'§2ffliiQ8k^Uto.JiMm^ O' ; ^M^i®W^M0MMM M I W$t& 1 CORSELL UNIVERSITY L1EEAE7. ;#V 'his book'ls .noMo be taken i from the Reading: Room. ^fc- -V <* "• -. :©:■ 1$ t>. WHiN DONE WiTH, RETURN AT QMCE TO fegi SHELF. <&g^=3~Z. /(LGj- m W ^£t*S W72. QB 521.P96T872 n ' VerS ' ,yLibrary The surmiler, fire, light, and life of 3 1924 005 025 519 DATE DUE NOV i 1083 PRINTED tN U.S.A. Cornell University Library The original of this book is in the Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924005025519 THE SUN. LONDON : PRINTED BY SFOTTISWOODB AND CO., NEW-STREET SQUARE AND PARLIAMENT STREET THE SUN: KULER, FIRE, LIGHT, AND LIFE OF THE PLANETARY SYSTEM. BY RICHARD AT gROCTOR, B.A. Camb., HON. SEC. R.A.S. AUTHOR OF * OTHER WORLDS THAN OURS,' * SATURN" AND ITS SYSTEM,' ' LIGHT SCIENCE FOR LEISURE HOURS,' ETC. Beyond expression bright Compared with aught on eanh, metal or stone ; Not all parts like, but all alike inform'd "With radiant light, as glowing iron with fire * * * * What wonder then if fields and regions there Breathe forth elixir pure, and rivers run Potable gold, when with one virtuous touch Th' arch-chemic Sun, so far from us remote, Produces, with terrestrial humour mix'd, Here in the dark so many precious things ? MrLTON. WITH TEN LITHOGRAPHIC PLATES {SEVEN COLOURED) AND <>NE HUNDRED AND SIX DRAWINGS ON WOOD, SECOND EDITION. LONDON : LONGMANS, GREEN, AND CO. 1872. 'CORNELL UNIVERSITY .LIBRARY^ tie / TO WILLIAM HITGGINS, D.C.L. LLD. F.RS. —THE HBBSCHEL OF THE SPECTROSCOPE- IS EECOQNITION OF THOSE LABOURS BY -WHICH THE WHOLE ASPECT OF MODERN ASTRONOMY HAS BEEN MODIFIED ffcis Maxk is glebkairb BY THE AUTHOE. PREFACE TO THE SECOND EDITION. The favourable reception accorded to the first edition of this treatise (two thousand copies of which were sold during less than a year from the date of publication) encourages me to hope that the work has been found a useful compendium of what is known or may be surmised respecting the great ruling luminary of our system. The present edition has been completely revised, and in some important respects modified. Several new illustrations, for which I am indebted to Mr. Brothers of Manchester, and to Professor Young, Mr. Willard, and Mr. Knight, of America, have been added, as well as an account and analysis of the observations made during the eclipse of December 1870. Some of the modifications in the present edition are due to suggestions made in reviews ; and in acknow- ledging this obligation, I cannot refrain from express- ing my sense of the kindly tone in which, with scarce viu PREFACE TO an exception, the present work has been treated by reviewers. It may be a dangerous admission to make, but it is simply the truth that their mildly expressed censures or corrections have often seemed to me better merited than their very generous commendations. Yet I have occasion to defend myself from a most serious charge occurring in a review, of which I certainly had no other reason to complain, and in a journal whose general fairness is beyond suspicion. It was remarked in the ' Athenaeum ' that I show myself in this work ' a little too niggardly in claiming every scrap of theory to which I may be entitled.' As I was not conscious of any passages deserving this rebuke, while I was exceedingly anxious to remove such blemishes if they existed, I sought for further information. The editor of the ' Athenaeum ' pointed out in reply that the sentence ' involved no reference to the justice of the re- clamations.' But I pressed for instances of niggardli- ness, or even of any reclamation of a scrap of theory, for I knew of none throughout the book, nor could I remember a single instance in which I had had occasion there or elsewhere to reclaim a ' scrap of theory ' from any fellow-worker in science. I was met with the re- joinder that the criticism was based on the general tone of the volume, not on any special instances of re- clamation ! If the criticism had not been specially limited to 'scraps of theory,' and explained by the THE SECOND EDITION. ix very just remark that ' the man who first enunciates a theory may have considered it much more crudely than one who has not mentioned it outside the range of pri- vate conversation,' I could have understood it to relate to the occasions, few and far between, when I have been forced to make reclamations respecting results worked out to their completion by myself. But so far as my memory serves, I have never reclaimed a theory, or a scrap of one, in any sort or fashion, niggardly or other- wise ; nor have I ever had occasion to do so, so far as I am aware. The second appendix to the first edition has been removed from the present, partly to make room for the added matter (which much exceeds it in extent), and partly as being somewhat too abstruse for a popular treatise. The chapters relating to the circumsolar region were in type before the news of the recent eclipse had reached England. If the present edition had been prepared a few months later, I should simply have substituted the overwhelming evidence derived from the late eclipse, for the equally complete but less striking evidence already available when I wrote. The results of the eclipse expeditions of December last can be readily summed up. In the first place, seventeen excellent photographs, agreeing admirably in all essen- tial respects, have been obtained at Bekul, Dodabetta, X PREFACE TO Avenashi, and Jaffna. Next, the existence of Young's complex and relatively shallow atmosphere, giving a spectrum of multitudinous bright lines, has been finally established. Thirdly, Eespighi has observed what Young had demonstrated, the fact, namely, that the gaseous matter of the inner corona extends upwards of 200,000 miles from the Suns surface ; for he saw three coloured spectral images of the corona with this de- gree of extension (two of these images being due to glowing hydrogen). Fourthly, Janssen has proved that besides Kirchhoff's 1474-line, the hydrogen lines and several other fainter lines exist in the gaseous spectrum of the corona, while he has been able to de- tect the solar dark lines (notably the sodium line) across the faint continuous spectrum of the corona. Lastly, the polarisation observations show that to a height of more than a million miles from the Sun the coronal luminosity contains reflected light. All these results agree perfectly with the theories enunciated in the present work. In particular my opposition to the ' atmospheric glare' theory, which some had regarded as over-confident and too earnest, has been abundantly justified, as well by the observational overthrow of that theory as by the feeling now entertained that its enunciation was unfortunate. To quote the words of Janssen, ' the question whether the corona is due to the terrestrial atmosphere is now disposed of (tranchee), THE SECOND EDITION. xi and we have before us the prospect of the study of the extra solar regions, which will be very interesting and fruitful.' Or, in Mr, Lockyer's words (and as I have been urging for the last two years), ' steps should be taken in order that the question may be more closely investigated and more rapidly worked out than it is now.' There is at present no erroneous theory to in- terfere with the prosecution of such enquiries ; and the only misfortune is that no total eclipse worth observing will now occur for several years. Eichaed A. Proctor. Brighton; February 1872. PREFACE THE FIRST EDITION". When I had completed my treatise on Saturn and its System, I formed the design of preparing a separate treatise on each of the planets Mars and Jupiter, and then another and larger treatise on the Suu. Circum- stances, which it is needless to particularise, prevented me from carrying out this design at that time, and indeed threatened to withdraw my attention altogether from scientific pursuits. That my plans, though delayed, have not been lost sight of during the last four years, is evidenced by the appearance of many papers of mine on Mars, Jupiter, and the Sun, in several quarterly, monthly, and weekly journals. These, if collected, would of themselves suffice to form volumes of no inconsiderable dimensions on these several orbs ; while my ' Other Worlds than Ours ' presents a sort of summary of my researches on these and other astronomical subjects. But it is only quite recently that I have been able to resume my original design. THE FIRST EDITION. xiii The delay has not been without its advantages, however. A work on the Sun has at the present time a far greater interest than it would have had four years since ; while I have been able to obtain a much wider and more complete view of the subject than I should probably have thought necessary had I completed the work at that time. My primary object in the present volume has been to furnish a full account of the remarkable discoveries which have been effected by observers of the Sun, whether by means of the telescope, the spectroscope, polariscopic analysis, or photography. It will be seen that Chapters IV., Y., and VI., in which I deal with these discoveries, together constitute more than one half of the main text. In these chapters the labours of the Herschels, Schwabe, Carrington, Secchi, De la Hue, Stewart, and others in examining the solar surface ; the later observations of Hugginsy Zollner, Respighi, Secchi, Lockyer, Young, and others in the study of the prominences and chromatosphere ; and the observations which have been made during the past two centuries on the phenomena presented by the solar corona, have been dealt with at considerable length. But it seemed desirable that a separate and complete explanation should be given of all those matters which specially appertain to the application of spectroscopic analysis to the study of solar physics. "Without an Xiv PREFACE TO account of these matters, many of the most interesting discoveries made in recent times would be almost un- intelligible, and it did not seem fitting to refer the general reader to the valuable but costly works of Roscoe and Schellen. Further, I think such a mode of treating spectroscopic analysis as I have adopted in Chapter III. of this work, more likely to be of use to the reader than a fuller but less simple account. In one respect, indeed, Chapter III, presents what is wanting in every treatise on the analysis that I have hitherto seen : the matter in pp. 128—156 exhibits what really happens when the light from the object studied is sent through a battery of prisms. In Chapter III. I give an account of the principles of Browning's auto- matic spectroscope, and exhibit a plan of my own by which this principle may be extended so as to include a second battery. I think that in future applications of spectroscopic analysis, the plan illustrated at p. 139 is likely to be found of considerable utility. Another large section of the book is devoted to the question of the Sun's distance. In Chapter I. will be found a very full account (the fullest popular account yet published, I believe) of the researches which have been made up to the present time into this subject: while in Appendix A the transits of 1874 and 1882 (already attracting much notice) are dealt with at length, and the best means for observing them effec- THE FIRST EDITION. XV tively are fully considered. The subject is one to which I have given much attention. In constructing the pictures from which Plates IX. and X. have been reduced (by photolithography), every circumstance of the transit of 1874 was taken most carefully into ac- count; and I think that I may safely say of these views, the four which accompanied them in Vol. xxix. of the ' Astronomical Society's Notices,' and Plate VIII., that they are far the most accurate graphic representations that have yet been made of this or any other transit. The views on pp. 449, 495, though small, illustrate the transit of 1882 more accurately than any views yet made, so far as I know. In Chapter II. a detailed account is given of the Sun's influence as ruler over the system of planets. Many of the relations here dealt with are novel and I think interesting. In the last three chapters I deal with the Sun's physical condition, his position as fire, life, and light of the solar system, and his place and motions among his fellow-suns. It will be seen that in Chapter VIII. I have presented reasons for considering that the most important work science has to accomplish is to show how the Sun's action can be more fully utilised than it is at present, so that before the Earth's stores of force are exhausted (as they must some day be), resources which can never be exhausted xvi PREFACE TO THE FIRST EDITION. — because unceasingly restored — may be rendered available. It only remains for me to point out that in some of the notes I deal with matters and employ methods of treatment which would not be suitable for the main text. The general reader can omit these notes alto- gether, as they are not necessary for the elucidation of the subject and have only been introduced for the benefit of those more advanced students of astronomy who might desire to see certain points more thoroughly dealt with than they could be in the body of a popular treatise like the present. Richard A. Proctor. Londok: December 1870. CONTENTS. PAGE Introduction 1 CHAPTER I. THE SUN'S DISTANCE AND DIMENSIONS. Nature of the problem ........ 7 Mode of measuring distance of inaccessible object ... 12 Applicable to the Moon but not to the Sun 15 Plan employed by Aristarchus of Samos 15 Plan devised by Hipparchus 17 Sun's parallax explained ....... 19 The Sun's distance determined by observations of Mars . . 20 Nearness of orbits of Venus and the Earth 26 Transits of Venus 27 118116/8 method of determining the Sun's distance . . 32 Its special difficulties 33 Delisle's method 34 Its difficulties 36 Transit of June 1761 and its results 37 Foot-note on transits eight years apart . . 37 Transit of June 1769 and its results ... 40 The ' black ligament ' difficulty 43 Encke's analysis of the observations of 1761 and 1769 ... 47 Other modes of estimating the Sun's distance : — Moon's parallactic inequality . . . . . .49 Earth's motion round centre of gravity of Earth and Moon 51 Eizeau's and Foucault's measurement of velocity of light . 53 Observations of Mars at a single station .... 59 a XV111 CONTENTS. PAGE Powalky re-examines transit of 1769 61 Stone finally solves the problem 61 Sun's dimensions "5 Summary . 66 CHAPTER H. THE SUN AS KTJLEK. The Sun's rule over planetary space Extent of the Sun's domain .... Measurement of the Sun's attractive influence The Earth's obedience to his rule, how estimated Increase of velocity necessary to free her Fatal effects of such freedom .... The decrease of Sun's mass necessary to free her . Then all the planets would suffer alike . . Sun's influence at the distances of the several planets At any given distance ...... Close by his surface ...... At and beyond the orbit of Neptune Interpretation of very great and very small meteoric velocities Summary 68 70 71 75 79 81 81 81 82 84 85 86 86 93 CHAPTER III. ANALYSING SUNLIGHT. Difficulties in the way of solar research Ilewton's analysis of light with the prism . Wollaston detects dark spaces in the solar spectrum Eraunhofer discovers multitudes of lines in the Sun's His work on other kinds of light Spectra given by terrestrial sources of light The heat rays and the actinic rays Spectra of incandescent solids and liquids Spectra of glowing vapours . Spectra of the metallic elements . Dark lines caused by absorbing vapours Kirchhoff interprets the spectral lines . spectrum 96 97 100 102 105 105 107 109 112 113 114 116 CONTENTS. XIX He shows that sodium, iron, and other elements exist in the Sim The constitution of the Sun as revealed by the spectroscope Principles of spectroscopic analysis Effect of increasing prismatic dispersion Browning's automatic spectroscope Direct-vision prisms ....... The author's plan for a double automatic battery The spectroscopic analysis of the Sun's disc . » „ of spots, faculae, &e. . How the spectroscope renders visible the prominence-lines » „ the prominences themselves How the spectroscope exhibits motions of approach or recess The method very reaflily applicable in solar spectroscopy Interpretation of peculiarities in the prominence-lines . Summary CHAPTER IV. STUDY OP THE SUN'S SUBFACE. Discovery of the spots Narrative of Fabricius Work of Galileo and Scheiner (and note from; JBurton's^.Anatomy of Melancholy') . Objections made by the Aristotelians Cassini's observations . Dr. Wilson's observations and theory Observations of Sir William Herschel His study of the faculae » The solar corrugations and nodules Sir William Herschel's theories respecting the Sun The solar spot-zones ...... Sir John Herschel's theory respecting them . Schwabe's discovery of the periodicity of spot-frequency Associated period of magnetic disturbance . . Solar outburst witnessed by Carrington and Hodgson Followed by magnetic disturbances Carrington's researches ..... Proper motion of the spots Elements of the solar rotation .... XX CONTENTS. Researches by De la Eue, Stewart, and Loewy . Probable level of the umbrae of spots Eelative position of Spots and faculae General results as to umbrae, penumbrae, and faculae Views as to planetary action on solar surface The Willow-leaves of Nasmyth .... Dawes.on the Willow-leaves , Views of other observers Dawes's discovery of nucleus within the umbrae of spots Views of Secchi ........ Formation, enlargement, and disappearance of spots Whirling motion in some spots Vast dimensions of Sun-spots . Rapid changes Spectroscopic analysis of spots, &c Evidence given by the polariscope Conclusion . PAGE 213 214 216 217 217 219 219 223 227 228 229 . 233 234 , 235 236. . 240 , 241 CHAPTER V. THE PROMINENCES AND THE SIERRA. Discovery of the red prominences in 1733 . Their rediscovery in 1842 .... Paye and others ascribe them to illusion, &c. The prominences seen during the eclipse of 1851 Still ascribed to illusion .... The eclipse of 1860 — Goldschmidt's observations Photographs taken by De la Eue and Secchi Conclusions arrived at by Secchi .Recognition of the chromatosphere Probable distribution of the prominences The vast dimensions of some of them . The eclipse of August 1868 Tennant's photographs . Other views of the prominences Spectroscopic analysis of the prominences They are shown to be gaseous . Jansscn sees the prominence-lines when Sun is not eclipsed Lockyer does thelike, later but independently Study of the prominence-spectrum 243 244 247 248 253 255 259 263 264 265 269 270 271 276 277 280 282 282 287 CONTENTS. xxi PAGE Secchi's results {foot-note) 289 Lockyer rediscovers chroma tosphere and (unaware of its prior re- cognition) gives it a name 290 The views of others respecting the chromatosphere {foot-note) . 291 The chromatosphere probably not a true solar atmosphere . . 292 Low atmospheric pressure near the photosphere .... 294 Solar storms — their exceeding violence 296 Observations by Professor Young, of America .... 299 Dr. Huggins succeeds in seeing the prominences themselves . . 301 Observations by Lockyer on Huggins's plan .... 302 Dr. Zollner's observations ...... 302 Respighi's daily views of the prominences and chromatosphere . 308 His account of the phenomena they present 309 The eclipse of August 1869 311 Secchi's observations of the prominences . . , . .316 Professor Young witnesses a solar eruption . . . . .317 Photographs a prominence when the Sun is not eclipsed . . 322 CHAPTEE VI. THE CORONA AND ZODIACAL LIGHT. Discovery of the corona ...... Observations by Clavius, Kepler, and Wyberd The phenomena seen at commencement of totality explained Observations by Plantade, Capies, and Maraldi . The eclipses of 1733, 1766, and 1778 .... Observations made by Airy, Baily, and others in 1842 Peculiar structure of the corona The eclipses of 1851, 1858, and 1860 .... In 1860 Secchi photographs the corona Eclipses of April 1865 and March 1867 Observations by Grrosch and Vidal in August 1867 The corona during the eclipse of August, 1868 Tennant observes its spectrum to be continuous . Seemingly discordant results obtained in August 1869 . These results reconciled Direct observations of the corona in August 1869 Mr. Gilman's observations .... Professor Eastman's Mr. Homer Lane's ■.,,... General Myer's 324 326 327 329 330 332 334 336 337 341 342 346 347 348 350 353 353 354 355 358 xxu CONTENTS. PAGB The corona successfully photographed in August 1869 . . . 361 Discarded theories of the corona ....... 365 The ' atmospheric glare ' theory considered and disproved . . 367 The corona not simply a solar atmosphere . . . . .371 Its structure perhaps meteoric and cometic ..... 373 Evidence as to meteoric systems ....... 374 As to cometic systems ......... 370 Leverrier's intra-Mercurial planets 376 BaxendelTs intra-Mercurial zone or disc ..... 377 Positive and negative evidence corroboratory .... 378 The radial beams considered ....... 378 Association between coronal, auroral, and cometic phenomena . 380 Further evidence deduced from the Zodiacal Light . . . 383 This radiance certainly not terrestrial 383 Summary of the evidence respecting the meteoric theory . . 387 Eclipse of 1870 388 American observations of 1869 confirmed 390 Photographic successes 394 Drawings of the corona 396 Descriptions of its figure ........ 399 Photographs by Brothers and Willard 400 Their agreement 402 Sir J. Herschel's views thereon ....... 404 Oudemann's theory negatived {foot-note) 404 Balfour Stewart's reasoning. ....... 406 Inferences from the photographic pictures ..... 409 Theory of solar eruptions as the cause of some coronal features . 413 Great velocities in eruption prominences . . . . .414 Especially in case of eruptions witnessed by Professor Young . 415 Corroborative evidence derived from meteoric phenomena . . 418 From the microscopical structure of meteors .... 420 From their chemical structure ....... 420 From the phenomena of Sun-spots 421 Summary 423 CHAPTER VII. PHYSICAL CONDITION OP THE SUN. Great difficulty of the subject 42g The elements exist in the Sun in conditions wb are not familiar with 427 CONTENTS. xxiil PAGE We are as yet but imperfectly acquainted with physical laws . 428 Amazing velocity with which matter is moving in the Sun , . 431 General consideration of the Sun's constitution .... 432 Variations in the condition of the solar elements at different levels 433 Evidence given by the spectroscope imperfect . . . .434 Does the outline of the Sun's disc indicate his real dimensions ? . 435 The solar spots present many perplexing problems . . . 437 We cannot as yet tell whether they are due to internal or external action 438 The origin of the prominences still a mystery .... 439 The corona's true nature also unknown 439 CHAPTER VIII. THE STJN CTOR FIRE, LIGIIT, AND LIFE. Extent of the Sun's influence on the Earth 441 Sir John Herschel's account of the Sun's action .... 441 His and Pouillet's measurements of the Sun's heat . . . 443 The Sun's light 445 The Sun's chemical activity . . .. 446 Action of the Sun on vegetation 440 Eventual exhaustion of the Earth's ' force-principal ' . . . 448 Duty of scientific men in this matter ...... 450 The direct utilisation of solar energies ...... 452 The enormous ' force-income' which may thus become available . 453 These ideas extended to other worlds than ours .... 455 Source of the solar energy 456 The Meteoric theory and the theory of Helmholtz . . . 458 Conclusion — reflections of Professor Tyndall .... 459 CHAPTER IX. THE SUN AMONG HIS PEEES. The Sun in the sidereal system 462 Distances of the stars ....•••■• 463 Structure of the sidereal system 464 Sir W. Herschel's star-gauging 465 Signs of aggregation among lucid stars 467 The lucid stars crowd along Milky Way 468 xxiv CONTENTS. PAGE Distribution of the nebulae 469 Milky "Way probably a spiral system of small stars . . . 470 Sun travelling in space from a rich region in the Southern to another in the Northern heavens 471 Dissimilar helicoidal paths of the planets 473 Has the Sun companion suns in his voyage through space ? . . 