, besides D, the citron 
 and green pairs, and sometimes the blue pair and the orange 
 pair, when hydrogen charged with sodium vapour was burn- 
 ing at different pressures in oxygen. In the case of sodium 
 vapours, therefore, and presumably in all other vapours and 
 gases, it is a matter of indifference whether the necessary 
 vibratory motion of the molecules is produced by electric 
 discharges or by flames. The presence of lines in the solar 
 spectrum which we can only produce electrically is an indi- 
 
398 HUGGINS 
 
 cation, however, as Stas points out, of the high tempera- 
 ture of the sun. 
 
 We must not forget that the light from the heavenly 
 bodies may consist of the combined radiations of different 
 layers of gas at different temperatures, and possibly be fur- 
 ther complicated to an unknown extent by the absorption 
 of cooler portions of gas outside. 
 
 Not less caution is needed if we endeavour to argue 
 from the broadening of lines and the coming in of a con- 
 tinuous spectrum as to the relative pressure of the gas in 
 the celestial atmospheres. On the one hand, it can not be 
 gainsaid that in the laboratory the widening of the lines 
 in a Plucker's tube follows upon increasing the density of 
 the residue of hydrogen in the tube, when the vibrations 
 are more frequently disturbed by fresh encounters; and that 
 a broadening of the sodium lines in a flame at ordinary 
 pressure is produced by an increase of the quantity of 
 sodium in the flame; but it is doubtful if pressure, as dis- 
 tinguished from quantity, does produce an increase of the 
 breadth of the lines. An individual molecule of sodium will 
 be sensibly in the same condition, considering the relatively 
 enormous number of the molecules of the other gases, 
 whether the flame is scantily or copiously fed with the 
 sodium salt. With a small quantity of sodium vapour the 
 intensity will be feeble except near the maximum of the 
 lines; when, however, the quantity is increased, the com- 
 parative transparency on the sides of the maximum will 
 allow the light from the additional molecules met with in 
 the path of the visual ray to strengthen the radiation of 
 the molecules farther back, and so increase the breadth of 
 the lines. 
 
 In a gaseous mixture it is found, as a rule, that at the 
 same pressure or temperature, as the encounters with simi- 
 lar molecules become fewer, the spectral lines will be 
 affected as if the body were observed under conditions of 
 reduced quantity or temperature. 
 
 In their recent investigation of the spectroscopic behav- 
 iour of flames under various pressures up to forty atmos- 
 
CELESTIAL SPECTROSCOPY 399 
 
 pheres, Professors Liveing and Dewar have come to the 
 conclusion that though the prominent feature of the light 
 emitted by flames at high pressure appears to be a strong 
 continuous spectrum, there is not the slightest indication 
 that this continuous spectrum is produced by the broad- 
 ening of the lines of the same gases at low pressure. On 
 the contrary, photometric observations of the brightness 
 of the continuous spectrum, as the pressure is varied, show 
 that it is mainly produced by the mutual action of the 
 molecules of a gas. Experiments on the sodium spec- 
 trum were carried up to a pressure of forty atmospheres 
 without producing any definite effect on the width of 
 the lines which could be ascribed to the pressure. In a 
 similar way the lines of the spectrum of water showed no 
 signs of expansion up to twelve atmospheres; though more 
 intense than at ordinary pressure, they remained narrow 
 and clearly defined. 
 
 It follows, therefore, that a continuous spectrum can 
 not be considered, when taken alone, as a sure indication 
 of matter in the liquid or the solid state. Not only, as in 
 the experiments already mentioned, such a spectrum may 
 be due to gas when under pressure, but, as Maxwell pointed 
 out, if the thickness of a medium, such as sodium vapour, 
 which radiates and absorbs different kinds of. light, be very 
 great, and the temperature high, the light emitted will be 
 of exactly the same composition as that emitted by lamp- 
 black at the same temperature, for the radiations which are 
 feebly emitted will be also feebly absorbed, and can reach 
 the surface from immense depths. Schuster has shown 
 that oxygen, even in a partially exhausted tube, can give a 
 continuous spectrum when excited by a feeble electric dis- 
 charge. 
 
 Compound bodies are usually distinguished by a banded 
 spectrum; but on the other hand such a spectrum does not 
 necessarily show the presence of compounds that is, of 
 molecules containing different kinds of atoms but simply 
 of a more complex molecule, w r hich may be made up of 
 similar atoms, and be therefore an allotropic condition of 
 
400 HUGGINS 
 
 the same body. In some cases, for example, in the diffuse 
 bands of the absorption spectrum of oxygen, the bands 
 may have an intensity proportional to the square of the 
 density of the gas, and may be due either to the formation 
 of more complex molecules of the gas with increase of pres- 
 sure, or it may be to the constraint to which the molecules 
 are subject during their encounters with one another. 
 
 It may be thought that at least in the coincidences of 
 bright lines we are on the solid ground of certainty, since 
 the length of the waves set up in the ether by a molecule, 
 say of hydrogen, is the most fixed and absolutely perma- 
 nent quantity in Nature, and is so of physical necessity, 
 for with any alteration the molecule would cease to be 
 hydrogen. 
 
 Such would be the case if the coincidence were certain; 
 but an absolute coincidence can be only a matter of greater 
 or less probability, depending on the resolving power em- 
 ployed, on the number of the lines which correspond and 
 on their characters. When the coincidences are very nu- 
 merous, as in the case of iron and the solar spectrum, or 
 the lines are characteristically grouped, as in the case of 
 hydrogen and the solar spectrum, we may regard the coin- 
 cidence as certain; but the progress of science has been 
 greatly retarded by resting important conclusions upon the 
 apparent coincidence of single lines, in spectroscopes of 
 very small resolving power. In such cases, unless other 
 reasons supporting the coincidence are present, the proba- 
 bility of a real coincidence is almost too small to be of any 
 importance, especially in the case of a heavenly body which 
 may have a motion of approach or of recession of unknown 
 amount. 
 
 But even here we are met by the confusion introduced 
 by multiple spectra, corresponding to different molecular 
 groupings of the same substance; and further, to the in- 
 fluence of substances in vapour upon each other; for when 
 several gases are present together, the phenomena of radia- 
 tion and reversal by absorption are by no means the same 
 as if the gases were free from each other's influence, and 
 
CELESTIAL SPECTROSCOPY 4 OI 
 
 especially is this the case when they are illuminated by an 
 electric discharge. 
 
 I have said as much as time will permit, and I think 
 indeed sufficient, to show that it is only by the laborious 
 and slow process of most cautious observations that the 
 foundations of the science of celestial physics can be surely 
 laid. We are at present in a time of transition when the 
 earlier and, in the nature of things, less precise observa- 
 tions are giving place to work of an order of accuracy much 
 greater than was formerly considered attainable with ob- 
 jects of such small brightness as the stars. 
 
 The accuracy of the earlier determinations of the spectra 
 of the terrestrial elements is in most cases insufficient for 
 modern work on the stars as well as on the sun. They fall 
 much below the scale adopted in Rowland's map of the 
 sun, as well as below the degree of accuracy attained at 
 Potsdam by photography in a part of the spectrum for 
 the brighter stars. Increase of resolving power very fre- 
 quently breaks up into groups, in the spectra of the sun 
 and stars, the lines which had been regarded as single, and 
 their supposed coincidences with terrestrial lines fall to 
 the ground. For this reason many of the early conclusions, 
 based on observations as good as it was possible to make 
 at the time with the less powerful spectroscopes then in use, 
 may not be found to be maintained under the much greater 
 resolving power of modern instruments. 
 
 The spectroscope has failed as yet to interpret for us the 
 remarkable spectrum of the aurora borealis. Undoubtedly 
 in this phenomenon portions of our atmosphere are lighted 
 up by electric discharges; we should expect, therefore, to 
 recognise the spectra of the gases known to be present in 
 it. As yet we have not been able to obtain similar spectra 
 from these gases artificially, and especially w r e do not know 
 the origin of the principal line in the green, which often 
 appears alone, and may have therefore an origin independ- 
 ent of that of the other lines. Recently the suggestion has 
 been made that the aurora is a phenomenon produced by 
 the dust of meteors and falling stars, and that near positions 
 26 
 
402 HUGGINS 
 
 of certain auroral lines to lines or flutings of manganese, 
 lead, barium, thallium, iron, etc., are sufficient to justify us 
 in regarding meteoric dust in the atmosphere as the origin 
 of the auroral spectrum. Liveing and Dewar have made a 
 conclusive research on this point by availing themselves of 
 the dust of excessive minuteness thrown off from the sur- 
 face of electrodes of various metals and meteorites by a 
 disruptive discharge and carried forward into the tube of 
 observation by a more or less rapid current of air or other 
 gas. These experiments prove that metallic dust, however 
 fine, suspended in a gas will not act like gaseous matter in 
 becoming luminous with its characteristic spectrum in an 
 electric discharge, similar to that of the aurora. Professor 
 Schuster has suggested that the principal line may be due 
 to some very light gas which is present in too small a pro- 
 portion to be detected by chemical analysis or even by the 
 spectroscope in the presence of the other gases near the 
 earth, but which at the height of the auroral discharges is 
 in a sufficiently greater relative proportion to give a spec- 
 trum. Lemstrom, indeed, states that he saw this line in the 
 silent discharge of a Holtz machine on a mountain in Lap- 
 land. The lines may not have been obtained in our labora- 
 tories from the atmospheric gases, on account of the diffi- 
 culty of reproducing in tubes with sufficient nearness the 
 conditions under which the auroral discharges take place. 
 In the spectra of comets the spectroscope has shown the 
 presence of carbon presumably in combination with hydro- 
 gen, and also sometimes with nitrogen; and in the case of 
 comets approaching very near the sun, the lines of sodium, 
 and other lines which have been supposed to belong to iron. 
 Though the researches of Professor H. A. Newton and of 
 Professor Schiaparelli leave no doubt of the close connec- 
 tion of comets with corresponding periodic meteor swarms, 
 and therefore of the probable identity of cometary matter 
 with that of meteorites, with which the spectroscopic evi- 
 dence agrees, it would be perhaps unwise at present to 
 attempt to define too precisely the exact condition of the 
 matter which forms the nucleus of the comet. In any 
 
CELESTIAL SPECTROSCOPY 403 
 
 case the part of the light of the comet which is not reflected 
 solar light can scarcely be attributed to a high tempera- 
 ture produced by the clashing of separate meteoric stones 
 set up within the nucleus by the sun's disturbing force. 
 We must look rather to disruptive electric discharges pro- 
 duced probably by processes of evaporation due to increased 
 solar heat, which would be amply sufficient to set free por- 
 tions of the occluded gases into the vacuum of space. May 
 it be that these discharges are assisted, and indeed pos- 
 sibly increased, by the recently discovered action of the 
 ultra-violet part of the sun's light? Hertz has shown that 
 ultra-violet light can produce a discharge from a negatively 
 electrified piece of metal, while Hallwachs and Righi have 
 shown further that ultra-violet light can even charge posi- 
 tively an unelectrified piece of metal phenomena which 
 Lenard and Wolf associate with the disengagement from 
 the metallic surfaces of very minute particles. Similar 
 actions on cometary matter, unscreened as it is by an ab- 
 sorptive atmosphere, at least of any noticeable extent, may 
 well be powerful when a comet approaches the sun, and 
 help to explain an electrified condition of the evaporated 
 matter which would possibly bring it under the sun's re- 
 pulsive action. We shall have to return to this point in 
 speaking of the solar corona. 
 
 A very great advance has been made in our knowledge 
 of the constitution of the sun by the recent work at the 
 Johns Hopkins University by means of photography and 
 concave gratings, in comparing the solar spectrum, under 
 the great resolving power, directly with the spectra of the 
 terrestrial elements. Professor Rowland has shown that 
 the lines of thirty-six terrestrial elements at least are cer- 
 tainly present in the solar spectrum, while eight others are 
 doubtful. Fifteen elements, including nitrogen as it shows 
 itself under an electric discharge in a vacuum tube, have 
 not been found in the solar spectrum. Some ten other ele- 
 ments, inclusive of oxygen, have not yet been compared 
 with the sun's spectrum. 
 
 Rowland remarks that of the fifteen elements named as 
 
404 HUGGINS 
 
 not found in the sun, many are so classed because they have 
 few strong lines, or none at all, in the limit of the solar spec- 
 trum as compared by him with the arc. Boron has only 
 two strong lines. The lines of bismuth are compound and 
 too diffuse. Therefore, even in the case of these fifteen ele- 
 ments, there is little evidence that they are really absent 
 from the sun. 
 
 It follows that if the whole earth were heated to the 
 temperature of the sun, its spectrum would resemble very 
 closely the solar spectrum. 
 
 Rowland has not found any lines common to several 
 elements, and in the case of some accidental coincidences 
 more accurate investigation reveals some slight difference 
 of wave-length or a common impurity. Further, the rela- 
 tive strength of the lines in the solar spectrum is generally, 
 with a few exceptions, the same as that in the electric arc, 
 so that Rowland considers that his experiments show 
 " very little evidence " of the breaking up of the terrestrial 
 elements in the sun. 
 
 Stas, in a recent paper, gives the final results of eleven 
 years of research on the chemical elements in a state of 
 purity, and on the possibility of decomposing them by the 
 physical and chemical forces at our disposal. His experi- 
 ments on calcium, strontium, lithium, magnesium, silver, 
 sodium, and thallium show that these substances retain their 
 individuality under all conditions, and are unalterable by 
 any forces that we can bring to bear upon them. 
 
 Professor Rowland looks to the solar lines which are 
 unaccounted for as a means of enabling him to discover 
 such new terrestrial elements as still lurk in raw minerals 
 and earths, by confronting their spectra directly with that 
 of the sun. He has already resolved yttrium spectro- 
 scopically into three compounds, and actually into two. 
 The comparison of the results of this independent analyt- 
 ical method with the remarkable but different conclusions 
 to which M. Lecoq de Boisbaudran and Mr. Crookes have 
 been led respectively, from spectroscopic observation on 
 these bodies when glowing under molecular bombardment 
 
CELESTIAL SPECTROSCOPY 405 
 
 in a vacuum tube, will be awaited with much interest. It 
 is worthy of remark that as our knowledge of the spectrum 
 of hydrogen in its complete form came to us from the stars, 
 it is now from the sun that chemistry is probably about to 
 be enriched by the discovery of new elements. 
 
 In a discussion in the Bakerian lecture for 1885 f what 
 we knew up to that time of the sun's corona, I was led to 
 the conclusion that the corona is essentially a phenomenon 
 similar in the cause of its formation to the tails of comets 
 namely, that it consists for the most part probably of mat- 
 ter going from the sun under the action of a force, possibly 
 electrical, which varies as the surface, and can therefore in 
 the case of highly attenuated matter easily master the force 
 of gravity even near the sun. Though many of the coronal 
 particles may return to the sun, those which form the long 
 rays or streamers do not return; they separate and soon 
 become too diffused to be any longer visible, and may well 
 go to furnish the matter of the zodiacal light, which other- 
 wise has not received a satisfactory explanation. And fur- 
 ther, if such a force exist at the sun, the changes of terres- 
 trial magnetism may be due to direct electric action, as the 
 earth moves through lines of inductive force. 
 
 These conclusions seem to be in accordance broadly 
 with the lines along which thought has been directed by 
 the results of subsequent eclipses. Professor Schuster takes 
 an essentially similar view, and suggests that there may be 
 a direct electric connection between the sun and the planets. 
 He asks further whether the sun may not act like a magnet 
 in consequence of its revolution about its axis. Professor 
 Bigelow has recently treated the coronal forms by the 
 theory of spherical harmonics, on the supposition that we 
 see phenomena similar to those of free electricity, the rays 
 being lines of force, and the coronal matter discharged 
 from the sun, or at least arranged or controlled by these 
 forces. At the extremities of the streams for some reasons 
 the repulsive power may be lost, and gravitation set in, 
 bringing the matter back to the sun. The matter which 
 does leave the sun is persistently transported to the equa- 
 
406 HUGGINS 
 
 torial plane of the corona; in fact, the zodiacal light may 
 be the accumulation at great distances from the sun along 
 this equator of similar material. Photographs on a larger 
 scale will be desirable for the full development of the con- 
 clusions which may follow from this study of the curved 
 forms of the coronal structure. Professor Schaeberle, how- 
 ever, considers that the coronal phenomena may be satis- 
 factorily accounted for on the supposition that the corona 
 is formed of streams of matter ejected mainly from the 
 spot zones with great initial velocities, but smaller than 
 382 miles a second; further, that the different types of 
 the corona are due to the effects of perspective on the 
 streams from the earth's place at the time relatively to the 
 plane of the solar equator. 
 
 Of the physical and the chemical nature of the coronal 
 matter we know very little. Schuster concludes, from an 
 examination of the eclipses of 1882, 1883, and 1886, that 
 the continuous spectrum of the corona has the maximum 
 of actinic intensity displaced considerably toward the red 
 when compared with the spectrum of the sun, which shows 
 that it can only be due in small part to solar light scattered 
 by small particles. The lines of calcium and of hydrogen 
 do not appear to form part of the normal spectrum of the 
 corona. The green coronal line has no known representa- 
 tive in terrestrial substances, nor has Schuster been able 
 to recognise any of our elements in the other lines of the 
 corona. 
 
 The spectra of the stars are almost infinitely diversified, 
 yet they can be arranged, with some exceptions, in a series 
 in which the adjacent spectra, especially in the photographic 
 region, are scarcely distinguishable, passing from the bluish- 
 white stars like Sirius, through stars more or less solar 
 in character, to stars with banded spectra, which divide 
 themselves into two apparently independent groups, ac- 
 cording as the stronger edge of the bands is toward the 
 red or the blue. In such an arrangement the sun's place 
 is toward the middle of the series. 
 
 At present a difference of opinion exists as to the direc- 
 
CELESTIAL SPECTROSCOPY 407 
 
 tion in the series in which evolution is proceeding: whether 
 by further condensation white stars pass into the orange 
 and red stages, or whether these more coloured stars are 
 younger and will become white by increasing age. The 
 latter view was suggested by Johnstone Stoney in 1867. 
 
 About ten years ago Ritter, in a series of papers, dis- 
 cussed the behaviour of gaseous masses during condensa- 
 tion, and the probable resulting constitution of the heav- 
 enly bodies. According to him, a star passes through the 
 orange and red stages twice, first during a comparatively 
 short period of increasing temperature which culminates in 
 the white stage, and a second time during a more prolonged 
 stage of gradual cooling. He suggested that the two 
 groups of banded stars may correspond to those different 
 periods, the young stars being those in which the stronger 
 edge of the dark band is toward the blue, the other banded 
 stars, which are relatively less luminous and few in number, 
 being those which are approaching extinction through age. 
 
 Recently a similar evolutional order has been sug- 
 gested, which is based upon the hypothesis that the nebulae 
 and stars consist of colliding meteoric stones in different 
 stages of condensation. 
 
 More recently the view has been put forward that the 
 diversified spectra of the stars do not represent the stages 
 of an evolutional progress, but are due for the most part 
 to differences of original constitution. 
 
 The few minutes which can be given to this part of the 
 address are insufficient for a discussion of these different 
 views. I purpose, therefore, to state briefly, and with re- 
 serve, as the subject is obscure, some of the considerations 
 from the characters of their spectra which appeared to me 
 to be in favour of the evolutional order in which T arranged 
 the stars from their photographic spectra in 1879. This 
 order is essentially the same as Vogel had previously pro- 
 posed in his classification of the stars in 1874, in which the 
 white stars, which are most numerous, represent the early 
 adult and most persistent stage of stellar life, the solar con- 
 dition that of full maturity and of commencing age; while 
 
408 HUGGINS 
 
 in the orange and red stars with banded spectra we see the 
 setting in and advance of old age. But this statement must 
 be taken broadly, and not as asserting that all stars, how- 
 ever different in mass and possibly to some small extent in 
 original constitution, exhibit one invariable succession of 
 spectra. 
 
 In the spectra of the white stars the dark metallic lines 
 are relatively inconspicuous and occasionally absent, at the 
 same time that the dark lines of hydrogen are usually strong, 
 and more or less broad, upon a continuous spectrum, which 
 is remarkable for its brilliancy at the blue end. In some of 
 these stars the hydrogen and some other lines are bright 
 and sometimes variable. 
 
 As the greater or less prominence of the hydrogen lines, 
 dark or bright, is characteristic of the white stars as a class, 
 and diminishes gradually with the incoming and increase 
 in strength of the other lines, we are probably justified in 
 regarding it as due to some conditions which occur natu- 
 rally during the progress of stellar life and not to a pecul- 
 iarity of original constitution. 
 
 To produce a strong absorption-spectrum a substance 
 must be at the particular temperature at which it is notably 
 absorptive; and, further, this temperature must be suf- 
 ficiently below that of the region behind from which the 
 light comes for the gas to appear, so far as its special rays 
 are concerned, as darkness upon it. Considering the high 
 temperature to which hydrogen must be raised before it 
 can show its characteristic emission and absorption, we 
 shall probably be right in attributing the relative feebleness 
 or absence of the other lines, not to the paucity of the 
 metallic vapours, but rather to their being so hot relatively 
 to the substance behind them as to show feebly, if at all, by 
 reversion. Such a state of things would more probably be 
 found, it seems to me, in conditions anterior to the solar 
 stage. A considerable cooling of the sun would probably 
 give rise to banded spectra due to compounds, or to more 
 complex molecules, which might form near the condensing 
 points of the vapours. 
 
CELESTIAL SPECTROSCOPY 409 
 
 The sun and stars are generally regarded as consisting 
 of glowing vapours surrounded by a photosphere where 
 condensation is taking place, the temperature of the photo- 
 spheric layer from which the greater part of the radiation 
 comes being constantly renewed from the hotter matter 
 within. 
 
 At the surface the convection currents would be too 
 strong, producing a considerable commotion, by which the 
 different gases would be mixed and not allowed to retain 
 the inequality of proportions at different levels due to their 
 vapour densities. 
 
 Now, the conditions of the radiating photosphere and 
 those of the gases above it, on which the character of the 
 spectrum of a star depends, will be determined, not alone 
 by the temperature, but also by the force of gravity in these 
 regions; this force will be fixed by the star's mass and its 
 stage of condensation, and will become greater as the star 
 continues to condense. 
 
 In the case of the sun the force of gravity has already 
 become so great at the surface that the decrease of the den- 
 sity of the bases must be extremely rapid, passing in the 
 space of a few miles from atmospheric pressure to a den- 
 sity infinitesimally small; consequently the temperature 
 gradient at the surface, if determined solely by expansion, 
 must be extremely rapid. The gases here, however, are ex- 
 posed to the fierce radiation of the sun, and unless wholly 
 transparent would take up heat, especially if any solid or 
 liquid particles were present from condensation or convec- 
 tion currents. 
 
 From these causes, within a very small extent of space 
 at the surface of the sun, all bodies with which we are ac- 
 quainted should fall to a condition in which the extremely 
 tenuous gas could no longer give a visible spectrum. The 
 insignificance of the angle subtended by this space as seen 
 from the earth should cause the boundary of the solar at- 
 mosphere to appear defined. If the boundary which we 
 see be that of the sun proper, the matter above it will have 
 to be regarded as in an essentially dynamical condition 
 
4 io HUGGINS 
 
 an assemblage, so to speak, of gaseous projectiles for the 
 most part falling back upon the sun after a greater or less 
 range of flight. But in any case it is within a space of rela- 
 tively small extent in the sun, and probably in the other 
 solar stars, that the reversion which is manifested by dark 
 lines is to be regarded as taking place. 
 
 Passing backward in the star's life, we should find a 
 gradual weakening of gravity at the surface, a reduction of 
 the temperature gradient so far as it was determined by 
 expansion, and convection currents of less violence pro- 
 ducing less interference with the proportional quantities of 
 gases due to their vapour densities, while the effects of 
 eruptions would be more extensive. 
 
 At last we might come to a state of things in which, if 
 the stars were hot enough, only hydrogen might be suf- 
 ficiently cool relatively to the radiation behind to produce 
 a strong absorption. The lower vapours would be pro- 
 tected, and might continue to be relatively too hot for their 
 lines to appear very dark upon the continuous spectrum; 
 besides, their lines might be possibly to some extent effaced 
 by the coming in under such conditions in the vapours 
 themselves of a continuous spectrum. 
 
 In such a star the light radiated toward the upper part 
 of the atmosphere may have come from portions lower 
 down of the atmosphere itself, or at least from parts not 
 greatly hotter. There may be no such great difference of 
 temperature of the low and less low portions of the star's 
 atmosphere as to make the darkening effect of absorption 
 of the protected metallic vapours to prevail over the illu- 
 minating effect of their emission. 
 
 It is only by a vibratory motion corresponding to a very 
 high temperature that the bright lines of the first spectrum 
 of hydrogen can be brought out, and by the equivalence 
 of absorbing and emitting power that the corresponding 
 spectrum of absorption should be produced; yet for a 
 strong absorption to show itself, the hydrogen must be 
 cool relatively to the source of radiation behind it, whether 
 this be condensed particles or gas. Such conditions, it 
 
CELESTIAL SPECTROSCOPY 411 
 
 seems to me, should occur in the earlier rather than in the 
 more advanced stages of condensation. 
 
 The subject is obscure, and we may go wrong in our 
 mode of conceiving of the probable progress of events, but 
 there can be no doubt that in one remarkable instance the 
 white-star spectrum is associated with an early stage of 
 condensation. 
 
 Sirius is one of the most conspicuous examples of one 
 type of this class of stars. Photometric observations, com- 
 bined with its ascertained parallax, show that this star emits 
 from forty to sixty times the light of our sun, even to the 
 eye, which is insensible to ultra-violet light, in which Sirius 
 is very rich, while we learn from the motion of its compan- 
 ion that its mass is not much more than double that of our 
 sun. It follows that unless we attribute to this star an 
 improbably great emissive power, it must be of immense 
 size, and in a much more diffuse and therefore an earlier 
 condition than our sun; though probably at a later stage 
 than those white stars in which the hydrogen lines are 
 bright. 
 
 A direct determination of the relative temperature of the 
 photospheres of the stars might possibly be obtained in 
 some cases from the relative position of maximum radia- 
 tion of their continuous spectra. Langley has shown that 
 through the whole range of temperature on which we can 
 -experiment, and presumably at temperatures beyond the 
 maximum of radiation, power in solid bodies gradually 
 shifts upward in the spectrum from the infra-red through 
 the red and orange, and that in the sun it has reached 
 the blue. 
 
 The defined character, as a rule, of the stellar lines of 
 absorption suggests that the vapours producing them do 
 not at the same time exert any strong power of general 
 absorption. Consequently we should probably not go far 
 wrong, when the photosphere consists of liquid or solid 
 particles, if we could compare select parts of the continuous 
 spectrum between the stronger lines or where they are few- 
 est. It is obvious that if extended portions of different 
 
4 I2 HUGGINS 
 
 stellar spectra were compared, their true relation would be 
 obscured by the line-absorption. 
 
 The increase of temperature, as shown by the rise in 
 the spectrum of the maximum of radiation, may not always 
 be accompanied by a corresponding greater brightness of 
 a star as estimated by the eye, which is an extremely im- 
 perfect photometric instrument. Not only is the eye blind 
 to large regions of radiations, but even for the small range 
 of light that we can see, the visual effect varies enormously 
 with its colour. According to Professor Langley, the same 
 amount of energy which just enables us to perceive light 
 in the crimson at A would in the green produce a visual 
 effect 100,000 times greater. In the violet the proportional 
 effect would be 1,600, in the blue 62,000, in the yellow 
 28,000, in the orange 14,000, and in the red 1,200. Cap- 
 tain Abney's recent experiments make the sensitiveness of 
 the eye for the green near F to be 750 times greater than 
 for red about C. It is for this reason, at least in part, that 
 I suggested in 1864, and have since shown by direct ob- 
 servation, that the spectrum of the nebula in Andromeda, 
 and presumably of similar nebulae, is in appearance only 
 wanting in the red. 
 
 The stage at which the maximum radiation is in the 
 green, corresponding to the eye's greatest sensitiveness, 
 would be that in which it could be most favourably meas- 
 ured by eye-photometry. As the maximum rose into the 
 violet and beyond, the star would increase in visual bright- 
 ness, but not in proportion to the increase of energy radi- 
 ated by it. 
 
 The brightness of a star would be affected by the nature 
 of the substance by which the light was chiefly emitted. 
 In the laboratory solid carbon exhibits the highest emis- 
 sive power. A stellar stage in which radiation comes, to a 
 large extent, from a photosphere of the solid particles of 
 this substance would be favourable for great brilliancy. 
 Though the stars are built up of matter essentially similar 
 to that of the sun, it does not follow that the proportion 
 of the different elements is everywhere the same. It may 
 
CELESTIAL SPECTROSCOPY 413 
 
 be that the substances condensed in the photospheres of 
 different stars may differ in their emissive powers, but prob- 
 ably not to a great extent. 
 
 All the heavenly bodies are seen by us through the 
 tinted medium of our atmosphere. According to Langley, 
 the solar stage of stars is not really yellow, but, even as 
 gauged by our imperfect eyes, would appear bluish-white 
 if we could free ourselves from the deceptive influences of 
 our surroundings. 
 
 From these considerations it follows that we can scarcely 
 infer the evolutional stages of the stars from a simple com- 
 parison of their eye-magnitudes. We should expect the 
 white stars to be, as a class, less dense than the stars in the 
 solar stage. As great mass might bring in the solar type 
 of spectrum at a relatively earlier time, some of the bright- 
 est of these stars may be very massive and brighter than 
 the sun for example, the brilliant star Arcturus. For these 
 reasons the solar stars should not only be denser than the 
 white stars, but perhaps, as a class, surpass them in mass 
 and eye-brightness. 
 
 It has been shown by Lane that, so long as a conden- 
 sing gaseous mass remains subject to the laws of a purely 
 gaseous body, its temperature will continue to rise. 
 
 The greater or less breadth of the lines of absorption 
 of hydrogen in the white stars may be due to variations 
 of the depth of the hydrogen in the line of sight, arising 
 from the causes which have been discussed. At the sides 
 of the lines the absorption and emission are feebler than 
 in the middle, and would come out more strongly with a 
 greater thickness of gas. 
 
 The diversities among the white stars are nearly as nu- 
 merous as the individuals of the class. Time does not 
 permit me to do more than to record that in addition to 
 the three subclasses into which they have been divided by 
 Vogel, Scheiner has recently investigated minor differences 
 as suggested by the character of the third line of hydrogen 
 near G. He has pointed out, too, that so far as his observa- 
 tions go the white stars in the constellation of Orion stand 
 
414 HUGGINS 
 
 alone, with the exception of Algol, in possessing a dark line 
 in the blue which has apparently the same position as a 
 bright line in the great nebula of the same constellation; 
 and Pickering finds in his photographs of the spectra of 
 the stars dark lines corresponding to the principal lines 
 of the bright-line stars, and the planetary nebulae with the 
 exception of the chief nebular line. The association of 
 white stars with nebular matter in Orion, in the Pleiades, 
 in the region of the Milky Way, and in other parts of the 
 heavens, may be regarded as falling in with the view that 
 I have taken. 
 
 In the stars possibly farther removed from the white 
 class than our sun, belonging to the first division of Vogel's 
 third class, which are distinguished by absorption bands 
 with their stronger edge toward the blue, the hydrogen lines 
 are narrower than in the solar spectrum. In these stars the 
 density-gradient is probably still more rapid, the depth of 
 hydrogen may be less, and possibly the hydrogen molecules 
 may be affected by a larger number of encounters with dis- 
 similar molecules. In some red stars with dark hydro- 
 carbon bands the hydrogen lines have not been certainly 
 observed; if they are really absent it may be because the 
 temperature has fallen below the point at which hydrogen 
 can exert its characteristic absorption; besides, some hydro- 
 gen will have united with the carbon. The coming in of 
 the hydrocarbon bands may indicate a later evolutional 
 stage, but the temperature may still be high, as acetylene 
 can exist in the electric arc. 
 
 A number of small stars more or less similar to those 
 which are known by the names of their discoverers, Wolf 
 and Rayet, have been found by Pickering in his photo- 
 graphs. These are remarkable for several brilliant groups 
 of bright lines, including frequently the hydrogen lines and 
 the line D 3 , upon a continuous spectrum strong in blue 
 and violet rays, in which are also dark lines of absorption. 
 As some of the bright groups appear in his photographs 
 to agree in position with corresponding bright lines in 
 the planetary nebulae, Pickering suggests that these stars 
 
CELESTIAL SPECTROSCOPY 415 
 
 should be placed in one class with them, although the 
 brightest nebular line is absent from these stars. The sim- 
 plest conception of their nature would be that each star is 
 surrounded by a nebula, the bright groups being due to 
 gaseous matter outside the star. Mr. Roberts, however, 
 has not been able to bring out any indication of nebulosity 
 by prolonged exposure. The remarkable star rj Argus may 
 belong to this class of the heavenly bodies. 
 
 In the nebulae the elder Herschel saw portions of the 
 fiery mist or " shining fluid " out of which the heavens and 
 the earth had been slowly fashioned. For a time this view 
 of the nebulae gave place to that which regarded them as 
 external galaxies, cosmical " sand heaps," too remote to be 
 resolved into separate stars; though, indeed, in 1858, Mr. 
 Herbert Spencer showed that the observations of nebulae 
 up to that time were really in favour of an evolutional 
 progress. 
 
 In 1864 I brought the spectroscope to bear upon them; 
 the bright lines which flashed upon the eye showed the 
 source of the light of a number of them to be glowing gas, 
 and so restored these bodies to what is probably their true 
 place, as an early stage of sidereal life. 
 
 At that early time our knowledge of stellar spectra was 
 small. For this reason, partly, and probably also under the 
 undue influence of theological opinions then widely preva- 
 lent, I unwisely wrote in my original papers in 1864 that 
 " in these subjects we no longer have to do with a special 
 modification of our own type of sun, but find ourselves in 
 presence of objects possessing a distinct and peculiar plan 
 of structure." Two years later, however, in a lecture before 
 this association, I took a truer position. " Our views of 
 the universe," I said, " are undergoing important changes; 
 let us wait for more facts, with minds unfettered by any 
 dogmatic theory, and therefore free to receive the obvious 
 teaching, whatever it may be, of new observations." 
 
 Let us turn aside for a moment from the nebulae in the 
 sky to the conclusions to which philosophers had been 
 irresistibly led by a consideration of the features of the solar 
 
4I 6 HUGGINS 
 
 system. We have before us in the sun and planets obvi- 
 ously not a haphazard aggregation of bodies, but a system 
 resting upon a multitude of relations pointing to a common 
 physical cause. From these considerations Kant and La- 
 place formulated the nebular hypothesis, resting it on gravi- 
 tation alone, for at that time the science of the conservation 
 of energy was practically unknown. These philosophers 
 showed how, on the supposition that the space now occu- 
 pied by the solar system was once filled by a vaporous mass, 
 the formation of the sun and planets could be reasonably 
 accounted for. 
 
 By a totally different method of reasoning, modern sci- 
 ence traces the solar system backward step by step to a 
 similar state of things at the beginning. According to 
 Helmholtz, the sun's heat is maintained by the contraction 
 of his mass, at the rate of about 220 feet a year. Whether 
 at the present time the sun is getting hotter or colder we 
 do not certainly know. We can reason back to the time 
 when the sun was sufficiently expanded to fill the whole 
 space occupied by the solar system, and was reduced to a 
 great glowing nebula. Though man's life, the life of the 
 race, perhaps, is too short to give us direct evidence of any 
 distinct stages of so august a process, still the probability is 
 great that the nebular hypothesis, especially in the more 
 precise form given to it by Roche, does express broadly, 
 notwithstanding some difficulties, the succession of events 
 through which the sun and planets have passed. 
 
 The nebular hypothesis of Laplace requires a rotating 
 mass of fluid which at successive epochs became unstable 
 from excess of motion, and left behind rings, or more prob- 
 ably perhaps lumps of matter, from the equatorial regions. 
 
 The difficulties to which I have referred have suggested 
 to some thinkers a different view of things, according to 
 which it is not necessary to suppose that one part of the 
 system gravitationally supports another. The whole may 
 consist of a congeries of discrete bodies, even if these bodies 
 be the ultimate molecules of matter. The planets may have 
 been formed by the gradual accretion of such discrete 
 
CELESTIAL SPECTROSCOPY 417 
 
 bodies. On the view that the material of the condensing 
 solar system consisted of separate particles or masses, we 
 have no longer the fluid pressure which is an essential part 
 of Laplace's theory. Faye, in his theory of evolution from 
 meteorites, has to throw over this fundamental idea of the 
 nebular hypothesis, and he formulates instead a different 
 succession of events in which the outer planets were formed 
 last a theory which has difficulties of its own. 
 
 Professor George Darwin has recently shown, from an 
 investigation of the mechanical condition of a swarm of 
 meteorites, that on certain assumptions a meteoric swarm 
 might behave as a coarse gas, and in this way bring back 
 the fluid pressure exercised by one part of the system on 
 the other, which is required by Laplace's theory. Our chief 
 assumption consists in supposing that such inelastic bodies 
 as meteoric stones might attain the effective elasticity of a 
 high order which is necessary to the theory through the 
 sudden volatilization of a part of their mass at an encoun- 
 ter, by which what is virtually a violent explosive is intro- 
 duced between the two colliding stones. Professor Darwin 
 is careful to point out that it must necessarily be obscure as 
 to how small a mass of solid matter can take up a very large 
 amount of energy in a small fraction of a second. 
 
 Any direct indications from the heavens themselves, 
 however slight, are of so great value that I should perhaps 
 in this connection call attention to a recent remarkable 
 photograph by Mr. Roberts of the great nebula in Androm- 
 eda. On this plate we seem to have presented to us some 
 stage of cosmical evolution on a gigantic scale. The photo- 
 graph shows a sort of whirlpool disturbance of the luminous 
 matter which is distributed in a plane inclined to the line 
 of sight, in which a series of rings of bright matter separated 
 by dark spaces, greatly foreshortened by perspective, sur- 
 round a large undefined central mass. The parallax of this 
 nebula has not been ascertained, but there can be little 
 doubt that we are looking upon a system very remote, and 
 therefore of a magnitude great beyond our power of ade- 
 quate comprehension. The matter of this nebula, in what- 
 27 
 
418 HUGGINS 
 
 ever state it may be, appears to be distributed, as in so 
 many other nebulae, in rings or spiral streams, and to sug- 
 gest a stage in a succession of evolutional events not in- 
 consistent with that which the nebular hypothesis requires. 
 To liken this object more directly to any particular stage 
 in the formation of the solar system would be to compare 
 things great with small, and might be indeed to introduce 
 a false analogy; but, on the other hand, we should err 
 through an excess of caution if we did not accept the re- 
 markable features brought to light by this photograph as 
 a presumptive indication of a progress of events in cos- 
 mical history following broadly upon the lines of Laplace's 
 theory. 
 
 The old view of the original matter of the nebulae, that 
 it consisted of a " fiery mist " 
 
 "... a tumultuous cloud 
 Instinct with fire and 'nitre " 
 
 fell at once with the rise of the science of thermodynamics. 
 In 1854 Helmholtz showed that the supposition of an 
 original fiery condition of the nebulous stuff was unneces- 
 sary, since in the mutual gravitation of widely separated 
 matter we have a store of potential energy sufficient to 
 generate the high temperature of the sun and stars. We 
 can scarcely go wrong in attributing the light of the nebulae 
 to the conversion of the gravitational energy of shrinkage 
 into molecular motion. 
 
 The idea that the light of comets and of nebulae may 
 be due to a succession of ignited flashes of gas from the 
 encounters of meteoric stones was suggested by Professor 
 Tait, and was brought to the notice of this association in 
 1871 by Sir William Thomson in his presidential address. 
 
 The spectrum of the bright-line nebulae is certainly not 
 such a spectrum as we should expect from the flashing by 
 collisions of meteorites similar to those which have been 
 analyzed in our laboratories. The strongest lines of the 
 substances which in the case of such meteorites would first 
 show themselves iron, sodium, magnesium, nickel, etc. 
 
CELESTIAL SPECTROSCOPY 419 
 
 are not those which distinguish the nebular spectrum. On 
 the contrary, this spectrum is chiefly remarkable for a few 
 brilliant lines, very narrow and denned upon a background 
 of a faint continuous spectrum, which contains numerous 
 bright lines, and probably some lines of absorption. 
 
 The two most conspicuous lines have not been inter- 
 preted; for though the second line falls near, it is not coin- 
 cident with a strong double line of iron. It is hardly neces- 
 sary to say that though the near position of the brightest 
 line to the bright double line of nitrogen, as seen in a small 
 spectroscope in 1864, naturally suggested at that early time 
 the possibility of the presence of this element in the nebulae, 
 I have been careful to point out, to prevent misapprehen- 
 sion, that in more recent years the nitrogen line and subse- 
 quently a lead line have been employed by me solely as 
 fiducial points of reference in the spectrum. 
 
 The third line we know to be the second line of the 
 first spectrum of hydrogen. Mr. Keeler has seen the first 
 hydrogen line in the red, and photographs show that this 
 hydrogen spectrum is probably present in its complete form, 
 or nearly so, as we first learned to know it in the absorption 
 spectrum of the white stars. 
 
 We are not surprised to find associated with it the line 
 D 3 near the position of the absent sodium lines, probably 
 due to the atom of some unknown gas, which in the sun 
 can only show itself in the outbursts of highest tempera- 
 ture, and for this reason does not reveal itself by absorption 
 in the solar spectrum. 
 
 It is not unreasonable to assume that the two bright- 
 est lines, which are of the same order as the third line, are 
 produced by substances of a similar nature, in which a 
 vibratory motion corresponding to a very high temperature 
 is also necessary. These substances, as well as that repre- 
 sented by the line D 3 , may be possibly some of the un- 
 known elements which are wanting in our terrestrial chem- 
 istry between hydrogen and lithium, unless, indeed, D 3 be 
 on the lighter side of hydrogen. 
 
 In the laboratory we must have recourse to the elec- 
 
420 HUGGINS 
 
 trie discharge to bring out the spectrum of hydrogen; but 
 in a vacuum tube, though the radiation may be great, from 
 the relative fewness of the luminous atoms or molecules or 
 from some other cause, the temperature of the gas as a 
 whole may be low. 
 
 On account of the large extent of the nebulae, a com- 
 paratively small number of luminous molecules or atoms 
 would probably be sufficient to make the nebulas as bright 
 as they appear to us. On such an assumption the average 
 temperature may be low, but the individual particles, which 
 by their encounters are luminous, must have motions cor- 
 responding to a very high temperature, and in this sense 
 be extremely hot. 
 
 In such diffuse masses, from the great mean length of 
 free path, the encounters would be rare but correspond- 
 ingly violent, and tend to bring about vibrations of com- 
 paratively short period, as appears to be the case if we may 
 judge by the great relative brightness of the more re- 
 frangible lines of the nebular spectrum. 
 
 Such a view may perhaps reconcile the high tempera- 
 ture which the nebular spectrum undoubtedly suggests with 
 the much lower mean temperature of the gaseous mass, 
 which we should expect at so early a stage of condensation, 
 unless we assume a very enormous mass ; or that the matter 
 coming together had previously considerable motion, or 
 considerable molecular agitation. 
 
 If the hydrogen shown by the spectroscope in the 
 nebulae and in the atmospheres of the stars is retained by 
 the bodies, we should be able to assign approximately an 
 inferior limit for the force of gravity at their surfaces, pro- 
 vided we assume that the gas is in the uncombined state, 
 and always exists in some greater proportion than in the 
 free space about them. 
 
 The inquisitiveness of the human mind does not allow 
 us to remain content with the interpretation of the present 
 state of the cosmical masses, but suggests the question 
 
 " What see'st thou else 
 In the dark backward and abysm of time? " 
 
CELESTIAL SPECTROSCOPY 
 
 421 
 
 What was the original state of things? how has it come 
 about that by the side of aging worlds we have nebula 
 in a relatively younger stage? Have any of them received 
 their birth from dark suns, which have collided into new 
 life, and so belong to a second or later generation of the 
 heavenly bodies? 
 
 During the short historic period, however, there is no 
 record of such an event; still, it would seem to be only 
 through the collision of dark suns, of which the number 
 must be increasing, that a temporary rejuvenescence of 
 the heavens is possible, and by such ebbings and flowings 
 of stellar life that the inevitable end to which evolution in 
 its apparently uncompensated progress is carrying us can, 
 even for a little, be delayed. 
 
 We can not refuse to admit as possible such an origin 
 for nebulae. In considering, however, the formation of the 
 existing nebulae, we must bear in mind that, in the part of 
 the heavens within our ken, the stars still in the early and 
 middle stages of evolution exceed greatly in number those 
 which appear to be in an advanced condition of condensa- 
 tion. Indeed, we find some stars which may be regarded 
 as not far advanced beyond the nebular condition. 
 
 It may be that the cosmical bodies which are still 
 nebulous owe the lateness of their development to some 
 conditions of the part of space where they occur, such as 
 conceivably a greater original homogeneity, in consequence 
 of which condensation began less early. In other parts of 
 space condensation may have been still further delayed or 
 even have not yet begun. It is worthy of remark that these 
 nebulae group themselves about the Milky Way, where we 
 find a preponderance of the white-star type of stars, and 
 almost exclusively the bright-line stars which Pickering 
 associates with the planetary nebulae. Further, Dr. Gill 
 concludes, from the rapidity with which they impress them- 
 selves upon the plate, that the fainter stars of the Milky 
 \Vay also, to a large extent, belong to this early type of 
 stars. At the same time other types of stars occur also 
 over this region, and the red hydrocarbon stars are found 
 
4 22 MUGGINS 
 
 in certain parts: but possibly these stars may be before 
 or behind the Milky Way, and not physically connected 
 with it. 
 
 If light matter be suggested by the spectrum of these 
 nebulae, it may be asked further, as a pure speculation, 
 whether in them we are witnessing possibly a later con- 
 densation of the light matter which had been left behind, 
 at least in a relatively greater proportion, after the first 
 growth of the worlds into which the heavier matter con- 
 densed, though not without some entanglement of the 
 higher substances. The wide extent and great diffuseness 
 of this bright-line nebulosity over a large part of the con- 
 stellation of Orion may be regarded perhaps as pointing 
 in this direction. The diffuse nebulous matter streaming 
 round the Pleiades may possibly be another instance, 
 though the character of its spectrum has not yet been 
 ascertained. 
 
 In the planetary nebulae, as a rule, there is a sensible in- 
 crease of the faint continuous spectrum, as well as a slight 
 thickening of the bright lines toward the centre of the 
 nebula, appearances which are in favour of the view that 
 these bodies are condensing gaseous masses. 
 
 Professor George H. Darwin, in his investigation of the 
 equilibrium of a rotating mass of fluid, found, in accord- 
 ance with the independent researches of Poincare, that when 
 a portion of the centra"! body becomes detached through 
 increasing angular velocity, the portion should bear a far 
 larger ratio to the remainder than is observed in the planets 
 and satellites of the solar system; even taking into account 
 heterogeneity from the condensation of the parent mass. 
 
 Now this state of things, in which the masses, though 
 not equal, are of the same order, does seem to prevail in 
 many nebulae, and to have given birth to a large class of 
 binary stars. Mr. See has recently investigated the evolu- 
 tion of bodies of this class, and points out their radical dif- 
 ferences from the solar system in the relatively large mass- 
 ratios of the component bodies, as well as the high eccen- 
 tricities of their orbits brought about by tidal friction which 
 
CELESTIAL SPECTROSCOPY 423 
 
 would play a more important part in the evolution of such 
 a system. Considering the large number of these bodies, he 
 suggests that the solar system should perhaps no longer 
 be regarded as representing celestial evolution in its normal 
 form 
 
 " A goodly Paterne to whose perfect mould 
 He fashioned them . . ." 
 
 but rather as modified by conditions which are exceptional. 
 
 It may well be that in the very early stages condensing 
 masses are subject to very different conditions, and that 
 condensation may not always begin at one or two centres, 
 but sometimes set in at a large number of points, and pro- 
 ceed in the different cases along very different lines of evo- 
 lution. Besides its more direct use in the chemical analysis 
 of the heavenly bodies, the spectroscope has given to us a 
 great and unexpected power of advance along the lines of 
 the older astronomy. In the future a higher value may, 
 indeed, be placed upon this indirect use of the spectroscope 
 than upon its chemical revelations. 
 
 By no direct astronomical methods could motions of 
 approach or of recession of the stars be even detected, much 
 less could they be measured. A body coming directly to- 
 ward us or going directly from us appears to stand still. In 
 the case of the stars we can receive no assistance from 
 change of size or of brightness. The stars show no true 
 disks in our instruments, and the nearest of them is so 
 far off that if it were approaching us at the rate of a hundred 
 miles in a second of time, a whole century of such rapid 
 approach would not do more than increase its brightness by 
 the one-fortieth part. 
 
 Still, it was only too clear that, so long as we were un- 
 able to ascertain directly those components of the stars' 
 motions which lie in the line of sight, the speed and direc- 
 tion of the solar motion in space, and many of the great 
 problems of the constitution of the heavens, must remain 
 more or less imperfectly known. Now, the spectroscope 
 has placed in our hands this power, which, though so essen- 
 tial, appeared almost in the nature of things to lie forever 
 
424 MUGGINS 
 
 beyond our grasp; it enables us to measure directly and 
 under favourable circumstances to within a mile per sec- 
 ond, or even less, the speed of approach or of recession of 
 a heavenly body. This method of observation has the great 
 advantage for the astronomer of being independent of the 
 distance of the moving body, and is therefore as applicable 
 and as certain in the case of a body on the extreme confines 
 of the visible universe, so long as it is bright enough, as in 
 the case of a neighbouring planet. 
 
 Doppler had suggested as far back as 1841 that the 
 same principle, on which he had shown that a sound should 
 become sharper or flatter if there were an approach or a 
 recession between the ear and the source of the sound, 
 would apply equally to light; and he went on to say that 
 the difference of colour of some of the binary stars might 
 be produced in this way by their motions. Doppler was 
 right in that the principle is true in the case of light, but 
 he was wrong in the particular conclusion which he drew 
 from it. Even if we suppose a star to be moving with a 
 sufficiently enormous velocity to alter sensibly its colour to 
 the eye, no such change would actually be seen, for the 
 reason that the store of invisible light beyond both limits 
 of the visible spectrum, the blue and the red, would be 
 drawn upon, and the light-waves invisible to us would be 
 exalted or degraded so as to take the place of those raised 
 or lowered in the visible region, and the colour of the star 
 would remain unchanged. About eight years later Fizeau 
 pointed out the importance of considering the individual 
 wave-lengths of which white light is composed. It is, in- 
 deed, Doppler's principle which underlies the early deter- 
 mination of the velocity of light by Roemer; but this 
 method, in its converse form, can scarcely be regarded as 
 of practical value for the motions in the line of sight of 
 binary stars. As soon, however, as we had learned to rec- 
 ognise the lines of known substances in the spectra of the 
 heavenly bodies, Doppler's principle became applicable as 
 the basis of a new and most fruitful method of investigation. 
 The measurement of the small shift of the celestial lines 
 
CELESTIAL SPECTROSCOPY 425 
 
 from their true positions, as shown by the same lines in the 
 spectrum of a terrestrial substance, gives to us the means 
 of ascertaining directly in miles per second the speed of 
 approach or of recession of the heavenly body from which 
 the light has come. 
 
 An account of the first application of this method of re- 
 search to the stars, which was made in my observatory in 
 1868, was given by Sir Gabriel Stokes from this chair at the 
 meeting at Exeter in 1869. The stellar motions deter- 
 mined by me were afterward confirmed by Professor Vogel 
 in the case of Sirius, and in the case of other stars by 
 Mr. Christie, now Astronomer Royal, at Greenwich; but 
 necessarily, in consequence of the inadequacy of the instru- 
 ments then in use for so delicate an inquiry, the amounts of 
 these motions were but approximate. 
 
 The method was shortly afterward taken up systematic- 
 ally at Greenwich and at the Rugby Observatory. It is to 
 be greatly regretted that, for some reasons, the results have 
 not been sufficiently accordant and accurate for a research 
 of such exceptional delicacy. On this account, probably, 
 as well as that the spectroscope at that early time had 
 scarcely become a familiar instrument in the observatory, 
 astronomers were slow in availing themselves of this new 
 and remarkable power of investigation. That this com- 
 parative neglect of so truly wonderful a method of ascer- 
 taining what was otherwise outside our powers of observa- 
 tion has greatly retarded the progress of astronomy during 
 the last fifteen years is but too clearly shown by the bril- 
 liant results which within the last two years have followed 
 fast upon the recent masterly application of this method by 
 photography at Potsdam, and by eye with the needful ac- 
 curacy at the Lick Observatory. At last this use of the 
 spectroscope has taken its true place as one of the most 
 potent methods of astronomical research. It gives us the 
 motions of approach and of recession, not in angular meas- 
 ures, which depend for their translation into actual veloci- 
 ties upon separate determinations of parallactic displace- 
 ments, but at once in terrestrial units of distance. 
 
426 HUGGINS 
 
 This method of work will doubtless be very prominent 
 in the astronomy of the near future, and to it probably we 
 shall have to look for the more important discoveries in 
 sidereal astronomy which will be made during the coming 
 century. 
 
 In his recent application of photography to this method 
 of determining celestial motions, Professor Vogel, assisted 
 by Dr. Scheiner, considering the importance of obtaining 
 the spectrum of as many stars as possible on an extended 
 scale without an exposure inconveniently long, wisely de- 
 termined to limit the part of the spectrum on the plate to 
 the region for which the ordinary silver-bromide gelatine 
 plates are most sensitive namely, to a small distance on 
 each side of G, and to employ as the line of comparison the 
 hydrogen line near G, and recently also certain lines of iron. 
 The most minute and complete mechanical arrangements 
 were provided for the purpose of securing the absolute 
 rigidity of the comparison spectrum relatively to that of 
 the star, and for permitting temperature adjustments and 
 other necessary ones to be made. 
 
 The perfection of these spectra is shown by the large 
 number of lines, no fewer than 250 in the case of Capella, 
 within the small region of the spectrum on the plate. 
 Already the motions of about fifty stars have been meas- 
 ured with an accuracy, in the case of the large number of 
 them, of about an English mile per second. 
 
 At the Lick Observatory it has been shown that ob- 
 servations can be made directly by eye with an accuracy 
 equally great. Mr. Keeler's brilliant success has followed 
 in great measure from the use of the third and fourth 
 spectra of a grating with 14,438 lines to the inch. The 
 marvellous accuracy attainable in his hands on a suitable 
 star is shown by observations on three nights of the star 
 Arcturus, the greatest divergence of his measures being 
 not greater than six tenths of a mile per second, while the 
 mean of the three nights' work agreed with the mean of 
 five photographic determinations of the same star at Pots- 
 dam to within one tenth of an English mile. These are 
 
CELESTIAL SPECTROSCOPY 427 
 
 determinations of the motions of a sun so stupendously 
 remote that even the method of parallax practically fails 
 to fathom the depth of intervening space, and by means of 
 light-waves which have been, according to Elkin's nominal 
 parallax, nearly two hundred years upon their journey. 
 
 Mr. Keeler with his magnificent means has accom- 
 plished a task which I attempted in vain in 1874, with the 
 comparatively poor appliances at my disposal, of measuring 
 the motions in the line of sight of some of the planetary 
 nebulae. As the stars have considerable motions in space, 
 it was to be expected that nebulas should possess similar 
 motions, for the stellar motions must have belonged to the 
 nebulae out of which they have been evolved. My instru- 
 mental means, limiting my power of detection to motions 
 greater than twenty-five miles per second, were insufficient. 
 Mr. Keeler has found in the examination of ten nebulas 
 motions varying from two miles to twenty-seven miles, with 
 one exceptional motion of nearly forty miles. 
 
 For the nebula of Orion, Mr. Keeler finds a motion of 
 recession of about ten miles a second. Now, this motion 
 agrees with what it should appear to have from the drift of 
 the solar system itself, so far as it has been possible at pres- 
 ent to ascertain the probable velocity of the sun in space. 
 This grand nebula, of vast extent and of extreme tenuity, is 
 probably more nearly at rest relatively to the stars of our 
 system than any other celestial object we know; still, it 
 would seem more likely that even here we have some mo- 
 tion, small though it may be, than that the motions of the 
 matter of which it is formed were so absolutely balanced 
 as to leave this nebula in the unique position of absolute 
 immobility in the midst of whirling and drifting suns and 
 systems of suns. 
 
 The spectroscopic methods of determining celestial mo- 
 tions in the line of sight has recently become fruitful in a 
 new but not altogether unforeseen direction, for it has, 
 so to speak, given us a separating power far beyond that 
 of any telescope the glass-maker and the optician could 
 construct, and so enabled us to penetrate into mysteries 
 
428 HUGGINS 
 
 hidden in stars apparently single, and altogether unsus- 
 pected of being binary systems. The spectroscope has not 
 simply added to the list of the known binary stars, but has 
 given to us for the first time a knowledge of a new class 
 of stellar systems, in which the components are in some 
 cases of nearly equal magnitude and in close proximity, 
 and are revolving with velocities greatly exceeding the 
 planetary velocities of our system. 
 
 The K line in the photographs of Mizar, taken at the 
 Harvard College Observatory, was found to be double at 
 intervals of fifty-two days. The spectrum was therefore not 
 due to a single source of light, but to the combined effect 
 of two stars moving periodically in opposite directions in 
 the line of sight. It is obvious that if two stars revolve 
 round their common centre of gravity in a plane not per- 
 pendicular to the line of sight, all the lines in a spectrum 
 common to the two stars will appear alternately single or 
 double. 
 
 In the case of Mizar and the other stars to be mentioned, 
 the spectroscopic observations are not as yet extended 
 enough to furnish more than an approximate determination 
 of the elements of their orbits. 
 
 Mizar especially, on account of its relatively long period, 
 about 105 days, needs further observations. The two stars 
 are moving each with a velocity of about 50 miles a second, 
 probably in elliptical orbits, and are about 143,000,000 
 miles apart. The stars of about equal brightness have to- 
 gether a mass about forty times as great as that of our sun. 
 
 A similar doubling of the lines showed itself in the Har- 
 vard photographs of fi Aurigae at the remarkably close in- 
 terval of almost exactly two days, indicating a period of 
 revolution of about four days. According to Vogel's later 
 observations, each star has a velocity of nearly 70 miles a 
 second, the distance between the stars being little more 
 than 7,500,000 miles, and the mass of the system 4.7 times 
 that of the sun. The system is approaching us at the speed 
 of about 1 6 miles a second. 
 
 The telescope could never have revealed to us double 
 
CELESTIAL SPECTROSCOPY 
 
 429 
 
 stars of this order. In the case of /9 Aurigse, combining 
 Vogel's distance with Pritchard's recent determination of 
 the star's parallax, the greatest angular separation of the 
 stars as seen from the earth would be ^-J-^ part of a second 
 of arc, and therefore very far too small for detection by the 
 largest telescopes. If we take the relation of aperture to 
 separating power usually accepted, an object-glass of about 
 80 feet in diameter would be needed to resolve this binary 
 star. The spectroscope which takes no note of distance 
 magnifies, so to speak, this minute angular separation 4,000 
 times; in other words, the doubling of the lines, which is 
 the phenomenon that we have to observe, amounts to the 
 easily measurable quantity of twenty seconds of arc. 
 
 There were known, indeed, variable stars of short period, 
 which it had been suggested might be explained on the 
 hypothesis of a dark body revolving about a bright sun in 
 a few days; but this theory was met by the objection that 
 no such systems of closely revolving suns were known to 
 exist. 
 
 The Harvard photographs of which we have been speak- 
 ing were taken with a slitless form of spectroscope, the 
 prisms being placed, as originally by Fraunhofer, before 
 the object-glass of the telescope. This method, though it 
 possesses some advantages, has the serious drawback of not 
 permitting a direct comparison of the star's spectrum with 
 the terrestrial spectra. It is obviously unsuited to a variable 
 star like Algol, where one star only is bright, for in such a 
 case there would be no doubling of the lines, but only a 
 small shift to and fro in the spectrum of the lines of the 
 bright star as it moved in its orbit alternately toward and 
 from our system, which would need for its detection the 
 fiducial positions of terrestrial lines compared directly with 
 them. 
 
 For such observations the Potsdam spectrograph was 
 well adapted. Professor Vogel found that the bright star 
 Algol did oscillate backward and forward in the visual direc- 
 tion in a period corresponding to the known variation of 
 its light. The explanation which had been suggested for the 
 
430 
 
 HUGGINS 
 
 star's variability, that it was partially eclipsed at regular in- 
 tervals of 68.8 hours by a dark companion large enough to 
 cut off nearly five sixths of its light, was therefore the true 
 one. The dark companion, no longer able to hide itself 
 by its obscureness, was brought out into the light of direct 
 observation by means of its gravitational effects. 
 
 Seventeen hours before minimum Algol is receding at 
 the rate of about 24^ miles a second, while seventeen hours 
 after minimum it is found to be approaching with a speed 
 of about 28^ miles. From these data, together with those 
 of the variation of its light, Vogel found, on the assumption 
 that both stars have the same density, that the companion, 
 nearly as large as the sun but with about one fourth his 
 mass, revolves with a velocity of about 55 miles a second. 
 The bright star, of about twice the size and mass, moves 
 about the common centre of gravity with the speed of 
 about 26 miles a second. The system of the two stars, 
 which are about 3,250,000 miles apart, considered as a 
 whole, is approaching us with a velocity of 24 miles a sec- 
 ond. The great difference in luminosity of the two stars, 
 not less than fifty times, suggests rather that they are in 
 different stages of condensation, and dissimilar in density. 
 
 It is obvious that if the orbit of a star with an obscure 
 companion is sufficiently inclined to the line of sight, the 
 companion will pass above or below the bright star and 
 produce no variation of its light. Such systems may be 
 numerous in the heavens. In Vogel's photographs, Spica, 
 which is not variable, by a small shifting of its lines reveals 
 a backward and forward periodical pulsation due to orbital 
 motion. As the pair whirl round their common centre of 
 gravity, the bright star is sometimes advancing, at others 
 receding. They revolve in about four days, each star mov- 
 ing with a velocity of about 56 miles a second in an orbit 
 probably nearly circular, and possess a combined mass of 
 rather more than two and a half times that of the sun. Tak- 
 ing the most probable value for the star's parallax, the 
 greatest angular separation of the stars would be far too 
 small to be detected with the most powerful telescopes. 
 
CELESTIAL SPECTROSCOPY 431 
 
 If in a close double star the fainter companion is of the 
 white-star type, while the bright star is solar in character, 
 the composite spectrum would be solar with the hydrogen 
 lines unusually strong. Such a spectrum would in itself 
 afford some probability of a double origin, and suggest the 
 existence of a companion star. In the case of a true binary 
 star the orbital motions of the pair would reveal themselves 
 in a small periodical swaying of the hydrogen lines rela- 
 tively to the solar ones. 
 
 Professor Pickering considers that his photographs 
 show ten stars with composite spectra; of these, five are 
 known to be double. The others are : r Persei, f Aurigae, 
 B Sagittarii, 31 Ceti, and Capricorni. Perhaps Kyrae 
 should be added to this list. 
 
 In his recent classical work on the rotation of the sun, 
 Duner has not only determined the solar rotation for the 
 equator, but for different parallels of latitude up to 75. 
 The close accordance of his results shows that these obser- 
 vations are sufficiently accurate to be discussed with the 
 variation of the solar rotation for different latitudes, which 
 had been determined by the older astronomical methods 
 from the observations of the solar spots. 
 