475 Groups of companion suns ........ 476 Conclusion 478 APPENDIX A. APPROACHING TRANSITS OP YENUS AND THE BEST MEANS FOR OBSERVING THEM. Interest of the subject ... 481 Theory of transits . 482 Transit of 1874 .484 Places suitable for applying Delisle's method : — At ingress ......... 488 At egress .......... 489 Places suitable for applying Halley's method .... 490 My reasons for dwelling earnestly on the points at issue (Joot-note) 491 Transit of 1882 493 Suggestions for applying the direct parallactic method . . 498 Tables referring to stations for observing transit of 1874 : — I. Places where ingress is accelerated .... 500/ II. „ „ „ retarded .... 500 III. „ „ egress is accelerated .... 501 IV. „ „ „ retarded . . .501 V. „ „ Halley's method is applicable . . 502 TABLE VI. Principal solar elements 503 TABLE VII. For determining the effect of changes in value of solar parallax on our estimate of the Sun's distance 503 ILLUSTRATIONS. PLATES. I. Part of the solar disc (coloured) . . . To face page 177 II. General view of the Sun (coloured). . . Frontispiece III. Six views of the eclipsed Sun (coloured) . To face page 273 IV. Prominences seen by Zollner (coloured) . „ „ 305 V. Two views of a prominence by Zollner (coloured) „ „ 308 VI. Daily views of the prominences by Kespighi (coloured) „ 309 VII. The eclipsed Sun and corona, drawn byGilman (colowed) „ 353 VIII. The Earth's passage through Venus's shadow-cone during the transit of 1874 To face page 487 XI. The transit of 1874, ingress "1 To face each other between X. „ „ egress I pages . . 488, 489 WOODCUTS. PIS. PAQE 1. The Earth's orbit round the Sun 9 2. Diagram 12 3. Diagram 14 4. Illustrating measurement of Moon's distance . . . .14 5. Aristarchus' method of measuring Sun's distance . . .16 6. Hipparchus' „ „ „ „ ... 17 7. Diagram 19 • 8. Orbits and conjunction-lines of Mars and the Earth . . 21 9. Measurement of Mars's distance 22 10. Orbits of Venus and the Earth 26 11. Transits of Venus (illustrating Bailey's method) . . .29 12. Dlustrating Delisle's method of observing transits . . .34 13. Diagram 38 b XXVI LIST OF ILLUSTRATIONS. 14. Diagram ...... 1 5. Venus pear-shaped, &c. .... 16. Diagram ...... 1 7. Venus at ingress (apparent internal contact) 18. „ „ (real contact) 19. Diagram 20. Diagram 2 1 . Prismatic dispersion of light 22. Wollaston's observation . 23. Fraunhofer's lines . 24. The heat, light, and actinic spectra . 25. Dispersion of light through a battery of prisms 23. Diagram illustrating effects of dispersion 27. Diagram 28. A battery of prisms 29. Browning's automatic contrivance 30. A modification of same . 31. Direct-vision prism 32. Twice-acting battery . . 33. The author's double automatic twice-acting battery 34. Prismatic analysis of the Sun's surface 35. „ „ of a Sun-spot 36. „ „ of a prominence (normal slit) 37. ,, „ „ „ (tangential slit) 38. How the spectroscope makes prominences visible 39. Spectroscopic exhibition of motions of recess or approach 40. Interpretation of distorted prominence-lines . 41. Dlustrating Dr. Wilson's theory of the Sun-spots 42 43. „ Kirehhoff's 44. View of Sun, showing corrugations (Secchi) 45. „ „ „ facula (Seccki) 46. Varying presentation of solar spot-zones 47. Sun-spot showing Nasmyth's willow-leaves 48. Large spot-group showing willow-leaves (Nastn, 49. Sun-spots showing penumbral rills (Secchi) 50. Sun-spots (Capocci) .... 51. Solar granules (Huggins) 52. Solar leaf-stalks (Secchi) 53. Remarkable Sun-spot (Secchi) 54. Views of the great spot of 1865 (Howlett) ;yth) LIST OF ILLUSTRATIONS. xxvii 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81. 82. 83. 84. 85. 86. 87. 88. 89. 90. 91. 92. 93. 94. 95. Faculse near a Sun-spot (Chacornac) Sun-spot indicating cyclonic action (Secchi) Peculiarities in spectrum of Sun-spot Prominences seen during the eclipse of 1851 {Airy) » >> » „ ,, (Dawes) „ „ „ „ „ (Hind) H i! ii .. „ (Lassell) .. ». ». .. „ (Gray) ii ii n ,i i. (Stephenson) Chandelier prominence seen in 1860 (Goldschmidt) Another prominence seen in 1860 (Goldschmidt) Photograph of eclipsed Sun in 1860 (De la Rue) ., ,, „ (-De la Sue) Illustrating distribution of prominences . ,, vast dimensions of „ . . Eclipsed Sun, August 1868, photographed at Aden . Spectrum of prominence and of solar limb "Widening of the hydrogen F-line in prominence-spectrum Spectroscopic indications of solar cyclones ii ii ii 'i The first prominence seen by aid of spectroscope (Hugging) Prominences seen by Huggins's method (Loekyer) Same prominences ten minutes later Prominences seen during American eclipse (1869) Eclipse of 1869, drawing by Knight Solar eruptions, Sept. 7, 1871 (Young) . Illustrating progress of Moon's shadow-cone during ecli; The corona during eclipse "of 1842 . The corona in 1 858 (Liais) „ as photographed by Secchi in 1860 „ in 1860 (Feilitsch) „ as drawn in 1868 at Mantawalok-Kekee „ as photographed by Whipple in 1869 . Diagram exhibiting incorrectness of ' atmospheric-glare ' The corona during the eclipse of 1870 (Brown) Willard's photograph of same Brothers's „ „ .... Diagram illustrating objections to Oudemann's theory pse 320 321 321 328 335 337 338 341 345 361 theory 368 . 396 . 401 . 401 . 405 XXV111 LIST OF ILLUSTRATIONS. 96. The Milky Way as a spiral . 97. Motion of the Earth's orbit through space 98. Earth's motion through space Proper motions of stars in Ursa Major, &c. . „ „ „ head of Aries . Diagram illustrating transits of Venus . Diagram „ ,, „ Explaining Plate Till. Transit of 1882 (ingress) 99. 100. 101. 102. 103. 104. 105. 106. Parallactic displacement of Venus on Sun's disc PAGE . 470 . 473 . 474 . 476 . 477 . 482 . 484 . 487 . 494 . 495 . 498 THE SUN. INTRODUCTION. It would be difficult to form an idea of the length of time during which the phenomena of day and night, and of the varying seasons, took place without attracting men's attention to the orb which governs both the day and the year. That the science now called Astronomy had its origin in the consideration of the Sun's apparent motions we can scarcely doubt. Clear indications remain, indeed, that the earliest efforts of men to determine the motions of the celestial bodies were directed to the great centre and ruler of the planetary scheme. But when those efforts were first made, what were the first conceptions of astronomers as to the nature of the solar motions, and how, in the process of time, those conceptions assumed the form described in the earliest records of astronomical re- search, we probably shall never know. Nor, so far as my purpose in these pages is con- cerned, is it of any great importance that the truth in B 2 THE SUN. these matters should be ascertained. It would doubt- less form an interesting subject of study to trace the first progress of men as they endeavoured to elucidate the secrets of the heavens. Even what we know of the early researches of astronomers is full of interest, and not wanting in instruction. Their failures as well as their successes teach us a useful lesson of patience and of perseverance. The confidence with which at times they insisted on adopting erroneous theories may serve to teach us a lesson of modesty and caution. And the gradual process by which observation, and thought- ful reasoning upon observation, led men to the success- ful solution of so many noble problems, is as full of , interest to the thoughtful student as the most spirit- stirring scenes of history. But where, as in the present instance, it is the object of the writer to exhibit the clearest possible picture of what is, it may be gravely questioned whether it is wise to present the full history of a series of researches which proceeded often on erroneous hypotheses. It is sufficiently difficult to con- vey in the compass of a single volume clear and accu- rate conceptions of a wide astronomical subject ; and the task is rendered much more difficult where the history of false or imperfect theories is mixed up with the account of recognised truths. But there is yet another reason for not undertaking to give in this work a history of the progress by which men attained their present conceptions respecting the Sun, nor even attempting the far easier task of showino- how, by a series of simple observations, the position of INTRODUCTION. 3 the Sun in the solar system, and especially with re- ference to our Earth, might hare been ascertained. The fact is, that such an undertaking would differ but little from an attempt to combine a complete history of astronomy — or at the least a complete discussion of all known astronomical relations — with that special discus- sion of the Sun's nature and condition which forms the essential object of this treatise. For there is not a single chapter of a treatise on general astronomy which is not more or less associated with the relations pre- sented by the Sun ; and it may even be said that there is not a single subject dealt with by astronomers which does not owe its chief interest to such an association. Therefore I find myself compelled to forego that mode of treating my subject which had seemed the best when I was dealing with the planet Saturn. One can, without prolixity, discuss the gradual progress of research by which the relations presented by a single planet have been, or might be, ascertained. But a preliminary research of this sort would require, in the case of the Sun, a volume — and no small one — to itself. The course I propose, therefore, to adopt is as follows : — I shall pass over all that portion of the his- tory of astronomy which relates to the determination of the Sun's central position in the system he governs. I shall give no account of the methods by which the nature of the Sun's diurnal motions was determined, nor shall I show how from this knowledge, combined with the gradual survey of the terrestrial globe and E 2 4 THE SUN. the recognition of the Sun's apparent annual progress around the sidereal heavens, the ancients recognised the fact that either the Sun travels yearly around the Earth, and is carried also daily round with the heavens, or else that this Earth on which we live speeds yearly around the Sun, rotating daily on her axis. The observations on the planets by which the true inter- pretation of these apparent motions of the Sun was eventually obtained must also remain undiscussed. We are to begin, then, by regarding the Sun as the recognised centre of the solar system, ruler over a scheme of worlds, on which' he pours forth abundant supplies of heat and light. So regarding the Sun, we shall first be led to inquire into the distance of the great luminary, in order that we may determine his real dimensions. The funda- mental problem of astronomy — the determination of the Sun's distance — a problem which has at the pre- sent time a special interest on account of those ap- proaching transits of Venus from the observation of which astronomers hope to obtain new and better measures — will therefore form the subject of the open- ing chapter of this work. This chapter will naturally include the consideration of the Sun's dimensions, and of the scale of the solar system generally. Then, next, the question of the Sun's mass, and of the influences which he exerts by reason of his mass, will come to be considered. We shall measure the might of the giant which rules the whole family of planets, and consider the limits of his domain. We INTRODUCTION. 5 shall inquire what motions it is in his power to control at this or that distance, and so determine the limits of his power to gather fresh materials from out the surrounding spaces, either in such sort as to recruit his own mass, or as to enlarge the crowd of relatively minute bodies which circle continually, as we know, around him. Then we shall proceed to discuss what the spectro- scope has taught, respecting the actual materials which constitute the Sun's substance, placing this inquiry before those chapters which deal with the telescopic aspect of the Sun's surface, the prominences, and the corona, in order that there may be no break in the narrative in passing from the era of unaided telescopic research to the recent era of mixed telescopic, spec- troscopic, photographic, and polariscopic observation. In other words, an account of the principles of spec- troscopic research, including so much of the history of spectroscopic analysis as is sufficient to make the subject clear (and therefore necessarily including an account of the Sun's general structure as indicated by the solar spectrum), will precede the narrative of that long series of researches which commenced with the discovery of the solar spots. The chapter on spectro- scopic analysis will close with an explanation of those special modes of spectroscopic research which have seemed unintelligible, or rather incredible, to many, but yet depend on principles of exceeding simplicity. Then will follow chapters describing the discoveries which have been made by aid of the telescope — and P THE SUN. its allies, the spectroscope, the polariscope, and photo- graphy — respecting the aspect and general condition of the Sun's photosphere, the coloured prominences, the corona, and so on. The physical condition of the Sun, the amount and probable source of his heat, and other like questions, ■will next be dealt with. And, finally, we shall consider the system of suns, and the position which our Sun holds in that system, so far as the researches yet made by astronomers enable us to deal with this noblest of all subjects of study. Thus we have before us a sufficiently wide range of research. Our progress will lead us to consider some of the most successful attempts yet made by man to resolve the mysteries of the universe. We shall have to deal with much that invites reflection and speculation — with much that may be explained by the thoughtful study of evidence already obtained ; — but also with much that continues, and may perchance continue for many years, altogether perplexing. We have, in fine, to deal with a subject which is full of interest, but whose real grandeur and significance, as well as its vast difficulties, are but now beginning to be rightly understood. CHAPTER I. THE SUNS DISTANCE AND DIMENSIONS. The determination of the Sun's distance is not only an important problem of general astronomy, but, so far as the subject of this treatise is concerned, it may be regarded as the very foundation of all our researches. For until we know the Sun's distance we can determine neither his bulk nor his weight ; and our views even as to his physical condition will be found to depend in an important degree on the estimate we form respecting those two elements. A minute error in the solution of the problems on which the determination of the Sun's distance depends would not only result in adding or withdrawing hundreds of millions of cubic miles from the Sun's volume, and many multiples of the Earth's mass from his weight, but our conceptions of the size of solar spots, the height of the coloured prominences, the velocity of solar currents and cyclones, and many other such matters, would be rendered proportionately erroneous. It is, therefore, of the utmost importance that we 8 THE SUN: should have accurate views respecting the modes of research by which the Sun's distance has been esti- mated, and that we should know the probable limits of error in the resulting determinations. The subject has a special interest at the present time, because preparations are even now being made for the appli- cation, in the winter of 1874, of one of the most effective methods at the disposal of astronomers for the solution of this most noble problem.* Let us consider the general nature of the problem, in order that we may the better appreciate its difficul- ties. Many are disposed to wonder that astronomers should not long since have mastered what seems to be an almost elementary problem of the science ; and it has appeared as a blot on the fair fame of astronomy that errors and differences of millions of miles should have followed the attempt to solve this particular problem. Then, too, when the resulting errors in the determination of the distances of the outer planets, and the still larger errors in the determination of the distances at which the fixed stars lie from us, are considered, the inquiry is suggested, ' Where is the boasted accuracy of the most exact of the sciences ? ' We shall see as we proceed that the great wonder is not that the estimates made by astronomers should differ, but that even the vaguest ideas should have been formed respecting the Sun's distance. The pro- blem, as presented in its simplicity (the simplicity of * See Appendix A, for an account of what is anticipated from the observations to be made on the transits of 1874 and 1882. ITS DISTANCE AND DIMENSIONS. 9 perfect difficulty), seems at first sight one that no human ingenuity could avail to solve. In fig. 1, let s represent the Sun, and e e' e" the orbit of the Earth. Then on the scale of the figure, a dot meant to represent the Earth should have dimen- sions so minute that it would be altogether invisible to the naked eye. The line representing the Earth's path is broad enough to obliterate more than a hundred such dots placed side by side across its width. Now, it is the inhabitants of the globe represented Fig. 1. by this tiny dot, who have to measure the distance separating the inaccessible globe s from e e' e". The globe on which they live is continually rotating on its axis, it is sweeping onward with inconceivable velocity on the path E e' e" ; yet from this rotating, onward-rushing, and relatively minute orb, the obser- vations are to be made by means of which the vast distance of s has to be determined. Still considering fig. 1, let us, in yet another way, picture to ourselves the nature of the problem to be dealt with. 10 THE SUN: Suppose the Earth on the orbit e e' e" to be re- presented by a globe one inch in diameter.* Then the Sun at S would be represented by a globe 9 feet in diameter, and his distance from the orbit E e' e" by no less than 320 yards. Let anyone who has noticed the 300 yards' range in rifle shooting consider how minute a disc one inch in diameter would appear at that distance, and he will at once recognise how diffi- cult a problem the astronomer has to solve in de- termining the Sun's distance; for that minute and scarcely perceptible disc subtends the very angle on whose exact measurement the solution of this problem depends. But even when anyone has pictured to himself the difficulty of determining the exact angle subtended by an inch disc at that distance, and how easily the angle might be over-estimated or under- estimated by a considerable fraction of its real value, — even then he will not have realised the actual diffi- culty of the problem. Let him reverse the illustration, and picture the difficulty of determining by obser- vations made from points within this inch disc the distance of a station 320 yards off. Yet even then he will have underrated the difficulty of the actual pro- blem astronomers have to solve. He must suppose the two sets of observations to be made by different observers, at different seasons, in different weather, with different instruments; that each set of obser- * It is convenient to remember that a bronze halfpenny is exactlv one inch in diameter ; so that an exact representation of a great circle of the earth, on the scale we are now considering, can be conveniently referred to. ITS DISTANCE AND DIMENSIONS. II vations has to be corrected for a -wholly different series of conditions ; that each observer's station is shifted continuously by two distinct forms of motion, which must both be taken into account (involving a careful reference to the question of time) before any satis- factory use can be made of the observed results. Such, in a general sense, is the nature of the problem astronomers have to deal with. The conditions of the problem are not of their fixing ; all they can do is to face as resolutely and skilfully as they can the diffi- culties which the problem presents to them. They have done this so well that the history of the problem has become to the thoughtful student of science as interesting as a romance. But they do not pretend to have secured a greater amount of accuracy than the nature of the problem and the means at their disposal render possible. Let this be distinctly understood beforehand, — Absolute accuracy in the solution of this problem is simply out of the question. And now let jus consider how the problem is to be attacked. In reality no less than six methods have been successfully applied ; but there is only one method which is strictly geometrical in its nature, and this method must be the first to engage our attention. In determining the distance of an inaccessible object, the geometer first measures a base-line, and then observes the bearing of the object from either end of that line. He thus has the means of determining the distance of the object with an exactness proportional to. the accuracy of his observations. 12 THE SUN: Suppose, for example, the object is at o (fig. 2), and that the observer measures the base-line a b and ob- serves the bearings E o and A o. Then, if he has done this accurately, he can draw a picture, as in the figure, accurately representing his observations, and he can measure either A O or B O in this picture, and, by com- paring these measurements with the length of a b in his picture, he can tell what relation the actual dis- tances a o or bo bear to the actual base-line a b. Fig. 2. Thus, if his base-line A B is 800 feet long, so that each of the divisions in the figure represents a length of 100 feet, then if B o be found to contain fifteen of these divisions, he will know that o is 1,500 feet from B. Usually, however, instead of merely drawing a figure (a process in which errors may creep in through the imperfection of rulers, compasses, and so on), the surveyor would apply trigonometrical calculation to determine B o or a o as accurately as his observations permitted. But now, if either his base-line or his bearings be wrongly determined, it is clear the distances A o, B o, will be wrongly estimated from them. The effect of a wrong measurement of the base-line is too obvious to need special discussion : clearly the error of B o or a o ITS DISTANCE AND DIMENSIONS. 13 will be precisely proportional to the error of a b. But the error resulting from wrong: estimates of the hear- ts o ings requires to be attentively considered. Suppose the bearing a o wrongly observed, and placed as A 1 or a 2. Then if the bearing b o be correctly observed, the resulting error will be o 1 or o 2 respectively. On the other hand, if the bearing a o be correctly observed but B o misplaced as b 3 or b 4, the error will be o 3 or o 4 respectively. If the bearings A o and bo be both misplaced outside, or both inside, the true direction of o, the place of the point o will be calculated as if at 5 or 6 respectively. And, finally, if the bearings are misplaced in different ways — that is, one inside o and the other outside — the point o will be calculated as if at 7 or 8, respectively. Now, under favourable conditions, a skilful observer, though he must needs make some error in estimating his bearings (for no instruments can be absolutely per- fect) would yet bring the estimated point relatively very near to O ; in other words, though he might set it at a point out of place in the same way as any of the points 1, 2, 3, . . . 