 Though I have already spoken incidentally of the in- 
 valuable aid which is furnished by photography in some of 
 the applications of the spectroscope to the heavenly bodies, 
 the new power which modern photography has put into the 
 hands of the astronomer is so great, and has led already, 
 within the last few years, to new acquisitions or knowledge 
 of such vast importance that it is fitting that a few sen- 
 tences should be specially devoted to this subject. 
 
 Photography is no new discovery, being about half a 
 century old; it may excite surprise, and indeed possibly 
 suggest some apathy on the part of astronomers, that 
 though the suggestion of the application of photography 
 to the heavenly bodies dates from the memorable occasion 
 when, in 1839, Arago, announcing to the Academic des Sci- 
 ences the great discovery of Niepce and Daguerre, spoke of 
 the possibility of taking pictures of the sun and moon by 
 
432 
 
 HUGGINS 
 
 the new process, yet that it is only within a few years that 
 notable advances in astronomical methods and discovery 
 have been made by its aid. 
 
 The explanation is to be found in the comparative un- 
 suitability of the earlier photographic methods for use in 
 the observatory. In justice to the early workers in astro- 
 nomical photography, among whom Bond, De la Rue, J. 
 W. Draper, Rutherfurd, and Gould hold a foremost place, 
 it is needful to state clearly that the recent great successes 
 in astronomical photography are not due to greater skill, 
 nor, to any great extent, to superior instruments, but to the 
 very great advantages which the modern gelatine dry plate 
 possesses for use in the observatory over the methods of 
 Daguerre, and even over the wet collodion film on glass 
 which, though a great advance on the silver plate, went but 
 a little way toward putting into the hands of the astronomer 
 a photographic surface adapted fully to his wants. 
 
 The modern silver-bromide gelatine plate, except for its 
 grained texture, meets the needs of the astronomer at all 
 points. It possesses extreme sensitiveness; it is always 
 ready for use; it can be placed in a^ny position; it can be 
 exposed for hours; lastly, it does not need immediate de- 
 velopment, and for this reason can be exposed again to the 
 same object on succeeding nights, so as to make up by 
 several instalments, as the weather may permit, the total 
 time of exposure which is deemed necessary. 
 
 Without the assistance of photography, however greatly 
 the resources of genius might overcome the optical and 
 mechanical difficulties of constructing large telescopes, the 
 astronomer would have to depend in the last resource upon 
 his eye. Now, we can not by the force of continued look- 
 ing bring into view an object too feebly luminous to be 
 seen at the first and keenest moment of vision. But the 
 feeblest light which falls upon the plate is not lost, but it 
 is taken in and stored up continuously. Each hour the 
 plate gathers up 3,600 times the light energy which is re- 
 ceived during the first second. It is by this power of accu- 
 mulation that the photographic plate may be said to in- 
 
CELESTIAL SPECTROSCOPY 
 
 433 
 
 crease, almost without limit, though not in separating 
 power, the optical means at the disposal of the astronomer 
 for the discovery or the observation of faint objects. 
 
 Two principal directions may be pointed out in which 
 photography is of great service to the astronomer. It en- 
 ables him within the comparatively short time of a single 
 exposure to secure permanently, with great exactness, the 
 relative positions of hundreds or even of thousands of stars, 
 or the minute features of nebulae or other objects, or the 
 phenomena of a passing eclipse, tasks which by means of 
 the eye and hand could only be accomplished, if at all, after 
 a very great expenditure of time and labour. Photography 
 puts it in the power of the astronomer to accomplish in the 
 short span of his own life, and so enter into their fruition, 
 great works which otherwise must, have been passed on by 
 him as a heritage of labour to succeeding generations. 
 
 The second great service which photography renders is 
 not simply an aid to the powers the astronomer already 
 possesses. On the contrary, the plate, by recording light- 
 waves which are both too small and too large to excite 
 vision in the eye, brings him into new regions of knowl- 
 edge, such as the infra-red and the ultra-violet parts of the 
 spectrum, which must have remained forever unknown but 
 for artificial help. 
 
 The present year will be memorable in astronomical his- 
 tory for the poetical beginning of the " Photographic Chart 
 and Catalogue of the Heavens/' which took their origin in 
 an International Conference which met in Paris, in 1887, 
 by the invitation of M. 1'Amiral Mouchex, director of the 
 Paris Observatory. 
 
 The richness in stars down to the ninth magnitude qf 
 the photographs of the comet of 1882, taken at the Cape 
 Observatory under the superintendence of Dr. Gill, and the 
 remarkable star charts of the brothers Henry which fol- 
 lowed two years later, astonished the astronomical world. 
 The great excellence of these photographs, which was due 
 mainly to the superiority of the gelatine plate, suggested to 
 these astronomers a complete map of the sky, and a little 
 28 
 
434 
 
 HUGGINS 
 
 later gave birth in the minds of the Paris astronomers to 
 the grand enterprise of an " International Chart of the 
 Heavens." The actual beginning of the work this year is 
 in no small degree due to the great energy and tact with 
 which the director of the Paris Observatory has conducted 
 the initial steps, through the many delicate and difficult 
 questions which have unavoidably presented themselves in 
 an undertaking which depends upon the harmonious work- 
 ing in common of many nationalities, and of no fewer than 
 eighteen observatories in all parts of the world. The three 
 years since 1887 have not been too long for the detailed 
 organization of this work, which has called for several elab- 
 orate preliminary investigations on special points in which 
 our knowledge was insufficient, and which have been ably 
 carried out by Professors Vogel and Bakhuyzen, Dr. Tre- 
 pied, Dr. Scheiner, Dr. Gill, the Astronomer Royal, and 
 others. Time also was required for the construction of the 
 new and special instruments. 
 
 The decisions of the conference in their final form pro- 
 vide for the construction of a great photographic chart of 
 the heavens, with exposures corresponding to forty min- 
 utes' exposure at Paris, which it is expected will reach 
 down to stars of about the fourteenth magnitude. As each 
 plate is to be limited to four square degrees, and as each 
 star, to avoid possible errors, is to appear on two plates, 
 over 22,000 photographs will be required. For the more 
 accurate determination of the positions of the stars, a reseau 
 with lines at distances of five millimetres apart is to be pre- 
 viously impressed by a faint light upon the plate, so that 
 the image of the reseau will appear together with the images 
 of the stars when the plate is developed. This great work 
 will be divided, according to their latitudes, among eighteen 
 observatories provided with similar instruments, though not 
 necessarily constructed by the same maker. Those in the 
 British dominions and at Tacubaya have been constructed 
 by Sir Howard Grubb. 
 
 Besides the plates to form the great chart, a second set 
 of plates for a catalogue is to be taken, with a shorter ex- 
 
CELESTIAL SPECTROSCOPY 435 
 
 posure, which will give stars to the eleventh magnitude 
 only. These plates, by a recent decision of the permanent 
 committee, are to be pushed on as actively as possible, 
 though, as far as may be practicable, plates for the charts 
 are to be taken concurrently. Photographing the plates 
 for the catalogue is but the first step in this work, and only 
 supplies the data for the elaborate measurements which 
 have to be made, which are, however, less laborious than 
 would be required for a similar catalogue without the aid 
 of photography. 
 
 Already Dr. Gill has nearly brought to conclusion, with 
 the assistance of Professor Kapteyn, a preliminary photo- 
 graphic survey of the southern heavens. 
 
 With an exposure sufficiently long for the faintest stars 
 to impress themselves upon the plate, the accumulating 
 action still goes on for the brighter stars, producing a great 
 enlargement of their images from optical and photographic 
 causes. The question has occupied the attention of many 
 astronomers whether it is possible to find a law connecting 
 the diameters of these more or less over-exposed images 
 with the relative brightness of the stars themselves. The 
 answer will come out undoubtedly in the affirmative, though 
 at present the empirical formulae which have been sug- 
 gested for this purpose differ from each other. Captain 
 Abney proposes to measure the total photographic action, 
 including density as well as size, by the obstruction which 
 the stellar image offers to light. 
 
 A further question follows as to the relation which the 
 photographic magnitudes of stars bears to those determined 
 by eye. Visual magnitudes are the physiological expres- 
 sion of the eye's integration of that part of the star's light 
 which extends from the red to the blue. Photographic 
 magnitudes represent the plate's integration of another part 
 of the star's light namely, from a little below where the 
 power of the eye leaves off in the blue to where the light 
 is cut off by the glass, or is greatly reduced by want of 
 proper corrections when a refracting telescope is used. It 
 is obvious that the two records are taken by different meth- 
 
436 HUGGINS 
 
 ods in dissimilar units of different parts of the star's light. 
 In the case of certain coloured stars the photographic 
 brightness is very different from the visual brightness; but 
 in all stars changes, especially of a temporary character, 
 may occur in the photographic or the visual region, unac- 
 companied by similar changes in the other part of the 
 spectrum. For these reasons it would seem desirable that 
 the two sets of magnitudes should be tabulated independ- 
 ently, and be regarded as supplementary of each other. 
 
 The determination of the distances of the fixed stars 
 from the small apparent shift of their positions when viewed 
 from widely separated positions of the earth in its orbit is 
 one of the most refined operations of the observatory. The 
 great precision with which this minute angular quantity, a 
 fraction of a second of arc only, has to be measured, is so 
 delicate an operation with the ordinary micrometer 
 though, indeed, it was with this instrument that the classi- 
 cal observations of Sir Robert Ball were made that a spe- 
 cial instrument, in which the measures are made by moving 
 the two halves of a divided object-glass, known as a heli- 
 ometer, has been pressed into this service, and quite re- 
 cently, in the skilful hands of Dr. Gill and Dr. Elkin, has 
 largely increased our knowledge in this direction. 
 
 It is obvious that photography might be here of good 
 service, if we could rely upon measurements of photo- 
 graphs of the same stars taken at suitable intervals of time. 
 Professor Pritchard, to whom is due the honour of having 
 opened this new path, aided by his assistants, has proved 
 by elaborate investigations that measures for parallax may 
 be safely made upon photographic plates, with, of course, 
 the advantages of leisure and repetition ; and he has already 
 by this method determined the parallax for twenty-one 
 stars with an accuracy not inferior to that of values pre- 
 viously obtained by purely astronomical methods. 
 
 The remarkable successes of astronomical photography, 
 which depend upon the plate's power of accumulation of a 
 very feeble light acting continuously through an exposure 
 of several hours, are worthy to be regarded as a new revela- 
 
CELESTIAL SPECTROSCOPY 437 
 
 tion. The first chapter opened when, in 1880, Dr. Henry 
 Draper obtained a picture of the nebula of Orion; but a 
 more important advance was made in 1883, when Dr. Com- 
 mon, by his photographs, brought to our knowledge details 
 and extensions of this nebula hitherto unknown. A further 
 disclosure took place in 1885, when the brothers Henry 
 showed for the first time in great detail the spiral nebulosity 
 issuing from the bright star Maia of the Pleiades, and 
 shortly afterward nebulous streams about the other stars 
 of this group. In 1886 Mr. Roberts, by means of a photo- 
 graph to which three hours' exposure had been given, 
 showed the whole background of this group to be nebulous. 
 In the following year Mr. Roberts more than doubled for 
 us the great extension of the nebular region which sur- 
 rounds the trapezium in the constellation of Orion. By his 
 photographs of the great nebula in Andromeda he has 
 shown the true significance of the dark canals which had 
 been seen by the eye. They are in reality spaces between 
 successive rings of bright matter, which appeared nearly 
 straight, owing to the inclination in which they lie relatively 
 to us. These bright rings surround an undefined central 
 luminous mass. I have already spoken of this photograph. 
 
 Some recent photographs by Mr. Russell show that the 
 great rift in the Milky Way in Argus, which to the eye is 
 void of stars, is in reality uniformly covered with them. 
 Also quite recently Mr. George Hale has photographed the 
 solar prominences by means of a grating, making use of the 
 lines H and K. 
 
 The heavens are richly but very irregularly inwrought 
 with stars. The brighter stars cluster into well-known 
 groups upon a background formed of an enlacement of 
 streams and convoluted windings and intertwined spirals 
 of fainter stars, which becomes richer and more intricate 
 in the irregularly rifted zone of the Milky Way. 
 
 We, who form part of the emblazonry, can only see the 
 design distorted and confused; here crowded, there scat- 
 tered, at another place superposed. The groupings due to 
 our position are mixed up with those which are real. 
 
438 MUGGINS 
 
 Can we suppose that each luminous point has no other 
 relation to those near it than the accidental neighbourship 
 of grains of sand upon the shore, or of particles of the wind- 
 blown dust of the desert? Surely every star from Sirius 
 and Vega down to each grain of the light-dust of the Milky 
 Way has its present place in the heavenly pattern from the 
 slow evolving of its past. We see a system of systems, for 
 the broad features of clusters and streams and spiral wind- 
 ings which mark the general design are reproduced in every 
 part. The whole is in motion, each point shifting its posi- 
 tion by miles every second, though from the august magni- 
 tude of their distances from us and from each other it is 
 only by the accumulated movements of years or of genera- 
 tions that some small changes of relative position reveal 
 themselves. 
 
 The deciphering of this wonderfully intricate constitu- 
 tion of the heavens will be undoubtedly one of the chief 
 astronomical works of the coming century. The primary 
 task of the sun's motion in space, together with the 
 motions of the brighter stars, has been already put well 
 within our reach by the spectroscopic method of the meas- 
 urement of star motions in the line of sight. 
 
 From other directions information is accumulating: 
 from photographs of clusters and parts of the Milky Way, 
 by Roberts in this country, Barnard at the Lick Observa- 
 tory, and Russell at Sydney; from the counting of stars, and 
 the detection of their configurations, by Holden and by 
 Backhouse; from the mapping of the Milky Way by eye, 
 at Parsonstown; from photographs of the spectra of stars, 
 by Pickering at Harvard and in Peru; and from the exact 
 portraiture of the heavens in the great international star 
 chart which begins this year. 
 
 I have but touched some only of the problems of the 
 newer side of astronomy. Of the many others which would 
 claim our attention if time permitted I may name the fol- 
 lowing: The researches of the Earl of Rosse on lunar radia- 
 tion, and the work on the same subject and on the sun by 
 Langley; observations of lunar heat with an instrument 
 
CELESTIAL SPECTROSCOPY 439 
 
 of his own invention by Mr. Boys, and observations of 
 the variation of the moon's heat with its phase by Mr. 
 Frank Very; the discovery of the ultra-violet part of the 
 hydrogen spectrum, not in the laboratory, but from the 
 stars; the confirmation of this spectrum by terrestrial hy- 
 drogen in part by W. H. Vogel, and in its all but complete 
 form by Cornu, who found similar series in the ultra-violet 
 spectra of aluminium and thallium; the discovery of a 
 simple formula for the hydrogen series by Balmer; the 
 important question as to the numerical spectral relationship 
 of different substances, especially in connection with their 
 chemical properties, and the further question as to the 
 origin of the harmonic and other relations between the lines 
 and the groupings of lines of spectra. On these points con- 
 tributions during the past year have been made by Rudolf 
 von Kovesligethy, Ames, Hartley, Deslandres, Rydberg, 
 Grunwald, Kayser and Runge, Johnstone Stoney, and 
 others; the remarkable employment of interference phe- 
 nomena by Professor Michelson for the determination of 
 the size, and distribution of light within them, of the images 
 of objects which, when viewed in a telescope, subtend an 
 angle less than that subtended by the light-wave at a dis- 
 tance equal to the diameter of the objective; a method 
 applicable not alone to celestial objects, but also to spectral 
 lines, and other questions of molecular physics. 
 
 Along the other lines there has not been less activity; 
 by newer methods, by the aid of larger or more accurately 
 constructed instruments, by greater refinement of analysis, 
 knowledge has been increased, especially in precision and 
 minute exactness. 
 
 Astronomy, the oldest of the sciences, has more than 
 renewed her youth. At no time in the past has she been 
 so bright with unbounded aspirations and hopes. Never 
 were her temples so numerous, nor the crowd of her 
 votaries so great. The British Astronomical Association, 
 formed within the year, numbers already about six hundred 
 members. Happy is the lot of those who are still on the 
 eastern side of life's meridian! 
 
440 HUGGINS 
 
 Already, alas! the original founders of the newer meth- 
 ods are falling out Kirchhoff, Angstrom, D' Arrest, Sec- 
 chi, Draper, Becquerel; but their places are more than 
 filled; the pace of the race is gaining, but the goal is not 
 and never will be in sight. 
 
 Since the time of Newton our knowledge of the phe- 
 nomena of Nature has wonderfully increased, but man asks, 
 perhaps more earnestly now than then, What is the ultimate 
 reality behind the reality of the perceptions? Are they only 
 the pebbles of the beach with which we have been playing? 
 Does not the ocean of ultimate reality and truth lie beyond? 
 
THE NEW ASTRONOMY: 
 A PERSONAL RETROSPECT 1 
 
 WHILE progress in all branches of knowledge has 
 been rapid beyond precedent during the past sixty 
 years, in at least two directions this knowledge 
 has been so unexpected and novel in character that two new 
 sciences may be said to have arisen : the new medicine, with 
 which the names of Lister and of Pasteur will remain asso- 
 ciated; and the new astronomy, of the birth and early 
 growth of which I have now to speak. 
 
 The New Astronomy, unlike the old astronomy to 
 which we are indebted for skill in the navigation of the 
 seas, the calculation of the tides, and the daily regulation 
 of time, can lay no claim to afford us material help in the 
 routine of daily life. Her sphere lies outside the earth. 
 Is she less fair? Shall we pay her less court because it is 
 to mental culture in its highest form, to our purely intel- 
 lectual joys, that she contributes? For surely in no part 
 of Nature are the noblest and most profound conceptions of 
 the human spirit more directly called forth than in the study 
 of the heavens and the host thereof. 
 
 " That with the glorie of so goodly sight 
 The hearts of men . . . 
 . . . may lift themselves up hyer." 
 
 May we not rather greet her in the words of Horace: " O 
 matre pulchra filia pulchrior "? 
 
 As it fell to my lot to have some part in the early de- 
 velopment of this new science, it has been suggested to me 
 1 From the " Nineteenth Century," June, 1897. 
 44* 
 
442 HUGGINS 
 
 that the present Jubilee year of retrospect would be a suit- 
 able occasion to give some account of its history from the 
 standpoint of my own work. 
 
 Before I begin the narrative of my personal observa- 
 tions it is desirable that I should give a short statement 
 of the circumstances which led up to the birth of the new 
 science in 1859, and also say a few words of the state of 
 scientific opinion about the matters of which it treats, just 
 before that time. 
 
 It is not easy for men of the present generation, familiar 
 with the knowledge which the new methods of research 
 of which I am about to speak have revealed to us, to put 
 themselves back a generation, into the position of the sci- 
 entific thought which existed on these subjects in the early 
 years of the Queen's reign. At that time any knowledge 
 of the chemical nature and of the physics of the heavenly 
 bodies was regarded as not only impossible of attainment 
 by any methods of direct observation, but as, indeed, lying 
 altogether outside the limitations imposed upon man by 
 his senses, and by the fixity of his position upon the earth. 
 
 It could never be, it was confidently thought, more than 
 a matter of presumption, whether even the matter of the 
 sun, and much less that of the stars, were of the same nature 
 as that of the earth, and the unceasing energy radiated from 
 it due to such matter at a high temperature. The nebular 
 hypothesis of Laplace at the end of the last century re- 
 quired, indeed, that matter similar to that of the earth 
 should exist throughout the solar system; but then this 
 hypothesis itself needed for its full confirmation the inde- 
 pendent and direct observation that the solar matter was 
 terrestrial in its nature. This theoretical probability in the 
 case of the sun vanished almost into thin air when the 
 attempt was made to extend it to the stellar hosts; for it 
 might well be urged that in those immensely distant regions 
 an original difference of the primordial stuff as well as other 
 conditions of condensation were present, giving rise to 
 groups of substances which have but little analogy with 
 those of our earthly chemistry. 
 
THE NEW ASTRONOMY 443 
 
 About the time of the Queen's accession to the throne 
 the French philosopher Comte put very clearly in his 
 " Cours de Philosophic Positive " the views then held of 
 the impossibility of direct observations of the chemical na- 
 ture of the heavenly bodies. He says: 
 
 " On congoit en effet, que nous puissions conjecturer, 
 avec quelque espoir de succes, sur la formation du systeme 
 solaire dont nous faisons partie, car il nous presente de 
 nombreux phenomenes parfaitement connus, susceptibles 
 peut-etre de porter un temoignage decisif de sa veritable 
 origine immediate. Mais quelle pourrait etre, au contraire, 
 la base rationnelle de nos conjectures sur la formation des 
 soleils eux-memes? Comment confirmer ou infirmer a ce 
 sujet, d'apres les phenomenes, aucune hypothese cosmo- 
 gonique, lorsqu'il n'existe vraiment en ce genre aucun 
 phenomene explore, ni meme, sans doute, EXPLOR- 
 ABLE? " (The capitals are mine.) 
 
 We could never know for certain, it seemed, whether 
 the matter and the forces with which we are familiar are 
 peculiar to the earth, or are common with it to the mid- 
 night sky, 
 
 " All sow'd with glistering stars more thicke than grasse, 
 Whereof each other doth in brightnesse passe." 
 
 For how could we extend the methods of the laboratory to 
 bodies at distances so great that even the imagination fails 
 to realize them? 
 
 The only communication from them which reaches us 
 across the gulf of space is the light which tells us of their 
 existence. Fortunately this light is not so simple in its 
 nature as it seems to be to the unaided eye. In reality it 
 is very complex; like a cable of many strands, it is made 
 up of light-rays of many kinds. Let this light-cable pass 
 from air obliquely through a piece of glass, and its sepa- 
 rate strand-rays all go astray, each turning its own way, 
 and then go on apart. Make the glass into the shape of 
 a wedge or prism, and the rays are twice widely scat- 
 tered. 
 
444 HUGGINS 
 
 "First the flaming red 
 
 Sprung vivid forth: the tawny orange next; 
 And next delicious yellow; by whose side 
 Fell the kind beams of all-refreshing green. 
 Then the pure blue, that swells autumnal skies, 
 Ethereal played; and then, of sadder hue, 
 Emerged the deepened indigo, as when 
 The heavy-skirted evening droops with frost; 
 While the last gleamings of refracted light 
 Died in the fainting violet away." 
 
 Within this unravelled starlight exists a strange cryptog- 
 raphy. Some of the rays may be blotted out, others may 
 be enhanced in brilliancy. These differences, countless in 
 variety, form a code of signals, in which is conveyed to us, 
 when once we have made out the cipher in which it is 
 written, information of the chemical nature of the celestial 
 gases by which the different light-rays have been blotted 
 out, or by which they have been enhanced. In the hands 
 of the astronomer a prism has now become more potent in 
 revealing the unknown than even was said to be " Agrippa's 
 magic glass." 
 
 It was the discovery of this code of signals, and of its 
 interpretation, which made possible the rise of the new 
 astronomy. We must glance, but very briefly, at some of 
 the chief steps in the progress of events which slowly led 
 up to this discovery. 
 
 Newton, in his classical work upon the solar spectrum, 
 failed through some strange fatality to discover the nar- 
 row gaps wanting in light, which, as dark lines, cross the 
 colours of the spectrum and constitute the code of symbols. 
 His failure is often put down to his using a round hole in 
 place of a narrow slit, through the overlapping of the 
 images of which the dark lines failed to show themselves. 
 Though Newton did use a round hole, he states distinctly 
 in his " Optics " that later he adopted a narrow opening in 
 the form of a long parallelogram that is, a true slit at 
 first one tenth of an inch in width, then only one twentieth 
 of an inch, and at last still narrower. These conditions un- 
 der which Newton worked were such as should have shown 
 him the dark lines upon his screen. Professor Johnson has 
 
THE NEW ASTRONOMY 445 
 
 recently repeated Newton's experiments under strictly 
 similar conditions, with the result that the chief dark lines 
 were well seen. For some reason Newton failed to discover 
 them. A possible cause may have been the bad annealing 
 of his prism, though he says that it was made of good glass 
 and free from bubbles. 
 
 The dark lines were described first by Wollaston in 
 1792, who strangely associated them with the boundaries 
 of the spectral colours, and so turned contemporary thought 
 away from the direction in which lay their true significance. 
 It was left to Fraunhofer, in 1815, by whose name the dark 
 lines are still known, not only to map some six hundred of 
 them, but also to discover similar lines, but differently ar- 
 ranged, in several stars. Further, he found that a pair of 
 dark lines in the solar spectrum appeared to correspond in 
 their position in the spectrum, and in their distance from 
 each other, to a pair of bright lines which were nearly 
 always present in terrestrial flames. This last observation 
 contained the key to the interpretation of the dark lines 
 as a code of symbols; but Fraunhofer failed to use it, 
 and the birth of astrophysics was delayed. An observation 
 by Forbes at the eclipse of 1836 led thought away from the 
 suggestive experiments of Fraunhofer, so that in the very 
 year of the Queen's accession the knowledge of the time 
 had to be summed up by Mrs. Somerville in the negation, 
 " We are still ignorant of the cause of these rayless bands." 
 
 Later on, the revelation came more oj less fully to many 
 minds. Foucault, Balfour Stewart, and Angstrom prepared 
 the way. Prophetic guesses were made by Stokes and by 
 Lord Kelvin. But it was KirchhofT who, in 1859, first fully 
 developed the true significance of the dark lines; and by his 
 joint work with Bunsen on the solar spectrum proved be- 
 yond all question that the dark lines in the spectrum of the 
 sun are produced by the absorption of the vapours of the 
 same substances, which when suitably heated give out cor- 
 responding bright lines; and, further, that many of the 
 solar absorbing vapours are those of substances found upon 
 the earth. The new astronomy was born. 
 
446 HUGGINS 
 
 At the time that I purchased my present house, Tulse 
 Hill was much more than now in the country and away 
 from the smoke of London. It was after a little hesitation 
 that I decided to give my chief attention to observational 
 astronomy, for I was strongly under the spell of the rapid 
 discoveries then taking place in microscopical research in 
 connection with physiology. 
 
 In 1856 I built a convenient observatory opening by a 
 passage from the house, and raised so as to command an 
 uninterrupted view of the sky except on the north side. It 
 consisted of a dome twelve feet in diameter, and a transit 
 room. There was erected in it an equatorially mounted 
 telescope by Dollond of five inches aperture, at that time 
 looked upon as a large rather than a small instrument. I 
 commenced work on the usual lines, taking transits, ob- 
 serving and making drawings of planets. Some of Jupiter 
 now lying before me, I venture to think, would not com- 
 pare unfavourably with drawings made with the larger in- 
 struments of the present day. 
 
 About that time Mr. Alvan Clark, the founder of the 
 American firm famous for the construction of the great 
 object-glasses of the Lick and the Yerkes Observatories, 
 then a portrait painter by profession, began, as an amateur, 
 to make object-glasses of large size for that time, and of 
 very great merit. Specimens of his earliest work came into 
 the hands of my friend Mr. Dawes and received the high 
 approval of that distinguished judge. In 1858 I purchased 
 from Mr. Dawes an object-glass by Alvan Clark of eight 
 inches diameter, which he parted with to make room for a 
 lens of a larger diameter by a quarter of an inch, which Mr. 
 Clark had undertaken to make for him. I paid the price 
 that it had cost Mr. Dawes namely, 200. This telescope 
 was mounted for me equatorially and provided with a clock 
 motion by Mr. Cooke, of York. 
 
 I soon became a little dissatisfied with the routine char- 
 acter of ordinary astronomical work, and in a vague way 
 sought about in my mind for the possibility of research 
 upon the heavens in a new direction or by new methods. 
 

 Photogravur 
 
 'immer. 
 
446 MS 
 
 At the time that 1 ny present house, Tulse 
 
 Hill was much mo; the country and away 
 
 from the smokt is after a little hesitation 
 
 that I decided to ^ ention to observational 
 
 astronomy. : iie spell of the rapid 
 
 scopical research in 
 coniH 
 
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 e Mr. Alvan Clark, the founder of the 
 
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 Yerkes Observatories, 
 
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 ti 1858 I purchased 
 
 ! van Clark of eight 
 
 i;ike room for a 
 
 mch, which Mr. 
 
 n. I paid the price 
 
 -'OO. This telescope 
 
 <d provided with a clock 
 
 . 
 
 isfied with the routine char- 
 
 actc Hnary astronomical work, and in a vague way 
 
 ut in my mind for the possibility of research 
 
 ii a new direction or by new methods. 
 
THE NEW ASTRONOMY 447 
 
 It was just at this time, when a vague longing after newer 
 methods of observation for attacking many of the problems 
 of the heavenly bodies rilled my mind, that the news reached 
 me of KirchhofFs great discovery of the true nature and 
 the chemical constitution of the sun from his interpretation 
 of the Fraunhofer lines. 
 
 This news was to me like the coming upon a spring of 
 water in a dry and thirsty land. Here at last presented itself 
 the very order of work for which in an indefinite way I 
 was looking namely, to extend his novel methods of re- 
 search upon the sun to the other heavenly bodies. A feel- 
 ing as of inspiration seized me: I felt as if I had it now in 
 my power to lift a veil which had never before been lifted; 
 as if a key had been put into my hands which would unlock 
 a door which had been regarded as forever closed to man 
 the veil and door behind which lay the unknown mystery of 
 the true nature of the heavenly bodies. This was especially 
 work for which I was to a great extent prepared, from be- 
 ing already familiar with the chief methods of chemical and 
 physical research. 
 
 It was just at this time that I happened to meet at a 
 soiree of the Pharmaceutical Society, where spectroscopes 
 were shown, my friend and neighbour, Dr. W. Allen Miller, 
 Professor of Chemistry at King's College, who had already 
 worked much on chemical spectroscopy. A sudden im- 
 pulse seized me to suggest to him that we should return 
 home together. On our way home I told him of what was 
 in my mind, and asked him to join me in the attempt I was 
 about to make, to apply Kirchhoff's methods to the stars. 
 At first, from considerations of the great relative faint- 
 ness of the stars, and the great delicacy of the work from 
 the earth's motion, even with the aid of a clockwork, he 
 hesitated as to the probability of our success. Finally he 
 agreed to come to my observatory on the first fine evening, 
 for some preliminary experiments as to what we might ex- 
 pect to do upon the stars. 
 
 At that time a star spectroscope was an instrument un- 
 known to the optician. I remember that for our first trials 
 
448 HUGGINS 
 
 we had one of the hollow prisms filled with bisulphide of 
 carbon, so much in use then, and which, in consequence of 
 a small leak, smelled abominably. To this day this pungent 
 odour reminds me of star spectra! 
 
 Let us look at the problem which lay before us. It is 
 difficult for any one, who has now only to give an order for 
 a star spectroscope, to understand in any true degree the 
 difficulties which we met with in attempting to make such 
 observations for the first time. From the sun, with which 
 the Heidelberg professors had to do which, even bright 
 as it is, for some parts of the spectrum has no light to spare 
 to the brightest stars is a very far cry. The light re- 
 ceived at the earth from a first-magnitude star, as Vega, is 
 only about the aooooiooocur P ar ^ f tnat received from 
 the sun. 
 
 Fortunately, as the stars are too far off to show a true 
 disk, it is possible to concentrate all the light received from 
 the star upon a large mirror or object-glass, into the tele- 
 scopic image, and so increase its brightness. 
 
 We could not make use of the easy method adopted by 
 Fraunhofer of placing a prism before the object-glass, for 
 we needed a terrestrial spectrum, taken under the same 
 conditions, for the interpretation, by a simultaneous com- 
 parison with it of the star's spectrum. Kirchhoff's method 
 required that the image of a star should be thrown upon a 
 narrow slit simultaneously with the light from a flame or 
 from an electric spark. 
 
 These conditions made it necessary to attach a spectro- 
 scope to the eye end of the telescope, so that it would be 
 carried with it, with its slit in the focal plane. Then, by 
 means of a small reflecting prism placed before one half 
 of the slit, light from a terrestrial source at the side of the 
 telescope could be sent into the instrument together with 
 the star's light, and so form a spectrum by the side of the 
 stellar spectrum, for convenient comparison with it. 
 
 This was not all. As the telescopic image of a star is a 
 point, its spectrum will be a narrow line of light without 
 appreciable breadth. Now, for the observation of either 
 
THE NEW ASTRONOMY 
 
 449 
 
 dark or of bright lines across the spectrum a certain breadth 
 is absolutely needful. To get breadth, the pointlike image 
 of the star must be broadened out. As light is of first 
 importance, it was desirable to broaden the star's image 
 only in the one direction necessary to give breadth to the 
 spectrum; or, in other words, to convert the stellar point 
 into a short line of light. Such an enlargement in one direc- 
 tion only could be given by the device, first employed by 
 Fraunhofer himself, of a lens convex or concave in one 
 direction only, and flat, and so having no action on the 
 light, in a direction at right angles to the former one. 
 
 When I went to the distinguished optician, Mr. Andrew 
 Ross, to ask for such a lens, he told me that no such lenses 
 were made in England, but that the spectacle lenses then 
 very occasionally required to correct astigmatism first 
 used, I believe, by the then Astronomer Royal, the late Sir 
 George Airy were ground in Berlin. He procured for 
 me from Germany several lenses; but not long afterward a 
 cylindrical lens was ground for me by Browning. By means 
 of such a lens, placed within the focus of the telescope, in 
 front of the slit, the pointlike image of a star could be 
 widened in one direction so as to become a very fine line 
 of light, just so long as, but no longer than, was necessary 
 to give to the spectrum a breadth sufficient for distinguish- 
 ing any lines by which it may be crossed. 
 
 It is scarcely possible at the present day, when all these 
 points are as familiar as household words, for any astron- 
 omer to realize the large amount of time and labour which 
 had to be devoted to the successful construction of the 
 first star spectroscope. Especially was it difficult to pro- 
 vide for the satisfactory introduction of the light for the 
 comparison spectrum. We soon found, to our dismay, 
 how easily the comparison lines might become instrumen- 
 tally shifted, and so be no longer strictly fiducial. As a 
 test we used the solar lines as reflected to us from the moon 
 a test of more than sufficient delicacy with the resolving 
 power at our command. 
 
 Then it was that an astronomical observatory began, 
 29 
 
450 
 
 HUGGINS 
 
 for the first time, to take on the appearance of a laboratory. 
 Primary batteries, giving forth noxious gases, were ar- 
 ranged outside one of the windows; a large induction coil 
 stood mounted on a stand on wheels so as to follow the 
 positions of the eye end of the telescope, together with a 
 battery of several Leyden jars; shelves with Bunsen 
 burners, vacuum tubes, and bottles of chemicals, especially 
 of specimens of pure metals, lined its walls. 
 
 The observatory became a meeting place where ter- 
 restrial chemistry was brought into direct touch with celes- 
 tial chemistry. The characteristic light-rays from earthly 
 hydrogen shone side by side with the corresponding radia- 
 tions from starry hydrogen, or else fell upon the dark lines 
 due to the absorption of the hydrogen in Sirius or in Vega. 
 Iron from our mines was line-matched, light for dark, with 
 stellar iron from opposite parts of the celestial sphere. 
 Sodium, which upon the earth is always present with us, 
 was found to be widely diffused through the celestial 
 spaces. 
 
 This time was, indeed, one of strained expectation and 
 of scientific exaltation for the astronomer, almost without 
 parallel; for nearly every observation revealed a new fact, 
 and almost every night's work was red-lettered by some dis- 
 covery. And yet, notwithstanding, we had to record that 
 " the inquiry in which we have been engaged has been more 
 than usually toilsome; indeed, it has demanded a sacrifice 
 of time very great when compared with the amount of in- 
 formation which we have been able to obtain." 
 
 Soon after the close of 1862 we sent a preliminary note 
 to the Royal Society, " On the Lines of some of the Fixed 
 Stars," in which we gave diagrams of the spectra of Sirius, 
 Betelgeux, and Aldebaran, with the statement that we had 
 observed the spectra of some forty stars, and also the spectra 
 of the planets Jupiter and Mars. It was a little remarkable 
 that on the same day on which our paper was to be read, 
 but some little time after it had been sent in, news arrived 
 there from America that similar observations on some of 
 the stars had been made by Mr. Rutherfurd. A very little 
 
THE NEW ASTRONOMY 451 
 
 later similar work on the spectra of the stars was undertaken 
 in Rome by Secchi, and in Germany by Vogel. 
 
 In February, 1863, the strictly astronomical character 
 of the observatory was further encroached upon by the 
 erection, in one corner, of a small photographic tent fur- 
 nished with baths and other appliances for the wet collodion 
 process. We obtained photographs, indeed, of the spectra 
 of Sirius and Capella; but from want of steadiness and more 
 perfect adjustment of the instruments, the spectra, though 
 defined at the edges, did not show the dark lines as we ex- 
 pected. The dry collodion plates then available were not 
 rapid enough; and the wet process was so inconvenient 
 for long exposures, from irregular drying, and draining 
 back from the positions in which the plates had often to 
 be put, that we did not persevere in our attempts to photo- 
 graph the stellar spectra. I resumed them with success in 
 1875, as we shall see further on. 
 
 At that time no convenient maps of the spectra of the 
 chemical elements, which were then but imperfectly known, 
 were available for comparison with the spectra of the stars. 
 Kirchhoff's maps were confined to a few elements, and 
 were laid down on an arbitrary scale, relatively to the solar 
 spectrum. It was not always easy, since our work had to 
 be done at night when the solar spectrum could not be 
 seen, to recognise with certainty even the lines included in 
 Kirchhoff's maps. To meet this want, I devoted a great 
 part of 1863 to mapping, with a train of six prisms, the spec- 
 tra of twenty-six of the elements; using as a standard scale 
 the spark-spectrum of common air, which would be always 
 at hand. The lines of air were first carefully referred to 
 those of purified oxygen and nitrogen. The spectra were 
 obtained by the discharge of a large induction coil furnished 
 with a condenser of several Leyden jars. I was much 
 assisted by specimens of pure metals furnished to me by 
 Dr. W. A. Miller and Dr. Matthiessen. My paper on this 
 subject, and its accompanying maps, appeared in the vol- 
 ume of the Transactions of the Royal Society for 1864. 
 
 During the same time, whenever the nights were fine, 
 
452 HUGGINS 
 
 our work on the spectra of the stars went on, and the re- 
 sults were communicated to the Royal Society in April, 
 1864, after which Dr. Miller had not sufficient leisure to 
 continue working with me. The general accuracy of our 
 work, so far as it was possible with the instruments at our 
 disposal, is shown by the good agreement of the spectra 
 of Aldebaran and Betelgeux with the observations of the 
 same stars made later in Germany by Vogel. 
 
 It is obviously unsafe to claim for spectrum compari- 
 sons a greater degree of accuracy than is justified by the 
 resolving power employed. When the apparent coinci- 
 dences of the lines of the same substance are numerous, 
 as in the case of iron; or the lines are characteristically 
 grouped, as are those of hydrogen, of sodium, and of mag- 
 nesium, there is no room for doubt that the same substances 
 are really in the stars. Coincidence with a single line may 
 be little better than trusting to a bruised reed; for the stellar 
 line may, under greater resolving power, break up into two 
 or more lines, and then the coincidence may disappear. As 
 we shall see presently, the apparent position of the star line 
 may not be its true one, in consequence of the earth's or 
 the star's motion in the line of sight. Our work, however, 
 was amply sufficient to give a certain reply to the wonder 
 that had so long asked in vain of what the stars were made. 
 The chemistry of the solar system was shown to prevail, 
 essentially at least, wherever a star twinkles. The stars were 
 undoubtedly suns after the order of our sun, though not at 
 all at the same evolutional stage, older or younger it may 
 be, in the life history of bodies of which the vitality is heat. 
 Further, elements which play a chief role in terrestrial 
 physics, as iron, hydrogen, sodium, magnesium, calcium, 
 were found to be the first and the most easily recognised of 
 the earthly substances in the stars. 
 
 Soon after the completion of the joint work of Dr. 
 Miller and myself, and then working alone, I was fortunate 
 in the early autumn of the same year, 1864, to begin some 
 observations in a region hitherto unexplored; and which, 
 to this day, remain associated in my memory with the pro- 
 
THE NEW ASTRONOMY 
 
 453 
 
 found awe which I felt on looking for the first time at that 
 which no eye of man had seen, and which even the sci- 
 entific imagination could not foreshow. 
 
 The attempt seemed almost hopeless. For not only are 
 the nebulae very faintly luminous as Marius put it, " like a 
 rush-light shining through a horn " but their feeble shin- 
 ing can not be increased in brightness, as can be that of 
 the stars, neither to the eye nor in the spectroscope, by 
 any optic tube, however great. 
 
 Shortly after making the observations of which I am 
 about to speak, I dined at Greenwich, Otto Struve being 
 also a guest, when, on telling of my recent work on the 
 nebulae, Sir George Airy said, " It seems to me a case of 
 ' Eyes and No Eyes.' ' Such work, indeed, it was, as we 
 shall see, on certain of the nebulae. 
 
 The nature of these mysterious bodies was still an un- 
 read riddle. Toward the end of the last century the elder 
 Herschel, from his observations at Slough, came very near 
 suggesting what is doubtless the true nature, and place in 
 the Cosmos, of the nebulae. I will let him speak in his own 
 words: 
 
 " A shining fluid of a nature unknown to us. 
 
 " What a field of novelty is here opened to our con- 
 ceptions! . . . We may now explain that very extensive 
 nebulosity, expanded over more than sixty degrees of the 
 heavens, about the constellation of Orion; a luminous mat- 
 ter accounting much better for it than clustering stars at 
 a distance. . . . 
 
 " If this matter is self-luminous, it seems more fit to 
 produce a star by its condensation than to depend on the 
 star for its existence." 
 
 This view of the nebulae as parts of a fiery mist out of 
 which the heavens had been slowly fashioned, began, a 
 little before the middle of the present century, at least in 
 many minds, to give way before the revelations of the giant 
 telescopes which had come into use, and especially of the 
 telescope, six feet in diameter, constructed by the late Earl 
 of Rosse at a cost of not less than 12,000. 
 
454 
 
 HUGGINS 
 
 Nebula after nebula yielded, being resolved apparently 
 into innumerable stars, as the optical power was increased; 
 and so the opinion began to gain ground that all nebulae 
 may be capable of resolution into stars. According to this 
 view, nebulae would have to be regarded, not as early stages 
 of an evolutional progress, but rather as stellar galaxies 
 already formed, external to our system cosmical " sand- 
 heaps " too remote to be separated into their component 
 stars. Lord Rosse himself was careful to point out that 
 it would be unsafe from his observations to conclude that 
 all nebulosity is but the glare of stars too remote to be 
 resolved by our instruments. In 1858 Herbert Spencer 
 showed clearly that, notwithstanding the Parsonstown reve- 
 lations, the evidence from the observation of nebulae up to 
 that time was really in favour of their being early stages 
 of an evolutional progression. 
 
 On the evening of the 2Qth of August, 1864, I directed 
 the telescope for the first time to a planetary nebula in 
 Draco. The reader may now be able to picture to him- 
 self to some extent the feeling of excited suspense, 
 mingled with a degree of awe, with which, after a few 
 moments of hesitation, I put my eye to the spectro- 
 scope. Was I not about to look into a secret place of 
 creation? 
 
 I looked into the spectroscope. No spectrum such as 
 I expected! A single bright line only! At first I suspected 
 some displacement of the prism, and that I was looking at 
 a reflection of the illuminated slit from one of its faces. 
 This thought was scarcely more than momentary; then the 
 true interpretation flashed upon me. The light of the 
 nebula was monochromatic, and so, unlike any other light 
 I had as yet subjected to prismatic examination, could not 
 be extended out to form a complete spectrum. After pass- 
 ing through the two prisms it remained concentrated into 
 a single bright line, having a width corresponding to the 
 width of the slit, and occupying in the instrument a posi- 
 tion at that part of the spectrum to which its light 
 belongs in refrangibility. A little closer looking showed 
 
THE NEW ASTRONOMY 455 
 
 two other bright lines on the side toward the blue, all 
 the three lines being separated by intervals relatively 
 dark. 
 
 The riddle of the nebulae was solved. The answer, 
 which had come to us in the light itself, read: Not an ag- 
 gregation of stars, but a luminous gas. Stars after the 
 order of our own sun, and of the brighter stars, would give 
 a different spectrum; the light of this nebula had clearly 
 been emitted by a luminous gas. With an excess of cau- 
 tion, at the moment I did not venture to go further than 
 to point out that we had here to do with bodies of an order 
 quite different from that of the stars. Further observations 
 soon convinced me that, though the short span of human 
 life is far too minute relatively to cosmical events for us 
 to expect to see in succession any distinct steps in so august 
 a process, the probability is indeed overwhelming in favour 
 of an evolution in the past, and still going on, of the 
 heavenly hosts. A time surely existed when the matter now 
 condensed into the sun and planets filled the whole space 
 occupied by the solar system, in the condition of gas, which 
 then appeared as a glowing nebula, after the order, it may 
 be, of some now existing in the heavens. There remained 
 no room for doubt that the nebulae, which our telescopes 
 reveal to us, are the early stages of long processions of 
 cosmical events, which correspond broadly to those re- 
 quired by the nebular hypothesis in one or other of its 
 forms. 
 
 Not, indeed, that the philosophical astronomer would 
 venture to dogmatize in matters of detail, or profess to be 
 able to tell you pat off by heart exactly how everything 
 has taken place in the universe, with the flippant tongue 
 of a Lady Constance after reading " The Revelations of 
 Chaos": 
 
 " It shows you exactly how a star is formed; nothing 
 could be so pretty. A cluster of vapour the cream of 
 'the Milky Way; a sort of celestial cheese churned into 
 light." 
 
 It is necessary to bear distinctly in mind that the old 
 
456 HUGGINS 
 
 view which made the matter of the nebulae to consist of an 
 original fiery mist in the words of the poet: 
 
 "... a tumultuous cloud 
 Instinct with fire and nitre " 
 
 could no longer hold its place after Helmholtz had shown, 
 in 1854, that such an originally fiery condition of the nebu- 
 lous stuff was quite unnecessary, since in the mutual gravi- 
 tation of widely separated matter we have a store of poten- 
 tial energy sufficient to generate the high temperature of 
 the sun and stars. 
 
 The solution of the primary riddle of the nebulae left 
 pending some secondary questions. What chemical sub- 
 stances are represented by the newly found bright lines? 
 Is solar matter common to the nebulae as well as to the 
 stars? What are the physical conditions of the nebulous 
 matter? 
 
 Further observations showed two lines of hydrogen; 
 and recent observations have shown associated with it the 
 new element recently discovered by Professor Ramsay, oc- 
 cluded in certain minerals, and of which a brilliant yellow 
 line in the sun had long been looked upon as the badge of 
 an element as yet unknown. The principal line of these 
 nebulae suggests probably another substance which has not 
 yet been unearthed from its hiding place in terrestrial rocks 
 by the cunning of the chemist. 
 
 Are the nebulae very hot, or comparatively cool? The 
 spectroscope indicates a high temperature that is to say, 
 that the individual molecules or atoms, which by their en- 
 counters are luminous, have motions corresponding to a 
 very high temperature, and in this sense are very hot. On 
 account of the great extent of the nebulae, however, a com- 
 paratively small number of luminous molecules might be 
 sufficient to make them as bright as they appear to us; tak- 
 ing this view, their mean temperature, if they can be said 
 to have one, might be low, and so correspond with what 
 we might expect to find in gaseous masses at an early 
 stage of condensation. 
 
THE NEW ASTRONOMY 
 
 457 
 
 In the nebulae I had as yet examined the condensa- 
 tion of nearly all the light into a few bright lines made the 
 observations of their spectra less difficult than I feared 
 would be the case. It became, indeed, a case of " Eyes and 
 No Eyes " when a few days later I turned the telescope to 
 the great nebula in Andromeda. Its light was distributed 
 throughout the spectrum, and consequently extremely 
 faint. The brighter middle part only could be seen, though 
 I have since proved, as I at first suggested might be the 
 case, that the blue and the red ends are really not absent, 
 but are not seen on account of their feebler effect upon the 
 eye. Though continuous, the spectrum did not look uni- 
 form in brightness, but its extreme feebleness made it un- 
 certain whether the irregularities were due to certain parts 
 being enhanced by bright lines, or the other parts enfeebled 
 by dark lines. 
 
 Out of sixty of the brighter nebulas and clusters, I found 
 about one third, including the planetary nebulae and that 
 of Orion, to give the bright-line spectrum. It would be 
 altogether out of place here to follow the results of my 
 further observations along the same lines of research, which 
 occupied the two years immediately succeeding. 
 
 I pass at once to a primary spectroscopic observation of 
 one of those rare and strange sights of the heavens, of 
 which only about nineteen have been recorded in as many 
 centuries: 
 
 "... those far stars that come in sight 
 Once in a century." 
 
 On the 1 8th of May, 1866, at 5 P. M., a letter came with 
 the address " Tuam, from an unknown correspondent, one 
 John Birmingham." Mr. Birmingham afterward became 
 well known by his observations of variable stars, and espe- 
 cially by his valuable catalogue of Red Stars in 1877. The 
 letter ran: 
 
 " I beg to direct your attention to a new star which I 
 observed last Saturday night, and which must be a most 
 
458 HUGGINS 
 
 interesting object for spectrum analysis. It is situated in 
 Cor. Bor., and is very brilliant, of about the second magni- 
 tude. I sent an account of it to the ' Times ' yesterday, 
 but as that journal is not likely to publish communications 
 from this part of the world, I scarcely think that it will find 
 a place for mine." 
 
 Fortunately the evening was fine, and as soon as it was 
 dusk I looked, with not a little scepticism, I freely confess, 
 at the place of the sky named in the letter. To my great 
 joy, there shone a bright new star, giving a new aspect to 
 the Northern Crown; of the order doubtless of the splen- 
 did temporary star of 1572, which Tycho supposed to be 
 generated from the ethereal substance of the Milky Way, 
 and afterward dissipated by the sun, or dissolved from some 
 internal cause. 
 
 I sent a messenger for my friend Dr. Miller, and an 
 hour later we directed the telescope, with spectroscope 
 attached, to the blazing star. Later in the evening a letter 
 arrived from Mr. Baxendale, who had independently dis- 
 covered the star on the I5th. 
 
 By this evening, the i8th, the star had already fallen in 
 brightness below the third magnitude. The view in the 
 spectroscope was strange, and up to that time unprece- 
 dented. Upon a spectrum of the solar order, with its num- 
 berless dark lines, shone out brilliantly a few very bright 
 lines. There was little doubt that at least two of these lines 
 belonged to hydrogen. The great brilliancy of these lines 
 as compared with the parts of the continuous spectrum 
 upon which they fell suggested a temperature for the gas 
 emitting them higher than that of the star's photosphere. 
 
 Few of days, as indeed had been its forbears appearing 
 at long intervals, the new star waned with a rapidity little 
 less remarkable than was the suddenness of its outburst, 
 without visible descent, all armed in a full panoply of light 
 from the moment of its birth. A few hours only before 
 Birmingham saw it blazing with second-magnitude splen- 
 dour, Schmidt, observing at Athens, could testify that no 
 outburst had taken place. Rapid was the decline of its 
 
THE NEW ASTRONOMY 459 
 
 light, falling in twelve days from the second down to the 
 eighth magnitude. 
 
 It was obvious to us that no very considerable mass of 
 matter could cool down from the high temperature indi- 
 cated by the bright lines in so short a time. At the same 
 time it was not less clear that the extent of the mass of the 
 fervid gas must be on a very grand scale indeed, for a star 
 at its undoubted distance from us, to take on so great a 
 splendour. These considerations led us to suggest some 
 sudden and vast convulsion, which had taken place in a star 
 so far cooled down as to give but little light, or even to be 
 partially crusted over, by volcanic forces, or by the disturb- 
 ing approach or partial collision of another dark star. The 
 essential character of the explanation lay in the suggestion 
 of a possible chemical combination of some of the escaping 
 highly heated gases from within, when cooled by the sud- 
 den expansion, which might give rise to an outburst of 
 flame at once very brilliant and of very short duration. 
 
 The more precise statement of what occurred during 
 our observations, as made afterward from the pulpit of one 
 of our cathedrals " that from afar astronomers had seen 
 a world on fire go out in smoke and ashes " must be put 
 down to an excess of the theological imagination. 
 
 From the beginning of our work upon the spectra of 
 the stars I saw in vision the application of the new knowl- 
 edge to the creation of a great method of astronomical ob- 
 servation which could not fail in future to have a powerful 
 influence on the progress of astronomy; indeed, in some 
 respects greater than the more direct one of the investiga- 
 tion of the chemical nature and the relative physical con- 
 ditions of the stars. 
 
 It was the opprobrium of the older astronomy though, 
 indeed, one which involved no disgrace, for a 1'impossible 
 nul n'est tenu that only that part of the motions of the 
 stars which is across the line of sight could be seen and di- 
 rectly measured. The direct observation of the other com- 
 ponent in the line of sight, since it caused no change of 
 place and, from the great distance of the stars, no appre- 
 
460 HUGGINS 
 
 ciable change of size or of brightness within an observer's 
 lifetime, seemed to lie hopelessly quite outside the limits of 
 man's powers. Still, it was only too clear that, so long as 
 we were unable to ascertain directly those components of 
 the star's motions which lie in the line of sight, the speed 
 and direction of the solar motion in space, and many of the 
 great problems of the constitution of the heavens, must 
 remain more or less imperfectly known. 
 
 Now, as the colour of a given kind of light, and the ex- 
 act position it would take up in a spectrum, depends directly 
 upon the length of the waves, or, to put it differently, upon 
 the number of waves which would pass into the eye in a 
 second of time, it seemed more than probable that motion 
 between the source of the light and the observer must 
 change the apparent length of the waves to him, and the 
 number reaching his eye in a second. To a swimmer strik- 
 ing out from the shore each wave is shorter, and the num- 
 ber he goes through in a given time is greater than would 
 be the case if he had stood still in the water. Such a change 
 of wave-length would transform any given kind of light, so 
 that it would take a new place in the spectrum, and from 
 the amount of this change to a higher or to a lower place, 
 we could determine the velocity per second of the relative 
 motion between the star and the earth. 
 
 The notion that the propagation of light is not instan- 
 taneous, though rapid far beyond the appreciation of our 
 senses, is due, not, as is sometimes stated, to Francis, but 
 to Roger Bacon. " Relinquitur ergo," he says, in his 
 " Opus Majus," " quod lux multiplicatur in tempore . . . 
 sed tamen non in tempore sensibili et perceptibili a visu, 
 sed insensibili. . . ." The discovery of its actual velocity 
 was made by Roemer, in 1675, from observations of the 
 satellites of Jupiter. Now, though the effect of motion in 
 the line of sight upon the apparent velocity of light under- 
 lies Roemer's determinations, the idea of a change of colour 
 in light from motion between the source of light and the 
 observer was announced for the first time by Doppler in 
 1841. Later, various experiments were made in connection 
 
THE NEW ASTRONOMY 461 
 
 with this view by Ballot, Sestini, Klinkerfues, Clerk Max- 
 well, and Fizeau. But no attempts had been made, nor 
 were indeed possible, to discover by this principle the mo- 
 tions of the heavenly bodies in the line of sight. For, to 
 learn whether any change in the light had taken place from 
 motion in the line of sight, it was clearly necessary to know 
 the original wave-length of the light before it left the star. 
 
 As soon as our observations had shown that certain 
 earthly substances were present in the stars, the original 
 wave-lengths of their lines became known, and any small 
 want of coincidence of the stellar lines with the same lines 
 produced upon the earth might safely be interpreted as 
 revealing the velocity of approach or of recession between 
 the star and the earth. 
 
 These considerations were present to my mind from the 
 first, and helped me to bear up under many toilsome disap- 
 pointments: " Studio fallente laborem." It was not until 
 1866 that I found time to construct a spectroscope of 
 greater power for this research. It would be scarcely pos- 
 sible, even with greater space, to convey to the reader any 
 true conception of the difficulties which presented them- 
 selves in this work, from various instrumental causes, and 
 of the extreme care and caution which were needful to dis- 
 tinguish spurious instrumental shifts of a line from a true 
 shift due to the star's motion. 
 
 At last, in 1868, I felt able to announce in a paper 
 printed in the " Transactions " of the Royal Society for that 
 year the foundation of this new method of research, which, 
 transcending the wildest dreams of an earlier time, enables 
 the astronomer to measure off directly in terrestrial units 
 the invisible motions in the line of sight of the heavenly 
 bodies. 
 
 To pure astronomers the method came before its time, 
 since they were then unfamiliar with Spectrum Analysis, 
 which lay completely outside the routine work of an ob- 
 servatory. It would be easy to mention the names of men 
 well known, to whom I was " as a very lovely song of one 
 that hath a pleasant voice." They heard my words, but for 
 
462 HUGGINS 
 
 a time were very slow to avail themselves of this new power 
 of research. My observations were, however, shortly after- 
 ward confirmed by Vogel in Germany, and by others the 
 principle was soon applied to solar phenomena. By mak- 
 ing use of improved methods of photography, Vogel has 
 recently determined the motions of approach and of reces- 
 sion of some fifty stars, with an accuracy of about an Eng- 
 lish mile a second. In the hands of Young, Duner, Keeler, 
 and others, the method has been successfully applied to a 
 determination of the rotation of the sun, of Saturn and his 
 rings, and of Jupiter. 
 
 It has become fruitful in another direction, for it puts 
 into our hands the power of separating double stars which 
 are beyond the resolving power of any telescope that can 
 ever be constructed. Pickering and Vogel have independ- 
 ently discovered by this method an entirely new class of 
 double stars. 
 
 Double stars too close to be separately visible unite in 
 giving a compound spectrum. Now, if the stars are in 
 motion about a common centre of gravity, the lines of one 
 star will shift periodically relatively to similar lines of the 
 other star, in the spectrum common to both; and such lines 
 will consequently, at those times, appear double. Even if 
 one of the stars is too dark to give a spectrum which can 
 be seen upon that of the other star, as is actually the case 
 with Algol and Spica, the whirling of the stars about each 
 other may be discovered from the periodical shifting of the 
 lines of the brighter star relatively to terrestrial lines of the 
 same substance. It is clear that as the stars revolve about 
 their common centre of gravity, the bright star would be 
 sometimes advancing, and at others receding, relatively to 
 an observer on the earth, except it should so happen that 
 the stars' orbit were perpendicular to the line of sight. 
 
 It would be scarcely possible, without the appearance 
 of great exaggeration, to attempt to sketch out even in 
 broad outline the many glorious achievements which 
 doubtless lie before this method of research in the imme- 
 diate future. 
 
THE NEW ASTRONOMY 463 
 
 Comets in the olden time were looked upon as the por- 
 tents of all kinds of woe: 
 
 " There with long bloody haire, a blazing star 
 Threatens the World with Famin, Plague, and War." 
 
 Though they were no longer, at the time of which I am 
 speaking, a terror to mankind, they were a great mystery. 
 Perhaps of no other phenomenon of Nature had so many 
 guesses at truth been made on different, and even on oppos- 
 ing principles of explanation. It was about this time that 
 a beam of light was thrown in, for the first time, upon the 
 night of mystery in which they moved and had their being, 
 by the researches of Newton of Yale College, by Adams, 
 and by Schiaparelli. The unexpected fact came out of the 
 close relationship of the orbits of certain comets with those 
 of periodic meteor-swarms. Only a year before the obser- 
 vations of which I am about to speak were made, Odling 
 had lighted up the theatre of the Royal Institution with 
 gas brought by a meteorite from celestial space. Two 
 years earlier, Donati showed the light of a small comet to 
 be in part self-emitted, and so not wholly reflected sun- 
 shine. 
 
 I had myself, in the case of three faint comets, in 1866, 
 in 1867, and January, 1868, discovered that part of their 
 light was peculiar to them, and that the light of the last one 
 consisted mainly of three bright flutings. Intense, there- 
 fore, was the great expectancy with which I directed the 
 telescope with its attached spectroscope to the much 
 brighter comet which appeared in June, 1868. 
 
 The comet's light was resolved into a spectrum of three 
 bright bands or flutings, each alike falling off in brightness 
 on the more refrangible side. On the evening of the 22d I 
 measured the positions in the spectrum of the brighter be- 
 ginnings of the flutings on the red side. I was not a little 
 surprised the next morning to find that the three cometary 
 flutings agreed in position with three similar flutings in the 
 brightest part of the spectrum of carbon. Some time before, 
 I had mapped clown the spectrum of carbon, from different 
 
464 HUGGINS 
 
 sources, chiefly from different hydrocarbons. In some of 
 these spectra, the separate lines of which the flutings are 
 built up are individually more distinct than in others. The 
 comet bands, as I had seen them on the previous evening, 
 appeared to be identical in character in this respect, as well 
 as in position in the spectrum, with the flutings as they 
 appeared when I took the spark in a current of olefiant 
 gas. I immediately filled a small holder with this gas, ar- 
 ranged an apparatus in such a manner that the gas could be 
 attached to the end of the telescope, and its spectrum, when 
 a spark was taken in it, seen side by side with that of the 
 comet. 
 
 Fortunately the evening was fine; and on account of 
 the exceptional interest of confronting for the first time the 
 spectrum of an earthly gas with that of a comet's light, I 
 invited Dr. Miller to come and make the crucial observa- 
 tion with me. The expectation which I had formed from 
 my measures was fully confirmed. The comet's spectrum 
 when seen together with that from the gas agreed in all 
 respects precisely with it. The comet, though " subtle as 
 Sphinx," had at last yielded up its secret. The principal 
 part of its light was emitted by luminous vapour of carbon. 
 
 This result was in harmony with the nature of the gas 
 found occluded in meteorites. Odling had found carbonic 
 oxide as well as hydrogen in his meteorite. Wright, ex- 
 perimenting with another type of meteorite, found that 
 carbon dioxide was chiefly given off. Many meteorites 
 contain a large percentage of hydrocarbons; from one of 
 such sky-stones a little later I observed a spectrum similar 
 to that of the comet. The three bands may be seen in 
 the base of a candle flame. 
 
 Since these early observations the spectra of many 
 comets have been examined by many observers. The close 
 general agreement as to the three bright flutings which 
 form the main feature of the cometary spectrum confirms 
 beyond doubt the view that the greater part of the light of 
 comets is due to the fluted spectrum of carbon. Some 
 additional knowledge of the spectra of comets, obtained 
 
THE NEW ASTRONOMY 465 
 
 by means of photography, will have its proper place 
 later on. 
 
 About this time I devoted some attention to spectro- 
 scopic observations of the sun, and especially to the modi- 
 fications of the spectrum which take place under the in- 
 fluence of the solar spots. 
 
 The aerial ocean around and above us, in which finely 
 divided matter is always more or less floating, becomes itself 
 illuminated, and a source of light, when the sun shines upon 
 it, and so conceals, like a luminous veil, any object less 
 brilliant than itself in the heavens beyond. From this cause 
 the stars are invisible at midday. This curtain of light 
 above us at all ordinary times shuts out from our view the 
 magnificent spectacle of red flames flashing upon a coronal 
 glory of bright beams and streamers, which suddenly bursts 
 upon the sight, for a few minutes only, when at rare inter- 
 vals the light curtain is lifted by the screening of the sun's 
 light by the moon, at a total eclipse. 
 
 As yet the spectrum of the red flames had not been 
 seen. If, as seemed probable, it should be found to be that 
 of a gas, consisting of bright lines only, it was conceivable 
 that the spectroscope might enable us so to weaken by dis- 
 persion the air-glare, relatively to the bright lines which 
 would remain undispersed, that the bright lines of the 
 flames might become visible through the atmospheric 
 glare. 
 