8, the area of error corresponding to the area 5 7 6 8 would be small compared with the area a O b. But now suppose that instead of such a triangle as a o b, our observer has to deal with a triangle shaped like a b in the next figure — an ill-con- ditioned triangle, to use an expression of Sir John Herschel's. It is at once seen that a very small error in either of his bearings will set the observer far wrong 14 THE SUN: in his estimate of the distance of o. Suppose he has rightly determined the position of a o, but has the bearing b o' or b o" in place of the true bearing b o. He has the large error o o' or o o", instead of the relatively small error o 4 or o 3 in the case pictured in fig. 2. Fio. 3. Now, the first important astronomical problem in dis- tance-measuring — a problem infinitely less difficult than that of determining the Sun's distance — involves this very difficulty to a degree far greater than is indi- cated in fig. 3. I refer to the measurement of the Moon's distance. If e (fig. 4) represent the Earth, the Moon would be placed somewhat as at M, and if it were possible to make use of two observatories situated as at a and b at opposite extremities of a diameter of the Earth, the actual difference of bearing of the Moon would be re- presented by the small angle a M b. As a matter Fig. 4. '(JE==^- o . of fact, however, even this small angle has to be re- duced considerably ; because from a or b, the Moon would be on the horizon, and the estimate of her posi- tion rendered unsatisfactory by atmospheric refraction. The angle a m b is about a degree and a quarter, and ITS DISTANCE AND DIMENSIONS. 15 it affords a very satisfactory idea of the skill with which ancient astronomers employed their relatively ineffective instrumental means, that their estimate of the Moon's distance differed from ijje truth by only a fiftieth part. Modern astronomy has so completely mastered the pro- blem of determining the Moon's distance, that the esti- mate now adopted can scarcely exceed or fall short of the truth by so much as twenty miles, or less than a ten-thousandth part of the whole. But when the method thus shown to be available in the case of the Moon is applied to the Sun, it is found to be absolutely ineffective. The nicest obser- vation fails to show any measurable difference in the Sun's position according as he is viewed from one or another part of the Earth's surface. It is true that there is a difference, and indeed a difference which is large compared with some quantities which astronomers are in the habit of dealing with ; but as a means of esti- mating the Sun's distance, this direct reference to what is called parallactic displacement may be regarded as wholly ineffective. Other methods, then, must be adopted. I proceed to consider two methods which suggested themselves to ancient astronomers. It is interesting to consider even those attempts which have failed ; for they show the real difficulty of the problem we are engaged upon. It occurred to Aristarchus of Samos (who flourished some twenty centuries ago) that the illumination of the Moon by the Sun affords a means of estimating the Sun's distance. i6 THE SUX. If »n m' M (fig. 5) represent the Moon's path about the Earth E, and S be the place of the Sun, we know that the Moon is half full when near m. But clearly it is not when the Moon has reached the point M, such that M E m is a right angle, that she is exactly half full, but when she is at the point m', such that E m' s is a right angle. If, then, we can only determine the arc m m', or find out how soon after new moon the Moon appears exactly half full, we can tell in what proportion the distance of the Sun exceeds the Moon's distance ; Fig. 5. for in that case we have the angle m' e s as well as the right angle at m', and thus the shape of the tri- angle E m' s is assigned, and with it the proportion of E S to E m', which is what we require. Let us pause to notice the ingenuity of this method. The point to be determined is, in reality, the distance between M and m', or the angle jTe m' which is the same as the angle at S. In other words, instead of estimating the angle which the Earth's radius subtends as seen from S, this plan requires that we should determine the angle which the Moon's distance sub- tends as seen from S ;— a much easier problem, first, because the latter angle is 60 times as great as the former, and secondly, because the necessary observa- tions can be made at one terrestrial station. ITS DISTANCE AND DIMENSIONS. 17 Ingenious as the plan was, however, it was totally inadequate to meet the real (but as yet unknown) difficulty of the problem. Aristarchus estimated the Sun's distance E s at nineteen times the Moon's, or (roughly) at a twentieth of its true value. However, we should perhaps regard the estimate by Aristarchus as corresponding to those modern esti- mates of certain stellar distances, regarding which astronomers only say that they do not fall short of a certain value, without claiming to know how far they may exceed it. The next plan of attack was devised by Hippar- chus. Let M (fig. 6) represent the Moon just entering the Fig. 6. shadow of the Earth E, s being the Sun. It is clear that if the Sun were just as large as the Earth, the shadow's width m m! would be exactly equal to the Earth's diameter. If the Sun were less than the Earth, the shadow at m m' would be wider than this ; and if the Sun were greater than the Earth, the shadow at m mf would be narrower than the Earth's diameter. Hipparchus reasoned that if m m' were known, then by combining this measure with our knowledge of the c l8. THE SUN: Moon's distance and the Sun's apparent diameter, we could determine the Sun's distance.* This method, like the former, was exceedingly inge- nious, because it promised to enable a single observer, by merely timing the duration of a lunar eclipse, to solve a problem which, attacked directly, requires very delicate observations, made at stations very far apart. Again, however, the as yet unknown vastness of the Sun's distance foiled the ingenuity of astronomers. We now know that the plan just described is utterly inadequate ; and we can readily understand how it was that Hipparchus and Ptolemy (who followed him in applying the method) arrived at a measure of the Sun's distance which lay no nearer to the truth than the determination made by Aristarchus of Samos. Thus it came to pass that until the time of Tycho Brahe the received estimate of the Sun's distance was no greater than five millions of miles ; nor is it too much to say that the methods applied by Aristarchus and Hipparchus might equally well have given any result whatever, from a million miles to infinity. In other words, the limits of error by these methods, and with the means available to ancient astronomers, actu- ally exceed the quantity to be determined. We come now, however, to the methods belonging to modern astronomy. Before dealing with them it * We can determine at once the angle included between e m and d m'; it is easily seen that the angle subtended by the Sun's semi-diameter exceeds this angle by twice the Sun's horizontal parallax. ITS DISTANCE AND DIMENSIONS. 19 will be convenient to indicate the quantity which — instead of the distance — is the direct object of the researches to be described. Of course the distance is what astronomers really require ; but this distance is determined (at least as far as direct surveying methods are concerned) by the measurement of the angle be- tween lines directed towards the Sun's centre from dif- ferent parts of the Earth. For convenience, one of the points is taken to be at the Earth's centre as E (fig. 7). Fig. 7. e (F Now, if E s' represent a line directed from E towards the Sun's centre, e s" a line directed to the Sun from a point e on the Earth's surface, so placed that e s" is an horizon-line (that is, square to E e), then the angle between the lines E s' and e s" is called the Sun's horizontal parallax,* and this is the quantity which astronomers set themselves to determine in the first place. Of course, the distance of the Sun becomes known so soon as this angle is determined ; and * As the Earth is not a perfect sphere, horizontal parallax is dif- ferent in different places. Further, the Earth's path is eccentric, and so there is a variation depending on her position .in her orbit. To secure uniformity, the results obtained by astronomers are always re- ferred to the horizontal parallax of the Sun at his mean distance and for a place on the Earth's equator — or the mean equatorial horizontal solar parallax, as it is called. It may perhaps be useful to remind the. reader that this expression means merely the apparent length of half the greater diameter of the Earth's disc as seen from the Sun (at his mean distance). c 2 20 THE SUN: throughout the remainder of this chapter, besides mentioning the parallax deduced by each method, I shall always mention the corresponding distance. Six several methods have been devised, each at least as ingenious as the methods of Aristarchus and Hip- parchus, and each requiring an exactness of observa- tion which would have seemed to the old astronomers altogether hopeless of attainment. The first two methods to be described are inti- mately associated with Kepler's laws of the planetary distances. So long as no known law associated the distances of the planets from the Sun, it did not seem advisable to attempt to measure the distance of any planet from the Earth as a preliminary to determining the Sun's distance ; for further observations were required in order to determine what relation the latter distance bore to the former. But so soon as Kepler proved that the distances and periods of the planets are asso- ciated by a simple law, it seemed a promising course to attack — instead of the Sun — some planet which approaches us within a less distance. Let us consider, for example, the case of the planet Mars, in order that we may judge how much is to be gained by the course suggested. In doing this we are still following the actual order of events, for the first determination of the Sun's distance by modern astro- nomers, and with modern instrumental means, was founded on observations made upon the Planet of War. In fig. 8, the orbits of Mars (m) and the Earth (e) ITS DISTANCE AND DIMENSIONS. 21 about the Sun (s) are accurately laid down,— the line S T representing a fixed line from which astronomers measure the motions of the planets around the Sun.* Now, it is seen at once that when near m, Mars is much nearer to the Earth's path than the Sun is ; so that when the Earth and Mars are in conjunction in this neighbourhood, it becomes easier {pro tanto) to determine the distance of Mars than that of the Sun. Fig. 8. To put the matter simply, the Earth at E looks larger as seen from m than as seen from S ; and to say she looks larger is the same as saying that she subtends a greater visual angle ; and the visual angle she subtends from Mars precisely measures the displacement which Mars will show as seen from different parts of the Earth. * This line is only introduced to explain the unsymmetrieal aspect of the two paths ; to show, iu fact, that these are intentionally eccentric. 22 THE SUN: Supposing Mars thus favourably situated, and that two observers, one at E and the other at e' (fig. 9), observe the planet whose centre really lies at m, to lie on different points, m and m' of the celestial sphere: Then this arc m m' , if measured accurately, would at once give the actual displacement of Mars corresponding to the distance E e' between the ob- servers ; for though in the figure m m' is closer to M than to E, yet in reality the celestial sphere on which we thus estimate the place of Mars may be regarded as infinitely far off, so that M E is as a mere point at the centre of this sphere ; and therefore the arc m m' , as estimated from the Earth, is precisely the same as though it were estimated from M, — or, in other words, Fig. 9. this arc measures the angle m M m' and therefore the equal angle E m e'. Thus we learn at once the angle e m e', and as we know the base-line E e', we deduce the distance e m by a very simple process of calculation. But what is essential to the accuracy of the result is that the arc m m' should be accurately measured. Whatever error we make in this measurement will produce a proportionate error in our final estimate of the distance of Mars. Now, if there were no stars in the background of the heavens, it would be absolutely impossible to mea- ITS DISTANCE AND DIMENSIONS. 23 sure m m? as accurately as our purpose requires. The problem would be quite as hopeless as the attempt to measure the Sun's distance by a similar process. But the presence of stars upon the celestial vault, and the certainty which the astronomer possesses that these stars are at a distance incomparably exceeding that of Mars, make the measurement of this arc m m' feasible. The stars serve as index-points. In fig. 9, for example, a star is supposed to be placed at s ; now while it would be hopeless for an astronomer to attempt to determine the direction of either line E M or e' m' in space, without reference to any star, it is quite easy to measure the arcs m s and m' s with a very considerable degree of accuracy, and so to determine the difference m m'.* And here one point in which the modern possesses an enormous advantage over the ancient astronomer, lies in the fact that spaces on the heavens which are blank so far as naked eye vision is concerned, are shown by the powers of the telescope to be occupied bv multitudes of minute stars, — and the minutest star serves quite as well as a large star for such observa- * Here I have supposed s, m, and m! to lie all on the same arc, which of course would not ordinarily be the case. It is easily seen, however, that it falls quite within the scope and bearing of ordinary astronomical observation, to measure not only the distances but the bearings of m and m' from s, and so, — two sides and an included angle of the triangle s m m' being determined, — to determine the third side mm'. I may notice here in passing that quite a large proportion of the details in- volved in the various processes applied to the problem considered in this chapter are necessarily left untouched, or are barely mentioned. A volume much larger than the present would be required to exhibit these details in full and in all their bearings. 24 THE SUN: tions as we are here considering. So that the astrono- mer need be under no anxiety lest Mars, during the period when he is nearest to us, should approach no star near enough to render the required measurements effective. So soon as the distance of Mars has been calculated the distance of the Sun can be determined by the application of Kepler's third law. There is a pre- liminary process depending on the circumstance that S E and S m are not the mean distances of the Earth and Mars ; but this process is perfectly simple, since observation has shown what is the true figure of each orbit. Thus Kepler's third law, by showing us the exact relation between the mean distances, shows us the exact relation between s M and s e ; and therefore between e m and s e. The plan here described was the first from which astronomers obtained any satisfactory estimate of the Sun's enormous distance. Kepler, after a careful study of Tycho Brahe's observations of Mars, had already confidently stated that the Sun's parallax is not greater than 1' (or in other words that the Sun's distance is not less than 13| million of miles). But Tycho Brahe's observations were such as we should now call altogether rough. Cassini proposed and carried out a much more exact series of observations. At his suggestion the Paris Academy of Sciences sent Richer to Cayenne, while Cassini himself, Romer, and Picard, observed Mars at different French stations. The parallax of Mars was indeed not measured, for ITS DISTANCE AND DIMENSIONS. 25 the instrumental means of the observers were insuffi- cient. But Cassini calculated that if the parallax had exceeded 25", the means employed ought to have exhibited its effects. A parallax of 25" in the case of Mars (situated as when Cassini's observations were made) corresponds to a solar parallax of 10". Cassini expressed his conviction that the solar parallax is not greater than 9" -5 — in other words, that the Sun's distance is not less than 85,500,000 miles.* The next application of this method involved the comparison of observations made by Lacaille at the Cape of Good Hope and by several astronomers at dif- ferent European stations. The parallax deduced was 10", corresponding to a distance of 81,700,000 miles. I shall presently have occasion to mention other and much more trustworthy results obtained by this method in recent times, f For the present, however, I pags on to other methods. * Flamsted, however, had already deduced a similar result by means of observations applied at a single station, according to the method described at p. 59. It would appear from his account that he had been the first to call the attention of the French astronomers to the favour- able position of Mars. The observations of Flamsted are remarkable as the first from which a definite value of the solar parallax was ever inferred. He assigned 10", not as the superior limit of the parallax, but as its actual value. This corresponds to a distance of 81,700,000 miles. t It might seem that as Mars comes into opposition at intervals averaging about 780 days, the method could be applied frequently, and so the results due to it could be rapidly improved upon. As a matter of fact, however, only those observations made when Mars is in opposi- tion near perihelion are of service. From fig. 8 it will be seen that favourable opportunities do not occur at short intervals. The figure shows the successive conjunction-lines of the Earth and Mars between 26 TSE SUN: The path of Venus lies even nearer to the Earth's orbit than that of Mars does. Fig. 10 represents the relation between the orbits E e and V v of the Earth and Venus ; S T as before representing the line from which astronomers measure the motions of the planets round the Sun. It will be seen by comparing fig. 8 with fi°\ 10 that when the Earth and Venus are in Fib. 10. conjunction as at E and v, the distance separating them bears a smaller proportion to the Earth's mean distance from the Sun than that separating the Earth and Mars when in conjunction. But then there is a circumstance in which Venus is less favourable for the purpose of astronomers than Mars. When the Earth the years 1856 and 1871. It is seen that only the conjunctions of I860 and 1862 .are favourable, and those not so near as they should be to perihelion. (The wide distances separating conjunction-lines in this neighbourhood, as compared with the opposite, are due to the relatively rapid motion of Mars near perihelion.) The opposition of 1877 will be exceptionally favourable, as the conjunction-line 'will fall nearly midway between those of 1860 and 1862. It is necessary for me to re- mark that fig. 8 is copied from a drawing of my own, illustrating a paper in the Popular Science Review for January 1867. ITS DISTANCE AND DIMENSIONS. 2J and Mars are in conjunction as at. E and M, fig. 8, the Sun is on the opposite side of the Earth, and so Mars is seen on a dark sky ; but when Venus and the Earth are in conjunction, as in fig. 10, Venus lies directly towards the Sun, and even though visible (in powerful telescopes), yet is seen under very unfavourable con- ditions. The background of the sky is bright, and none but the chief stars can be discerned, unless the telescope is very large and powerful, in which case it is not so well adapted as a more manageable one would be, for the class of observations required. Even the leading stars are but faintly seen ; and as Venus may not lie near any of them, the kind of measurement which was available in the case of Mars becomes too precarious for the purpose of determining any paral- lactic displacement of the planet. Hence the direct observation of Venus, when near- est to us, after the manner applied to Mars, is not a very valuable method of determining the Sun's dis- tance. It may yet be applied successfully (according to the plan proposed in 1848 by Dr. Gerling of Mar- burg) ; but even if it should, the method now to be considered is preferable. When Venus and the Earth are in conjunction, she is not commonly on a direct line between the Earth arid the Sun. She would be so, if the path V v' v lay in the same level with the path e e' e ; but this is not the case. If we suppose the path E e' e to lie in the plane of the paper, then the path v v' v must be sup- posed to intersect that plane in the line v' V, the half 28 THE SUN: of v v v' lying slightly below the plane of the paper, and the half V v v' slightly above, — the short lines near v and v showing the greatest amount of separation between the two planes on account of this tilt. Hence, unless the conjunction happen to take place when Venus is at v' or v', Venus will not be on a direct line joining the Earth and the Sun. But when Venus is within a certain distance of these two points,* — the * It is easy to calculate how near Venus must be to a node in order that she may be visible on the Sun's disc. The extreme limits will be those corresponding to the case when the disc of Venus as seen from all the Earth, save one point only, lies outside the Sun's disc, but as seen from that one point just touches the disc on the outside. Further, on account of the ellipticity of the two orbits, the exact extent of these limits will be different for conjunctions occurring when Venus is at v" and at«\ To estimate these limits strictly, the exact values of the distances of Venus and the Earth from the Sun when in longitudes corresponding to s e' and s d may be taken from the Nautical Almanac, and combined with the estimated diameter of Venus. The formulae to be used are suffi- ciently simple, and will occur at once to the mathematical reader, — or may be deduced from my paper on the transit of Venus in the Monthly Notices of the Astronomical Society for March 1869 (vol. xxix.). But for our present purpose such exactitude is not needed. It may be well, however, to determine in a general way the limits in question ; for which purpose we may assume that no transit need be considered in which the line joining the centres of Venus and the Earth does not meet the Sun. Hence, Venus must not have a greater geocentric latitude than the Sun's apparent semi-diameter. Now the distance of Venus from the Earth, when she is in inferior conjunction, is to the Earth's distance from the Sun as 277 to 1,000. So that when her apparent latitude is equal to 16' (which we may take as the Sun's semi-diameter) her heliocentric latitude is to 16' as 277 to 723, oris about ff-1. The inclination of her orbit being 3° 24', or 204', it is easily calculated that she must be within about 1§° of a node, at the epoch of inferior conjunction, in order that a transit may occur. Thus at each node there is an arc of about 3g° along any part of which Venus, if in inferior conjunction, will be projected on the Sun's disc ; and as there are two nodes, the total range out of the 360° of her orbit along which transits can occur is about 6f °, or a 54th part of the whole. Hence, on the average there will be one transit in ITS DISTANCE AND DIMENSIONS. 29 nodes of her orbit, as they are called,— she will be so nearly on the line joining the Earth and the Sun, that as seen from the Earth she will appear on the Sun's disc. Now when a conjunction of this sort takes place, the observation of Venus's apparent place is rendered much easier, since the disc of the Sun forms a sort of index^ plate, as it were, on which we can estimate her position. So that if Venus's distance is to be determined at all from observations of her parallactic displacement as seen from different parts of the Earth's surface, it is Fig. 11. clear that the proper time for attacking the problem is when Venus is in transit. Let us see, however, how this may be most effectually contrived. Suppose that as seen from the point e (fig. 11) on the Earth at any moment, Venus (v) is seen at v on the Sun's disc ; whereas as seen from e' she appears to be at v'. Then it seems at first sight as though nothing coidd be simpler than to determine the distance v v' ; and then having the arc v v' and the length of the base-line E e' we could at once determine the distance 54 conjunctions, and as conjunctions occur at average intervals of 583-920 days, there will he on the average one transit in 86j years. But this average would only correspond to the case where a very large number of conjunctions was considered. 