 The historic sequence of events is as follows. In No- 
 vember, 1866, Mr. Lockyer asked the question: " May not 
 the spectroscope afford us evidence of the existence of the 
 red flames, which total eclipses have revealed to us in the 
 sun's atmosphere; though they escape all other methods of 
 observation at other times? " 
 
 In the " Report of the Council of the Royal Astronomi- 
 cal Society," read in February, 1868, occurs the following 
 statement, furnished by me, in which the explanation is 
 fully given of the principle on which I had been working 
 to obtain the spectrum of the red flames without an eclipse: 
 
 " During the last two years Mr. Huggins has made 
 
 i *=> 
 30 
 
466 HUGGINS 
 
 numerous observations for the purpose of obtaining a view, 
 if possible, of the red prominences seen during an eclipse. 
 The invisibility of these objects at ordinary times is sup- 
 posed to arise from the illumination of our atmosphere. If 
 these bodies are gaseous, their spectra would consist of 
 bright lines. With a powerful spectroscope the light re- 
 flected from our atmosphere near the sun's limb edge would 
 be greatly reduced in intensity by the dispersion of the 
 prisms, while the bright lines of the prominences, if such be 
 present, would remain but little diminished in brilliancy. 
 This principle has been carried out by various forms of 
 prismatic apparatus, and also by other contrivances, but 
 hitherto without success." 
 
 At the total eclipse of the sun, August 18, 1868, sev- 
 eral observers saw the light of the red flames to be resolved 
 in their spectroscopes into bright lines, among which lines 
 of hydrogen were recognised. The distinguished astron- 
 omer, Janssen, one of the observers in India, saw some of 
 the bright lines again the next day, by means of the prin- 
 ciple described above, when there was no eclipse. 
 
 On October 2Qth Mr. Lockyer sent a note to the Royal 
 Society to say that on that day he had succeeded in ob- 
 serving three bright lines, of a fine prominence. 
 
 About the time that the news of the discovery of the 
 bright lines at the eclipse reached this country, in Sep- 
 tember, I was altogether incapacitated for work for some 
 little time through the death of my beloved mother. We 
 had been all in all to each other for many years. The first 
 day I was sufficiently recovered to resume work, Decem- 
 ber iQth, on looking at the sun's limb with the same spec- 
 troscope I had often used before, now that I knew exactly 
 at what part of the spectrum to search for the lines, I saw 
 them at the first moment of putting my eye to the instru- 
 ment. 
 
 As yet, by all observers the lines only of the promi- 
 nences had been seen, and therefore to learn their forms 
 it was necessary to combine in one design the lengths of the 
 lines as they varied, when the slit was made to pass over a 
 
THE NEW ASTRONOMY 
 
 467 
 
 prominence. In February of the following year it occurred 
 to me that by widening the opening of the slit the form 
 of a prominence, and not its lines only, might be directly 
 observed. This method of using a wide slit has been since 
 universally employed. 
 
 It does not fall within the scope of this article to de- 
 scribe an ingenious photographic method by which Hale 
 has been able to take daily records of the constantly vary- 
 ing phenomena of the red flames and the bright faculae, 
 upon and around the solar disk. 
 
 The purpose of this article is to sketch, in very broad 
 outline only, the principal events, in the order of their suc- 
 cession in time, quorum pars magna fui, which contributed 
 in an important degree to the rise of the new astronomy. 
 As a science advances it follows naturally that its further 
 progress will consist more and more in matters of detail, 
 and in points which are of technical rather than of general 
 interest. 
 
 It would, therefore, be altogether out of place here to 
 carry on in detail the narrative of the work of my observa- 
 tory, when, as was inevitable, it began to take on the 
 character of a development only, along lines of which I 
 have already spoken namely, the observation of more 
 stars, and of other nebulae, and other comets. I pass on at 
 once, therefore, to the year 1876, in which by the aid of 
 the new dry plates, with gelatine films, introduced by Mr. 
 Kennett, I was able to take up again, and this time with 
 success, the photography of the spectra of the stars, of 
 my early attempts at which I have already spoken. 
 
 I was now better prepared for work. My observatory 
 had been enlarged from a dome of twelve feet in diameter 
 to a drum having a diameter of eighteen feet. This altera- 
 tion had been made for the reception of a larger telescope 
 made by Sir Howard Grubb, at the expense of a legacy to 
 the Royal Society, and which was placed in my hands on 
 loan by that society. This instrument was furnished with 
 two telescopes: an achromatic of fifteen inches aperture, 
 and a Cassegrain of eighteen inches aperture, with mirrors 
 
468 HUGGINS 
 
 of speculum metal. At this time, one only of these tele- 
 scopes could be in use at a time. Later on, in 1882, by a 
 device which occurred to me of giving each telescope an 
 independent polar axis, the one working within the other, 
 both telescopes could remain together on the equatorial 
 mounting, and be equally ready for use. 
 
 By this time I had the great happiness of having se- 
 cured an able and enthusiastic assistant by my marriage 
 in 1875. 
 
 The great and notable advances in astronomical meth- 
 ods and discoveries by means of photography since 1875 
 are due almost entirely to the great advantages which the 
 gelatine dry plate possesses for use in the observatory, over 
 the process of Daguerre, and even over that of wet collo- 
 dion. The silver-bromide gelatine plate, which I was the 
 first, I believe, to use for photographing the spectra of 
 stars, except for its grained texture, meets the need of the 
 astronomer at all points. This plate possesses extreme sen- 
 sitiveness; it is always ready for use; it can be placed in 
 any position; it can be exposed for hours; lastly, imme- 
 diate development is not necessary, and for this reason, as 
 I soon found to be necessary in this climate, it can be ex- 
 posed again to the same object on succeeding nights; and 
 so make up by successive instalments, as the weather may 
 permit, the total long exposure which may be needful. 
 
 The power of the eye falls off as the spectrum extends 
 beyond the blue, and soon fails altogether. There is there- 
 fore no drawback to the use of glass for the prisms and 
 lenses of a visual spectroscope. But while the sensitiveness 
 of a photographic plate is not similarly limited, glass, like the 
 eye, is imperfectly transparent, and soon becomes opaque, 
 to the parts of the spectrum at a short distance beyond the 
 limit of the visible spectrum. To obtain, therefore, upon 
 the plate a spectrum complete at the blue end of stellar 
 light, it was necessary to avoid glass, and to employ instead 
 Iceland spar and rock crystal, which are transparent up to 
 the limit of the ultra-violet light which can reach us through 
 our atmosphere. Such a spectroscope was constructed and 
 
THE NEW ASTRONOMY 469 
 
 fixed with its slit at the focus of the great speculum of the 
 Cassegrain telescope. 
 
 How was the image of a star to be easily brought, and 
 then kept, for an hour or even for many hours, precisely at 
 one place on a slit so narrow as about the y^ of an inch? 
 For this purpose the very convenient device was adopted 
 of making the slit-plates of highly polished metal, so as to 
 form a divided mirror, in which the reflected image of a 
 star could be observed from the eye end of the telescope 
 by means of a small telescope fixed within the central hole 
 of the great mirror. A photograph of the spectrum of 
 a Lyrae, taken with this instrument, was shown at the Royal 
 Society in 1876. 
 
 In the spectra of such stars as Sirius and Vega there 
 came out in the ultra-violet region, which up to that time 
 had remained unexplored, the completion of a grand rhyth- 
 mical group of strong dark lines, of which the well-known 
 hydrogen lines in the visible region form the lower mem- 
 bers. Terrestrial chemistry became enriched with a more 
 complete knowledge of the spectrum of hydrogen from the 
 stars. Shortly afterward Cornu succeeded in photograph- 
 ing a similar spectrum in his laboratory from earthly hy- 
 drogen. 
 
 I presented in 1879 a paper, with maps, to the Royal 
 Society, on the photographic spectra of the stars, which was 
 printed in their " Transactions " for 1880. In this paper, 
 besides descriptions of the photographs, and tables of the 
 measures of the positions of the lines, I made a first at- 
 tempt to arrange the stars in a possible evolutional series 
 from the relative behaviour of the hydrogen and the me- 
 tallic lines. In this series Sirius and Vega are placed at 
 the hotter and earlier end; Capella and the sun at about 
 the same evolutional stage, somewhere in the middle of 
 the series; while at the most advanced and oldest stage of 
 the stars which I had then photographed came Betelgeux, 
 in the spectrum of which the ultra-violet region, though 
 not wanting, is very greatly enfeebled. 
 
 Shortly afterward I directed the photographic arrange- 
 
470 HUGGINS 
 
 ment of combined spectroscope and telescope to the nebula 
 in Orion, and obtained for the first time information of the 
 nature of its spectrum beyond the visible region. One line 
 a little distance on in the ultra-violet region came out very 
 strongly on the plate. If this kind of light came within the 
 range of our vision it would no doubt give the dominant 
 colour to the nebula, in place of its present blue-greenish 
 hue. Other lines of the hydrogen series, as might be ex- 
 pected, were seen in the photograph, together with a num- 
 ber of other bright lines. 
 
 In 1 88 1, for the first time since the spectroscope and 
 also suitable photographic plates had been in the hands of 
 astronomers, the coming of a bright comet made it possible 
 to extend the examination of its light into the invisible re- 
 gion of the spectrum at the blue end. On the 22d of June, 
 by leaving very early a banquet at the Mansion House, I 
 was able, after my return home, to obtain, with an exposure 
 of one hour, a good photograph of the head of the comet. 
 It was under a great tension of expectancy that the plate 
 was developed, so that I might be able to look for the first 
 time into a virgin region of Nature, as yet unexplored by 
 the eye of man. 
 
 The plate contained an extension and confirmation of 
 my earlier observations by eye. There were the combined 
 spectra of two kinds of light a faint continuous spectrum, 
 crossed by Fraunhofer lines which showed it to be reflected 
 solar light. Upon this was seen a second spectrum of the 
 original light emitted by the comet itself. This spectrum 
 consisted mainly of two groups of bright lines, character- 
 istic of the spectra of certain compounds of carbon. It will 
 be remembered that my earlier observations revealed the 
 three principal flutings of carbon as the main feature of a 
 comet's spectrum in the visible region. The photograph 
 brought a new fact to light. Liveing and Dewar had shown 
 that one of these bands consisted of lines belonging to a 
 nitrogen compound of carbon. We gained the new knowl- 
 edge that nitrogen, as well as carbon and hydrogen, exists 
 in comets. Now, nitrogen is present in the gas found 
 
THE NEW ASTRONOMY 
 
 471 
 
 occluded in some meteorites. At a later date, Dr. Flight 
 showed that nitrogen formed as much as seventeen per cent 
 of the occluded gas from the meteorite of Cranbourne, 
 Australia. 
 
 I have now advanced to the extreme limit of time within 
 which the rise of the New Astronomy can be regarded as 
 taking place. At this time, in respect of the broad lines 
 of its methods, and the wide scope of the directions in 
 which it was already applied, it had become well estab- 
 lished. Already it possessed a literature of its own, and 
 many observatories were becoming, in part at least, devoted 
 to its methods. 
 
 In my own observatory work has gone on whenever our 
 unfavourable climate has permitted observations to be 
 made. At the present moment more than one research is 
 in progress. It would be altogether beyond the intention 
 and limited scope of the present article to follow this later 
 work. 
 
 We found the New Astronomy newly born in a labora- 
 tory at Heidelberg; to astronomers she was 
 
 "... a stranger, 
 Born out of their dominions." 
 
 We take leave of her in the full beauty of a vigorous youth, 
 receiving homage in nearly all the observatories of the 
 world, some of which, indeed, are devoted wholly to her 
 cult. So powerful is the magic of her charms that gifts have 
 poured in from all sides to do her honour. It has been by 
 such free gifts that Pickering, at Cambridge, United States, 
 and in the southern hemisphere, has been able to give her 
 so devoted a service. In this country, where from almost 
 the hour of her birth she won hearts, enthusiastic wor- 
 shippers have not been wanting. By the liberality of the 
 late Mr. Newall, and the disinterested devotion of his son, 
 a well-equipped observatory is now wholly given up to her 
 worship at Cambridge. This Jubilee year is red-lettered at 
 Greenwich by the inauguration of a magnificent double 
 telescope, laid at her feet by Sir Henry Thompson. Next 
 
472 HUGGINS 
 
 year the Royal Observatory at the Cape will be able to add 
 to its devotion to the old astronomy a homage not less sin- 
 cere and enthusiastic to the New Astronomy, by means of 
 the splendid instruments which Mr. McClean, who person- 
 ally serves under her colours, has presented to that ob- 
 servatory. In Germany, the first National Observatory 
 dedicated to the New Astronomy in 1874, under the direc- 
 tion of the distinguished astrophysicist, Professor Vogel, is 
 about to be furnished by the Government with new and 
 larger instruments in her honour. 
 
 In America many have done liberally, but Mr. Yerkes 
 has excelled them all. This summer will be celebrated the 
 opening of a palatial institution on the shore of Lake Ge- 
 neva founded by Mr. Yerkes, and dedicated to our fair lady, 
 the New Astronomy. This observatory, in respect of the 
 great size of its telescope, of forty inches in aperture, the 
 largest yet constructed, its armoury of instruments for spec- 
 troscopic attack upon the heavens, and the completeness 
 of its laboratories and its workshops, will represent the 
 most advanced state of instrument-making; and at the same 
 time render possible, under the most favourable conditions, 
 the latest and the most perfect methods of research of the 
 New Astronomy. Above all, the needful men will not be 
 wanting. A knightly band, who have shown their knight- 
 hood by prowess in discovery, led by Professor Hale in 
 chivalrous quest of Truth, will surely make this palace of 
 the New Astronomy worthy to be regarded as the Urani- 
 borg of the end of the nineteenth century, as the Danish 
 Observatory, under Tycho and his astronomers, repre- 
 sented the highest development of astronomy at the close 
 of the sixteenth. 
 
AN ASTRONOMER'S WORK IN A 
 MODERN OBSERVATORY 
 
 BY 
 
 DAVID GILL 
 
AN ASTRONOMER'S WORK IN A 
 MODERN OBSERVATORY 1 
 
 THE work of astronomical observatories has been di- 
 vided into two classes viz., astrometry and astro- 
 physics. The first of these relates to astronomy of 
 precision that is, to the determination of the position of 
 celestial objects; the second relates to the study of their 
 physical features and chemical construction. 
 
 Some years ago the aims and objects of these two 
 classes of observatories might have been considered per- 
 fectly distinct, and in fact were so considered. But I hope 
 to show that in more recent years their objects and their 
 processes have become so interlaced that they can not with 
 advantage be divided, and a fully equipped modern ob- 
 servatory must be understood to include the work both of 
 astrometry and astrophysics. 
 
 In any such observatory the principal and the funda- 
 mental instrument is the transit circle. It is upon the posi- 
 tion in the heavens of celestial objects, as determined with 
 this instrument or with kindred instruments, that the whole 
 fair superstructure of exact astronomy rests that is to say, 
 all that we find of information and prediction in our nau- 
 tical almanacs, all that we know of the past and can pre- 
 dict for the future motions of the celestial bodies. 
 
 Here is a very small and imperfect model, but it will 
 serve to render intelligible the photographs of the actual 
 instrument which will be subsequently projected on the 
 screen. (Here the lecturer described the adjustments and 
 mode of using a transit circle.) 
 
 1 From the " Proceedings of the Royal Institution of Great Britain," 
 i8oo-'92. 
 
 475 
 
476 GILL 
 
 We are now in a position to understand photographs 
 of the instrument itself. But first of all as to the house in 
 which it dwells. Here, now on the screen, is the outside 
 of the main building of the Royal Observatory, Cape of 
 Good Hope. I select it simply because, being the observa- 
 tory which it is my privilege to direct, it is the one of 
 which I can most easily procure a series of photographs. 
 It was built during the years 1824-^28, and like all the 
 observatories built about that time, and like too many built 
 since, it is a very fair type of most of the things which an 
 observatory should not be. It is, as you see, an admirably 
 solid and substantial structure, innocent of any architec- 
 tural charm, and so far as it affords an excellent dwelling- 
 place, good library accommodations, and good rooms 
 for computers, no fault can be found with it. But these 
 very qualities render it undesirable as an observatory. An 
 essential matter for a perfect observatory should be the 
 possibility to equalize the internal and external tempera- 
 ture. The site of an instrument should also be free from 
 the immediate surroundings of chimneys or other origin 
 of ascending currents of heated air. Both these conditions 
 are incompatible with thick walls of masonry and the chim- 
 neys of attached dwelling-houses, and therefore, as far as 
 possible, I have removed the instruments to small detached 
 houses of their own. But the transit circle still remains 
 in the main building, for, as will be evident to you, it is 
 no easy matter to transport such an instrument. 
 
 The first two photographs show the instrument in one 
 case pointed nearly horizontally to the north, the other 
 pointed nearly vertical. Neither can show all parts of the 
 instrument, but you can see the massive stone piers, weigh- 
 ing many tons each, which, resting on the solid blocks ten 
 feet below, support the pivots. Here are the counter- 
 weights, which remove a great part of the weight of the 
 instrument from the pivots, leaving only a residual pres- 
 sure sufficient to enable the pivots to preserve the motion 
 of the instrument in its proper plane. Here are the micro- 
 scopes by which the circle is read; here the openings 
 
WORK IN A MODERN OBSERVATORY 
 
 477 
 
 through which the instrument views the meridian sky. 
 The observer's chair is shown in this diagram. His work 
 appears to be very simple, and so it is, but it requires spe- 
 cial natural gifts patience and devotion, and a high sense 
 of the importance of his work to make a first-class me- 
 ridian observer. Nothing apparently more monotonous 
 can be well imagined if a man is not " to the manner born." 
 
 Having directed this instrument by means of the setting 
 circle to the required altitude, he clamps it there and waits 
 for the star which he is about to observe to enter the field. 
 
 As the star enters the field it passes wire after wire, and 
 as it passes each wire he presses the key of his chrono- 
 graph and records the instant automatically. As the star 
 passes the middle wire he bisects it with the horizontal 
 web, and again similarly records on his chronograph the 
 transit of the star over the remaining webs. Then he reads 
 off the microscopes by which the circle is read, and also 
 the barometer and thermometer, in order afterward to be 
 able to calculate accurately the effect of atmospheric re- 
 fraction on the observed altitude of the star; and then his 
 observation is finished. Thus the work of the meridian 
 observer goes on, star after star, hour after hour, and night 
 after night; and, as you see, it differs very widely from the 
 popular notion of an astronomer's occupation. It presents 
 no dreamy contemplation, no watching for new stars, no 
 unexpected or startling phenomena. On the contrary, 
 there is beside him the carefully prepared observing-list for 
 the night, the previously calculated circle setting for each 
 star, allowing just sufficient time for the new setting for 
 the next star after the readings of the circle for the pre- 
 vious observation. 
 
 After four or five hours of this work the observers have 
 had enough of it; they have, perhaps, observed fifty or 
 sixty stars, they determine certain instrumental errors, and 
 betake themselves to bed, tired but (if they are of the right 
 stuff) happy and contented men. At the Cape we employ 
 two observers the one to read the circle and the other to 
 record the transit. Four observers are employed, and they 
 
478 GILL 
 
 are thus on duty each alternate night. Such is the work 
 that an outsider would see were he to enter a working 
 meridian observatory at night; but he would find out if 
 he came next morning that the work was by no means 
 over. By far the largest part has yet to follow. An ob- 
 servation that requires only two or three minutes to make 
 at night requires at least half an hour for its reduction by 
 day. Each observation is affected by a number of errors, 
 and these have to be determined and allowed for. Al- 
 though solidly founded on massive piers resting on solid 
 rock, the constancy of the instrument's position can not 
 be relied upon. It goes through small periodic changes 
 in level, in collimation, and in azimuth, which have to be 
 determined by proper means, and the corresponding cor- 
 rections have to be computed and applied; and also there 
 are other corrections for refraction, etc., which involve 
 computation and have to be applied. But these matters 
 would fall more properly under the head of a special lec- 
 ture upon the transit instrument. I mention them now 
 merely to explain why so great a part of an astronomer's 
 work comes in the daytime, and to dispel the notion that 
 his work belongs only to the night. 
 
 One might very well occupy a special lecture in an ac- 
 count of the peculiarities of what is called personal equa- 
 tion that is to say, the different time which elapses for 
 different observers between the time when the observer 
 believes the star to be upon the wire and the time when 
 the finger responds to the message which the eye has con- 
 veyed to the brain. Some observers always press the key 
 too soon, some always too late. Some years ago I dis- 
 covered, from the observations to which I will subsequently 
 refer, that all observers press the chronograph key either 
 too soon for bright stars or too late for faint ones. 
 
 Other errors may, and I am sure do, arise both at 
 Greenwich and the Cape from the impossibility of securing 
 uniformity of outside and inside temperature in a building 
 of strong masonry. The ideal observatory should be as solid 
 as possible as to its foundations, but as light as possible as to 
 
WORK IN A MODERN OBSERVATORY 479 
 
 its roof and walls say, a light framework of iron covered 
 with canvas. But it would be undesirable to cover a valu- 
 able and permanent instrument in this way. 
 
 But here is a form of observatory which realizes all 
 that is required, and which is eminently suited for per- 
 manent use. The walls are of sheet iron, which readily 
 acquire the temperature of the outer air. The iron walls 
 are protected from direct sunshine by wooden louvres, and 
 small doors in the iron walls admit a free circulation of 
 air. The revolving roof is a light framework of iron cov- 
 ered with well-painted papier-mache. 
 
 The photograph now on the screen shows the interior 
 of the observatory, and this brings me to the description 
 of observations of an entirely different class. In this ob- 
 servatory the roof turns round on wheels, so that any part 
 of the sky can be viewed from the telescope. This is so 
 because the instrument in this observatory is intended for 
 purposes which are entirely different from those of a tran- 
 sit circle. The transit circle, as we have seen, is used to 
 determine the absolute positions of the heavenly bodies; 
 the heliometer to determine with greater precision than is 
 possible by the absolute method the relative positions of 
 celestial objects. 
 
 To explain my meaning as to absolute and relative posi- 
 tions: It would, for example, be a matter of very little im- 
 portance if the absolute latitude of a point on the Royal 
 Exchange or the Bank of England were one tenth of a 
 second of arc (or ten feet) wrong in the maps of the 
 Ordnance Survey of England that would constitute a 
 small absolute error common to all buildings on the same 
 map of a part of the city, and common to all the adjoining 
 maps also. Such an error, regarded as an absolute error, 
 would evidently be of no importance if every point on the 
 map had the same absolute error. There is no one who 
 can say at the present moment whether the absolute lati- 
 tude of the Royal Exchange nay, even of the Royal Ob- 
 servatory, Greenwich is known to ten feet. But it would 
 be a very serious thing indeed if the relative positions on 
 
4 8o GILL 
 
 the same map were ten feet wrong here and there. For 
 example, if of two points marking a frontage boundary on 
 Cornhill one were correct, the other ten feet in error 
 what a nice fuss there would be! what food for lawyers! 
 what a bad time for the Ordnance-Survey Office! Well, 
 it is just the same in astronomy. 
 
 We do not know, we probably never shall know with 
 certainty, the absolute places of even the principal stars 
 to one tenth of a second of arc. But one tenth of a second 
 of arc in the measure of some relative position would be 
 fatal. For example, in the measurement of the sun's paral- 
 lax an error of one tenth of a second of arc means an error 
 of one million miles, in round numbers, in the sun's dis- 
 tance; and it is only when we can be quite certain of our 
 measures of much smaller quantities than one tenth of a 
 second of arc that we are in a position to begin seriously 
 the determination of such a problem as that of the distance 
 of the fixed stars. For these problems we must use differ- 
 ential measures that is, measures for the relative positions 
 of two objects. The most perfect instrument for such pur- 
 poses is the heliometer. 
 
 Lord McLaren has kindly sent from Edinburgh, for 
 the purposes of this lecture, the parts of his heliometer 
 which are necessary to illustrate the principles of the in- 
 strument. 
 
 This instrument is the same which I used on Lord 
 Crawford's expedition to Mauritius in 1874. It was also 
 kindly loaned me by Lord Crawford for an expedition to 
 the Island of Ascension to observe the opposition of Mars 
 in 1877. In 1879, when I went to the Cape, I acquired 
 the instrument from Lord Crawford, and carried out cer- 
 tain researches with it on the distances of the fixed stars. 
 
 In 1887, when the Admiralty provided the new heli- 
 ometer for the Cape Observatory, this instrument again 
 changed hands. It became the property of Lord McLaren. 
 I felt rather disloyal in parting with so old a friend. We 
 had spent so many happy hours together, we had shared 
 a good many anxieties together, and we knew each other's 
 
WORK IN A MODERN OBSERVATORY 481 
 
 weaknesses so well! But my old friend has fallen into good 
 hands, and has found another sphere of work. 
 
 There is now on the screen a picture of the new heli- 
 ometer of the Cape Observatory, which was mounted in 
 1887, and has been in constant use ever since. It is an 
 instrument of the most refined modern construction, and 
 is probably the finest apparatus for refined measurements 
 of celestial angles in the world. 
 
 (Here were explained the various parts of the instru- 
 ment in relation to the model, and the actual processes of 
 observation were illustrated by the images of artificial stars 
 projected on a screen.) 
 
 Here, again, there is little that conforms to the popular 
 idea of an astronomer's work; there is no searching for 
 objects, no contemplative watching, nothing sensational of 
 any kind. On the contrary, every detail of his work has 
 been previously arranged and calculated beforehand, and 
 the prospect that lies before him in his night's work is 
 simply more or less of a struggle with the difficulties which 
 are created by the agitation of the star images, caused by 
 irregularities in the atmospheric refraction. It is not upon 
 one night in a hundred that the images of stars are per- 
 fectly tranquil. You have the same effect in an exagger- 
 ated way when looking across a bog on a hot day. Thus, 
 generally, as the images are approached, they appear to 
 cross and recross each other, and the observer must either 
 seize a moment of comparative tranquility to make his 
 definitive bisection, or he may arrive at it by gradual ap- 
 proximations till he finds that the vibrating images of the 
 two stars seem to pass each other as often to one side as 
 to the other. So soon as such a bisection has been made 
 the time is recorded on the chronograph, then the scales 
 are pointed on and printed off, and so the work goes on, 
 varied only by reversals of the segments and of the posi- 
 tion circle. Generally I now arrange for thirty-two such 
 bisections, and these occupy about an hour and a half. 
 By that time one has had about enough of it; the nerves 
 are somewhat tired, so are the muscles of the back of the 
 31 
 
482 GILL 
 
 neck, and, if the observer is wise and wishes to do his 
 best work, he goes to bed early and gets up again at two 
 or three o'clock in the morning and goes through a similar 
 piece of work. In fact, this must be his regular routine 
 night after night, whenever the weather is clear, if he is 
 engaged, as I have been, on a large programme of work 
 on the parallaxes of the fixed stars, or on observations to 
 determine the distance of the sun by observations of minor 
 planets. 
 
 I will not speak now of these researches, because they 
 are still in progress of execution or of reduction. I would 
 rather, in the first place, endeavour to complete the pic- 
 ture of a night's work in a modern observatory. 
 
 We pass on to celestial photography, where astrometry 
 and astrophysics join hands. The observer's work duijjng 
 the exposure is simply to direct the telescope to the re- 
 quired part of the sky, and then the clockwork nearly does 
 the rest but not quite so. The observer holds in his 
 hand a little electrical switch with two keys; by pressing 
 one key he can accelerate the velocity of the driving screw 
 by about one per cent, and by pressing the other he can 
 retard it one per cent. In this way he keeps one of the 
 stars in the field always perfectly bisected by the cross-wires 
 of his guiding telescope, and thus corrects the small errors 
 produced partly by changes of refraction, partly by small 
 unavoidable errors in cutting the teeth of the arc into which 
 the screw of the driving shaft of the clockwork gears. 
 
 The work is monotonous rather than fatiguing, and the 
 companionship of a pipe or cigar is very helpful during 
 long exposures. A man can go on for a watch of four or 
 five hours very well, taking plate after plate, exposing each, 
 it may be, forty minutes or an hour. If the night is fine 
 a second observer follows the first, and so the work goes 
 on the greater part of the night. Next day he develops 
 his plate. 
 
 Working just in this way, but with the more humble 
 apparatus which you see imperfectly in the picture now 
 on the screen, we have, with a rapid rectilinear lens by Ball- 
 
WORK IN A MODERN OBSERVATORY 483 
 
 meyer of six inches aperture, photographed at the Cape 
 during the past six years the whole of the southern hemi- 
 sphere from 20 of south declination to the south pole. 
 
 The plates are being measured by Professor Kapteyn, 
 of Groningen, and I expect that in the course of a year the 
 whole work containing all the stars to 9^ magnitude (be- 
 tween 200,000 and 300,000 stars) in that region will be 
 ready for publication. This work is essential as a pre- 
 liminary step for the execution in the southern hemisphere 
 of the great work inaugurated by the Astrophotographic 
 Congress at Paris in 1887, the last details of which were 
 settled at our meeting at Paris in April last. What we 
 shall do with the new apparatus perhaps I may have the 
 honour to describe to you some years hence, after the work 
 bas been done. 
 
 We now come to an important class of astronomical 
 work more purely astrophysical, for the illustration of 
 which I can no longer appeal to the Cape, because I re- 
 gret to say that we are not yet provided with the means 
 for its prosecution. I refer to the use of the spectroscope 
 in astronomy, and especially to the latest developments of 
 its use for the accurate measurement of the velocity of the 
 motions of stars in the line of sight. 2 
 
 It is beyond the province of this lecture to enter into 
 history, but it is impossible not to refer to the fact that 
 the chief impulse to astronomical work in this direction 
 was given by Dr. Huggins, our chairman to-night; nay, 
 more, except for the early contributions of Fraunhofer to 
 the subject, Dr. Huggins certainly is the father of sidereal 
 spectroscopy, and that not in one but in every branch of 
 it. He has devised the means, pointed the way, and, while 
 in many branches of the work he still continues to lead 
 the way, he has of necessity left the development of other 
 branches to other hands. 
 
 From an astrometer's point of view the most important 
 
 9 The older methods enabled us to measure motions at right angles 
 to the line of sight, but till the spectroscope came we could not measure 
 motions in the line of sight. 
 
484 GILL 
 
 advance that has been made in spectroscopy of recent years 
 is the sudden development of precision in the measures of 
 star motion in the line of sight. The method remained for 
 fifteen or sixteen years quite undeveloped from the condi- 
 tion in which it left the hands of Dr. Huggins, and cer- 
 tainly no progress in the accuracy attained by Dr. Hug- 
 gins was made till the matter was taken up by Dr. Vogel 
 at Potsdam. At a single step Dr. Vogel has raised the 
 precision of the work from that of observations in the days 
 of Ptolemy to that of the days of Bradley from the days 
 of the old sights and pinnules to the days of telescopes. 
 Therefore I take Potsdam observation as the best type of 
 a modern spectroscopic observation for description, espe- 
 cially as I have recently visited Dr. Vogel at Potsdam, and 
 he has kindly given me a photograph of his spectroscope, 
 as well as of some of the work done with it. 
 
 The method of observation consists simply in inserting 
 a small photographic plate in the dark slide, directing the 
 telescope to the star, and keeping the image of the star 
 continuously on the slit during an exposure of about an- 
 hour; and this is what is obtained on development of the 
 picture. 
 
 If the star remained perfectly at rest between the jaws 
 of the slit the spectrum would be represented by a single 
 thread of light, and of course no lines would be visible upon 
 such a thread; but the observer intentionally causes the 
 star image to travel a little along the slit during the time 
 of exposure, and so a spectrum of sensible width is ob- 
 tained. 
 
 You will remark how beautifully sharp are the faint 
 lines in this spectrum. Those who have tried to observe 
 the spectrum of Sirius in the ordinary way know that many 
 of these fine lines can not be seen or measured with cer- 
 tainty. The reason is that on account of irregularities in 
 atmospheric refraction, the image of a star in the telescope 
 is rarely tranquil; sometimes it shines brightly in the centre 
 of the slit, sometimes barely in the slit at all, and the eye 
 becomes puzzled and confused. But the photographic eye 
 
WORK IN A MODERN OBSERVATORY 485 
 
 is not in the least disturbed; when the star image is in the 
 slit, the plate goes on recording what it sees, and when 
 the star is not in the slit the plate does nothing, and it 
 is of no consequence whatever how rapidly these alternate 
 appearances and disappearances recur. The only differ- 
 ence is that when the air is very steady and the star's image, 
 therefore, always in the slit, the exposure takes less time 
 than when the star is unsteady. 
 