30 THE SUN: of Venus. For v v' is the. arc between two lines from Venus containing the angle v v v', which is equal to E v e'. So that we know the angle E v e'. For example, if the arc v »' were shown to be thirty-five seconds as seen from the Earth, then the angle v v v' would be greater (because v is nearer to v v' than e is) in the proportion of E v to v v, or roughly as 7 to 5, so that the angle v v v' (or E V e') is an angle of about forty-nine seconds. Now suppose that the stations E, e' are so placed on the Earth's surface that the base- line E e' is known to be about 6,000 miles in length : then the distance e v exceeds 6,000 miles in the pro- portion that the radius of an arc of forty-nine seconds exceeds that arc, or roughly as 4,200 to 1. Hence E v is about 25,200,000 miles, and e s exceeds this distance in the proportion of about 7 to 2, or comes out equal to nearly 90,000,000 miles. But for many reasons this direct method of solving the problem of the Sun's distance has not been hitherto applied. In the first place, it is absolutely necessary that the observations made at E and e' should either be made exactly at the same moment, or that the difference of time should be exactly known, so that the two observations may be fairly compared together. But for this purpose we must know the exact position of the stations E and e' on the Earth, so as to be able from the apparent time at these stations to infer the true time at Greenwich or some other fixed station. It is easily seen that a very slight error in the determination of the longitude of either station would make the whole ITS DISTANCE AND DIMENSIONS. 31 series of observations useless. For example, suppose the observer at e recorded the place of Venus on the Sun's disc at apparent noon for his station ; and that the longitude of the station was supposed to be such that this epoch corresponded exactly with the epoch when the observer at e' recorded the Sun's position. Then if the supposition were correct, the above process would be available. But if e were ten or twelve mi T es to the east of the supposed longitude, apparent noon would occur a minute or so * earlier than at a place in that longitude. But, in one minute, Venus, as seen in transit, moves over an arc of about two seconds on the Sun's face, so that the observer at E noting her place a minute or so too soon (so far as the comparison with the other observer's record is concerned) would set her two seconds of arc out of place. But our problem is one in which seconds of arc are all-important. But this is not all. The determination of the exact place of Venus on the Sun's disc at any epoch would be a matter of extreme difficulty. It would be necessary to determine, not merely her distance from the Sun's centre, but her bearing from that point; and a very slight error in either determination would (in so delicate an inquiry) cause a considerable error in the determi- nation of the Sun's distance. There is, indeed, a way ' of getting over this difficulty which I touch upon in Appendix A ; but though it gives, in my opinion, the very best method of determining the Sun's distance now * The exact difference of time would of course depend on the latitude of the station. 32 THE SUN: available to us, it requires (as will be seen) a prelimi- nary knowledge which was not possessed when the observation of Venus in transit was first proposed as a means for solving the problem we are upon. Accordingly Halley proposed (in 1716) a plan for evading the observational difficulties. He suggested, that instead of attempting to estimate the position of Venus on the Sun's disc at any moment, the observers at two stations, such as e and e', should time the passage of Venus along her chord of transit. Neglect- ing for a moment the consideration of the Earth's rota- tion, Venus would seem to the observer at e to describe such a path as I v m, while to the observer at e' she would seem to describe such a path as /' v' m'. Now if we know the length of time she takes in describing these chords, we know the length of the chords, since the rate of Venus's motion across the Sun's disc (the same of course for both stations on the assumption that the Earth is not rotating) is known from the tables independently of her actual distance. Hence it is a very simple problem in geometry to determine the distance separating the chords I m and I' m' , and thence, as in the former case, to determine the distance of Venus, and so that of the Sun. Nor does the fact that the Earth is rotating prevent us from applying this method ; though it causes the problem to be somewhat more complicated. Venus in fact does not describe quite a straight chord across the Sun as seen from any station ; nor does she move quite uniformly ; nor again is her rate of motion across the ITS DISTANCE AND DIMENSIONS. 33 Sun's face exactly the same as seen from different stations. But all these points are such as the astrono- mer is quite accustomed to take into account, nor do they in themselves detract one whit from the certainty with which Halley's method can be applied. But there is one effect of the Earth's rotation which has to be very carefully considered in weighing the value of Halley's method. It is absolutely necessary (since the duration of the transit is to be timed) that at each station the beginning and end of the transit should be visible. Now a transit may last a consider- able time — as long indeed as eight hours ; and it may not always be easy to find two stations — one far to the north, and the other far to the south, at each of which both the beginning and end of the transit will be favour- ably seen. For it must be remembered that a large part of the Earth is unfitted for the observer's purposes. We must Dot place our observers on the open sea, nor in regions where bad weather ordinarily prevails. And this question of the weather is in itself a great difficulty. Eor transits of Venus can only occur in December or in June, as is obvious from a consideration of fig. 10, where e' and e' — the points near which the Earth must be that Venus in conjunction may be near a node — cor- respond to the Earth's position on about December 8 and June 6. Now, at a northern station in June, or at a southern station in December, fair weather may be commonly expected ; but the reverse holds as respects the northern station in a December transit, and the southern station in a June transit. So that the diffi- x> 34 THE SUN: culty of finding two stations, one northern the other southern, each well suited for observing both the beginning and end of the transits, and at each of which there is a fair prospect of clear weather at both epochs, is a very serious one. In Appendix A we shall see more about the circum- stances here considered, which it will be understood are not such, ordinarily, as to prevent Halley's method from being applied, though they call for the most cautious exercise of judgment in the selection of stations for the purpose. At present it suffices to say that the difficulty led Delisle, in anticipation of the transit of Venus in 1761, to propose another method. Reverting to fig. 11, it will be clear that as v moves Pig. 12. onward in the direction of the arrow,* the time must come when the transit is just beginning at some point on the Earth's surface from whence the first view (as it were) is obtained of Venus in transit. Some interval must elapse before the transit has begun for the whole Earth — or at least for all that hemisphere which is turned towards the Sun. And at some point on the Earth's surface — which will clearly be nearly opposite * We suppose the Earth at rest, or — which is the same thing — con- sider only Venus's motion relatively to a line constantly joining the centres of the Sun and Earth. ITS DISTANCE AND DIMENSIONS. 35 the point just referred to — the transit will seem to begin later than at any other station. Now, neglect- ing matters of detail, and considering Venus as a point for the moment, we may reason in this way on obser- vations of the kind considered: — Suppose Venus at v (fig. 12) when the transit first begins, so that a line bt* just touches both the Earth and Sun, and that Venus is at Y when the transit has begun for the whole Earth, so that e' V s just touches both the Earth and Sun. Then we know the length of E e', and therefore we know the length of v Y, which is less than e e' in the proportion before used, of about five to seven. We also know exactly how long Venus has taken to traverse this arc V v', and there- fore, since we know how long she takes to complete the circuit of her orbit, we know what proportion the known length v Y bears to the circumference of her orbit. This gives the circumference, and thence the radius of her orbit, whence, as before, we learn the radius of the Earth's orbit. Here we have supposed the Earth at rest. But as the motions both of the Earth and Venus are known, the relative motion of Venus is known, and so the conditions of the problem are as fully ascertained as in the simpler case actually dealt with. "We thus see that the phenomena presented at the commencement of the transit are sufficient for deter- mining the Sun's distance. So also are the phenomena presented at the end of the transit ; since it is obvious d 2 36 TEE SUN: that as Venus passes from v to v' similar relations will be presented, but in a contrary order. All that is required, then, for a successful application of Delisle's method is that one observer should have a favourable view of the commencement (or end) of the transit from some place on the Earth where the transit begins (or ends) nearly at the earliest, and that another should obtain a favourable view of the same phase at some place where the transit begins (or ends as the case may be) nearly at the latest. Both observers must time the commencement (or end) of the transit most carefully. Then to compare the two observations, in order to tell the absolute interval of time between the two, we must know the exact longitude of the two places ; for the observations will of course be referred to local time, so that in order to compare them, we must refer them to some standard time, as that of Greenwich or Paris. Here, then, are the difficulties in Delisle's method : unless the longitude of each station is accurately known, and furthermore, the exact local time at which transit begins or ends, the determination of the Sun's distance will be inexact. As respects the former point there is little difficulty, only the observers must stay some time at their respective stations, making suitable ob- servations to determine the longitude of the station. This can be done either before or after the transit, as may be convenient. But as respects the determination of the local time at which the transit begins (or ends as the case may be), there will be a difficulty if bad ITS DISTANCE AND DIMENSIONS. 37 weather precede and follow the epoch at which the phase occurs. For we can only determine local time exactly when the weather is clear, so that we can make suitable observations on the stars ; and during a few days of cloudy weather, the best astronomical time-pieces will get a second or two wrong. In Halley's method the clock may be altogether wrong, yet if its rate be fairly good, the duration of the transit will be accurately determined; but in Delisle's the clock must show absolutely correct time. Here again, however, the difficulties, so far from being insuperable, are only such as astronomers are in the habit of dealing with and mastering. Both methods were applied during the two transits of the eighteenth century. Of these one took place in June 1761, and the other in June 1769. Both occur- ring during the summer of the northern hemisphere, the Earth's northern pole was bowed towards the Sun; and in this respect the circumstances of the transits differed importantly from those of the transits which are to occur in 1874 and 1882, for both these will take place in December, when the southern pole of the Earth is bowed towards the Sun. The transit of 1761 was not observed in a very satisfactory manner. It was a transit for the observation of which Delisle's method was somewhat better suited than Halley's,* and the astronomers of the eighteenth * It is worthy of notice that in the case of two transits separated by an interval of eight years, the former is commonly best suited for Delisle's method, the latter for Halley's. Both the transits will occur at the same season, that is, Venus will either be near her ascending 38 THE SUN: century were not so well prepared to deal with the difficulties of that method as those of our time will node at both transits, or near her descending node at both. For an interval of eight years corresponds almost exactly to thirteen revolu- tions of Venus, so that, supposing Venus near a node when in inferior conjunction, she will be near that node at the conjunction occurring eight years later (or after five synodical revolutions). Now, it so happens, that the line of these successive conjunctions in the same neighbourhood continually regredes round the ecliptic, one interval being in fact about 2J days less than eight years. Thus the latitude of Venus at successive eight yearly conjunctions near a node differs by the amount of Venus's motion in latitude (near a node) in the course of 2£ days. Thus the apparent path of Venus across the Sun's disc (as supposed to be seen from the Earth's centre) would be as 1 1, 2 2, fig. 13 (i), when she is near her Fig. 13. rising node or when both the transits are December ones ; and as 3 3, 4 4, fig. 13 (n), when she is at a descending node, or when both the transits are June ones. [The distance between 1, 1 and 2, 2 — and be- tween 3 , 3 and 4, 4, would be about that shown in the figure, and the slope with reference to the ecliptic is of course the same (approximately) in all transits. But the pair of lines may have any position whatever as respects distance from the Sun's centre. Now, if either line in either figure falls very near the centre of the Sun's disc, the other will not fall on the Sun, and there will be only one transit during those years when this conjunction is travelling past the node. This will happen in no inconsiderable proportion of these passages — a circumstance requiring notice because our treatises on astronomy commonly assert that transits of Venus occur at successive intervals of 8, 121^, 8, 105i, 8, 12l£, &c. years, which is far from being strictly correct. In fact, instead of two transits occurring at every such passage, very nearly half the passages would supply only one transit. The list in Lalande's Astronomy is wholly untrustworthy in this respect, as any one will find who will cal- culate the distance of A 7 enus from her nodes at the conjunctions referred to in that list.] To resume ; — Taking the case of December ITS DISTANCE AND DIMENSIONS. 39 prove themselves. Yet the result of the observations then made, which were interpreted as giving a solar transits illustrated by fig. 1 3, we see that at the first transit when the path is as 1, 1, the northern station from which the path will appear lowest down on the Sun's disc will give the longest interval ; and the advantage of applying Bailey's method will depend on the greatness of this interval as compared with the shorter interval during which the tran- sit lasts as seen from some southern station. Now the Earth is rotating, and the effect of the Earth's rotation considered alone is to give Venus a continual slight westerly displacement in all places where the Sun is moving from east to west — that is, at all places save those close by the south pole, at which the Sun (being above the horizon all day, or nearly all day) moves through part of the day from west to east. Hence at northern stations, where Venus's path is longest, she is hastened on her path by this westerly displacement ; and so the lengthening of her period of transit is diminished and the value of Halley's method pro tanto reduced. At southern stations the shortening will be increased at places where the transit occurs during the mid-day hours, and di- minished where the transit occurs during the midnight (nominal) hours. But at the former stations the apparent path of Venus will not be thrown so far south as at the latter ; so that at the southern stations also we find that the greatest possible shortening due to parallax can- not be combined with an additional shortening due to the Earth's motion of rotation. But at the second transit of this set, when Venus appears to follow such a path as 2, 2 (fig. 13), the reverse is the case. At the northern station Venus's path is thrown southwards and so shortened, while her motion across the Sun's disc is hastened (by the effects of the Earth's rotation) and therefore also shortened ; whereas at the southern stations, where Venus's path is most lengthened, her motion across the Sun's face is retarded and so lengthened. Halley's method is then applicable under the most favourable conditions for securing a considerable time-difference. Similarly, it may be shown that at a June transit, when Venus's path is as 3, 3 (the first of a pair), Halley's method is not so favourably applicable as at a June transit when her path is as 4, 4 (the second of a pair). Theoretically this is just, but practically, especially in December transits, the difficulty of securing suitable stations near the pole which is turned towards the Sun, may altogether change the conditions. As a matter of fact, indeed, the approaching transits of 1874 and 1882 are exceptions to the rule ; and I have been able to demonstrate that, so far from Halley's method being most favourably applicable in 1882 (as the Astronomer-Eoyal had inferred from reasoning resembling the above), 4-0 THE SUN: parallax of 8""65, corresponding to a mean distance of about 94,500,000 miles,* was a great improvement on any before obtained — and better, in fact (though this was due to chance), than that deduced from the more complete and satisfactory observations made in 1769. The transit of June 1769 attracted an amount of attention both in England and on the Continent which afforded very creditable evidence of the scientific en- thusiasm of the men of the last century. The Royal Society presented a memorial to King George III., requesting that a vessel might be fitted out at Govern- there is no reasonable chance of its being applied at all in 1882, the only two southern stations where it is possible to apply the method being such that the Sun will be barely 5° above the horizon, — a state of things preventing all exact observation, and assuredly not justifying expeditions to stations so near the south pole that the observing parties would inevitably have to winter there. On the other hand, I have also been able to demonstrate thatHalley's method, besides all the advantages arising from its simplicity, will be applicable under more favourable circumstances than Delisle's, in 1874. It is necessary to observe that there is nothing hypothetical about this conclusion. The difference be- tween my conclusions and those before adopted arises simply from my having taken into consideration facts which had been (mistakenly) imagined to be such as might safely be neglected. Since my results were published, papers by Peters, of Altona, and by Hansen, the eminent German mathematician, have confirmed all the views I had insisted upon. See further, Appendix A. * It is convenient to notice that if the solar parallax were 10", the distance of the Sun would be 81,738,420 ; and the distance correspond- ing to any other value of the parallax can be deduced by simply dividing 817,384,200 by the number expressing such parallax. The table in Ferguson's Astronomy, complacently quoted in Chambers's Handbook, at p. 248, is incorrect, owing to the erroneous estimate of the Earth's mean diameter on which the table is based. Oddly enough, Mr. Cham- bers has combined the correct estimate for the parallax at present adopted, with Ferguson's incorrect values. It would almost appear as though the figures had been simply quoted without being tested in any way, were not such an idea incredible. ITS DISTANCE AND DIMENSIONS. 41 ment expense to convey skilful observers to one of the stations which had been judged suitable for observing the phenomenon. The petition was complied with, and after some difficulty as to the choice of a leader, the ship Endeavour, of 370 tons burden, was placed under the command of Captain Cook. Continental astronomers also betook themselves to the most advan- tageous posts for observing the transit, and when at length Yenus came between the Earth and the Sun, her arrival and passage were watched by observers at Wardhus, Kola, and Kajeneberg, at St. Petersburg, Orenberg, Yakutsk, Pekin, Manilla, Batavia, Otaheite, St. Joseph (in California), Kola, Hudson's Bay, and other well-chosen stations. Most of the observations of the transit were well and skilfully conducted; but a circumstance, which now for the first time attracted serious attention, caused a certain difficulty in their interpretation. The observers had to determine the exact interval of time occupied by Venus in crossing the Sun's disc — or else in certain cases the exact moment of time when she began to cross the solar disc. Now, if Venus presented the appearance of a mathematical point, the observation would be simply described in the words I have just used. But as Venus has a disc of appreciable dimen- sions, the question arises, whether the commencement or end of transit shall be considered with reference to the moment when the disc of Venus just touches the Sun's disc on the outside, or to the moment when her disc just touches his on the inside, or, finally, to the 42 THE SUN: moment when her centre is crossing the Sun's edge. As to the last of these cases, we may dismiss it at once from consideration, because no observer, however experienced, could pretend to determine within a second or two when exactly one half of the disc of Yenus was upon the Sun's. Either, then, the observer must note when Venus is, as at A (fig. 14), just touching the edge of the Sun's disc, s s', on the outside, or, as at B, just touching that edge (the limb as astronomers call it) on the outside. Now, taking the case of ingress, if the observer knew exactly where Venus would begin to Fig. 14. cross the Sun's disc, as at a, and very nearly the true instant of such ingress, he might, if the weather were very favourable, determine within a second or two the moment when the uniformity of the limb S s' was marred by the encroachment of the disc of Venus. But as a matter of fact neither one condition nor the other is fulfilled, and so, even under favourable circum- stances of weather, he might not detect the exact commencement of ingress. Nor is this all. By favour- able weather, I mean something more than clear weather. In weather which seems perfectly clear the telescopist will often see the edge of the Sun's disc ITS DISTANCE AND DIMENSIONS. 43 absolutely rippled through the effects of atmospheric disturbance, and when the Sun is near the horizon, even in good observing weather, the outline of the disc is often egregiously disturbed. Under such cir- cumstances nothing would be more difficult than to assign the exact moment when external contact took place. But these are the very circumstances to be looked for in the observations necessary for determin- ing the Sun's distance. The Sun must be low down at all the stations most suitable (in other respects). For the essential point both in Halley's method and Delisle's — nay, in any method whatever for determin- ing the Sun's parallax — is that two observers shall be as far as possible from each other on the illuminated hemisphere of the Earth ; so that they must be near the rim of that hemisphere. In other words, they must be near that great circle of the Earth along which the Sun is seen exactly on the horizon. The Sun then will be nearly on the horizon at such stations, and will be viewed necessarily under somewhat unfavourable con- ditions. It was this consideration which led, and probably will always lead, to the selection of internal contact as the proper phase to be observed. In attempting to time the moment when Venus is just within the Sun's disc, either at ingress or egress, there will, of course, be difficulties arising from atmospheric disturbances ; but on the whole the observation of this phase is easier than the observation of external contact. But, unfortunately, a certain peculiarity affects the 44 THE SUN: appearance of Venus when just within the Sun's disc. Instead of appearing circular, she assumes (just before she leaves the Sun's edge at ingress, or just as she reaches it at egress) a pear-shaped aspect, as at 1 (fig. 15), or such an aspect, as at 2 or 3, according to the nature of the telescope made use of, the conditions of the atmosphere, and the visual power of the observer. Thus the question arises, whether the assumption of this aspect is to be regarded as the true moment of internal contact, or whether that is the true moment when the circular part of Venus's apparent outline, if Fig. 15. w w w continued so as to form a complete circle, would just touch the Sun's outline. Between one moment and the other several seconds occur, and the whole question ■ is one of seconds. Here is a difficulty of grave importance ; for it is to be noticed that a practised eye would be needed to determine the moment when the outline of Venus would just touch the Sun's if undisturbed, and even a practised eye might well be deceived on such a point. On the other hand, though an observer might time the exact moment when the outline of Venus is just clear of the Sun's (and within it), either at ingress or egress, yet as this moment depends (as already mentioned) on the qualities of the telescope and on the observer's ITS DISTANCE AND DIMENSIONS. 45 visual powers — probably also on the state of the weather — there is room for a considerable error to creep in. I shall discuss further on the interpretation of the peculiarity just considered. At present the point to be attended to is the actual effect produced by this peculiarity upon the trustworthiness of the results obtained in 1769. The observers were for the most part unprepared for the serious difficulty which the peculiarity actually introduces. Although in 1761 Venus had been distorted at ingress and egress, while Halley himself had in 1753 noticed that Mercury exhibited a similar distortion, it had not occurred to astronomers to discuss the phenomenon, and so to be enabled to point out how the difficulty might be got over. In fact, the whole matter had been so completely overlooked, that, as I have said, few of the observers in 1769 were prepared for the peculiarities which Venus presents when in interior contact with the Sun.* The result was that much difficulty was experienced in interpreting the observations. It will be understood * There is nothing more remarkable in the history of astronomical observations than the little preparation made for important occurrences, such as transits and eclipses, so far as the actual observation of the phenomena is concerned. Abundant preparation is made as far as instrumental means are concerned ; but again and again the history of astronomy exhibits cases where the actual phenomena of transits and, eclipses take the observer by surprise, and so are not observed as well as they might have been, even when abundant information has been in' reality available for the instruction of those who are to take part in watching such occurrences. There is much, for example, in the history of eclipse-observations that is exceedingly painful to the real lover of science, more particularly in the wearisome repetition of observations which have already revealed all they can reveal, and in the apparent dearth of invention as respects the devisal of new modes of research. 46 THE SUN: that more observations were made than were actually- necessary for the solution of the problem. One pair of observations, if absolutely exact, would have suf- ficed to determine the Sun's distance ; but as errors must to a greater or less extent affect all observations, it was necessary that many sets should be made in order that the mean result should be taken. Now, in all such cases the amount of reliance which is to be placed on -the final or mean result depends on the closeness with which the several results aggregate round the mean. Precisely as we should place little reliance on the deduced mean of a series of ordinary measurements, which differed considerably inter se, so astronomers would not be prepared to accept with confidence a value of the Sun's distance which was the mean of several discordant results.* Now, the observations made in 1769 were in this respect much more satisfactory than those made in 1761; but, owing to the peculiarity I have mentioned, they did not accord so well by any means as might have been anticipated. It is not too much to say that in the attempts first made to determine the Sun's distance by means of the transit observations of 1769, no settled principle was adopted. It was not much to be wondered at, there- fore, that the estimates of the solar parallax varied * The total number of observers in 1761 was no less than 63— thus distributed: 13 in North Europe, 8 in England, 15 in Prance, 6 in Spain, Portugal, and Italy, 16 in Germany, and 3 in other places. In 1769 there were observers at 50 stations in Europe, 6 in Asia, 17 in America, and at one station in Polynesia. • ITS DISTANCE AND DIMENSIONS. 47 between somewhat wide limits ; so that whereas some mathematicians made the solar parallax as great as 9" - 2, others obtained a value of only 7""5. The corre- sponding values of the Sun's distance are respectively 87,890,780 and 108,984,560 miles,— a range compared with which recent discrepancies seem insignificant.* During the years 1822 and 1824 Encke re-examined the whole series of observations made on the transits of 1761 and 1769. With wonderful patience, espe- cially when we consider the nature of his materials, he combined together the results of no less than 149 obser- vations made during the former transit, and as many made (at 75 stations) during the second. From the transit of 1761 he deduced for the Sun's parallax the value 8" - 49, and from the transit of 1769 he deduced the value 8""60 ; f while, by combining the two, he arrived at 8 //, 5776, the value which was employed during so many years in the ' Nautical Almanac ' and * The results of the transit of 1761 are thus summed up in Dr. Bruhn's Life of Encke (Johann Franz Enc/ce, sein Leben und Wirken) : — Short obtained a parallax of between 8"-47 and 8"-52; Pingre, 10"; Bumowski, 8'''35 ; Planmann, 8"-2 ; Audefredy, 9"-2. From the transit of 1769, adds Bruhn, ¥m, Smith deduced a parallax of 7"'5 ; Hornby, 8"78; Pingre, 9"-2, 8"-88, and 8"-43; Lalande, 8"'8 ; Lexell, by Euler's method, between 8"-65 and 8"'86, whence he finally adopted 8"'8. The value of the Sun's distance corresponding to all these results can be at once deduced from Table II., at the end of this work ; save only the distance corresponding to 7""5 (in Bruhn's work it is written 7''5) which is stated above. t The actual results deduced by Encke were— from the transit of 1761, 8"-490525 + 0-060712"; from that of 1769, 8"'6030 + 0"-0460; from the two combined, 8"'5776 + 0"-0370—3-0120Sp, where Sp is the correction for the Sun's semi-diameter (estimated by Encke at 958'H24). Later, Encke deduced the value 8"'5716, by introducing some corrections in his work. 48 THE SUN: other like works. The corresponding distance, viz., 95,274,000 miles, held its ground during all those years in popular treatises on astronomy. It is somewhat surprising, considering the evidence which was afforded by the discrepancies between the observations made in 1761 and 1769, that this result should have been regarded with such confidence, since it needed but a brief examination of the basis on which Encke's result was founded to see that no faith what- ever could be placed in three at least out of the five numerals in the expression 8""5776. Delambre re- garded 8""6, very justly, as the most probable value of the solar parallax half a century ago. It was rightly admitted that the observations of Venus in transit afforded the most reliable results ; but Delambre, Bes- sel, and other astronomers of eminence were far from adopting the value 8"*5776 with that implicit confi- dence which caused the corrections made in recent times to attract so much notice, and so greatly to sur- prise the general public. The observation of Venus in transit being admit- tedly the most trustworthy method of determining the Sun's distance, it might have been supposed that no new results could serve to throw doubt on those de- duced by Encke, until the time should come when Venus would again cross the Sun's face — that is, until the year 1874. But the rapid progress of science during the past half-century, though it has not served to alter the relative value of different methods for deter- mining the Sun's distance, has enabled astronomers to ITS DISTANCE AND DIMENSIONS. 49 apply some of the less powerful methods in so much more effective' a manner than of old, as to obtain more trustworthy results than had followed from the best method of all when less skilfully applied. Amongst these relatively inferior methods, no less than four are novel ; but a very brief description must suffice for each. It had been noticed by Laplace, towards the close of the last century, that among the perturbations of the Moon there is one which depends on the Sun's distance. Suppose m, m 2 m 3 M 4 to represent the Moon's path Fig. 16. around the Earth, E. Then clearly as the Moon moves from M 4 through m, to M 2 , she is disturbed by the Sun's action, which is here greater on the Moon than on the more distant Earth. On the other hand, while the Moon is moving from M 2 through M 3 to M 4 , she is dis- turbed, because the Sun's action is greater on the Earth than on her. Without entering into an exact investi- gation of the effects thus produced, it is abundantly evident that the Sun's perturbing effect in the former case will be greater than in the latter, because the radius of the orbit M, m 2 m 3 m 4 clearly bears a greater proportion to Mj s than to M 3 s. The Moon's orbit is E 50 THE SUN: indeed so minute compared with the Sun's distance, that the difference is very slight ; but still there is a dif- ference. "When the Moon is at m, the Sun tends to pull her more strongly away from the Earth than he tends to pull the Earth away from the Moon when the latter is at M 3 ; and a similar preponderance holds for other and corresponding positions of the Moon and Earth. It follows that there is a slight variation in the Moon's motion depending on this cause alone, and readily admitting of being estimated theoretically, while the continued observations made by astronomers on the Moon's motions suffice to show how great the perturbation really is. It is only necessary to com- pare the theoretical with the observed value to deduce the Sun's distance ; only, of course, the accuracy of the result will depend on the number and accuracy of the observations. Laplace, with the best observations available in his time, deduced for the Sun's parallax the value 8""6, corresponding closely with the value subsequently deduced by Encke from the transits of Venus. ' It is remarkable,' wrote Laplace, ' that an astronomer, without leaving his observatory, by merely comparing his observations with analysis, has thus been enabled to determine the distance of the Earth from the Sun — an elenjent the knowledge of which has been the fruit of long and troublesome voyages in both hemispheres.' But this method is clearly one which modern astro- nomers can apply much more effectively, because the observations at their disposal are so much more ITS DISTANCE AND DIMENSIONS. 51 numerous and so much more exact. Accordingly, Hansen, the eminent mathematician and lunarian, announced in 1854, in a letter addressed to the Astro- nomer Royal, that this method, applied with the aid of his new tables of the Moon, gave a solar parallax considerably exceeding Encke's estimate. In 1863, he assigned the value 8""9159, corresponding to a dis- tance of 91,659,000 miles. Another method, depending on the apparent motions of the Sun, was applied (with a very similar result) by Leverrier. If the Earth had no satellite she would travel on her elliptic orbit round the Sun, with no other perturba- tions than those produced by the planets. But since she has a satellite, whose mass is an appreciable though small aliquot part of her own, she is disturbed precisely in the same way, though not to the same extent, that the Moon is disturbed. The Moon travels once in a lunar month around her orbit, but the point round which the Moon moves is not the centre of the Earth, but the centre of gravity of the Earth and Moon ; and around that centre of gravity the Earth also travels once in a lunar month. Now, precisely as an observer on the Moon would have in effect the range of the Moon's orbit around this centre of gravity, as a base- line by which to estimate the Sun's distance, so the observer on the Earth has the range of the Earth's orbit around the same centre of gravity for the same purpose. The diameter of this last-named orbit is indeed very small — little more, in fact, than three-fourths of the E 2 52 THE SUN: Earth's own diameter ; but by the radius of this small orbit the Earth is sometimes in advance and sometimes behind her mean position. In other words, her motion in longitude (that is, her angular motion round the Sun) is not equable. Over and above the variation of her velocity due to the ellipticity of her path, there is this alternate advance and (relative) retrogression, having for its period a lunar month. Obviously, the observed effect, so far as the astronomer is concerned, is an apparent irregularity in the Sun's motion, having the same period of one lunation. The effect is ex- ceedingly minute : it is less than the displacement of the Sun as seen from different parts of the Earth ; and, as we have seen, this effect could never be employed to deter- mine the Sun's distance. Why then, it may be asked, is the other and smaller effect available ? For this reason simply, that the daily observations made on the Sun in the meridian supply a fund of materials for estimating the effect in question. Such observations are made (severally) at one station by one telescope, and if not by one observer, yet by a series of observers who are always working together, so that their relative modes and powers of observation are comparable to- gether.* Leverrier, by the careful study of an enormous number of observations on the Sun, made at the * The great point, however, is that all the observations are meri- dional. Were extra-meridional observations of the Sun as trustworthy as those made on the meridian, the Sun's distance could have been long since determined through those effects of the Earth's rotation which depend on the length of her diameter. ITS DISTANCE AND DIMENSIONS. 53 principal observatories in Europe, came to the con- clusion that the- Sun's parallax is 8"-95, corresponding ■ to a mean distance of 91,330,000 miles. Mr. Stone, however, has detected a numerical error in M. Leverrier's calculations ; and when this error is cor- rected the value 8" -91 results, corresponding to a distance of 91,739,000 miles.* Mr. Simon Newcomb, of America, has, by the application of the same method, deduced the parallax 8" "81, corresponding to a distance of about 92,800,000 miles. MM. Fizeau and Foucault applied a method differing wholly in character from any that had before been thought of. It seems at first sight incredible that the ingenious combination of revolving wheels or mirrors should serve to determine the Sun's distance ; but such * Every method of solving the problem of the Sun's distance has its special difficulties. In Leverrier's method, the accuracy of the result is wholly dependent on the accuracy of our estimate of the Moon's mass ; for clearly on this estimate depends the extent we are to assign to the Earth's monthly orbital motion around the common centre of gravity of the Earth and Moon. But the Moon's mass is only measurable by observations determining the amount of the nutation of the Earth's axis, a quantity of the same minute order as the solar parallax itself. Still , this method has the advantage of depending on a very large number of observations, both as respects the determination of the Moon's nutation, and that of the inequality of the Sun's motion. It is obvious that the hitter inequality may be employed either to determine the Moon's mass when the Sun's distance is known, or vice versd. It has been employed both ways, Delambre having deduced the value of the Moon's mass by this mode. The way in which the inequality is applied will depend on the question whether the Moon's mass or the Sun's distance is supposed to be best known by other methods. At present it is assumed that the Moon's mass is the more accurately determined element ; but doubtless after the transits of 1874 and 1882 the inequality in the Sun's motion in longitude will be applied to determine the Moon's mass from the known distance of the Sun. 54 THE SUN: is the case. The essential point in the new method is the direct measurement of the velocity with which light travels. This velocity had been determined in two ways by astronomers, or rather it had been dis- covered in one way, and the deduced result had been confirmed in another. When Jupiter is in opposition — at which time he is nearest to ns — the eclipses and occultations of his satellites were found to occur a few minutes earlier than had been calculated ; whereas when he is near conjunction these phenomena occur after the calculated time. Homer first pointed out the meaning of this observation. He showed that the phenomena really occur at the calculated epochs, but that the light which brings to us the account of those phenomena reaches us more quickly when Jupiter is nearer to us than when he is farther away. Bradley afterwards found in this discovery the explanation of the aberration of the fixed stars. If light travelled with infinite velocity we should see the stars in the same direction whether the Earth was at rest or in motion. But as the velocity of light, though very great, is yet not infinite, the apparent direction in which the light from a star reaches the terrestrial observer is affected by the motion of the Earth, according to a law precisely similar to that which causes the apparent direction of the wind to be affected by the motion of an observer who is rapidly carried onwards in a carriage or other vehicle. And when Bradley determined the amount of the aberration of the fixed stars at different seasons, he found — and astronomers have since abun-p ITS DISTANCE AND DIMENSIONS. 55 dantly confirmed the result — that the precise velocity- assigned to light by Romer was that required to account for the peculiarity which affects the apparent place of every star in the heavens, as the Earth sweeps onward on her yearly orbit. But it will be seen that neither observation supplied the means of directly determining the velocity of light in miles per second. All that was known was — first, that light takes a certain interval of time in crossing the Earth's Orbit (or some known chord of that orbit) ; and, secondly, that the velocity of light bears a certain pro- portion to the Earth's velocity in her orbit. Until the exact dimensions of the Earth's orbit are known, neither of these facts informs us of the real velocity of light. Judging, however, from Encke's estimate of the Sun's distance, astronomers concluded that light travels at the rate of no less than 192,000 miles in a single second of time. It might seem altogether hopeless to attempt to estimate directly a velocity so enormous as this. Re- membering how the velocity of sound has been measured, and considering only the application of a similar method to the case of light, how utterly futile does the very thought of such an attempt appear ! We can make a signal when a sound is heard at one station, and observers at another station can note how long the sound takes in reaching them ; because where the stations are at a considerable distance, an appre- ciable time elapses before the sound travels from one to the other. But the very best signal we can use is some 56 THE SUN: visible signal (in preference, I mean, to some electric signal), i.e., a light-message, which travels so quickly that we can wholly neglect the time it has taken, in comparison with that taken by the sound. Obviously, we have no such means of measuring the passage of light, for what is our signal to be if the velocity with which it is conveyed is so largely to exceed the velocity of light that we can neglect the time occupied in its transmission ? But if we had even a satisfactory answer to this question (instead of having none whatever), the problem would yet be insoluble in this manner. Suppose the light were shown at a distance of 500 miles — about the limit at which any terrestrial light could be seen, even under the most favourable atmo- spheric conditions — yet the time occupied by the light- waves in traversing this distance would be but about the 360th part of a second. What instrument or what observer could note such an interval — to say nothing of measuring it, which would yet be absolutely essen- tial to the successful solution of the problem ? I shall not here enter into a full account of the means by which Foucault and Fizeau solved a problem appa- rently so intractable, referring the reader to ' Pouillet's Physics' and other works in which the subject of light is dealt with. The general principle of the method employed by Fizeau may be thus presented. Suppose we see an object by light-rays which have been caused to traverse a long path by means of several reflections. Now conceive that the continuity of the long path is simultaneously broken at regular intervals at twq ITS DISTANCE AND DIMENSIONS. S7 points, one near, the beginning the other near the end of the path, the path being broken — re-made — broken — re-made, and so on. Then if light travelled with infinite velocity, the light-rays, which at any instant traversed the first part of the path at a time when the path was made there, would traverse the last part also, because at that same instant the path would be com- plete there also. But light not travelling with infinite velocity, the light-rays which pass the first part of the path may be stopped by the break in the second part, if the interval between the making and breaking be but short enough. Now, Fizeau had a revolving toothed wheel, and matters were so arranged that when a tooth of this wheel was opposite a certain small aperture, the path of light was broken both at its beginning and end, for the light had to pass through this aperture, and then, after pursuing a long course, to pass out again through the same aperture. Thus, when the re- volution was moderately rapid, light-rays which passed through the aperture found the aperture open when they came back again to it ; but by causing the revo- lution to be very rapid indeed, so that a very minute fraction of a second elapsed between the passage of successive teeth across the aperture, it was possible to cause the light-rays which had entered while the aper- ture was open to be prevented from passing out again by the interference of a tooth of the wheel. It is easily seen that when the wheel revolved at this parti- cular rate there would be a total eclipse so far as light coming through the aperture was concerned. For light 58 THE SUN: which went in when any portion of the aperture was free, would return to the aperture when just that por- tion of the aperture was closed. Then, as Fizeau had the means of telling at what rate the wheel was revolv- ing when total eclipse thus occurred, he could tell pre- cisely what fraction of a second elapsed between the passage of tooth after tooth across the aperture ; and knowing the length of the path traversed by the light- rays he could measure the velocity of light with a con* siderable degree of accuracy. Foucault adopted an arrangement in which a plane mirror was caused to rotate very rapidly, and the principle of his plan (which, to be fully explained, would require more space than is here available) depends on the duration of visual impressions.