 That is one reason why the Potsdam results are accu- 
 rate; and there are many other reasons besides, into which 
 I can not now enter. What, however, it is very important 
 to note is this: that we have here a method which is to a 
 great extent independent of the atmospheric disturbances 
 which in all other departments of astronomical observation 
 have imposed a limit to their precision. Accurate astro- 
 spectroscopy, therefore, may be pushed to a degree of per- 
 fection which is limited only by the optical aid at our dis- 
 posal and by the sensibility of our photographic plates. 
 
 And now I think we have sufficiently considered the 
 ordinary processes of astronomical observation to illustrate 
 the character of the work of an astronomer at night; the 
 picture should be completed by an account of his work 
 by day. But to go into that matter in detail would cer- 
 tainly not be within the limits of this lecture. It is better 
 that I should in conclusion touch upon some recent re- 
 markable results of these day and night labours. It is 
 these, after all, that most appeal to you; it is for these that 
 the astronomer labours; it is the prospect of them that 
 lightens the long watches of the night and gives life to 
 the otherwise dead bones of mechanical routine. 
 
 Let us take first some spectroscopic results. To ex- 
 plain their meaning let me remind you for a moment of 
 the familiar analogy between light and sound. 
 
 The pitch of a musical note depends on the rapidity of 
 the vibrations communicated to the air by the reed or 
 string of the musical instrument that produces the note, a 
 low note being given by slow vibrations and a high one 
 by quick vibrations. 
 
486 GILL 
 
 Just in the same way red light depends on relatively 
 slow vibrations of ether, and blue or violet light on rela- 
 tively quick vibrations. Well, if there is a railway train 
 rapidly approaching one, and the engine sounds its whis- 
 tle, more waves of sound from that whistle will reach the 
 ear in a second of time than would reach the ear were the 
 train at rest. On the other hand, if the train is travelling 
 at the same rate away from the observer, fewer waves of 
 sound will reach his ears in a second of time. Therefore 
 an observer beside the line should observe a distinct change 
 of pitch in the note of the engine whistle as the train passes 
 him, and as a matter of fact such a change of pitch can be 
 and has been observed. 
 
 Just in the same way, if a source of light could be 
 moved rapidly enough toward an observer it would become 
 bluer, or if away from him it would become red in colour; 
 only it would require a change of velocity in the moving 
 light of some thousands of miles per second in order to 
 render the difference of colour sensible to the eye. The 
 experiment is, therefore, not likely to be frequently shown 
 at this lecture table! 
 
 But the spectroscope enables such changes of colour 
 to be measured with extreme precision. Here on the 
 screen is the most splendid illustration of this that exists 
 at present viz., copies of three negatives of the spectrum 
 of a Aurigae, taken at Potsdam in October and December 
 of 1888, and in March, 1899. 
 
 The white line (the picture being a positive) represents 
 the bright line Hy given by the artificial light of hydrogen ; 
 the strong black line in the picture of the star spectrum 
 corresponds to the black absorption line which is due to 
 hydrogen in the atmosphere of the star. 
 
 Why is it that the artificial hydrogen line does not cor- 
 respond with the stellar line in these three pictures? The 
 answer is, either the star is moving toward or from the 
 earth in the line of sight, or the earth is moving from or 
 toward the star. But in December the earth in its motion 
 round the sun is moving at right angles to the direction 
 
WORK IN A MODERN OBSERVATORY 487 
 
 of a Aurigse; why, then, does the stellar hydrogen line 
 not agree in position with the terrestrial hydrogen line? 
 The simple explanation is that a Aurigse is moving with 
 respect to the sun. 
 
 In what way is it moving? Well, that is also clear; the 
 stellar line is displaced toward the red end of the spectrum 
 that is to say, the star light is redder than it should be in 
 consequence of a motion of recession; this proves that the 
 star is moving away from us, and measures of the photo- 
 graph show the rate of this motion to be fifteen and a half 
 miles per second. We also know that in October the earth 
 in its motion round the sun is moving toward a Aurigae 
 nearly at the same rate as we have just seen that a Aurigae 
 is running away from the sun. Consequently, at that time, 
 their relative motions are nearly insensible, because both 
 are going at the same rate in the same direction, and we find 
 accordingly, in October, that the positions of the stellar 
 and artificial hydrogen lines perfectly correspond. Finally, 
 in March, the earth in its motion round the sun is moving 
 away from a Aurigae, and as a Aurigae is also running away 
 from the sun, the starlight becomes so much redder than 
 normal that the stellar hydrogen line is shifted completely 
 to one side of the hydrogen and artificial line. 
 
 The accuracy of these results may be proved as follows: 
 
 If we measure all the photographs of a Aurigae which 
 Dr. Vogel has obtained, we can derive from each a deter- 
 mination of the relative velocity of the motion of the star 
 with respect to our earth. 
 
 Of course these velocities are made up of the velocity 
 of motion of a Aurigae with respect to the sun (which we 
 may reasonably assume to be a uniform velocity) and the 
 velocity of the earth due to its motion round the sun. But 
 the velocity of the earth's motion in its orbit is known with 
 an accuracy of about one five-hundredth part of its amount, 
 and therefore, within that accuracy, we can allow precisely 
 for its effect on the relative velocity of the earth and 
 a Aurigse. When we have done so we get the annexed 
 results for the velocity of the motion of a Aurigae with re- 
 
488 
 
 GILL 
 
 spect to the sun. You see by the annexed table how beau- 
 tifully they agree in the Potsdam results, and how com- 
 paratively rough and unreliable are the results obtained 
 by the older method of Greenwich. 8 
 
 I believe that in a few years at least in a period of 
 time that one may hope to see we shall not be content 
 merely to correct our results for the motion of the earth 
 in its orbit only, and so test our observations of motion 
 
 9 a Auriga Potsdam 
 
 DATS. 
 
 Observed relative 
 motion of earth 
 and star. Miles 
 per second. 
 
 Motion of earth. 
 
 Concluded motion. 
 Star relative to sun. 
 
 1888. 
 
 + 2.5 
 
 13.0 
 
 + 15.5 
 
 
 + 3- r 
 
 12.4 
 
 + 15.5 
 
 
 -T-3-I 
 
 12.4 
 
 + 15.5 
 
 " 28th . . 
 
 + 2.5 
 
 ii. 8 
 
 + 14.3 
 
 November oth 
 
 + 6.8 
 
 8.7 
 
 + 15.5 
 
 
 + n. 8 
 
 3. i 
 
 + 14.9 
 
 " i3th . ... 
 
 + 14.. Q 
 
 + 0.6 
 
 + I4-3 
 
 1889. 
 
 + 2O.5 
 
 + 0.8 
 
 + 1^7 
 
 
 + 32.0 
 
 + 14. a 
 
 + 18.6 
 
 
 + 34.2 
 
 + 16.8 
 
 + 17.4 
 
 
 
 
 
 a Auriga Greenwich 
 
 DATB. 
 
 Observed relative 
 motion of earth 
 and star. Miles 
 per second. 
 
 Motion of earth. 
 
 Concluded motion. 
 Star relative to sun. 
 
 1887. 
 January 26th . . 
 
 + 16.4 
 
 + 12 6 
 
 +-1 8 
 
 February i6th 
 
 + ^4-4 
 
 + 11 Q 
 
 +18 5 
 
 
 + SQ.8 
 
 I*}. 5 
 
 + 52. ^ 
 
 " 25th 
 
 + 25.4 
 
 T7 O 
 
 + ^8 4 
 
 " 2Qth 
 
 + 40.6 
 
 12. 1 
 
 + 52 7 
 
 1888. 
 
 + 2Q.O 
 
 1.2 
 
 + 16.2 
 
 l88q. 
 
 February i$th 
 
 + 2^.8 
 
 + 16 o 
 
 + 7 8 
 
 March 5th 
 
 + 2O.^ 
 
 + 17. 1 
 
 +1 2 
 
 September I7th 
 
 + 18 6 
 
 jo <i 
 
 + *?'? 7 
 
 " igth 
 
 + 21.8 
 
 16 7 
 
 + ^8 5 
 
 " 25th . 
 
 + 24 8 
 
 16 5 
 
 + AT 3 
 
 November 25th . , 
 
 + 24.5 
 
 4..O 
 
 + 20.4 
 
WORK IN A MODERN OBSERVATORY 489 
 
 in the line of sight, but that we shall have arrived at a 
 certainty and precision of working which will permit the 
 process to be reversed, and that we shall be employing the 
 spectroscope to determine the velocity of the earth's mo- 
 tion in its orbit, or, in other words, to determine the 
 fundamental unit of astronomy, the distance of the sun 
 from the earth. 
 
 I will take as another example one recent remarkable 
 spectroscopic discovery. Miss Murray, in examining a 
 number of photographs of stellar spectra taken at Harvard 
 University, discovered that in every spectrum of ft Aurigae 
 certain lines doubled themselves every two days, becoming 
 single in the intermediate days. Accurate Potsdam obser- 
 vations confirmed the conclusion. 
 
 The picture on the screen shows the spectrum of 
 P Aurigse photographed on November 226. and 25th of last 
 year. In the first the lines are single, in the other every 
 line is doubled. Measures and discussions of a number of 
 these photographs have shown that the doubling of the 
 lines is perfectly accounted for by the supposition of two 
 suns revolving round each other in a period of four days, 
 each moving at a velocity of about seventy miles a second 
 in its orbit. 
 
 When one star is approaching us and the other reced- 
 ing, the lines in the spectrum formed by the light of the first 
 star will be moved toward the blue end of the spectrum, 
 those in the spectrum of the second star toward the red 
 end of the spectrum; then, as the two stars come into the 
 same line with us, their motions become at right angles to 
 the line of sight, and their two spectra, not being affected 
 by motion, will perfectly coincide; but then, after the stars 
 cross, their spectra again separate in the opposite direction, 
 and so they go on. 
 
 Thus by means of their spectra we are in a position to 
 watch and to measure the relative motions of two objects 
 that we can never see apart; nay, more, we can determine 
 not only their period of revolution, but also the velocity 
 of their motions in their orbits. Now, if we know the time 
 
490 GILL 
 
 that a body takes to complete its revolution, and the 
 velocity at which it moves, clearly we know the dimensions 
 of the orbit; and if we know the dimensions of an orbit 
 we know what attractive force is necessary to compel the 
 body to keep in that orbit, and thus we are able to weigh 
 these bodies. The components of ft Aurigse are two suns, 
 which revolve about each other in four days; they are only 
 between 7,000,000 and 8,000,000 miles (or one twelfth of 
 our distance from the sun) apart, and if they are of equal 
 weight they each weigh rather over double the weight of 
 the sun. 
 
 I have little doubt that these facts do not represent a 
 permanent condition, but simply a stage of evolution in the 
 life history of the system, an earlier stage of which may 
 have been a nebular one. 
 
 Other similar double stars have been discovered, both 
 at Potsdam and at Cambridge, United States stars that 
 we shall never see separately with the eye aided by the most 
 powerful telescope; but time does not permit me to enter 
 into any account of them. 
 
 I pass now to another recent result that is of great 
 cosmical interest. The Cape photographic star charting 
 of the southern hemisphere has been already referred to. 
 In comparing existing eye estimates of magnitude by Dr. 
 Gould with the photographic determinations of these mag- 
 nitudes, both Professor Kapteyn and myself have been 
 greatly struck with a very considerable systematic discord- 
 ance between the two. In the rich parts of the sky that 
 is, in the Milky Way the stars are systematically photo- 
 graphically brighter by comparison with the eye observa- 
 tions than they are in the poorer part of the sky, and that 
 not by any doubtful amount but by half or three fourths 
 of a magnitude. One of two things was certain: either 
 that the eye observations were wrong, or that the stars of 
 the Milky Way are bluer or whiter than other stars. But 
 Professor Pickering, of Cambridge, America, has lately 
 been making a complete photographic review of the 
 heavens, and by placing a prism in front of the telescope 
 
WORK IN A MODERN OBSERVATORY 491 
 
 he has made pictures of the whole sky like this. (Here two 
 examples of plates of Pickering's spectroscopic " Durch- 
 musterung " were exhibited on the screen.) He has dis- 
 cussed the various types of the spectra of the brighter stars 
 as thus revealed, according to their distribution in the sky. 
 He finds thus that the stars of the Sirius type occur chiefly 
 in the Milky Way, while stars of other types are fairly 
 divided over the sky. 
 
 Now, stars of the Sirius type are very white stars, very 
 rich relative to other stars in the rays which act most 
 strongly on a photographic plate. Here, then, is the ex- 
 planation of the results of our photographic star-charting, 
 and of the discordance between the photographic and visual 
 magnitudes in the Milky Way. 
 
 The results of the Cape charting further show that it is 
 not alone to the brighter stars that this discordance ex- 
 tends, but it extends also, though in a rather less degree, 
 to the fainter stars of the Milky Way. Therefore we may 
 come to the very remarkable conclusion that the Milky 
 Way is a thing apart, and that it has been developed perhaps 
 in a different manner, or more probably at a different and 
 probably later epoch from the rest of the sidereal universe. 
 
 Here is another interesting cosmical revelation which 
 we owe to photography. You all know the beautiful con- 
 stellation Orion, and many in this theatre have before seen 
 the photograph of the nebula which is now on the screen, 
 taken by Mr. Roberts. Here is another photograph of the 
 same object taken with a much longer exposure. You see 
 how over-exposed in fact, burned out the brightest part 
 of the picture is, and yet what a wonderful development of 
 faint additional nebulous matter is revealed. But I do 
 not think that many persons in this room have seen this 
 picture, and probably very few have any idea what it repre- 
 sents. It is from the original negative taken by Professor 
 Pickering, with a small photographic lens of short focus, 
 after six hours' exposure in the clear air of the Andes, 
 10,000 feet above sea level. The field embraces the three 
 well-known stars in the belt of Orion on the one hand, 
 
4Q2 GILL 
 
 and fi Orionis (Rigel) on the other. You can hardly rec- 
 ognise these great white patches as stars; their ill-defined 
 character is simply the result of excessive over-exposure. 
 But mark the wonders which this long exposure with a 
 lens of high intrinsic brilliancy of image has revealed. Here 
 is the great nebula, of course terribly over-exposed, but 
 note its wonderful fainter ramifications. See how the whole 
 area is more or less nebulous, and surrounded, as it were, 
 with a ring fence of nebulous matter. This nebulosity 
 shows a special concentration about y9 Orionis. 
 
 Well, when Professor Pickering got this wonderful pic- 
 ture, knowing that I was occupied with investigations on 
 the distance of the fixed stars, he wrote to ask whether 
 I had made any observations to determine the distance of 
 & Orionis, as it would be of great interest to know from 
 independent evidence whether this very bright star was 
 really near to us or not. It so happens that the observa- 
 tions were made, and their definitive reduction has shown 
 that Orionis is really at the same distance from us as are 
 the faint comparison stars. Orionis is, therefore, prob- 
 ably part and parcel of an enormous system in an ad- 
 vanced but incomplete state of stellar evolution, and that 
 what we have seen in this wonderful picture is all a part 
 of that system. 
 
 I should explain what I mean by an elementary or by 
 an advanced state of stellar evolution. There is but one 
 theory of celestial evolution which has so far survived the 
 test of time and comparison with observed facts viz., the 
 nebular hypothesis of Laplace. Laplace supposed that the 
 sun was originally a huge gaseous or nebulous mass of a 
 diameter far greater than the orbit of Neptune. I say 
 originally; do not misunderstand me. We have finite 
 minds; we can imagine a condition of things which might 
 be supposed to occur at any particular instant of time, 
 however remote, and at any particular distance of space, 
 however great; and we may frame a theory beginning at 
 another time still more remote, and so on. But we can 
 never imagine a theory beginning at an infinite distance 
 
WORK IN A MODERN OBSERVATORY 493 
 
 of time or at an infinitely distant point in space. Thus, 
 in any theory which man with his finite mind can devise, 
 when we talk of " originally " we simply mean at or during 
 the time considered in our theory. 
 
 Now, Laplace's theory begins at a time, millions on 
 millions of years ago, when the sun had so far disentangled 
 itself from chaos, and its component gaseous particles had 
 by mutual attraction so far coalesced as to form an enor- 
 mous gaseous ball, far greater in diameter than the orbit 
 of the remotest planet of our present system. The central 
 part of this ball revolved. There is nothing improbable in 
 this hypothesis. If gaseous matter came together from 
 different parts of space such coalition would unquestion- 
 ably occur, and as in the meeting of opposite streams of 
 water or of opposite currents of wind, vortices would be 
 created and revolution about an axis set up, such as we 
 are familiar with in the case of whirlpools or cyclones. 
 The resultant would be rotation of the whole globular 
 gaseous mass about an axis. 
 
 Now, this gaseous globe begins to cool, and as it cools 
 it necessarily contracts. Then follows a necessary result 
 of contraction viz., the rotation becomes more rapid. 
 This is a well-known fact in dynamics, about which there 
 is no doubt. Thus, the cooling and the contracting go on, 
 and simultaneously the velocity of rotation becomes greater 
 and greater. At last the time arrives when, for the outside 
 particles, the velocity of rotation becomes such that the 
 centrifugal force is greater than the attractive force, and 
 so the outside particles break off and form a ring. Then, 
 as the process of cooling and contraction proceed still 
 further, another ring is formed, and so on until we have 
 finally a succession of rings and a condensed central ball. 
 If from any cause the cooling of any of these rings does 
 not go on uniformly, or if some of the gaseous matter of 
 the ring is more easily liquefied than others, then probably 
 a simple nucleus of liquid matter will be formed in that 
 ring, and this nucleus will finally by attraction absorb the 
 whole of the matter of which the ring is composed at 
 
494 
 
 GILL 
 
 first as a gaseous ball with a condensed nucleus, and tfiis 
 will finally solidify into a planet. Or, meanwhile, this yet 
 unformed planet may repeat the history of its parent sun. 
 By contraction, and consequent acceleration of its rota- 
 tion, it may throw off one or more rings, which in like 
 manner condense into satellites like our moon, or those of 
 Jupiter, Saturn, Uranus, or Neptune. Such, very briefly 
 outlined, is the celebrated nebular hypothesis of Laplace. 
 No one can positively say that the hypothesis is true; still 
 less can any one say that it is untrue. Time does not 
 permit me to enter into the very strong proofs which 
 Laplace urged in favour of its acceptance. 
 
 But I beg you for one moment to cast your imagina- 
 tion back to a period of time long antecedent to that when 
 our sun had begun to disentangle itself from chaos, and 
 when the fleecy clouds of cosmic stuff had but begun 
 to rush together. What should we see in such a case were 
 there a true basis for the theory of Laplace? Certainly, 
 in the first place, we should have a huge whirlpool or 
 cyclone of cosmic gaseous stuff, the formation of rings, 
 and the condensation of these rings into gaseous globes. 
 
 Remembering this, look now on this wonderful photo- 
 graph of the nebula in Andromeda, made by Mr. Roberts. 
 In the largest telescopes this nebula appears simply as an 
 oval patch of nearly uniform light, with a few dark canals 
 through it, but no idea of its true form can be obtained, 
 no trace can be found of the significant story which this 
 photograph tells. It is a picture that no human eye un- 
 aided by photography has ever seen. It is a true picture, 
 drawn without the intervention of the hand of infallible 
 man, and uninfluenced by his bias or imagination. Have 
 we not here, so at least it seems to me, a picture of a 
 very early stage in the evolution of a star cluster or sun- 
 system a phase in the history of another star-system simi- 
 lar to that which once occurred in our own millions and 
 millions of years ago when our earth nay, even our sun 
 itself " was without form and void," and " darkness was 
 on the face of the deep " ? 
 
WORK IN A MODERN OBSERVATORY 495 
 
 During this lecture I have been able to trace but very 
 imperfectly the bare outlines of an astronomer's work in 
 a modern observatory, and to give you a very few of its 
 latest results results which do not come by chance, but 
 by hard labour, and to men who have patience to face dull, 
 daily routine for the love of science to men who realize 
 the imperfections of their methods, and are constantly on 
 the alert to improve them. 
 
 The mills of the astronomer grind slowly, and he must 
 be infinitely careful and watchful if he would have them, 
 like the mills of God, to grind exceedingly small. I think 
 he may well take for his motto these beautiful lines: 
 
 " Like the star 
 Which shines afar, 
 Without haste, 
 Without rest, 
 Let each man wheel 
 With steady sway 
 Round the task 
 Which rules the day, 
 And do his best." 
 
THE SYSTEM OF THE WORLD 
 THE NEBULAR HYPOTHESIS 
 
 BY 
 
 PIERRE SIMON, MARQUIS DE LAPLACE 
 (Born 1749 J d i ed 1827) 
 
THE SYSTEM OF THE WORLD 1 
 
 WE have five phenomena, as follows, from which 
 to ascend to the cause of the primitive motions 
 of the planetary system: The motions of the 
 planets are in the same direction and almost in the same 
 plane; the motions of the satellites are in the same direc- 
 tion as the planets; the motions of rotation of these sev- 
 eral bodies and of the sun are in the same direction as 
 their motions of translation, and in planes only slightly 
 inclined to each other; the eccentricity of the orbits of the 
 planets and of the satellites is very small; and, lastly, the 
 eccentricity of the orbits of the comets is large, although 
 the inclinations of these bodies may have been the result of 
 chance. 
 
 Buffon is the only philosopher known to me who, since 
 the discovery of the true system of the world, 2 has at- 
 tempted to explain the origin of the planets and of the 
 satellites. He supposes that a comet, falling upon the 
 sun, has expelled from it a torrent of matter which has 
 coalesced once more, at a distance, into various globes, 
 larger or smaller, and nearer or farther from that heav- 
 enly body; and that these globes, having become opaque 
 and solid as they cooled, are the planets and their sat- 
 ellites. 
 
 This hypothesis explains the first of the five phenomena 
 
 1 " (Euvres Completes de Laplace " (reprint of 1884, from fifth edi- 
 tion, 1835), publiees sous les auspices de 1' Academic des Sciences par 
 MM. les Secretaires perpetuels. Paris, 1884, vol. vi. " Systeme du 
 Monde," note vii, pp. 498-509. Translated by SARA CARR UPTON. 
 
 1 By Copernicus, Kepler, and Newton. EDITOR. 
 
 499 
 
5oo 
 
 LAPLACE 
 
 previously cited,- for it is clear that all the bodies thus 
 formed must move very nearly in the plane which passes 
 through the centre of the sun, and in the direction of the 
 torrent of matter which has produced them; the other four 
 phenomena seem to me inexplicable by his method. In 
 truth, the absolute motions of the molecules of a planet 
 would then be in the same direction as the motion of its 
 centre of gravity. But it does not at all follow that the 
 rotation-motion of the planet would have the same direc- 
 tion; thus the earth might turn from east to west, and 
 yet the absolute motion of each of its molecules might be 
 directed from west to east. This would also apply to the 
 motion of revolution of the satellites, the direction of which, 
 under the hypothesis in question, is not necessarily the 
 same as the direction of the motion of translation of the 
 planets in their orbits. 
 
 A phenomenon not only very difficult to explain under 
 this hypothesis, but one even contrary to it, is the small 
 eccentricity of the planetary orbits. It is known from the 
 theory of central forces that if a body moving in an orbit 
 revolving around the sun touches the surface of that 
 heavenly body, it will constantly return to the same point 
 there at each of its revolutions; whence it follows that, had 
 the planets been detached from the sun at their origin, they 
 would touch it upon each of their returns toward that 
 body, and that their orbits, far from being circular, would 
 be very eccentric. It is true that a torrent of matter ex- 
 pelled from the sun can not be exactly compared to a 
 globe which grazes its surface; the impulsion which the 
 parts of such a torrent would receive from each other, and 
 the reciprocal attraction they would exert among them- 
 selves as they changed their direction of motions, might 
 force their perihelions farther from the sun. But their 
 orbits would always be very eccentric, or at least they 
 could not all of them have small eccentricities, except by 
 the most extraordinary chance. Lastly, under Buffon's 
 hypothesis it can not be explained why the orbits of more 
 than one hundred comets already observed are all much 
 
THE SYSTEM OF THE WORLD 
 
 501 
 
 elongated. This hypothesis, therefore, is very far from ex- 
 plaining the preceding phenomena. Let us see if it is pos- 
 sible to rise to their veritable cause. 
 
 Whatever may be the nature of this cause, since it 
 has produced or directed the motions of the planets it must 
 have comprehended all such bodies, and in view of the 
 prodigious distances which separate them one from an- 
 other, it can only have been a fluid of immense extension. 
 To have given the man almost circular motion in the 
 same direction around the sun, this fluid must have sur- 
 rounded that body like an atmosphere. The considera- 
 tion of the planetary motions thus leads us to think 
 that by reason of excessive heat the atmosphere of the 
 sun originally extended beyond the orbits of all the 
 planets, and that it has successively shrunk to its pres- 
 ent limits. 
 
 In the primitive condition that we have ascribed to the 
 sun it resembled the nebulae, which the telescope shows 
 to be composed of a nucleus more or less brilliant, sur- 
 rounded by nebulosity which, as it condenses on the sur- 
 face of the nucleus, transforms it into a star. If, by analogy, 
 we conceive all stars to be formed in this manner, we may 
 imagine their former state of nebulosity which was itself 
 preceded by other states in which the nebulous matter was 
 still more diffused, the nucleus still less luminous. We thus 
 reach, going backward as far as possible, a nebulosity so 
 diffused that its existence is barely manifest. 
 
 For a long time the special configuration of certain 
 stars visible to the naked eye has struck philosophical ob- 
 servers. Mitchel has already remarked how improbable it 
 is that the stars of the Pleiades, for example, should have 
 been clustered together in the small region containing 
 them as the result of mere chance, and he concludes there- 
 from that this group of stars and other similar groups dis- 
 played before us in the sky are the effects of some primi- 
 tive cause, or of a general law of Nature. Such groups 
 are a necessary result of the condensation of nebulae with 
 several nuclei, for it is plain that the nebulous matter, being 
 
502 LAPLACE 
 
 constantly attracted by these different nuclei, must finally 
 form a group of stars like the Pleiades. The condensation 
 of nebulae with two nuclei similarly will form stars in very 
 close proximity, which will turn one around the other, 
 similar to those double stars whose relative motions have 
 already been determined. 
 
 But how did the solar atmosphere determine the mo- 
 tions of rotation and revolution of the planets and of the 
 satellites? Had those bodies penetrated far into that at- 
 mosphere its resistance would have caused them to fall 
 upon the sun. It may therefore be conjectured that the 
 planets were formed at the limits of the solar atmosphere 
 by the condensation of the zones of vapours, which, as it 
 cooled, the atmosphere successively abandoned in the plane 
 of its equator. 
 
 The atmosphere of the sun can not extend outward 
 indefinitely; its limit is the point where the centrifugal 
 force due to its motion of rotation balances its weight. 
 Now, as the cooling contracts the atmosphere by de- 
 grees, and condenses the neighbouring molecules on the 
 surface of the body, the motion of rotation augments; 
 for, according to the principle of " conservation of areas," 
 the sum of the areas described by the radius-vector of 
 each molecule of the sun and of its atmosphere, pro- 
 jected upon the plane of its equator, being always the 
 same, the rotation must be more rapid when such mole- 
 cules are near the sun's centre. The centrifugal force due 
 to this motion thus becoming greater, the point where the 
 weight is equal to it is nearer to that centre. Supposing, 
 therefore, which it is natural to admit, that the atmosphere 
 reached at some epoch to its extreme limit, it must, in 
 cooling, have abandoned the molecules situated at that 
 limit and at the successive limits due to the increase in 
 the rotation of the sun. These abandoned molecules have 
 continued to circulate around that body, since their centrif- 
 ugal force was balanced by their weight. But this equality 
 not existing for the atmospheric molecules situated upon 
 parallels of the solar equator, the latter molecules have 
 
THE SYSTEM OF THE WORLD 503 
 
 been drawn by their weight nearer to the solar atmosphere 
 as it progressively condensed, and they have not ceased to 
 belong to it, until by such motion they have drawn close 
 to that equator. 
 
 Let us consider now the zones of vapours that have 
 been successively abandoned. Such zones must, in all 
 probability, have formed, by their condensation and the 
 mutual attraction of their molecules, various concentric 
 rings of vapours revolving around the sun. The mutual 
 friction of the molecules of each ring must have accelerated 
 some and retarded others until all acquired a like angular 
 motion. Thus the real velocities of the molecules farthest 
 from the centre of the sun were the greatest. The follow- 
 ing cause must also have contributed to this difference of 
 velocity: The molecules farthest from the sun, and those 
 which from the effects of cooling and condensation have 
 drawn near the sun to form the upper part 3 of the ring, con- 
 stantly described areas proportional to the time, since the 
 central force which animated them was constantly directed 
 toward the sun. Now, this conservation of areas necessi- 
 tates an increase of velocity in proportion to their prox- 
 imity to the sun. We see that the same cause must have 
 diminished the velocity of the molecules which have moved 
 to form its lower part of the ring. 4 
 
 If all the molecules of a ring of vapours continued to 
 condense without disuniting, they would finally form a 
 liquid or solid ring. But the regularity in all the portions 
 of the ring and in their cooling, demanded by such a forma- 
 tion, must have rendered such a phenomenon extremely 
 rare. Thus the solar system presents but a single example 
 of it the rings of Saturn. Almost always each ring of 
 vapours must have broken into several masses which, mov- 
 ing by very slightly differing velocities, continued to re- 
 volve at the same distance around the sun. Such masses 
 would have assumed a spheroidal form, with a motion of 
 rotation in the same direction as that of their revolution, 
 
 8 The part most distant from the sun. 
 4 That is, the part nearest to the sun. 
 
504 
 
 LAPLACE 
 
 since their lower 5 molecules had less real velocity than 
 their upper 6 ones; they have found thus so many planets 
 in a vaporous state. But if one has been powerful enough 
 to reunite successively by its attraction all the others 
 around its own centre, the vaporous ring would have been 
 thus transformed into a single spheroidal mass of vapours, 
 revolving around the sun with a rotation in the direction 
 of its revolution. The latter case has been the most com- 
 mon; nevertheless, the solar system presents the first case 
 in the four small planets which move between Jupiter and 
 Mars, 7 unless we suppose with M. Olbers that they origi- 
 nally formed a single planet that has been divided by 
 some great explosion into several portions having differ- 
 ent velocities. 
 
 Now, if we follow the changes that subsequent cooling 
 would have produced in the planets in a vaporous state, 
 the formation of which we have been conceiving, we shall 
 see produced in the centre of each of them a nucleus that 
 will constantly increase by the condensation of the atmos- 
 phere surrounding it. In this state the planet would ex- 
 actly resemble the sun in the nebulous state in which we 
 have just been considering it: the process of cooling would 
 therefore have produced at the various limits of its atmos- 
 phere phenomena similar to those which we have described 
 that is to say, rings and satellites revolving around its 
 centre in the direction of its motion of rotation and turn- 
 ing in the same direction upon themselves. The regular 
 distribution of the mass of the rings of Saturn around its 
 centre, and in the plane of its equator, follows naturally 
 from this hypothesis and without it is inexplicable; these 
 rings seem to me to be ever-existing proofs of the original 
 extension of the atmosphere of Saturn and of its successive 
 recessions. Thus the singular phenomena of the slight ec- 
 centricity of the orbits of the planets and of the satellites, 
 of the slight inclination of those orbits to the solar 
 equator, and of the identity of direction of the motions of 
 rotation and of revolution of all those bodies with that of 
 8 Nearer the sun. * Farther from the sun. T The asteroids. 
 