* Fizeau's method, in some respects in- ferior to Foucault's, resulted in assigning to light a velocity of 194,600 miles per second ; but Foucault's gave a velocity of only 185,300 miles per second, fall- * It may be thus exhibited : — An image of a certain wire is seen directly, and when a certain plane mirror which can be rapidly revolved is in a certain position, the image of the wire is caused to appear in coincidence with the wire seen directly. The mirror is so rotated as to take up once in each rotation the required position ; and 'so long as the rotation is slow the reflected .image makes successive appear- ances. With an increase in the velocity of rotation the image appears continuously in one place — owing to the continuance of visible impres- sions. Now, for moderate velocities of rotation, the position thus taken up by the image coincides with that of the wire seen directly. But with a very rapid rotation the position of the mirror suited for causing the image to be visible (after reflection), no longer accords appreciably with the position required when the mirror is at rest. Accordingly, the image appears appreciably separated from the wire seen directly ; and the amount of separation, combined with the known velocity of rotation, supplies the means of estimating the velocity of light.- ITS DISTANCE AND DIMENSIONS. 59 ing considerably short of the estimate of 192,000 miles above referred to. So satisfactory were Foucault's experiments, that this discrepancy was held gravely to affect the estimate of the Sun's distance, on which the latter value is based. It would follow, if Foucault's experiments were held to indicate truly the velocity of light in the interplanetary spaces (as well as in air), that the Sun's mean distance would be but 91,400,000 miles, his parallax 8""942. The Astronomer Royal also suggested the applica- tion of a method already employed by Flamsted, and later by Bond, of America. He pointed out that in- stead of comparing the position of Mars on the sky (when the plauet is near opposition) as seen from dif- ferent stations, an even more satisfactory estimate of the planet's distance, and so of the Sun's, might be obtained by observing how far the diurnal rotation of the Earth, by shifting the place of any fixed station, affected the apparent position of the planet. It is clear that if the station e (fig. 9) is supposed to be carried by the Earth's rotation to e', the observer can as effec- tively compare the distances at which the observed places of Mars, m and m', lie from a fixed star s, as though there were two observers, one at e and the other at e' at the same instant of time ; for astronomers know well how to take into due account the motion of Mars during the interval. In 1862 this method was employed, as well as the former method of treating observations of Mars. The result was to confirm the impression that the Sun lies 60 THE SUN: nearer to us than had been so long imagined. Stone, of Greenwich, by combining the two methods, deducing the solar parallax first from observations of Mars made at Greenwich alone, then from observations made at Greenwich and Capetown, then from observations made at Greenwich and Williamstown, and combining all these results, deduced a solar parallax of 8"*943, with a probable error of 0"'051. This corresponds to a distance of about 91,400,000 miles, with a probable error of about 500,000 miles. Winnecke, by combin- ing observations of Mars made at Poulkowa and Cape- town, deduced a parallax of 8" - 964, corresponding to a distance of about 9 1 ,200,000 miles. Newcomb, from the observations of Mars in the same year, deduced a parallax of 8""855, corresponding to a distance of about 92,300,000 miles. Besides these, there were estimates by Leverrier, Stone, Pogson, and several others, founded on the re-examination of processes already referred to, or on observations enabling those processes to be applied anew with more or less chance of exactitude in the results. By the year 1864 it had become abundantly clear that the accepted estimate of the Sun's distance was too great. All the new values clustered around a value of about 8""9 for the parallax, corresponding to a distance of about 91,850,000 miles. Thus astronomers were led to re-examine the obser- vations of the transit of 1769 in order to see whether they could be so interpreted as to correspond with the ITS DISTANCE AND DIMENSIONS. 6 1 result pointed to by so many independent researches. Powalky subjected these observations to a new dis- cussion and deduced the value 8""832, corresponding much more closely than Encke's with the recent de- terminations. But astronomers were not satisfied with his labours, because he rejected some of the more im- portant observations made in 1769, without assigning sufficient cause, and also because the agreement be- tween those observations which he retained was by no means exhibited satisfactorily. As Admiral Manners said, when addressing the Astronomical Society in 1862, ' The weight of Encke's value could scarcely be said to be affected by Powalky's discussion.' It was reserved for Mr. Stone, of Greenwich, to solve in a satisfactory manner a difficulty which had long perplexed astronomers. We have seen that there are two phases which an observer of Venus in transit might try to catch — the moment when the connection between the disc of Venus and that of the Sun appears to break at ingress or form at egress, and the moment when the outline of Venus, if undisturbed, would just touch the outline of the Sun. Now, if we consider why Venus is distorted so peculiarly near the time of internal contact, we shall see which of these phases corresponds most closely to the real moment of internal- contact. The matter is sufficiently simple. Owing to a peculiarity of vision, every bright object appears some- what larger than it really is, the borders of the image on the retina affecting the neighbouring part of the retina. It follows obviously that every dark object will 62 THE SUN: appear somewhat smaller than it really is, for the hack- ground on which the dark object is depicted will be relatively bright, and so its image on the retina will encroach on that of the dark object. Thus, suppose the arc s s' v' (fig. 17) to represent part of the true outline of the Sun, then, to the observer, the outline would lie as shown outside this circle ; but if the circle s' v v' represents the true outline of Venus, her apparent out- line would lie as shown inside this circle. It is clear, Fio. 17. Pig- 18. then, that the part of the outline of Venus which lies upon the Sun seems to belong to a smaller circle than the part of the outline lying outside the Sun. This is true until the actual moment when the true outlines touch, when the appearance would be that shown in fig. 18, the connecting ligament being the finest con- ceivable line ; and obviously (referring to ingress), the undistorted part of the apparent outline of Venus would have seemed to belong to a circle touching the Sun's outline some seconds earlier. Similar remarks apply to egress. So that if an observer could note the moment when the discs of Venus and the Sun were apparently united by a fine line, as in fig. 18, he would have ascertained ITS DISTANCE AND DIMENSIONS. 63 the moment of real contact ; whereas if he had noticed the time when the circular part of Venus's outline would touch the Sun's, if continued undistorted, he would have caught quite a distinct phase, which we may call apparent contact. As a matter of fact, only the finest possible telescope, and great care and observ- ing skill on the part of the astronomer, would render it possible to notice the moment when such a ligament as is shown in fig. 18 broke or formed. Usually a much coarser ligament only would be noticed ; but the observation of the sudden breaking or formation of the ligament would be regarded as an approximate obser- vation of real contact.* Now, Mr. Stone (after carefully inferring, from the account actually given by the diiferent observers, whether real or apparent contact was noticed) intro- duced a definite correction,! according to the best estimate we have of the effects of irradiation in en- larging the solar disc and diminishing that of Venus. By adopting this definite rule, Mr. Stone not only deduced a solar parallax corresponding closely with that obtained by the various other methods described * In Appendix A will be found some further remarks on this point, in relation to the expectations connected with the approaching transits. It has seemed to me that the view taken by Mr. Stone of the value of those expectations is altogether too despondent. It is quite true, as he points out, that different observers will see a ligament of different breadth — in other words, will catch a different phase ; but we need not therefore fear that a corresponding error will be introduced. If each observer does but note the apparent breadth of the ligament, there will be the means of in large part eliminating this cause of error. + It was in this respect that Mr. Stone's work differed fromthat of his predecessors in the same process of research. 64 THE SUN: above, but (which is much more important) he brought the observations of the transit into most satisfactory accordance. To quote the words of Admiral Manners' address on presenting to Mr. Stone the Gold Medal of the Royal Astronomical Society — ' Mr. Stone's in- vestigation includes every complete observation from internal contacts of the transit, and yet, notwithstand- ing this circumstance, all the observed durations are represented with a degree of accuracy which is, beyond all doubt, within the limits of probable errors of obser- vation. Mr. Stone has effected this object by simple interpretation of the words of the observer, inferring from his language, and from the accompanying circumstances of the case, whether the phenomenon noticed by him referred to an apparent or a real contact.' The result thus deduced assigned to the Sun a parallax of 8"*91, with a probable error of 0"*03, cor- responding to a distance of about 91,730,000 miles, with a probable error of about 300,000 miles. In other words, the Sun's distance, according to this latest, and in many respects most satisfactory deter- mination, is found to lie between about 92,030,000 miles and about 91,430,000 miles. If we assume 8" - 9 as, on the whole, the best value for the parallax, im- plying, as it does, that as yet we have no knowledge of the next decimal figure, we have for the Sun's mean distance 92,000,000 miles ; * as we ought not to admit * The very value, be it noted, which Professor Smyth, Astronomer Eoyal for Scotland, considers to be recorded in the dimensional features ITS DISTANCE AND DIMENSIONS. 65 any significant figures beyond the second. Doubtless, after the observations which are to be made on the transits of 1874 and 1882, astronomers will be able to extend the value of the parallax to a second decimal figure, and so there will be three significant figures in the expression for the distance. ( See Appendix A. ) It remains to be noticed that the corresponding value of the Sun's diameter is fairly represented by 850,000 miles, nor is anything gained by expressing this diameter, as is so commonly done, with significant figures down to the units' place. Certainly if the true solar parallax is 8" "94000, for instance, then the exact value of the Sun's diameter is 852,908 miles. But when we see that the value of the solar parallax can only fairly be presented to the first decimal place, or as 8"*9, we cannot pretend to know the Sim's diameter with greater exactness. The limits of error in our estimate of the Sun's distance being probably four or five hundred thousand miles at the least, it must be admitted that the limits of error in our estimate of the Sun's diameter cannot be less than four thousand miles. The surface of the Sun is equal to four times the area of a circle having this diameter of 850,000 miles, or in round numbers to 2,285,000 millions of miles. His volume is about 1,250,000 times that of the Earth. of the Great Pyramid. See also his Life and Work at the Great Pyramid, which, though not dealing with this particular feature— or supposed fea- ture — of the pyramid's dimensions, yet goes far to demonstrate that a number of the facts regarded as recent acquisitions of astronomy were known to the architects of that most remarkable erection. V 66 THE SUN: Of his mass and his resulting might as a ruler, I shall speak at length in the following pages. In drawing to a close this chapter on the most dif- ficult and important problem of astronomy— a chapter which, long as it is, has barely sufficed to convey so much of the history of investigation as seemed essen- tial to my purpose — I would remark that there is no branch of research with which astronomers have better reason to feel satisfied. I cannot consent to speak of recent successes as though they had removed a reproach from astronomy. There is no single part of the history of this problem of which astronomers have not abundant reason to be proud. By long and patient labours they have been able to overcome dif- ficulties which might fairly have been thought in- superable. Availing themselves at every step of the best means they could secure for approximating more and more closely to the truth, they have necessarily had during their advance to pass from ground which they had formerly occupied. But every such change of ground has been an advance towards more complete, success. Sir John Herschel has said that the recent correction in the value of the Sun's parallax, cor- responds to the apparent breadth of a human hair at 125 feet, or of a sovereign at eight miles off; ' and that, moreover, the error has been detected and the cor- rection applied ; and that the detection and correction have originated with the friends and not with the enemies of science.' But I would go even farther than this, since in place of regarding the recent change as ITS DISTANCE AND DIMENSIONS. 6 J involving the detection and correction of error, I would speak of it rather as a new approximation, more successful indeed than former ones, but not therefore changing those former successes into defeats. The astronomy of half a century since had as good reason to be proud of Encke's work as the astronomy of the present day has to rejoice at the successes of Hansen, Foucault, Leverrier, and Stone ; or as the astronomy of future ages will have to boast of those labours by which the results now accepted will inevitably be improved upon. F 2 68 THE SUN. CHAPTEE II. THE SUN AS SULES. One of the most important of the Sun's functions is that by virtue of which he rules the motions of his family of planets. By the exercise of his mighty at- tractive influence he continually controls the tendency which they have to rush tangentially far out into space beyond the influence of his illuminating, heating, and actinic rays. Their swift orbital motions, combined with the relative stability of their axial pose, result in producing the orderly succession of the seasons. But this succession would come to an end, were it not for the stability of their orbital motion ; and this stability is due to the Sun's overmastering attraction. To this it is due that the paths of the planets though under- going continual processes of variation, yet suffer no sudden changes as respects their distance from him, or, therefore, as respects the period necessary for a complete revolution. Nay, so perfect is the whole scheme of governance that even the processes of slew change take place within limits, and those limits not very wide. Not merely can the orbits of the planets suffer no sudden change, but they can neither suffer a great change nor a permanent change. THE SUN AS RULER. 69 "We might dwell much farther on the importance of the Sun's influence as the most massive portion of the scheme of which he is the centre. On the one hand we might point to the possibly even vital importance of the action which causes the terrestrial equinoxes to circuit the ecliptic in their grand processional year of 25,868 solar years, or of the slowly-exerted influence which changes the eccentricity of the .Earth's orbit. It is very far from unlikely that but for these influences the Earth would long since have been rendered unfit, through a species of exhaustion, for being the abode of living creatures. But as it is, continents become oceans, and oceans continents, one hemisphere inter- changes with another the office of supplying the chief proportion of land surface ; activity is followed by rest and rest by activity ; and so through countless cycles this globe has been and will continue to be a fit abode for innumerable races. On the other hand, we might dwell on the influence which the Sun's mighty attrac- tive influences exert in gathering in from all sides abundant supplies of motive energy, to recruit it may well be, his seemingly exhaustless stores of heat and light and chemical activity. But without dwelling further in this place on themes of which some will find a place in other chapters, while others — interesting and fascinating though they be — must yet be regarded as lying outside the range of our subject, let us proceed at once to consider what the Sun's true influence is, by virtue of that principle of gravitation which causes every particle of his mass to 70 THE SUN. aid in attracting all bodies within the sphere of his influence. In one sense it may be said that the sphere of the Sun's influence is all space. According to the present conceptions of the power of gravitation, there is no particle of matter throughout the whole universe which does not feel the attractive influence of the Sun (as indeed of every particle of matter). But to all intents and purposes the Sun's reign may be regarded as limited. His influence on the stars is not merely minute so far as the amount of motion it is capable of producing in any given interval is concerned, but it is to be regarded as the influence of a peer among peers, not of a king over his subjects. The results of the mutual attractions of the stars may be, and doubtless are, of the utmost importance, but they do not belong to the history of the Sun as a ruler. On every side, then, the Sun's rule is limited — for in all directions there are stars, and the sphere over which each star rules, is as definite as that governed by the Sun, so that in each direction we come upon regions where his influence is subordinate to the influence of some other orb.* * It is sometimes said that a, body like a comet can pass from the sway of one star to come permanently under the dominion of our Sun or of another star, and vice versa ; but setting aside the ease of interference with such a body through the action of a planet, or by reason of atmo- spheric resistance near the Sun, or the like, this can never happen. For let us suppose that a comet is passing from the sphere of one star's influence to that of the Sun's. Then it cannot be moving on a closed orbit around the Sun, at this time, for if so 'it is already subject to the solar dominion, contrary to our supposition. It must then be travelling on a hyperbolic or parabolic orbit around the Sun, and so must eventually pass THE SUN AS RULER. ?I But it is desirable to know what sort of influence the Sun would exert even at distances beyond the limits of his direct control. Let us set ourselves to form adequate conceptions of the Sun's energy of gravitation. The measure of all gravitation is that force which the Earth exerts as we know to draw bodies to her surface. We must therefore first ascertain what pro- portion the energy of the Sun's attraction bears to that familiar attraction exerted by the Earth. For this purpose we may proceed either directly by comparing the amount of velocity which the Earth's attraction communicates to falling bodies, with the actual motion towards himself which the Sun causes in the case of any planet ; or indirectly by comparing the motion of any planet round the Sun with the Moon's motion round the Earth. I select the second method as being the simplest.* I adopt also a way of wholly away from his dominion. It follows that, setting aside the above- mentioned influences, a, comet which travels in an unclosed orbit round one star can never travel in a closed orbit round any star, but will con- tinue to flit from star to star through all time. * It is worthy of notice, however, that the other method, though seldom applied, is in effect quite as available as the method in which reference is made to the Moon's motion. Thus we know that in one second a body falls about 16 - 1 feet towards the Earth, acquiring a velo- city of twice this number of feet, or 322 feet per second. Now the Earth circuits the Sun at a rate of about eighteen miles per second, and supposing e ef (fig. 19) to represent this distance on the orbit e d E, then e l, obtained by drawing a perpendicular from ^ on the diameter e s b, represents the amount by which in effect the Earth has been drawn to- wards s, and a velocity of twice e l per second measures the Sun's gravity at the distance s e. But by Euclid vi. 8, e e' is a mean proportional between e l and e E ; in other words, c L is equal to ii-'i-; and therefore 7% THE SUN. applying it by which certain difficulties of conception are removed. The Earth at a distance from the' Moon of about 238,800 miles has power to change the direction of the Moon's motion through four right angles in 27*322 days, the Moon moving in her orbit round the Earth (e e 1 ) 2 the Sun's gravity at e is measured by a velocity of — - per second, or in feet i — ' . Hence at a distance equal to the Earth's 91,500,000 ^ radius, the Sun (supposing all his mass collected at one point) would exert «. force represented by a velocity of about * — '- ! ' _ (4,000) 2 feet per second, which reduces to 9,783,180 feet (or about 1,853 miles per second). Comparing this with the measure of the force of terres- Fio. 19. trial gravity, or 322 feet, we see that the Sun's mass must be, according to this rough procesSj fully 300,000 times as great as the Earth's. As a matter of fact it is estimated at 315,000 times the Earth's, a result we should have closely approximated to had we taken the Earth's radius at 3,960 instead of 4,000. The following general theorem is often useful. Take r the Earth's radius, e her mass and g terrestrial gravity, b the radius of a circular orbit described by a body of mass m, about a larger body of mass m with velocity v (in feet per second). Then the attraction between the bodies, or the gravity of m towards m, is represented by the expression — and the mass m + m is equal to e . -— . If m is relatively very small THE SUN AS RULER. 73 with a velocity which we may represent by f^°.