THE SYSTEM OF THE WORLD 
 
 505 
 
 the rotation of the sun, flow from the hypothesis which we 
 propose, and give it a great probability, which may be still 
 more augmented by the following consideration: 
 
 All the satellites that revolve around a planet having 
 been formed, according to this hypothesis, by the zones 
 successively abandoned by the planet's atmosphere, and its 
 motion of rotation having become more and more rapid, 
 the duration of its motion of rotation must be less than the 
 durations of the revolutions of these various bodies. The 
 case is the same for the sun as compared with the planets. 8 
 All this is confirmed by observations. The duration of the 
 revolution of Saturn's nearest ring is, according to Her- 
 schel's observations, -^nnr of a day, and that of the rotation 
 of Saturn is but $?$. The difference, yHir> is very slight, 
 as it should be, because the portion of Saturn's atmosphere 
 which the diminution of heat has deposited on its surface 
 since the formation of the ring having been small, and 
 coming from a small distance above the planet, it could 
 have only augmented the rotation of the planet in a small 
 degree. 
 
 Had the solar system been formed with perfect regu- 
 larity, the orbits of the bodies which compose it would have 
 been circles whose planes, as well as the planes of the sev- 
 eral equators and rings, would have coincided with the 
 plane of the solar equator. But it is readily conceivable 
 that the numberless varieties of temperature and of den- 
 sity of the various parts, which must have existed in these 
 great masses, have produced the eccentricities in their or- 
 bits and the deviations in their motions from the plane of 
 that equator. 
 
 * Kepler, in his work, " De motibus stellae Martis," has explained 
 the motion of all the planets in a like direction, as caused by imma- 
 terial substances emanating from the surface of the sun, which, pre- 
 serving the rotation motion that they had at the surface, gave like mo- 
 tions to the planets. He concludes therefrom that the sun turns upon 
 itself in a period less than that of the revolution of Mercury, which 
 Galileo determined to be the case by observation soon afterward. Kep- 
 ler's hypothesis is doubtless inadmissible, but it is noteworthy that he 
 should have made the identity of direction of the planetary motions de- 
 pend on this rotation of the sun, so natural would such tendency appear. 
 
5o6 LAPLACE 
 
 Under our hypothesis, comets are foreign to the plan- 
 etary system. In considering them, as we have done, as 
 small nebulae wandering about from solar system to solar 
 system, and formed by the condensation of the nebulous 
 matter scattered with so much profusion throughout the 
 universe, we see that when they arrive in that part of space 
 where the attraction of the sun predominates, it forces 
 them to describe elliptical or hyperbolical orbits. But their 
 velocities being equally possible in all directions, they must 
 move indifferently in all directions and with every degree 
 of inclination to the ecliptic, which conforms with what is 
 actually observed. Thus the condensation of the nebulous 
 matter by which we have just explained the motions of 
 rotation and of revolution of the planets and satellites in 
 the same direction and in slightly differing planes, explains 
 equally why the motions of the comets depart from the 
 general law. 
 
 The great eccentricity of the cometary orbits is also a 
 result of our hypothesis. If these orbits are elliptical they 
 are very elongated, since their major axes are at least equal 
 to the radius of the sphere of activity of the sun. But 
 these orbits may be hyperbolic, and if the axes of these 
 hyperbolas are not very large in relation to the mean dis- 
 tance of the sun from the earth, the motion of the comets 
 which describe them will appear sensibly hyperbolic. How- 
 ever, out of at least a hundred comets whose elements we 
 know, none has appeared to move in a hyperbola; it must 
 be then that the circumstances which produce a sensible 
 hyperbola are extremely rare in relation to the contrary 
 circumstances. Comets are so small that they do not be- 
 come visible until their perihelion distance is inconsider- 
 able. Up to the present time this distance for any comet 
 has only twice surpassed the diameter of the terrestrial 
 orbit, and oftenest it has been less than the radius of that 
 orbit. It is obvious that to approach so near to the sun 
 their velocity at the moment of their entrance into its 
 sphere of activity must have an amount and a direction 
 comprised within narrow limits. In determining by anal- 
 
THE SYSTEM OF THE WORLD 507 
 
 ysis of the probabilities the ratio of the chances which, in 
 such limits, give a sensible hyperbola to the chances which 
 give an orbit likely to be confounded with a parabola, I 
 have found that there are at least six thousand chances to 
 one that a nebula which penetrates into the sphere of 
 activity of the sun, so as to be observed, will describe either 
 a very elongated ellipse or a hyperbola that from the size 
 of its axis will be sensibly confounded with a parabola in 
 the portion of the orbit that is observed. It is not sur- 
 prising, therefore, that so far hyperbolic motions have not 
 been recognised. 
 
 The attraction of the planets, and perhaps also the re- 
 sistance of the ethereal media, must have changed many 
 cometary orbits into ellipses whose major axes are much 
 smaller than the radius of the sphere of activity of the sun. 
 This change may also result from the meeting of these 
 heavenly bodies; for it follows, from our hypothesis of their 
 formation, that there must have been a prodigious number 
 of them in the solar system, it having been possible to 
 observe only those which approach near enough to the 
 sun. It is to be believed that such a change took place 
 for the orbit of the comet of 1759, whose major axis only 
 exceeded thirty-five times the distance from the sun to 
 the earth. A still greater change happened to the orbits of 
 the comets of 1770 and 1805. 
 
 If a certain number of comets penetrated the atmos- 
 pheres of the sun and of the planets at the epoch of their 
 formation, they must have fallen upon those bodies in spiral 
 orbits, and by their fall have removed the planes of the 
 orbits and of the equators of the planets from the plane of 
 the solar equator. 
 
 If in the zones abandoned by the atmosphere of the 
 sun there have been molecules too volatile to unite among 
 themselves or with the planets, they must, while they con- 
 tinue to revolve around the sun, offer every appearance 
 of the zodiacal light, without opposing any sensible re- 
 sistance to the various bodies of the planetary system, 
 because of their extreme rarity, or because their motion 
 
5o8 
 
 LAPLACE 
 
 is very nearly the same. as that of the planets which they 
 meet. 
 
 The closer examination of all the circumstances of this 
 system still further increases the probability of our hypoth- 
 esis. The primitive fluidity of the planets is clearly indi- 
 cated by the flattening of their figures, in conformity with 
 the laws of the mutual attraction of their molecules. It is, 
 moreover, proved in regard to the earth, by the regular 
 diminution of gravity from the equator to the poles. This 
 state of original fluidity to which we are led by astronomical 
 phenomena ought to be manifested in the phenomena 
 which natural history presents to us. But to find it there 
 it is necessary to take into consideration the immense 
 variety of combinations formed by all the terrestrial sub- 
 stances mingled in a vaporous state, when the lowering of 
 the temperature has allowed their elements to unite. We 
 must next consider the prodigious changes which such 
 lowering of temperature must have brought about succes- 
 sively in the interior and at the surface of. the earth, in all 
 its products, in the constitution and pressure of the atmos- 
 phere, in the ocean and in the bodies which it has held in 
 solution. Lastly, the abrupt changes, such as the great 
 volcanic eruptions which must have disturbed at various 
 times the regularity of the changes, must be noted. Ge- 
 ology, from this point of view, which links it with astron- 
 omy, might, with respect to many objects, acquire the pre- 
 cise and certain knowledge of the latter science. 
 
 One of the most singular phenomena of the solar sys- 
 tem is the strict equality which has been observed between 
 the angular motions of rotation and of revolution of each 
 satellite. It is a wager of infinity against one that this 
 is not the result of chance. The theory of universal gravi- 
 tation causes the infinity to disappear from this improba- 
 bility, and shows us that for the phenomenon to exist it 
 suffices that at the beginning the two motions shall differ 
 but slightly. Then the attraction of the planet establishes 
 a perfect equality between them, but at the same time it 
 gives rise to a periodic oscillation in that axis of the satel- 
 
THE SYSTEM OF THE WORLD 509 
 
 lite which is directed toward the planet, an oscillation the 
 extent of which depends on the original difference of the 
 two motions. The observations of Mayer upon the libra- 
 tion of the moon and those which Messrs. Bouvard and 
 Nicollet have just made upon the subject, at my request, 
 not having shown this oscillation, the difference on which 
 it depends must be very small. This points with extreme 
 probability to a special cause, which at first has confined 
 that difference within very narrow limits, where the attrac- 
 tion of the planet has been able to establish a strict equality 
 between the mean motions of rotation and revolution, and 
 which afterward ended by destroying the oscillation which 
 was produced by such an equality. Both of these effects 
 are results of our hypothesis, for it is obvious that the 
 moon in a vaporous state formed, under the powerful at- 
 traction of the earth, an elongated spheroid, whose major 
 axis must have been constantly directed toward that planet, 
 owing to the readiness with which vapours yield to the 
 slightest forces acting upon them. The terrestrial attrac- 
 tion continuing to act in the same manner so long as 
 the moon was in a fluid state must at last, by constantly 
 causing the two motions of that satellite to approximate, 
 have made their difference fall within the limits where their 
 strict equality begins to establish itself. Afterward this 
 attraction must have, little by little, destroyed the oscilla- 
 tion which that equality produced in the major axis of the 
 spheroid directed toward the earth. It is thus that the 
 fluids which cover this planet have destroyed by their fric- 
 tion and by their resistance the original oscillations of its 
 axis of rotation which now is subject only to the nutation 
 resulting from the actions of the sun and of the moon. 
 It is easy to convince one's self that the equality of the 
 motions of rotation and of revolution of the satellites 
 must have placed an obstacle to the formation of rings 
 and of secondary satellites by the atmospheres of those 
 bodies. Thus, so far, observation has indicated nothing 
 of the sort. 
 
 The motions of the first three satellites of Jupiter pre- 
 
5io LAPLACE 
 
 sent a phenomenon still more extraordinary than the pre- 
 ceding. It consists in the fact that the mean longitude 
 of the first, less three times that of the second, plus twice 
 that of the third, is constantly equal at two right angles. 
 There is infinity against one to be wagered that this equal- 
 ity is not due to chance. 
 
 But we have just seen that, in order to produce it, it 
 was sufficient that the mean motions of these three bodies 
 should approximately satisfy the ratio which makes the 
 mean motion of the first, less three times that of the sec- 
 ond, plus twice that of the third, equal to zero. Their 
 mutual attraction has subsequently established this relation 
 rigorously, and furthermore has made the mean longitude 
 of the first satellite less three times that of the second plus 
 twice that of the third equal to a semi-circumference. At 
 the same time a periodic inequality has arisen which de- 
 pended upon the small quantity by which the mean motions 
 originally deviated from the relation we have just stated. 
 However much care Delambre has taken to prove this 
 inequality by observations, he has not succeeded, which 
 proves its extreme smallness, and which consequently 
 shows with great probability a cause which has made it 
 disappear. According to our hypothesis the satellites of 
 Jupiter, immediately after their formation, did not move 
 in a perfect vacuum; the least condensable molecules of 
 the original atmospheres of the sun and of the planet 
 formed then a rare medium whose resistance, differing for 
 each of the heavenly bodies, caused their mean motions of 
 the relation in question to approximate little by little. 
 When these motions have thus attained the conditions re- 
 quired in order that the mutual attraction of the three 
 satellites should establish this relation rigorously, the same 
 resistance constantly diminished the inequality which the 
 relation caused at first, and finally rendered it insensible. 
 We can not better compare these effects than to the motion 
 of a pendulum moving by great velocity in a medium of 
 small resistance. At first it will describe a great number 
 of circumferences, but at last its motion of revolution, con- 
 
THE SYSTEM OF THE WORLD 511 
 
 stantly decreasing, will be changed into a motion of oscil- 
 lation, which, itself diminishing more and more by the 
 resistance of the medium, will finally be destroyed; then 
 the pendulum, having come to rest, will forever remain 
 in that condition. 
 
ARE THE PLANETS HABITABLE 
 
 BY 
 
 GEORGE M. SEARLE 
 
 33 
 
ARE THE PLANETS HABITABLE 1 
 
 HAVING completed our survey of the planetary sys- 
 tem in which we live, a question naturally occurs to 
 us, which has occurred to every inquiring mind 
 since the real dimensions of the orbs belonging to it were 
 known. To the great majority of mankind it is, and is 
 rightly, a question of greater interest than any one with 
 which mathematics or physics has to deal; of greater in- 
 terest, since life is a much higher and nobler thing than 
 machinery, and the spiritual far above the material. This 
 question is, " Are these planets which, like our earth, move 
 in their appointed paths around the sun, and on which 
 there is certainly ample room for a population far greater 
 than what our globe could support, actually inhabited by 
 beings in any way like ourselves? " 
 
 Almost every astronomer has probably been asked what 
 his views are on this question, and whether his science has 
 anything to tell us about it. At each successive increase 
 in the size of telescopes, men vaguely hope that with the 
 new optical power it may be possible to discover some signs 
 of sentient, and perhaps even of intelligent, life in the 
 celestial worlds. " How much does this telescope mag- 
 nify? " is always the interesting question to the popular 
 mind. The professional astronomer perhaps is not looking 
 so much for that. He wants to get more light; to see and 
 to delineate faint nebulae, to follow a comet as far as he 
 can into the darkness of space, in order to determine its 
 orbit as well as possible; but the world in general has com- 
 paratively little sympathy with him in this. The discovery 
 
 1 A lecture delivered before the Catholic University of America. 
 515 
 
516 SEARLE 
 
 of one intelligent being outside this planet of ours would 
 be more interesting to most men here than all the comets 
 which ever have been or ever will be seen. 
 
 Is it then possible that the power of telescopes will at 
 any time be so increased that any discovery of this kind can 
 be made? That is what people would like to know. Let 
 us answer this question in the first place. 
 
 The moon is our nearest neighbour. If we can magnify 
 enough to see an object the size of a man on any of the 
 planetary orbs, we must first be able to see such an object 
 on the moon. Is it possible to obtain a magnifying power 
 sufficient for this? 
 
 It is possible, we answer, to have such a magnifying 
 power; but the difficulty is to avail ourselves of such a 
 power when we have got it. The great and turbulent sea 
 of atmosphere which lies above us is a seemingly insuper- 
 able difficulty. To some extent, of course, we can get free 
 from this by placing our telescope on some high mountain; 
 but thefe is no mountain high enough to place us alto- 
 gether out of the atmosphere, and if there were one, we 
 could not live or carry a telescope there. At the highest 
 point at which observations would be possible, which prob- 
 ably would be a good deal below the summit of the Hima- 
 layas, enough air still would remain above us to prevent 
 our using a power high enough to discern men like our- 
 selves on the face of our satellite. The tremulousness and 
 waviness produced in the telescopic image by the air, 
 which is, of course, increased the more we magnify, would 
 hopelessly obscure outlines so delicate as those here con- 
 cerned, and make of such small points a simple invisible 
 blur. 
 
 Even for the moon, then, the direct discovery of animal 
 life by increased optical power would seem to be a dream 
 which will never be realized. The difficulty, of course, is 
 immensely increased for any other celestial object. No 
 other planet comes nearer to us than about one hundred 
 times the moon's distance; and, moreover, in examining 
 them, we should have to contend with the confusion of 
 
ARE THE PLANETS HABITABLE 517 
 
 outlines coming from their atmospheres as well as from 
 our own. 
 
 We may then as well give up hope of trying to answer 
 the question, " Are the planets inhabited? " as one which 
 never will be solved for us in this world by any natural 
 means; and fall back on another, on which science, cer- 
 tainly, can give us some light namely, " Are they habit- 
 able; are the physical conditions such in them, so far as 
 we can ascertain, that the life of man or of any highly 
 organized animal could there subsist? " 
 
 Now, I say the " planets " ; for it seems to me that we 
 may as well put the great central body of our system, the 
 sun itself, out of the question. I think it is pretty clear 
 that the surface at least of this enormous globe is in such 
 a state as to make it absolutely impossible for us to con- 
 ceive of any organized life existing there. It is true that 
 we do not know exactly how much complexity of struc- 
 ture is required in matter as a basis of life; but we can 
 hardly consider life in the proper sense as belonging to a 
 chemical molecule, and everything would indicate that on 
 the surface of the sun matter is reduced to its simply 
 chemical or molecular state. Any structures or organisms 
 which we call alive would instantly be destroyed in that 
 intense flame; even inanimate shapes, like those of crystals, 
 would not survive its action for a moment. 
 
 But may there not be a cooler region below the sun's 
 surface, protected in some way from the intense heat of 
 the exterior? Such a theory was entertained in the last 
 century and even in this; but it is pretty safe to say that 
 no one now would hold it. That it should have held its 
 ground so long is due perhaps, in great measure, to the 
 authority of Sir William Herschel. I do not think it was 
 ever satisfactorily explained just how the interior was pro- 
 tected from the immense radiation of its envelope; cer- 
 tainly it is hard for us to see nowadays, knowing as we 
 do the radiating power of, the surface (10,000 horse power 
 per square foot, as we find it to be) how such a blaze as 
 this could even be supposed to be cut off from any point 
 
SEARLE 
 
 within. To suggest a cool place in the interior of the sun 
 is much as if one should advise a person suffering from 
 the heat of a furnace to wrap himself up well and take a 
 seat inside. Moreover, we know from spectroscopic in- 
 dications now, particularly from those of oxygen in the 
 sun, that the farther in we go, the hotter it gets; and 
 this also would follow from the only theory which can rea- 
 sonably account for the formation of the sun, and the main- 
 tenance of its heat. 
 
 We may pretty certainly say, then, that in any com- 
 mon-sense way of using the word, the sun is not habitable. 
 Absolutely speaking, of course, all space is habitable; there 
 is no conclusive reason why an organized being should 
 require nutriment or air, and hence an animal might be 
 conceived as being launched into space as a planet on his 
 own account. But what we mean by a place being habit- 
 able is, that it should furnish the requisites and conven- 
 iences belonging to a life similar in its principal features 
 to that with which we are acquainted. It is not a thing 
 which can be strictly defined; nevertheless, we know well 
 enough for practical purposes what we are talking about, 
 and we know that such a place as this empty space is not 
 " habitable." 
 
 From the consideration of the sun we will pass to that 
 of the next most conspicuous object to us in the planetary 
 system that is to say, the moon. I have already expressed 
 in a previous lecture the views generally entertained by 
 astronomers about the moon. It is pretty certain that the 
 side of it which we see offers nothing in the way of a con- 
 venience of life except mere standing-room. There is 
 hardly a doubt that its surface consists simply of bare rock, 
 unvaried by water, soil, or any kind of vegetation; that if 
 there be any atmosphere upon it, it is so excessively rare- 
 fied as to be, for purposes of life, practically equivalent 
 to none. 
 
 As to the other side, of course, we can say nothing 
 positively. It may perhaps in some way be different from 
 this. But taking the ordinary and (to say the least) very 
 
THE MOON'S SURFACE, 
 Photogravure from a photograph taken at the Lick Observatory, 
 
SEARLE 
 
 iin. To suggest at e in the interior of the sun 
 
 men as if one sho?; person suffering from 
 
 the heat of a furnace t- up well and take a 
 
 seat inside. Moreo f rom spectroscopic in- 
 
 dications now, of oxygen in the 
 
 sun, that tl !e hotter it gets; and 
 
 this also wo which can rea- 
 
 sonably i, and the main- 
 
 ny com- 
 v habitable, 
 .able; there 
 ould 
 
 r'ut \yhat we mean by a place being h:-. 
 
 es belonging to a life similar in its principal features 
 
 a which we are acquainted. It is not a thing 
 
 ">e strictly defined; nevertheless, we know well 
 
 ;tical purposes what we are talking about, 
 
 such a place as this empty space is not 
 
 that 
 
 ^e plan* - 
 
 entertained by 
 
 momers about the tty certain that the 
 
 side of it which n the way of a con- 
 
 venience of lifv- i.g-room. Ther 
 
 hardly a doubt >imply of bare rock 
 
 unvaried by vegetation; that if 
 
 there be an it, it is so excessively rare- 
 
 fied as to be, for purp- life, pracdcally equivalent 
 
 to none, 
 
 As to the other side, of course, we can say nothing 
 
 tively. It may perhaps in some way be different from 
 
 But taking the ordinary and (to say the least) very 
 
ARE THE PLANETS HABITABLE 519 
 
 probable view as to the method of formation of the plan- 
 etary masses, by cooling from a liquid condition, it is hard 
 to see how there could possibly be any considerable differ- 
 ence of shape or of density between the half of the lunar 
 sphere which is turned toward us and that which is turned 
 away. And unless there be such a difference, the other 
 side must be as destitute of atmosphere as this; and if of 
 atmosphere, of water as well; for the water or other fluid, 
 if existing in any quantity, wo.uld form an atmosphere, if 
 none previously existed. 
 
 The moon then hardly seems to present the condition 
 required for what we should call a habitable planet; though 
 it fails in a very different way from the sun. The moon is 
 dead; the sun is too much alive. The moon may have 
 been habitable and inhabited once; the sun may be in the 
 future. 
 
 So far, our survey has not been very encouraging. But 
 we have not yet considered the planets properly so called. 
 
 In considering them from this point of view, let us pro- 
 ceed in the contrary order to that which we followed in 
 describing them in detail. Let us start at the outer limit, 
 with the great twin planets, as we may call them, on ac- 
 count of their great similarity, widely separated in space 
 as they are namely, Uranus and Neptune. 
 
 These would perhaps generally be imagined as very 
 cheerless habitations for intelligent beings, on account of 
 their distance from the sun, and the comparatively small 
 amount of light and heat which that great central fire sends 
 to them, if that which the earth receives be taken as the 
 standard. Particularly would this impress us in the case 
 of Neptune. Its distance from the sun is about thirty times 
 ours, and, according to the oft-repeated law of the inverse 
 squares of the distances, the light and heat which it gets 
 from the sun is only one nine-hundredth part of that which 
 we receive. But let us not give up the matter as hopeless 
 on this account. One nine-hundredth part of sunlight is 
 not such a faint illumination, after all. It is nearly seven 
 hundred times the light of the full moon, and indeed equal 
 
5 20 SEARLE 
 
 to that given by a large electric arc-lamp at a distance of 
 a few feet. There would be no difficulty about reading by 
 means of it; it would be quite sufficient for all the ordi- 
 nary practical purposes for which sunlight is used here. 
 And then there is another consideration which is of very 
 great weight. 
 
 It is this: You know that, as I have said, what astrono- 
 mers increase the size of telescopes for is to gather more 
 light, rather than to get greater magnifying power. A 
 telescope of two inches diameter, or aperture, as it is 
 technically called, will give four times as much light as 
 one of only one inch; one of ten inches will give twenty- 
 five times as much as the two-inch, or a hundred times as 
 much as the one-inch. The great Lick telescope, of three 
 feet aperture, makes a star look about thirteen hundred 
 times as bright as a one-inch spy-glass, and enables us to 
 see stars about twenty thousand times fainter than any 
 which can be seen with the naked eye. And the same rule 
 would hold for the eye itself. If we should increase the 
 size of the pupil of the eye, we should see fainter objects 
 than we do now; and we indeed actually do this when we 
 go from bright light into a dark room. We can easily see 
 how the pupil dilates to accommodate itself to reduced 
 light, by simply examining another person's eye in these 
 changed conditions, or our own before a looking-glass. 
 The eye of a cat changes much more. If the retina of 
 the cat's eye is as sensitive as our own, she must habitually 
 see stars five or six times fainter than any which we can 
 discern without a glass, and the heavens must present to 
 her a magnificent appearance, if she cares to look at them. 
 Probably she actually uses this increased light rather to 
 discover mice than stars; but her astronomical oppor- 
 tunities are there all the same, though she may not avail 
 herself of them. 
 
 It is true that this increased light is obtained in the eye 
 at some sacrifice of definition, or sharpness of vision in 
 detail; but still an inhabitant of Neptune might have a 
 good deal larger pupil than ours in proportion to the size 
 
ARE THE PLANETS HABITABLE 
 
 521 
 
 of his eye than ours. And then, again, there is no reason 
 why the retina itself should not be made much more sen- 
 sitive to light than ours; and here we have an increase 
 which has no limit, so far as we can tell. It would be an 
 injury to us to have our optic nerve more sensitive; the 
 strong sunlight to which we are exposed would hurt us. 
 But there is no reason why the Neptunians should not 
 have what .would be a benefit to them. 
 
 The whole question, then, of light in the solar system 
 is one of little consequence; eyes could easily in any planet 
 be such as to suit the exigencies of the case. 
 
 With regard to heat, the question is a little more dif- 
 ficult, but not very much. If we should assume that the 
 500 Fahrenheit by which our temperature here is raised 
 above that of space are simply due to our distance from 
 the sun, and that Neptune could only have one nine-hun- 
 dredth part of that, of course the temperature there would 
 practically be that of space itself, or 460 below the Fahren- 
 heit zero. But we know that, in fact, the genial warmth of 
 the earth is in a great measure due to its atmospheric gar- 
 ment or blanket; and we can not be at all sure that an 
 atmosphere may not exist on Neptune which may make 
 the absorption so much greater than the radiation that an 
 equality between the two would not be reached before the 
 planet had accumulated from its scanty solar supply enough 
 to make its temperature equal to ours. 
 
 And, besides, there is no certainty that these great outer 
 planets may not still retain a great deal of their own in- 
 trinsic heat; that they may yet be warm enough, even on 
 the surface, to act as a source of heat to their inhabitants. 
 Indeed, the danger here is rather that they are too hot 
 than too cold. Yes, that is the trouble with all the great 
 outer planets, with Jupiter and Saturn, as well as Uranus 
 and Neptune, as we shall shortly see. As far as atmosphere 
 is concerned, the spectroscope would indicate rather a dense 
 one on both Uranus and Neptune, and of the same char- 
 acter on each. Uranus shows belts on its surface similar 
 to those seen on Jupiter; but we can not be sure that this 
 
522 SEARLE 
 
 indicates a similar constitution in the two planets. On the 
 whole, we may say that there is quite what we may call 
 a probability that Uranus and Neptune are in a habitable 
 condition; the probability is, however, as we may say, 
 rather negative than positive; we can not give any cer- 
 tain reason why they should not be; but there are really 
 no positive indications to show that they are fit to be the 
 abode of life. The arguments against habitability become 
 much stronger in the case of the two giants of the plan- 
 etary system, Saturn and Jupiter, which come next in order 
 as we proceed toward the sun. The brilliancy of Jupiter's 
 surface, and the rapidity of the changes which we see there, 
 exceeding what the moderate light and heat which it re- 
 ceives from the sun would be likely to produce, seem to be 
 quite strong arguments that it is still in a condition to 
 emit light and heat to a considerable extent on its own 
 account; and, indeed, that its temperature is still suffi- 
 cient to keep it in a fluid state. If its surface be indeed 
 in the condition of molten metal, it certainly becomes unin- 
 habitable in the common-sense view of the subject; for in 
 melted metal no organism composed of ordinary chemical 
 elements could possibly subsist. 
 
 These arguments apply with somewhat diminished force 
 to Saturn. Another, however, which may perhaps be de- 
 rived from the lightness or small density of all the four 
 great exterior planets of which we have been speaking, is 
 strongest in the case of this one. This lightness may indi- 
 cate that they have not yet shrunk to their proper dimen- 
 sions; for it seems reasonable enough to suppose that the 
 chemical constituents throughout the solar system are the 
 same; that all the planets are chips out of the same block; 
 and that when all are reduced to the physical state of the 
 earth they would have about the same density. But this 
 does not seem to amount to much; for though it holds well 
 enough in the cases of Mars and Venus, it notably fails 
 in that of Mercury, if the determinations of the mass of 
 that planet can be considered as trustworthy. The density 
 of Mercury would appear, it will be remembered, to be 
 
ARE THE PLANETS HABITABLE 523 
 
 twice that of the earth; which would prove most undoubt- 
 edly that it was made of decidedly heavier materials, unless 
 we maintain that it is very much more solidified than the 
 earth, which would seem to be improbable. When a planet 
 has once become, like the earth, solid on the surface, no 
 further perceptible shrinkage is possible except by a com- 
 plete breaking up of the crust, which could hardly result 
 except from a collision. 
 
 But to return to the great planets of which we have 
 been speaking. I think few, if any, astronomers believe 
 them to be habitable in their present condition; for, though 
 the case is more doubtful for Uranus and Neptune, still 
 they have, in their general features, so much resemblance 
 to Jupiter and Saturn, that it is usually presumed that they 
 are in the same state. But no one could pretend to be cer- 
 tain with regard to the matter. 
 
 Before we leave this portion of our system, however, we 
 must not omit a part of it which is eminently worth con- 
 sidering with reference to the present question. I mean the 
 numerous satellites, which are such a striking feature in it. 
 
 Let us consider specially those of Jupiter, about which 
 we know the most. The four moons of Jupiter are all quite 
 considerable bodies, ranging in size from that of our moon 
 to that of the planet Mars. There is plenty of room on 
 them for a very large population; the surface of the largest 
 does not fall far short of that of the land part of our own 
 globe. There is no reason why they should not be in the 
 same general physical state as the earth is ; we have already 
 seen that, as far as light and heat are concerned, they may 
 be considered as amply provided; perhaps, indeed, even 
 better than we; for the great planet itself, round which they 
 circulate, would probably serve as a much better luminary 
 by night than our own moon, and may very probably con- 
 tribute not a little to keeping them comfortably warm, if 
 it is indeed still in a melted and glowing condition. We 
 may well believe that it is indeed a second sun to them, 
 and if the satellites of Jupiter keep, like our own moon, the 
 same side always turned toward the primary planet, that 
 
524 
 
 SEARLE 
 
 favoured side would enjoy a continual warmth, which might 
 indeed be excessive. 
 
 Similar remarks may, of course, be made of all the other 
 satellites which we find in this great region, revolving 
 round Saturn, Uranus, and Neptune. Much has been said 
 of the splendour of the Saturnian sky as seen from the 
 planet itself, with the great ring arching over the heavens 
 and the satellites circling along it. It is far more likely 
 that, if this splendour is seen at all, it is from the satellites, 
 from which, especially from Japetus, the most remote, 
 whose orbit lies outside of the plane of the ring, a most 
 magnificent view of the noble planet, with its rings and 
 the other satellites, could be had. Saturn from Japetus 
 would look as it does to us with a magnifying power of 
 about three hundred and fifty diameters; or, to use another 
 illustration, the ball of the planet would look about three 
 and a half times the diameter of the moon, and the rings 
 nearly nine times that diameter. 
 
 We come next, in our inward course, to the planet 
 Mars. Here, for the first time, we begin to see positive 
 signs, instead of mere negative possibilities, of what we 
 have been looking for. 
 
 We have noticed, as we passed this planet on our way 
 outward from the sun, the similarity of its surface to that 
 of the earth, the permanent configurations on it of what 
 we have a good right to assume to be land and water. We 
 have seen its polar ice-caps, its green seas, and red earth; 
 and we know that it has an atmosphere which, though not 
 as dense as our own, is still enough, as it would seem, for 
 life. We know that it has a day almost exactly the same 
 as ours, and not only this, but seasons substantially like our 
 own, as far as the varying angle is concerned at which the 
 sun's rays strike its surface, though it is true that these are 
 a good deal interfered with by the considerable variation 
 in the sun's heat, depending on the eccentricity of its orbit; 
 still this would not amount to so very much. In this lati- 
 tude, for instance, on the earth, we receive more than three 
 times the heat from the sun in one day in the middle of 
 
ARE THE PLANETS HABITABLE 525 
 
 June than we get in the middle of December, on any given 
 area, say a square mile or a square yard, owing to the com- 
 bined influence of the greater height of the sun above the 
 horizon and the greater length of the daylight. About the 
 same would be the case in the same latitude on Mars. The 
 effect of the eccentricity would be quite considerable, mak- 
 ing the sun's heat once and a half as great at the nearest 
 point as at the farthest; still, if we can sustain the three- 
 fold multiplication, a half as much again might be added, 
 without the variation becoming intolerable. Moreover, this 
 great variation would only occur when the summer solstice 
 of one of the hemispheres coincided with the point of near- 
 est approach to the sun. During half the time, the eccen- 
 tricity would tend to moderate, instead of to accentuate, the 
 seasons, as it does with us here in the northern hemi- 
 sphere now. 
 