* Now the Sun at a distance from the Earth of about 91,500,000 miles has power to change the direction of her motion through four right angles in 365*256 days, the Earth moving in her orbit with a velocity which we may represent by 9J |p!f. Now clearly, since gravity varies inversely as the square of the distance, the Sun would require (were other things equal) to have an attractive power greater than the Earth's in the ratio C'JST)* to produce the same effect on her that she produces on the Moon ; and secondly, since to change the direction of a body's motion through any angle is a work which will be done at a rate proportioned to the force which operates, it is clear that the Sun's attractive power would have (were other things equal) to be less than the Earth's, in the ratio ^'^, to accomplish in one sidereal year what the Earth accomplishes in one synodical month ; while lastly, since the faster a body moves the greater the force necessary to deflect its (as in the case of the Earth compared with the Sun), and the radius of the larger mass is p. then gravity at the surface of the larger mass is equal to — —. P It may be added that if v and v he the velocities with which bodies coming from an infinite distance reach the surface of orbs having radii n, r, and mean densities A, 5, then - = - / ^_ v ry 8 The same relation holds if v and v are the velocities with which bodies circle round these orbs just at their surface. * We need not trouble ourselves to determine the velocity in miles per second, or minute, or hour ; because relative and not absolute velocities are in question. Hence we can represent the Moon's velocity by the radius of her orbit divided by the period, provided we represent the Earth's velocity in like manner. 74 THE SUN. course through a given angle, it is obvious that the Sun's attractive power would have (were other things equal) to be greater than the Earth's in the proportion f 91.500,000 +n 238.800 .flint i« in tW 1-flfin 9IJIOO.000xir3M tf) 01 365-256 t0 27-322 TIlaT 1S ' 1D tIle 1 aX1 ° 238,800x365-256 lU produce a given change on the quickly moving Earth in the same time that the Earth produces such a change on the less swiftly moving Moon. Now we have only to combine these three proportions, which take into account every circumstance in which the Sun's action on the Earth differs from the Earth's action on the Moon, in order to deduce the relation between the Sun's real attractive energy, and the Earth's (at equal distances from the centre of either). This gives the proportion <*#*m? x ^ 7 - 322 '!, which reduces to 314,798, in L r (238,800r (365-256)-' which proportion the Sun's mass exceeds the Earth's. We may take 315,000 (the value given in tables of the elements) as representing in round numbers the true proportion, which as it depends on the Sun's distance, cannot be determined so accurately that the last three figures of the number can be regarded as significant. At equal distances, then, the Sun exerts 315,000 times as much force on any body as the Earth. So that if the Earth's mass were as great as the Sun's, her dimensions remaining unchanged, a mass which now weighs one pound, would weigh more than 14^- tons. A man now of average weight would be crushed down by a weight of more than 20,000 tons. A body, if raised but a single inch and let fall, would strike the ground with a velocity three times as great as that of the swiftest express train. THE SUN AS RULER. 75 But now that we have thus ascertained the propor- tion which the Sun's attractive energy bears to that exerted by the Earth, and so are able to measure the Sun's might as ruler over his system by direct com- parison with the familiar force of terrestrial gravity, we must endeavour to form an estimate of the extent of force exerted by the Sun at different distances. We are to inquire what are the limits of the Sun's effective reign, not as regards distance alone, but as regards also the activity of matter, — that is, the velocity with which it is travelling. We may begin with the Earth. We know that the Earth is completely subject to the Sun's attraction. Notwithstanding the inconceivable velocity with which she moves, and therefore the inconceivable energy of the tendency she has to travel onwards in a right line, and so to free herself from the Sun's control, she is compelled to travel in a nearly circular course around him. At one time her velocity has reached its maxi- mum and she has power for awhile to increase her distance from the Sun. But she has derived that very power from him. Anon her speed is reduced to its minimum, and then she is compelled slowly to ap- proach the ruling centre. But throughout her course there is one constant relation from which there is no escape. The Earth's velocity and distance are the two quantities which measure the extent of the Earth's partial freedom. When one is reduced, the other is increased, and vice versa, in such sort that there is absolutely no change in her condition regarded as 76 THE SUN. depending on these two combined relations. The one law from which there is no escape, let distance and ■velocity change as they may during the Earth's circuit around the Sun, is that her period of revolution con- tinues unchangeable, — and therefore her mean distance also.* We may then take the Earth's mean distance as measuring her freedom from complete solar control — complete control being understood to be such an over- whelming influence on the Earth as would force her to fall directly upon the Sun. And while the Earth's mean distance thus measures her partial freedom, the shortness of the period in which she completes her circuit measures the amount of the Sun's power over her. Her mass may be regarded as having nothing to do with either relation. Increase of mass, so far as it would be effective at all, would tend to increase the strength of the bond uniting the Earth and the Sun, and to diminish the period of the Earth's circuit. But re- membering that the Earth's mass is a very minute fraction of the Sun's, it may be disregarded wholly. A body no larger and heavier than a peppercorn, if projected with the same velocity and on the same course as the Earth, would continue to travel in pre- cisely the same path and period around the Sun. * Or we may say in preference, that the one law from which there is no escape, is the law connecting the Earth's velocity v at any time, with her distance e from the Sun at that time, and a certain fixed quantity a which is her mean distance. This law is thus expressed mathematically : (2 1 \ J, where /i is the accelerating force of the Sun at the unit of distance. THE SUN AS RULER. 77 We have next a law for our guidance which is of a very remarkable character, and the recognition of which will be found to throw a most important light on all the relations of the planetary scheme. It is this : Given the distance of a body from the Sun, and the velocity with which the body is travelling, then — let the course of the body be what it may (so long only as it does not bring the body into actual contact with the Sun), the period of the body's revolution is assigned. The Earth's greatest velocity and her least corre- spond therefore — as truly as her mean velocity — with her period ; and further, we need not trouble ourselves about the direction of her motion at any time, for if this direction were altered by the action of some external force, while yet her velocity remained unchanged, she would continue to travel in the same periodic time around the Sun, and at the same mean distance. So that if we take the Earth's greatest velocity when she is in perihelion (18*5 miles per second), we have the velocity which is necessary in order that a body about 90,000,000 miles from the Sun may travel once in a year, or at a mean distance of 91,500,000 miles, round the Sun ; and if we take her least velocity (17"9 miles per second) when she is in aphelion, we have the velocity which is necessary in order that a body about 93,000,000 miles from the Sun may travel once in a year round the Sun ; while the Earth's mean velocity (18*2 miles per second) at her mean distance is the velocity necessary in order that a body at that distance may have a period of one year. 78 THE SUN. We will now take only the mean distance and the mean velocity. We see that at a distance of 9 1 ,500,000 miles a body requires a velocity of 18*2 miles per second if it is to have a mean distance of the same amount. So that clearly if it were projected square to a line from the Sun it would never change its dis- tance ; for it would have exactly the right velocity and exactly the right direction for travelling in a circle around the Sun. The Earth when at this distance, though travelling with the right velocity for the re- quired mean distance, is not travelling square to a line from the Sun, and so does not travel in a circle around him. But we have learned from her motion what is the just rate at which a body should be pro- jected so as to travel in a circle round the Sun at a distance of 91,500,000 miles. Now how much must this velocity be increased in order to enable a body at this distance of 91,500,000 miles to pass wholly from the control of the Sun ? If we can determine this we shall have determined the limits of the Sun's influence at this particular distance. Over bodies moving with a velocity below that limiting velocity he is completely master; let them travel onwards as they may, increasing their distance from him more and more, there is yet a limit to this increase. Their absolute velocity will become less and less ; it will become at last such, that if their direction of motion were but changed they would thenceforth con- tinue to describe a, circle around the Sun; still it will go on diminishing, until at length they reach their THE SUN AS RULER. 79 extreme range of distance, after which they will be brought back through all the orders of distance they have passed through, and finally return to the place they started from, to pursue the same round for ever. But if their velocity do but equal or exceed the limit we are dealing with, they will travel onwards — with ever-diminishing velocity, it is true, but still — with ever-increasing distance, for ever. It might be supposed that a very great increase of velocity would be required in order that the Earth should be thus (unfortunately for her inhabitants) re- leased from the Sun's service and sent to wander freely through space. But in reality it would not even be necessary that her velocity should be doubled ; an in- crease by one-half would be more than sufficient to free the Earth for ever from enforced periodic revolution around the centre of our planetary scheme. The exact proportion of increase necessary to effect this is repre- sented by the proportion in which the diagonal of a square exceeds the side,* which we know to be repre- * The following simple formula conveniently expresses the 'relation between the mean dis u ances a and a' of bodies which at a distance r from the Sun are travelling with the velocities v and v 1 respectively: n! (2a- r) : a (2a!— r):: 11 1 : v' 2 . Now supposing the velocity v to be such that a circle would be described about the centre of motion, if the body travelled square to the line from that centre, then obviously a is equal tor; so that the above relation becomes a' : Id ' — rwv 2 : v' 1 ; and therefore a' : r::v 2 : Iv'-v' 1 . Now clearly if we increase if until t/ 2 is equal to 2« 3 , we make the fourth term gradually diminish until it vanishes ; so that the third at last- 80 THE SUN. sented numerically by the proportion in which 1414 (pretty nearly) exceeds 1000. bears an infinitely great proportion to the fourth. But in this case the first will bear an infinitely great proportion to the second : in other words, a 1 will be infinitely great. Hence the period of the body will become also infinite (by Kepler's third law) : or in other words the body will never return to its starting-place. Therefore, it follows that if v be the velocity with which a body will describe a circle at a distance r from the Sun, the greatest possible velocity which a body can have at a distance r from the Sun, so as to travel on a closed orbit around him, is v V2 ; and if a body is observed to travel with any greater velocity at such a distance we know with certainty that that body has entered the domain of the Sun, with a velocity imparted to it by extra-solar influences. If in the above proportion if* is greater than 2v 2 , we see that a' must have a negative value. This means that the centre of the path described by the body lies in the direction contrary to that in which the Sun lies. And knowing that the path must needs be a conic section with the Sun 'in a focus, it follows that the path of the body must be hyperbolic, the Sun lying at that focus which is next to the branch traversed by the body. Of course in this case as in the former the body will never return. But there is a noteworthy distinction between the two cases. When the axis of the orbit is infinite, the body describes a parabola, and if it could be traced from the Sun as it approached from an indefinitely great distance to its nearest point and then passed away again to an infinitely great distance, the point to which it seemed to pass away would be pre- cisely the same as that from which it seemed to come, and these coinci- dent points would lie directly opposite the point of nearest approach. (This is obvious, because if lines be drawn from the focus of the parabola to two points of the curve equally and enormously removed from the vertex, these lines will enclose an indefinitely small angle, and will approach indefinitely near to coincidence with the axis.) On the other hand, a body approaching and then passing away on a hyper- bolic orbit will seem to come from one point of the heavens and to pass away to a different point, the bisection of the celestial arc between these points lying directly opposite the point at which the body makes its nearest approach ; and for a given distance of this nearest point, the arc separating the two former will be the greater as the velocity of the body when at its nearest is greater ; becoming equal to two right angles when this velocity is infinitely great. THE SUN AS RULER. 8 1 So that if the Earth when at her mean distance had her velocity suddenly increased from 18 "2 miles per second, to 257 miles or thereabouts, we should be carried thenceforth continually farther and farther away from the light and life of the planetary scheme. From Mars and Jupiter, and perchance from Saturn, Uranus, and Neptune, the unhappy career of the Earth might be traced for many a long year (though years — at least terrestrial ones — would then be no more). But long before the Earth crossed the confines of those distant regions along which the outer planets pursue their career, all the higher forms of life would have vanished from her surface. She would still rotate; day and night would still succeed each other on her surface ; but the orderly sequence of the seasons would be re- placed by the continual diminution of solar light and heat, until a cold more intense than that of the bitterest Arctic winter would bind the world in everlasting frost. A similar fate would befall us if the Sun's mass were suddenly reduced by one-half ; the only difference being that in this case we should have companions in our troubles ; for Mars and Venus and Mercury would all forthwith start on parabolic paths, carrying them away to infinite distances from the Sun. Nor would the larger planets escape. From their distant orbits they would rush off into outer space, carrying their systems of satellites with them ; so that if I have been right in regarding these orbs as acting the part of secondary suns to their satellites, the latter would be less unfortunate in their fate than the four minor G 82 THE SUN. planets, for these would have no sun at all, while the former would still enjoy such heat and light as their ruling centres could supply to them. It is not without a purpose that I have thus dwelt on the general result of a sudden diminution of the Sun's mass. The consideration that all the planets would thus at once be freed from their allegiance if the Sun's mass were reduced, leads us to the consideration that each planet's velocity need but be increased in the proportion of about 1,414 to 1,000, to lead to a similar result. And thus we see that the Sun's in- fluence at the distances of the successive planets is limited to the control of bodies moving with a velocity bearing such a relation to the velocity of the respective planets. So that we have only to draw up a table of the distances and mean velocities of the planets, and to increase the latter quantities in the proportion of about 1,414 to 1,000, in order to have a representation of the gradual diminution of the Sun's influence at greater and greater distances. The table runs thus : — Planet Mean distance in miles Mean velo- city in miles per second Velocity in- creased as 1,414 to 1,000 The Earth The Asteroids .... 35,392,000 66,134,000 91,430,000 139,311,000 250,000,000 475,692,000 872,137,000 1,753,869,000 2,745,998,000 29-3 21 -t 18-3 14-7 11-0 8-0 59 4-2 3-3 41-4 30-3 259 208 155 11-3 83 5-9 4-7 We see from this table, that if the three outermost planets could only have imparted to them the velocity THE SUN AS RULER. 83 of the minor planets, they would be freed from their allegiance to the Sun, and pass away on hyperbolic orbits. But with the exception of Uranus there is no planet which would be thus freed (absolutely) if it had imparted to it the velocity of the next inner one.* But the above table has only been presented by way of introducing a more general law. What the table teaches us respecting special distances we can determine for all distances from the Sun, by a simple application of Kepler's third law and its results. * It is a rather singular circumstance that the maximum velocity which the Sun can control at the distance of any one of the four outer planets should correspond so closely as it does with the actual Telocity of the planet whose orbit lies next within. We see that if Neptune could have the velocity of Uranus, he would be almost wholly freed from his allegiance ; Uranus would be just freed if he had the velocity of Saturn ; Saturn would be almost wholly freed if he had the velocity of Jupiter ; while Jupiter would be almost wholly freed if he had the velocity of those asteroids which travel at a mean distance. And there is a tendency, though less marked, to the same relation among the remaining planets. Remembering that the velocity a planet would require for freedom is that with which a body approaching on a para- bolic orbit from an infinite distance would pass the mean distance of that planet, we have throughout the solar system a. tendency (very marked among the outer members) to this remarkable relation, that the velocity with which a body approaching from infinity would cross the orbit of any planet should be the same as the actual velocity of the next inner planet. It need hardly be said perhaps that this relation directly results in the law to which Bode's law approximates for the outer planets. Thus, if the outermost planet had a distance d and a velocity v, the next inner planet would have a velocity \*/2 corre- D sponding to a mean distance ,;, and so on. But in the law of the duplication of the distances outwards there is no direct physical signifi- cance, whereas it is possible to conceive that the law as presented above — that is, regarded as associated with the velocities — may be associated also with the processes by which the solar system has reached its pre- sent condition. o 2 84 THE SUN. Thus, suppose we wish to determine the maximum velocity which the Sun can control at a distance half that of the planet Mercury. Then the law that the cubes of the distances are as the squares of the periodic times shows us that the period of a planet at such a distance would be to Mercury's as 1 to the square root of 8 (or twice the square root of 2). Since then, the actual circuit of such a planet would be half that of Mercury, its velocity would exceed Mercury's in the proportion which the square root of 2 bears to 1,* or about 1,414 to 1,000. This would correspond to a velocity of 4T4 miles per second; and the greatest velocity the Sun could control at this distance would be obtained by increasing this velocity of 41 "4 miles per second in the proportion of 1,414 to 1,000. It would therefore be 59 - 6. This shows how we can measure the Sun's controlling energy for any distance. But it also establishes a very important general rela- tion. It appears that when we halve the distance, we have, in order to determine the velocity which the Sun can control, to increase the velocity at the greater distance in the proportion of about 1,414 to 1,000. And therefore when we take one-fourth of the distance, we must increase the velocity twice in this proportion. But this amounts to doubling the velocity, since this * This illustrates the general law that if two planets have mean distances d and d' respectively, and mean velocities v and v' respectively, then v : v' : : d'* : d s . Clearly this is so, since the periodic times are as d'' to d'*, and the velocities, therefore, as d-i-d' to d'-i-d'*. THE SUN AS RULER. 85 proportion is that of the square root of 2 to unity. Hence we have this general rule, that the velocity which the Sun can control is doubled when the distance is reduced to one-fourth, and we can see at once how enormously the velocity must increase in the Sun's immediate neighbourhood.* Thus, at a distance of 8,848,000 miles (one-fourth that of Mercury), the velocity the Sun can control, so as to compel a body to move in a closed orbit around him, is 82 - 8 miles per second; at a distance of 2,212,000 miles it is 165*6 miles per second; at a distance of 553,000 miles it is 33T2 miles per second. But this brings us very close to the Sun's surface — since it is from his centre all our distances are measured — and his radius is about 425,000 miles. The actual velocity at his surface — that is, the velocity which he could just control so as to compel a body to travel in a closed orbit just touching his surface — is easily obtained from the formula given in the preceding note. It is no less * It need hardly be said that this result might have been obtained directly from a consideration of the law according to which gravity diminishes with distance. But apart from the fact that the mere dry reasoning by which the result would have been established would have had little interest to the general reader, the particular path which I have selected to follow has the advantage of introducing a number of independent relations, and of showing how the various matters dealt with bear upon each other and upon the general subject of the chapter. The general law connecting distance with the velocity which the Sun can control is as follows :— Let d represent the Earth's mean distance ; 11 her velocity at that distance in miles per second ; D any other dis- tance. Then the velocity which the Sun can control (so as to compel a body to travel in a closed orbit round him) at a distance d is ,U 86 THE SUN. than 378'9 miles per second; and this is the least velo- city with which a body must be projected from the Sun in order that it may never return to his globe again. Thus the Sun, which at the distance of Neptune can control a velocity of but 4*7 miles per second, can control close by his own surface a velocity eighty times as great. At a distance four times as great as Neptune's, a velocity of 2*4 miles per second would suffice to enable a body to pass away to an infinite distance from the Sun. But even this velocity is enormous and almost inconceivable. If we seek to know in what' regions the Sun could barely control velocities such as we are familiar with (by which I mean such velocities as the eye can appreciate, not velocities — as of cannon-balls or the like — which we speak of without appreciating), we shall find that such regions lie at enormous dis- tances. A body moving at the rate of our swiftest express trains (say 60 miles per hour) would be compelled to travel in a closed orbit around the Sun, unless its distance from him were no less than 220,507,500,000,000 miles, a distance some ten or twelve times exceeding that of the star Alpha Cen- tauri.