 Mars is certainly the most favourable case for those who 
 would believe the planets to be habitable. It really seems 
 that it might be inhabited by men like ourselves. As re- 
 marked on a previous occasion, its climate seems, from the 
 small size of the polar ice-caps, to be warmer than that of 
 the earth, in spite of its greater distance from the sun. 
 
 As to Venus and Mercury, we can hardly form any de- 
 cided opinion. They seem to be surrounded by dense, 
 cloudy atmospheres, which may tend, in a great measure, 
 to keep off the intense heat of the sun. A rather singular 
 thing has lately been observed, or at least thought to be 
 observed, by Schiaparelli, with regard to Mercury that is, 
 that some markings on it seem to indicate that its period 
 of rotation round its axis is the same as that of its revolu- 
 tion round the sun; or, in other words, that it acts as our 
 moon does, keeping always the same face toward the centre 
 round which it revolves. This would seem to be borne out 
 by the white spot on the black disk of the planet, which 
 has been reported by various observers as regularly visible 
 at the time of its transits across the sun's face. If this 
 white spot is a real object, it would seem that it is always 
 turned away from the sun. If this can be accepted, it would 
 
5 26 
 
 SEARLE 
 
 be, of course, to some extent, an argument against the 
 habitability of Mercury, as its inhabitants would be de- 
 prived of the vicissitude of day and night, and the side 
 turned constantly toward the sun would probably, in spite 
 of everything, become uncomfortably warm. 
 
 Now that we have though quite hurriedly completed 
 our consideration of the planets as to their suitableness for 
 habitation, what answer shall we give to the question with 
 which we started? Before giving it, another reflection must 
 be made, which will brighten the prospect a good deal for 
 those who would fain believe all these magnificent orbs to 
 be the abode of life like ours. 
 
 It is this: Will it not suffice to satisfy the minds of those 
 who can not believe that these great globes, similar in so 
 many respects to ours, can be tenantless, to hold that they 
 are habited for a portion, though not for the whole, of their 
 history? For myself, I do not feel the craving for the 
 plurality of worlds, as it is called, which seems to be gen- 
 eral. I must confess that I have never been able, per- 
 sonally, to feel the force of the argument which strikes most 
 minds so powerfully, that these habitations could not have 
 been made by their Creator except to be actually inhabited. 
 The mere size and mass of an object seem to me to amount 
 to little. Jupiter itself, or Saturn, with its beautiful ring 
 and satellite system, simply as a mass of matter or a me- 
 chanical construction, is a far less noble creation of God 
 than a single human soul; nor does it seem to me that 
 the mere size of these planets makes them much more 
 remarkable, or requires more reason for their formation, 
 than if they were only a few feet in diameter. The technical 
 study of astronomy, no doubt, has the effect of reducing 
 the impression made by mere magnitude on the mind; 
 whether this is a delusion or the removal of a delusion, of 
 course I can not say. That the mere size of a body itself 
 does not require inhabitants for it, seems plain from the 
 generally confessed impossibility of inhabiting the sun, the 
 surface of which far exceeds that of all the planets put to- 
 gether that is to say, that it does not require them at 
 
ARE THE PLANETS HABITABLE 527 
 
 every moment; but it may be, if you will, that it does re- 
 quire that at some time or other it should be used for such 
 a purpose. The general belief is, we may say, an argu- 
 ment for the fact. 
 
 And, of course, the argument for the plurality of worlds 
 is strengthened if, besides size or standing-room, as we may 
 say, we see some other conditions indicating conveniences 
 for life, though they be imperfect or incomplete. If we see 
 a house with only its framework up, we say, " Nobody lives 
 there now, but it is being built for some one "; and if we 
 see a house in ruins, we say, " Somebody lived there once." 
 
 Now, this is certainly very plausible; and I think that 
 the history of our own earth, so far as it can be learned 
 from science, increases the probability of the opinion that 
 the planets, and perhaps even the sun itself, were made to 
 be inhabited at some time or other. The teaching of 
 geology is that our own earth was for a long time unin- 
 habitable; that it subsequently became fitted to be the 
 abode of the inferior and simpler forms of life, and finally 
 became ready for the reception of man ; and we can hardly 
 shut our eyes, either, to the scientific conclusion that, from 
 the operation of natural causes alone, it would at some time 
 in the distant future become uninhabitable again, though 
 in a different way; that it would become, simply from the 
 changes which must come from the gradual progress of 
 cooling necessarily going on in the solar system, no longer 
 a building which its Creator is forming, but a cold and 
 desolate ruin like the moon. 
 
 The history of this earth is probably the history of the 
 other planets, if they are to be allowed to develop in a 
 natural way. Some, like the moon, seem to have passed 
 farther along the road than our own planet. This is 
 probably the case with Mars, the most habitable in appear- 
 ance of them all. As a rule, of course, the smaller a planet 
 is, other things being equal, the more rapidly it will cool 
 from its originally incandescent state; Mars then should 
 be older that is, have passed through more of its succes- 
 sive changes than we. It looks so, besides. The seas 
 
528 SEARLE 
 
 seem to be drying up, the air thinning away. On the other 
 hand, the great superior planets, Jupiter, Saturn, Uranus, 
 and Neptune are young, and have the best part of their 
 life before them. 
 
 What portion of the total life of a planet is that in 
 which it becomes habitable by beings like ourselves we 
 can not very well determine. If we accept the estimates 
 of geology, the time that the human race has been here is 
 a very small part of our world's history. But how much 
 longer this earth would naturally remain a possible resi- 
 dence for us we can not say with accuracy. It would seem 
 probable, however, that the period in which all the neces- 
 sary conditions of life would simultaneously exist can 
 hardly be a very considerable part of the whole. The in- 
 habitants of a planet in the stage of decadence from its 
 most perfect state could, no doubt, on the principle of the 
 " survival of the fittest," accommodate themselves to their 
 more unfavourable circumstances for a good while; but 
 the time would come when the struggle would have to be 
 abandoned. 
 
 If it is true that the period of habitability by the high 
 organisms is a small part of a planet's life, obviously the 
 chance is small for any planet in particular of its being in 
 that period now or at any particular time. We must say 
 that it probably is not, unless we have, as in the case of 
 Mars, some positive indications that it is. So far as we 
 can trust such positive indications, Venus and Mercury are 
 approaching that part of their life that the earth is in at 
 present; the earth seems at one time to have had the very 
 dense and vaporous atmosphere that apparently surrounds 
 them now. 
 
 To sum up now, briefly, the results to which our exam- 
 ination has led us: In the first place, our observations 
 should probably be modified by the very plausible theory, 
 now generally adopted, that all the bodies of our system, 
 sun and planets, have passed and are passing through a 
 series of changes, beginning with a state of great heat and 
 expansion, in which and for a long time no life is possible 
 
ARE THE PLANETS HABITABLE 
 
 529 
 
 on their surfaces, and in a great part of which indeed, as 
 in the case of the sun at present, they can hardly be said 
 to have a surface at all. As the changes due to the gradual 
 cooling and contraction proceed, life in its simpler forms 
 becomes possible, and in course of time a state is reached 
 like that of this globe at present, in which the conditions 
 for highly organized life are at their best. 
 
 Assuming this, the question of fact becomes, Is there 
 any other planet or satellite in the system in which this 
 state of maximum habitability, if we may so call it, now 
 exists? We can say with great confidence that it does not 
 on Jupiter and Saturn; that the chances are much against 
 it on Uranus and Neptune; that Venus and Mercury are 
 probably still too young for it; but that there is a reason- 
 able probability for it on Mars, though this planet seems 
 to be passing into the decline, the steps of which we do 
 not clearly understand, but of which we see perhaps the 
 final result in the torn, scarred, and desolate surface of our 
 own satellite. With regard to the satellites of the great 
 planets, we have absolutely to suspend judgment. As the 
 period of habitability is probably less than that of develop- 
 ment, though of this we are far from certain, the chances 
 are perhaps against any particular one of them being in 
 that state just now; but as they number at least seventeen 
 altogether, the probability that some one of them may be 
 habitable is not so inconsiderable. As to the satellites of 
 Mars, and the swarm of asteroids, they seem to be too 
 small to retain an atmosphere sufficient for the support of 
 beings like ourselves. If they had a course to run, it has 
 probably been concluded long ago. 
 
 In speaking of the natural life and development of the 
 planets, we are, of course, looking at the matter merely 
 from a scientific point of view. Of course, most Christians 
 believe that long before the natural life of this earth would 
 be concluded, it will suffer a final catastrophe which will 
 at least close the history of the human race on it as it 
 exists now. Such catastrophes may, of course, occur to 
 any planet by natural as well as supernatural causes; by 
 
 34 
 
530 
 
 SEARLE 
 
 collision with some other body, for instance; or to the 
 whole planetary system, by some large body striking on 
 the sun. One thing which we may perhaps look forward 
 to is a time when, after the death or destruction of all the 
 planets, the sun itself ceasing to be a luminary and furnace 
 for bodies circulating round it, may itself become the great 
 seat and home of life. In theorizing on this point we 
 have no past experience or history to guide us. We shall 
 see as we go on to discuss the stellar systems that we have 
 at least one case, perhaps more than one, of a body sunlike 
 in dimensions, which has either ceased to give light or 
 never gave it. It is only in exceptional cases that we have 
 any means of recognising the existence of such bodies; 
 they may be very numerous. Neither can we tell whether 
 the other innumerable brilliant suns scattered through 
 space have attendant planets like our own. But it would 
 be strange if they had not. If any considerable proportion 
 of them have, evidently the chance that there are other 
 habitable worlds in the universe becomes very great. 
 
INDEX 
 
 Aberration, 203; chromatic, 347, 
 
 350; spherical, 347, 350. 
 Acidalium, Mare, Martian sea, 
 
 145, 151, 152. 
 
 Ackeron, Canal of Mars, 156. 
 ^Ethra, the asteroid, 243. 
 Airy, geodetic values of, 108. 
 Aldebaran (o Tauri), 259; proper 
 
 motion of, 279; spectrum of, 
 
 373- 
 Algol (0 Persei), 462; spectrum 
 
 of, 429. 
 
 "Almagest" of Ptolemy, 256, 258. 
 Andromeda, nebula in, 169, 417, 
 
 437, 494; spectrum of, 386. 
 Aonius, Sinus, Martian estuary, 
 
 I54-. 
 
 Aquarius, nebula in, 384. 
 
 Arcturus (a Bootis), proper mo- 
 tion of, 272, 279, 426. 
 
 Argelander, "Durchmusterungen" 
 of, 259, 264. 
 
 Argyre, Martian island, 154. 
 
 Aristarchus of Samos, method of 
 determining solar parallax, 188. 
 
 Asteroids, 216; discovery of, 303; 
 celestial photography first used 
 in discovery of, 242; origin of, 
 Olbers's theory, 249; value in 
 determining parallax, 197, 244. 
 
 Astronomical publications, 313. 
 
 Astronomy, American, 309; origin 
 of, 255; sidereal, 255. 
 
 Astrophysics, 291, 363, 441. 
 
 Atlantis, Martian peninsula, 150. 
 
 " Atlas Ccelestis," Flamsteed's, 
 258. 
 
 Aurigae, a, see Capella. 
 
 Aurigae, 3, spectrum of, 428, 489. 
 
 Aurigae, C spectrum of, 431. 
 
 Aurora borealis, spectrum of the, 
 401. 
 
 Aurorae, Sinus, Martian estuary, 
 154- 
 
 Ausonia, Martian peninsula, 150. 
 
 Australe, Mare, Martian sea, 149, 
 
 iSi, 154, 155- 
 Auzout, great telescope of, 344. 
 
 Baltia, a district of Mars, 151. 
 Barnard, celestial photographs 
 
 taken by, 22. 
 Berlin Academy, star maps of the, 
 
 262. 
 Bessel, geodetic values of, 108; 
 
 parallax of 61 Cygni, 323. 
 Betelgeux (a Orionis), spectrum 
 
 of, 373, 379- 
 
 Bond, William Cranch, 318. 
 Bootis, o, see Arcturus. 
 Bowditch, Nathaniel, 314. 
 Bradley, observations of stellar 
 
 parallax, 325. 
 
 Brashear, methods of lens-mak- 
 ing, 357- 
 Brewster, Sir David, 204; theory 
 
 of life in other worlds, 83. 
 Brucia, the asteroid, 243. 
 Buffon, system of the world, 499. 
 
 Campani, improvement of the 
 
 telescope, 344. 
 Canals of Mars, 153; gemination 
 
 of, 156. 
 Capella (a Aurigae), 256; spectrum 
 
 of, 395, 469, 486. 
 Capricorni, ft, spectrum of, 431. 
 Cassini, discoveries of, 345; early 
 
 determinations of parallax, 189. 
 Castor (a Geminorum), proper 
 
 motion of, 279. 
 Centauri, o, parallax of, 331. 
 Cephei, /i spectrum of, 379. 
 Ceres, the asteroid, discovery of, 
 
 241. 
 
 Ceti, 31; spectrum of, 431. 
 Chandler, investigation of polar 
 
 movements, 32. 
 Chauvenet, 312. 
 Cimmerium, Martian sea, 150. 
 Clark, Alvan, 353. 
 Clark, geodetic values of, 108. 
 Clusters, star, 385. 
 Coal, decrease in supply of, 76. 
 Coast Survey, United States, 313. 
 Comets, 22; Laplace's theory of, 
 
 506; connection with meteoric 
 
532 
 
 INDEX 
 
 swarms, 182, 402, 463; photo- 
 graphs of, 299; spectra of, 387, 
 402, 463, 470. 
 
 Constellations, the, 258. 
 
 Copernicus, system of astronomy, 
 24, 187. 
 
 Corona, of the sun, 405, 465. 
 
 Crumpling, investigations of 
 earth-, 122. 
 
 Cygni, 0, spectrum of, 378. 
 
 Cygni, 61, parallax of, 280, 323, 
 327. 
 
 Cygnus, constellation of, 166. 
 
 Darwin, George Howard, mete- 
 oric nebular theory, 126, 163, 
 417; tidal theory of satellite for- 
 mation, 250. 
 
 Deucalionis, Regio, peninsula of 
 Mars, 151, 154. 
 
 Divini, improvement of the tele- 
 scope, 344. 
 
 Dog Star, see Sirius. 
 
 Dolland, John, improvement of 
 the telescope, 351. 
 
 Draconis, a, 28. 
 
 " Durchmusterungen," 264. 
 
 Earth, the, 216; astronomical ap- 
 pearance of, 148, 1 60; density of. 
 method of determining, 199; 
 evolution of, 64; effects of cool- 
 ing of, 71, 115, 120, 122; form 
 of, controversy of the Newto- 
 nians and Cassinians, 106; mass 
 of, distribution of the, in; 
 mathematical theories of, 103; 
 orbit of, 222; past and future of, 
 55; physical condition of, 39; 
 rotation of, 30; shape of, 198; 
 size of, methods of determining, 
 107. 
 
 Earthquakes, 124; cause of, 77. 
 
 Eclipses, photographs of, 295. 
 
 Elysium, district of Mars, 149. 
 
 Encke's value of solar parallax, 
 209. 
 
 Equation, astronomical meaning 
 of, 203; personal, 478. 
 
 Equilibrium, kinds of, 235. 
 
 Eros, the asteroid, discovery of, 
 241, 243; value of, in determin- 
 ing parallax, 245. 
 
 Erythraeum, Mare, Martian ocean, 
 
 144, 151, 154, 155- 
 Eskimos, astronomy of the, 256. 
 Ether, luminiferous, 204. 
 
 Eudoxus, celestial globe of, 256. 
 Euler, theories of, in regard to 
 aberration, 350. 
 
 Faraday, magnetization of light, 
 
 204. 
 Fizeau, measurement of the ve- 
 
 locity of light, 192, 201. 
 Flamsteed, " Atlas Ccelestis," 258; 
 
 determinations of parallax, 189. 
 Fons Juventae, Martian lake, 153. 
 Forests, devastation of, 76. 
 Foucault, method of determining 
 
 velocity of light, 202. 
 Fournier, theory of heat diffusion, 
 
 US- 
 
 Franklin, Benjamin, 309. 
 Fraunhofer, improvement of the 
 
 telescope, 352; discovery of the 
 
 lines of the solar spectrum, 445. 
 
 Galileo, astronomical discoveries 
 
 of, 342; telescope, claim to the 
 
 invention of the, 189, 342. 
 Gas, meteorites acting as a (Dar- 
 
 win's theory), 126, 163, 417; 
 
 molecular theory of, 10, 15. 
 Gascoigne, William, inventor of 
 
 the micrometer, 189. 
 Geminorum, o, see Castor; j8, see 
 
 Pollux. 
 
 Geodesy, development of, 107. 
 Geological time, Lord Kelvin's 
 
 limits to, 121. 
 Gilliss, James M., 317. 
 Glass, optical, manufacture of, 
 
 351- 
 Godfrey, inventor of the sextant, 
 
 310. 
 Gould, " Uranometria Argentina," 
 
 259- 
 Gravitation, force of, at surface of 
 
 the sun, 17; law of, 218; use of, 
 
 in determining parallax, 192; 
 
 variation of, with latitude, no. 
 Guinaud, improvements in the 
 
 manufacture of optical glass, 
 
 Hadley, reflector of, 348. 
 Hadriacum, Martian ocean, 150. 
 Halley, observation of transits of 
 
 Mercury and Venus, 190. 
 Heat, effect of, upon molecules, 
 
 9, 13, 15; internal, of earth, 70, 
 
 115; specific, 46 note; sun's, see 
 
 Sun. 
 
INDEX 
 
 533 
 
 Helmholtz, theory of the sun's 
 heat, 3, 51. 
 
 Henderson, observations of stellar 
 parallax, 326, 331. 
 
 Henry, Joseph, 316. 
 
 Hercules, star cluster in, 386. 
 
 Herculis, a, spectrum of, 377. 
 
 Herschel, Sir William, discover- 
 ies of, 241, 348. 
 
 Hesperia, Martian peninsula, 150. 
 
 Hilda, the asteroid, 243. 
 
 Hipparchus, astronomical discov- 
 eries of, 25, 256; method of de- 
 termining solar parallax, 189. 
 
 Holmes's comet, 22. 
 
 Huygens, discovery of the rings 
 of Saturn, 345; reflector of, 
 348. 
 
 Hydaspes, canal of Mars, 156. 
 
 Hydrogen, spectrum of, 16. 
 
 Hyperboreus, Lacus, Martian 
 lake, 145, 152. 
 
 Ingeborg, the asteroid, 243. 
 
 Iris, the asteroid, 246. 
 
 Ismenius, Lacus, Martian lake, 
 
 152, 154. 
 Isostasy, theory of, 123. 
 
 Jupiter, the planet, habitability of, 
 522; mass of, 61; size and dis- 
 tance, 216; spectrum of, 372. 
 
 Jupiter, satellites of, 84, 232, 233; 
 discovered by Galileo, 342; fifth 
 satellite discovered by Barnard, 
 22; habitability of, 523; rota- 
 tion of, 509; use in determining 
 light equation, 203. 
 
 Kant, nebular hypothesis of, 167, 
 170. 
 
 Kelvin, Lord (Sir William Thom- 
 son), terrestrial temperatures, 
 119; molecular theory, 14. 
 
 Kirchhoff, interpretation of the 
 spectrum, 445; development of 
 the spectroscope, 366. 
 
 Kyrae, 0, spectrum of, 431. 
 
 Laplace, distribution of earth's 
 mass, in; see Nebular Hypoth- 
 esis. 
 
 Latitude, method of determining, 
 34; variations in, due to polar 
 changes, 36; value of, in astro- 
 nomical investigation, 38. 
 
 Leibnitz, " consistentior status " 
 of, 115, 117. 
 
 Lens-making, 354. 
 
 Leonis, a, see Regulus. 
 
 Leverrier, observations of Mer- 
 cury, 133. 
 
 Libration, 135; of Mercury, 136; 
 of the moon, 135. 
 
 Lick Observatory, 306. 
 
 Life, sources of, 70 note, 72. 
 
 Light, composite character of, 
 443; corpuscular theory of, 203; 
 magnetization of, 204; velocity 
 of, measurement of, 192, 201, 
 use in determining parallax, 
 192, 201. 
 
 Lilienthal, observations of Mer- 
 cury, 133. 
 
 Lippersheim, see Lippershey. 
 
 Lippershey, Franz, inventor of 
 the telescope, 344. 
 
 Liquids, molecular theory of, 10, 
 12. 
 
 Lunae, Lacus, Martian lake, 152, 
 
 154- 
 
 Lunar inequality, 197. 
 Lyra, nebula in, 385, 388. 
 Lyne, , see Vega. 
 
 Magnitudes, scale of, 257. 
 
 Margaritifer Sinus, Martian es- 
 tuary, 154. 
 
 Mars, the planet, 84, 216; atmos- 
 phere of, 147; canals of, 153, 
 gemination of, 156; climate of, 
 147; habitability of, 524; paral- 
 lax of (1672), 189, 191; polar 
 caps of, 143; Schiaparelli's ob- 
 servations of, 143; spectrum of, 
 372; topography of, 150. 
 
 Maupertuis, the "Earth-flattener," 
 107. 
 
 Maxwell, Clerk, magnetism and 
 light, 204; investigations of Sat- 
 urn's rings, 170. 
 
 Measuring machines, 294. 
 
 Mercury, the planet, 84, 215: 
 atmosphere of, 138; habitability 
 of, 525; libration of, 136; mo- 
 tions of, 45; observations of 
 Leverrier and Lilienthal, 133; 
 Schiaparelli's observations of, 
 133; diurnal parallax of (1677), 
 190; rotation of, 135; transit of, 
 190, 191. 
 
 Meteor-streams, 181 ; Schiaparel- 
 li's theory, 292. 
 
 Meteorites, as sources of life, 70 
 note; of lunar craters, 70; of 
 
534 
 
 INDEX 
 
 the sun's heat, 44, 51; increase 
 of earth's mass by falling of 
 meteors, 64; as origin of stellar 
 systems (Darwin's theories), 59, 
 126,163, 173, 417; spectra of, 171, 
 389; nature and composition of, 
 170; connection of comets with, 
 402, 463. 
 
 Micrometer, invention of, 189. 
 
 Milky Way, 22, 277, 421, 437, 490. 
 
 Mitchel, O. M., 311. 
 
 Mizar, spectrum of, 428. 
 
 Moeris, Lacus, Martian lake, 152. 
 
 Molecular theory, 5 et seq.; of 
 solids, 8, 13; of gases, 10, 15, 
 171 ; of liquids, 10, 12. 
 
 Moon, discoveries of Galileo in 
 regard to the, 342; present con- 
 dition of the, 78; habitability of 
 the, 518; light and heat of the, 
 289; photographs of the, 294; 
 rotation of the, 66, 233; spec- 
 trum of the, 371; use in deter- 
 mining parallax, 192. 
 
 Motion in the line of sight, as 
 determined by the spectroscope, 
 278, 423, 461, 484, 485. 
 
 Motion, proper, 272. 
 
 Navigation, early American, 314. 
 
 Nebula, annular, in Lyra, 385, 
 388; dumb-bell, 385, 388; in An- 
 dromeda, 169, 386, 417, 437, 494; 
 in Aquarius, 384; in Orion, 60, 
 385, 427, 437, 491. 
 
 Nebulae, 60, 167; brightness of the, 
 388; first photographs of, 297; 
 spectra of, 170, 382, 415, 453. 
 
 Nebular hypothesis of Laplace, 
 59, 125, 167, 170, 416, 492, 497. 
 
 Neptune, the planet, 217; discov- 
 ery of, 263; distance from the 
 sun, 211 ; habitability of, 519; 
 satellites of, habitability of the, 
 524. 
 
 Nerigos, district of Mars, 151. 
 
 Newton, Sir Isaac, 106, 347. 
 
 Niliacus, Lacus, Martian lake, 
 151- 
 
 Nilosyrtis, canal of Mars, 153, 
 155, 158. 
 
 Noachis, Martian island, 154. 
 
 Nodes, lines of, 229. 
 
 Nutation, 192; defined, 197; use in 
 
 Observatories, American, early, 
 319. 
 
 Observatory, work in a modern, 
 473- 
 
 Olbers, theory of the origin of 
 asteroids, 249. 
 
 Orion, constellation of, 258. 
 
 Orion, nebula in, 60, 297, 300, 385, 
 437, 49i; motion of, 427; spec- 
 trum of, 470. 
 
 Orionis, o, see Betelgeux. 
 
 Orionis, j8, see Rigel. 
 
 Parallactic inequality, 196. 
 
 Parallax, hypothetical, 286; meth- 
 ods of determining, 192, 205, 
 246, 326; ancient observations 
 of, 188; stellar, 321, 436, 279. 
 
 Pegasi, 0, spectrum of, 375, 379. 
 
 Peirce, Benjamin, 315. 
 
 Persei, ft, see Algol. 
 
 Persei, T, spectra of, 431. 
 
 Phoenicis, Lacus, Martian lake, 
 154- 
 
 Photography, celestial, 22, 165, 
 242, 248, 290, 350, 431, 482. 
 
 Photography, spectroscopic, 391, 
 426, 468. 
 
 Photometry, 435; at Harvard Uni- 
 versity, 260, 298. 
 
 Photo-tachymetry, 192, 200. 
 
 Piazzi, 241. 
 
 Planets, analogies between the, 86; 
 distances of the, 187; habitabil- 
 ity of the, 83, 513; magnitudes, 
 etc., 215; orbits of the, varia- 
 tions in the, 222; mutual per- 
 turbations of the, 200, 218; 
 spectra of the, 371. 
 
 Plumb-line, deflection of the, no. 
 See Theorem of Stokes. 
 
 Poisson, studies of terrestrial 
 temperature, 116. 
 
 Polar caps, of Mars, 143. 
 
 Pole, celestial, Barnard's photo- 
 graph of, 22; changes in posi- 
 tion of the, 24. 
 
 Pole, terrestrial, 25; see North 
 Pole. 
 
 Poles, magnetic, changes in, how 
 caused, 70. 
 
 Pole, North, physical and astro- 
 nomical conditions at the, 25; 
 wanderings of the, 22. 
 
 Pole Star, 23, 25, 28. 
 
 Pollux (0 Geminorum), proper 
 motion of, 279. 
 
 Polynesians, astronomy of the, 
 256. 
 
INDEX 
 
 535 
 
 Precession, 192, 197; use in deter- 
 mining parallax, 197. 
 
 Prominences, solar, 465. 
 
 Propontis, Martian lake, 152. 
 
 Ptolemaic system of astronomy, 
 187. 
 
 Ptolemy, "Almagest" of, 256, 
 258. 
 
 Pyrois, 151 note. 
 
 Pyrrhse, Regio, district of Mars, 151. 
 
 Pythagoras, astronomical system 
 of, 187. 
 
 Refractors and reflectors, 347; see 
 
 Telescope. 
 
 Regulus (a Leonis), 258. 
 Rigel (jSOrionis), parallax of, 492. 
 Roche, investigation of Saturn's 
 
 rings, 170; hypothesis of, as to 
 
 the distribution of the earth's 
 
 mass, 114. 
 Rutherfurd, Lewis, 294. 
 
 Sabaeus, Sinus. Martian estuary, 
 
 154- 
 
 Sagittarii, 5, spectrum of, 431. 
 
 Sappho, the asteroid, 246. 
 
 Satellites, distribution of, 84; ro- 
 tation of the, 141, 508; stability 
 of satellite systems, 232; of the 
 great planets, habitability of 
 the, 524; see Moon. 
 
 Saturn, the planet, 61, 216; habita- 
 bility of, 522; photographs of, 
 301; rings of, 83, 170, 503, dis- 
 covery of the, 345, stability of 
 the, 234; satellites of, 84, dis- 
 covery of the, 345, habitability 
 of the, 524; spectrum of, 372. 
 
 Schoenfeld, " Durchmusterung " 
 of, 265. 
 
 Sextant, invention of the, 310. 
 
 Simois, canal of Mars, 156. 
 
 Sirenum, Martian sea, 150. 
 
 Sirius (o Canoris Majoris), the 
 Dog Star, 256, 257; distance of, 
 280; proper motion of, 272; 
 spectrum of, 469. 
 
 Snow, melting of, 140; apparent 
 existence of, on Mars, 143. 
 
 Solar system, magnitude of the, 
 183; origin of, 59; stability of 
 the, 213; see Nebular Hypoth- 
 esis, Sun, Planets, Satellites, 
 etc. 
 
 Solids, molecular theory of, 8, 13. 
 
 Solis, Lacus, Martian sea, 150. 
 
 Spectra of chemical elements, 451 ; 
 of meteorites, 171; of nebulae, 
 170; of the stars, see Stars. 
 
 Spectroscope, 278; development of 
 the, 365, 391; Muggins's first 
 stellar, 447. 
 
 Spectroscopy, celestial, 292, 363, 
 391, 44i, 483- 
 
 Spica (o Virginis), spectrum of, 
 430, 462. 
 
 Star gauging, 284. 
 
 Star maps, of the Berlin Academy, 
 262; photographic, 299, 433, 482, 
 490. 
 
 Stars, age of the, 406, 492; binary, 
 286, spectroscopic discovery of, 
 427, 462, 489; catalogues of, 269; 
 colours of the, 376; distances of 
 distribution of the, 282; magni- 
 tudes of, 257; masses of, 282; 
 naming of, 258; number of, 260; 
 parallax of the, 324; early pho- 
 tographs of the, 296, see Celes- 
 tial Photography; spectra of the 
 367, 406, 450, 484; temporary, 
 379, 457J variable, 289, 378, 429. 
 
 Stars, shooting, see Meteorites. 
 
 Stilbon, ancient name for Mer- 
 cury, 134. 
 
 Stokes, theorem of, no. 
 
 Struve, observations of stellar 
 parallax, 326, 331. 
 
 Sun, the, chemical composition of, 
 403; distance of, 211; force of 
 gravity at surface of, 17; habi- 
 tability of, 517; heat of, I, 41, 
 origin and total amount of, 50, 
 sources of, 47; present tem- 
 perature of the, 49; Professor 
 Young's views concerning the 
 present condition of, 67; secular 
 cooling of, 44; magnitude of, 
 215; movement of, in space, 
 273; photographs of, 295; rota- 
 tion of, 431; spectroscopic ob- 
 servations of, 465; weight of, 
 176; star cluster to which, be- 
 longs, 287. 
 
 Sun spots, 290, 344. 
 
 Swift's comet, 22. 
 
 Syrtis Major, Martian gulf, 155. 
 
 Tauri, o, see Aldebaran. 
 
 Telescope, Galileo's, 189; history 
 of the, 339; mounting of the, 
 353' power of vision attainable 
 in the, 516. 
 
536 
 
 INDEX 
 
 Tempe, district of Mars, 149. 
 
 Thule, the asteroid, 243. 
 
 Tides, 39; causes of the, 192; effect 
 
 of, on rotation, 289. 
 Torricelli, improvement of the 
 
 telescope, 344. 
 Transit circle, 475. 
 Triton, a canal of Mars, 156. 
 Trivium Charontis, Martian lake, 
 
 152, 154- 
 Tyrrhenum, Martian sea, 150. 
 
 " Uranometria Nova," 259; "Ar- 
 gentina," 259. 
 
 Uranus, the planet, 217; discovery 
 of, 241, 349; habitability of, 519; 
 satellites of, 84, habitability of 
 the, 524. 
 
 Vega (o Lyrae), 25, 257; parallax 
 of, 280, 331; proper motion of, 
 279; spectrum of, 375, 469. 
 
 Venus, the planet, 84, 216; discov- 
 ery of the phases of, by Galileo, 
 342; habitability of, 525; paral- 
 lax of (1681), 190; spectrum of, 
 372; transits of, 190, 191, 209, 
 244. 
 
 Victoria, the asteroid, 246. 
 
 Virginis, a, see Spica. 
 
 Volcanoes, 124; cause of, 77. 
 
 Whewell, theory of life in other 
 worlds, 84. 
 
 Zodiacal light, 181. 
 
 THE END 
 

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