* We have seen that the Sun can control the motions * At half this distance a planet travelling in a circle round the Sun would have a Telocity equal to that of an express train ; but if such a planet travelled in the plane of the ecliptic it would be unable to com- plete a circuit round the Sun on account of the disturbing influences of the star Alpha Centauri, which, being at only one-fifth of its distance from the Sun, would lie nearer to some parts of its circular orbit than the Sun does. THE SUN AS RULER. 87 of a body travelling with a velocity of less than 378*9 miles per second close by his surface in such sort as to compel that body to travel in a closed orbit around him, always coming close by him at each return. That is the maximum velocity a body can have under such circumstances. It is also the greatest velocity a body can acquire in approaching the Sun, under his attrac- tion alone, from an infinite distance and starting from rest. Now, the least velocity a body can have so as to travel close by the Sun is clearly that which would allow the body to travel in a complete circle around the Sun and close to his surface. We obtain this by simply reducing 378*9 in the proportion of 1,000 to 1,414 ; the required velocity is therefore 268 "0 miles per second.* Such are the limits between which the velocities of all bodies travelling around the Sun so as just to graze his surface must necessarily lie.f Nor can any mass reach * It may perhaps be necessary to point out that in a statement of this sort, 268-0 is not strictly the same as 268. The former implies that the velocity lies between 268'05 and 267'95 ; the latter would imply that the velocities lie between 268'5 and 2675. f It may be interesting to inquire how far from the Sun a body would reach if projected vertically with a velocity which would just enable a body projected horizontally to complete the circuit of the Sun. The problem is exceedingly easy. Such a body would move as if travelling on an orbit having a mean distance equal to the solar radius. Conceiving the Sun's mass all collected at its centre, the body would, after passing to its greatest distance, return to the centre, round which it would circle with (for a moment) an infinite velocity, and so return ' to its aphelion. The aphelion distance, therefore, from the centre must be equal to the Sun's diameter, and the greatest distance attained from the Sun's real surface must be equal to the solar radius, or more than 425,000 miles The rotation is hot here considered. It would not, however, affect the distance attained by the body, though it would affect the real path by which that distance would be attained. We can 88 THE SUN. him from without, whether gathered directly out of space by his attraction, or reaching him after passing in like manner determine the distance to which a hody projected with any other velocity would reach. For we can determine by means of the formula in the note at page 84, what the mean distance correspond- ing to this velocity at such a distance from the Sun would he, and so (as before) we can determine the greatest height attained by the projectile. But there is a simple and convenient formula for this purpose, thus : — Let v be the velocity of projection, g the accelerating force of gravity at the Sun's surface, E the Sun's radius, and h the height attained by the body; then v 2 = 2oh H . R T H If the reader prefer a formula in which the quantity g does not directly appear — so as to avoid the necessity of considering the value of I errestrial gravity (from which g is deduced) measured with reference to feet and seconds — we can easily get a convenient formula. Thus, we have v 2 = 2yq E + H and therefore from what has just been shown it follows that (2680) 2 = 25'K^?— (a mile being supposed to be the unit of length in the value of g and a second the unit of time). Therefore, dividing, we have v 2 = 2(268-0)2 H B + H v = 378-9 /- (very convenient for finding v when h is given) ; and since 2(268-0)' is equal to 143,648, we have, for finding H when v is given _ V 2 E H_ 1437648 -v 2 ' It follows, see p. 73, that if v and v be the velocities with which two bodies falling from heights h and h respectively, reach the surface of two globes having radii b and r, and mean densities A and 8, then e-^A V r + h _r /H(r + A)A rvT ' / k !•' */ A(k + h)5 V 7+~h THE SUN AS RULER. 89 through any number of forms of orbital motion due to the perturbing influences of the planets, with a velocity- exceeding 378*9 miles per second, unless such a mass had already had motion communicated to it before the Sun began to act upon it. In like manner, no body can reach any given distance from the Sun with a velocity greater than that deduced, according to the above-discussed considerations, as the limiting velocity the Sun can control, unless that body had started on its journey towards the Sun with a velocity communi- cated beforehand, and by the influence of other attract- ing orbs. So that if it has been demonstrated, as many believe, that some of the meteors which reach our atmosphere travel through it with a velocity greater than 25*9 milesper second, superadded to the velocity with which the Earth meets them (which, according to the direction of impact, may have any value up to 18"5 miles per second), then that body comes to us with an inter-sidereal velocity, so to speak, in addition to such velocity as the Sun has communicated. For example, if a meteor penetrate our atmosphere with a velocity of 50 miles per second, coming full tilt against the Earth when she is near perihelion, then 5*6 miles per second at least of that velocity was communicated to the meteor by other orbs than our Sun. It will naturally occur to the reader to inquire at Also if V = R /A. v r^f 5 These formulae are often useful. It may be mentioned, however, that with the assnmed units (a mile for length and a second for time), g= -169. 90 THE SUN. this point what is the least velocity with which a meteor can travel when at the Earth's distance from the Sun. For clearly if meteors moving with velo- cities exceeding the greatest which the Sun's attractive energies can give, afford information of an extra-solar force to which such meteors have been subjected, so also meteors moving more slowly than any which could circle around the Sun would have to be regarded as indicating the action of some other force than solar attraction. Now, if we suppose a body travelling in the Earth's orbit, and as fast as the Earth, to have its velocity increased, we have seen that its orbit must needs have its greater axis increased, and will become parabolic if the increase of velocity be great enough. In fact, an increase of velocity makes the point where the increase takes place the perihelion of the new orbit. And obviously a decrease of velocity will make the point where the decrease takes place the aphelion of the new orbit. Thus, suppose the body is at e (fig. 20) and Fig. 20. travelling along the Earth's orbit E e' about the Sun at s, when it suddenly receives an impulse onwards, then its new orbit will have such a shape as E e, e being THE SUN AS RULER. 91 the perihelion. But if the body is checked when at e, the new orbit will be of such a figure as E e', the aphelion being at e. And the question we have to determine is, how great the reduction of velocity can be in order that the revolution of the body in an orbit may not be prevented. Clearly there is no limit to the reduction except that resulting from the size of the Sun. If the body can but pass clear round the Sun's globe, it will return to the point e, and (neglect- ing perturbations or resistance as it passes through the Sun's atmosphere) it will continue to circle in this way for ever. We have to determine what velocity at E is necessary for this purpose ; and obviously, as a body moving more slowly at E would not pass clear of the Sun at s, so also, tracing its path back, it could not have arrived along a course clear of the Sun's globe ; and the only interpretation of its motions would be that it had been projected directly from the Sun's sur- face with sufficient velocity to reach the Earth. It is not difficult to calculate the least velocity with which a body could travel when at the Earth's distance on an orbit just grazing the Sun's surface.* This velocity is * There are many ways of attacking the problem. The best, perhaps, for our present purpose is the following : — It is obvious that if a body be projected tangentially to the Sun's surface with such Telocity as to just reach the Earth's orbit when at its greatest distance from the Sun, it would continue to revolve in such an orbit as we are inquiring into. Now, in order tu determine the velocity of projection requisite for such » result, we may apply the formula in the preceding note, remembering that if a body projected vertically upwards from the Sun would reach to a height H, a body projected horizontally with the same velocity would travel in an orbit having a mean distance equal to ^H-rii) where e is the Sun's radius; so that 92 THE SUN. about one-tenth of that with which the Earth moves in her orbit, or 1'85 miles per second: and no body can possibly reach the Earth with a smaller real velocity than this, unless actually projected from the Sun. A smaller relative velocity might very well be observed however. For example, a meteor travelling very little faster than the Earth might overtake her, and so seem to enter her atmosphere very slowly. And observa- we require our body to be projected tangentially with such velocity as would be required to project a body vertically to a height equal to the Earth's mean distance d (say) from the Sun's centre, since the required orbit is to have a mean distance equal to £(d + r). Hence, by the formula for v in the preceding note we have 2 »««. „> 2 91, 500,000 , . . , ., v 2 = 2(268-0) 9192 - uou (approximately); whence we find the required velocity of taDgential projection equal to 359 miles per second ; and we note in passing that such ». velocity as this would be required to project a body from the Sun to the Earth's distance. Now this is the perihelion velocity of our projectile when at a distance of 425,000 miles from the Sun's centre. In aphelion, its distance is 91,500,000 miles (approximately) ; hence from Kepler's first law its velocity there will be „.„ 425,000 ., , 3o9 '— — miles per second, 91,500,000 t or about 1"85 miles per second, which is almost exactly one-tenth of the Earth's velocity in her orbit, but considerably exceeds the Moon's, This velocity would, however, be somewhat increased by the Earth's attraction. It is to be noticed, that the Moon's velocity in her orbit being about Jths of a mile per second, the greatest velocity the Earth can control at the Moon's distance is but about Jjjths of a mile per second ; so that no body moving to the Earth's neighbourhood under the influence of the Sun's attraction can by any possibility be compelled by the Earth to travel in an orbit around her unless it comes much nearer than the Moon. The maximum velocity which the Earth can control close by her surface is, however, about seven miles per second ; so that bodies having the aphelion of their orbit nearly at the Earth's distance and the perihelion close to the Sun's surface, might, if they happened to come close by the Earth, be compelled to circuit in an orbit round her. THE SUN AS RULER. 93 tions seeming to indicate that the real velocity of a meteor was less than T85 miles per second, would need to be very strongly confirmed before they would be accepted by astronomers ; since to reach the Earth a body must be projected from the Sun with a velocity of about 360 miles per second. Here, having already passed the limits I had pro- posed to myself in dealing with the subject of this chapter, I draw it somewhat regretfully to a close. It would have been easy to extend the chapter so as to have occupied a volume twice the size of the present, and yet to have discussed no well-worn facts. The whole subject of the Sun's rule over the space sur- rounding him has remained in a sense almost untouched, until lately its significance began to be recognised by Mayer, Thomson, Waterston, and others. Even these, however, have dealt rather with the limits of activity possessed by bodies close to the Sun's surface than with the velocities which measure his influence at greater distances. For my own part, I find a wonderful interest in the ideas suggested by the Sun's activity throughout the whole range of his wide domain. The velocities to which I have referred in this chapter as those which the Sun can control are also those which he can generate. We have to think of him, therefore, as capable' of drawing towards himself all such cosmical matter as comes under his exclusive attraction either by leaving the domain of some other star, or on account of his own motion through space. In so drawing cosmical materials towards himself, he imparts to them 94 THE SUN. velocities such as we have been considering in the present chapter. The vaster the distances from which they come, the greater the velocities he imparts to them. As they sweep onward in their course they are subject to the influences of the planets — the patrols of the solar system — and under such disturbing in- fluences large numbers must be compelled to follow, either temporarily or permanently, true orbits (that is, closed curves) around the Sun. But the majority of such visitants, whether comets or meteors, must return to the sidereal depths after once paying their respects — in the full rush of their perihelion swoop — to the giant ruler of our system. It seems probable that in this continual rush of matter, this continual interchange of attendants on suns and stars, we may recognise the progress of processes exercising a most important in- fluence on the welfare of planetary systems. It is still more probable that the bodies which are finally drawn into the solar domain perform highly important functions in the economy of our own particular plane- tary scheme. But unless I mistake, the real significance of the considerations we have dealt with in the present chapter lies in their bearing on the past history of the solar system. The rush of matter which we now re- cognise affords perhaps but the faintest indication of the amazing conflicts in which our system had its birth. Tracing back the history of that system, we seem to recognise a time when the Sun's supremacy was still incomplete, when the planets struggled with him for the continually inrushing materials from which his sub- THE SUN AS RULER. 95 stance as well as theirs was to be recruited. We can see him by the mighty energy of his attraction clearing a wide space around him of all save such relatively liny orbs as Venus and the Earth, Mars, Mercury, and the asteroids. With more distant planets the struggle was less unequal. The masses which flowed in towards the centre of the scheme swept with comparatively slow motion past its . outer bounds, so that the subordinate centres there forming were able to grasp a goodly pro- portion of material to increase their own mass or to form subordinate systems around them. And so the giant planets, Jupiter and Saturn, Uranus and distant Neptune, grew to their present dimensions; and became records at once of the Sun's might as a ruler — for with- out his overruling attraction the material which formed these planets would never have approached the system — and of the richness of the chaos of matter from which his bulk and theirs were alike evolved. Nor is the consideration without a mysterious attraction, that in thus looking back at the past history of our system we have passed after all but a step towards that primal state whence the conflict of matter arose. We are looking into a vast abysm, and as we look we fancy we recognise strange movements, and signs as if the depths were shaping themselves into definite forms. But in truth those movements show only the vastness of the abysm, those depths speak to us of far mightier depths within which they are taking shape. ' Lo ! these are but a portion of His ways ; they utter but a whisper of His glory.' 96 THE SUN. CHAPTER III ANALYSING SUNLIGHT. The researches of telescopists have revealed many im- portant facts respecting the Sun's constitution. Studied thoughtfully, these researches enable us to answer many questions which at first sight seem to require more powerful modes of inquiry. But, undoubtedly, the science of solar physics is too vast and too difficult to be satisfactorily treated by telescopic research alone. The condition of the Sun is so different from that of any bodies we can experiment on, that his mere aspect - — and the telescope can show us nothing more — is in- sufficient to tell us what his constitution may be. The picture of the Sun presented by the most powerful telescope resembles a booh, full of meaning indeed, but written in an unknown language. The spectro- scope is the means by which that unknown language has been in part interpreted. Let us consider what spectroscopic analysis really is. It is scarcely possible to treat of any astronomical subject in the present day without describing the most powerful of all instruments of astronomical research ; but in the case of the Sun it would be hopeless to ANALYSING SUNLIGHT. 97 attempt such a course. Spectroscopic analysis forms the very basis of all our ideas respecting solar physics. We must thoroughly understand the mode in which it teaches, the meaning of its teaching, and the extent to which its teaching may be relied on, otherwise our views will be vague and unsatisfactory, depending rather on the statements of others, than on any clear apprehension of their truth on our own part. For this reason I shall spare no pains to make the explanation of spectroscopic analysis which follows as simple, and, at the same time, as complete as possible. There is, in reality, nothing difficult in the subject ; but it cannot be denied that, considering its simplicity, it is not nearly so well or so widely understood as it might be, — a circumstance the more to be regretted because the whole history of recent scientific re- searches is a sealed book to those who do not clearly recognise the nature of the instrument by which those researches have been effected. Newton was the first who showed that white light is a compound of light of many diiFerent colours.* He proved this by his investigation of an experiment of Grimaldi's — illustrated in fig. 21. Here a b represents the course of a pencil of solar lightf passing through * Grimaldi was, however, the first who discovered the effect of passing sun-light through a prism. (See his Physko-mathesis de luminc, prop. xxx. et seq.) f I have purposely modified Newton's figure, because many mis- understand a figure in which the pencil A b is shown with its proper divergency. They confuse the dispersing effect of the prism with the optical effects produced on a diverging pencil of pure light. It need hardly he said that it would be quite as great a mistake to neglect the H 98 THE SUN. a circular aperture in a screen s &'. The prism P is so placed as to intercept the light. It will be well to consider the prism as placed with the base E F upper- most and horizontal.* Now, if the prism were removed, the light would fall at i and make a small elliptical image there. And if the solar light were simple instead of being composed of rays of many different colours, it would follow such Fro. 21. a course as is indicated by the bent bright line, and form a small elliptical image at i' — this image being white like that at i, and resembling the latter image in shape, f effect of divergency altogether. In fact, many important optical con- siderations are associated with the divergency (however small this may be made) of the pencil analysed by the prism. But the two matters are best kept apart, at least in the case of the beginner. In the above description the object has been to direct the reader's special attention to the dispersive action of the prism, and therefore no attention is paid to the divergency of the pencil. * e 6 is supposed equal to G f, so that these lines are equally inclined to the vertical. f It would indeed be absolutely identical in shape with the image at i if the exact course indicated in the figure were followed; but if the ANALYSING SUNLIGHT. 99 But instead of this, Newton found, as Grimaldi had found before him, that a streak of light was formed as v r (v e is exaggerated in length), the streak being violet at the highest point and thence changing through indigo, blue, green, yellow, and orange, to red at the lowest point. Neither above nor below was the streak well defined, but passed gradually into dark- ness. At the sides, however, the streak was well defined, and in breadth equal to the horizontal breadth of the figure at i. It thus formed a rainbow-tinted streak or ribbon. It appears from this experiment that light consists of rays of all the colours of the rainbow, that the violet rays are the most bent by the action of a prism, the red rays least, the others in the order named above. This happens with prisms of all refracting angles and of whatever substance. Hence the rays forming the violet part of the spectrum are often called — without further description — the most refrangible rays, while the red rays are called in the same way the least re- frangible rays. This mode of speaking, and the ex- pressions arising from it, should be carefully noted. Now, the streak of light seen by Newton showed no breach of continuity. Newton appears to have sus- pected the possibility that by a change in the con- ditions of his experiment the streak would show gaps. In other words, he suspected that light of all degrees of single image fell at v or B this would not be the case. It need hardly be remarked that for pure light the course of the beam would depend on the refracting angle a of the prism. h 2 ioo the sun. refrangibility, between the light which forms the ex- treme violet and the extreme red of the streak, may not be present in the solar beam. But he did not succeed in proving this, though he employed apertures of different shape whereby to admit the light. It is clear that if there were simple violet light, and simple indigo light, and so on in the solar beam, a succession of small coloured images would be formed as shown in the figure at v, i, &c, and between these images dark spaces would be seen. Newton's experiments led him to the conclusion that an infinite number of images, shifting by indefinite gradations from v to E, exist along the streak, and so cause the colour to vary insensibly from violet to red as observed in his first experiment. "Wollaston was the first who succeeded in showing that there are gaps in the spectral streak. It is clear that the circular aperture in the above experiment is not suitable for determining whether rays of all degrees of refrangibility are included in a beam of solar light. In fig. 21 an image of the aper- ture is represented at v, another at I, another at B, and so on ; though of course the spectrum in Newton's experiment showed no such separate images. Now, it is perfectly obvious that if instead of the seven images represented in the figure there were twenty or thirty along the spectrum v B, there would be no means of knowing that the spectrum was made up of these twenty or thirty distinct images, for they would over- lap, and so show a continuous streak of light. Wollas- ton found that when, instead of a circular, triangular, ANALYSING SUNLIGHT. 101 or oblong aperture, a very narrow slit is employed, light of certain degrees of refrangibility is absent from the solar beam. He admitted the light through a narrow slit (parallel to the refracting angle of the prism). With this arrangement the spectrum seen by Wollaston was not continuous, but crossed by two dark lines parallel to the slit — or, in other words, at right angles to the length of the spectrum. These two lines — two gaps in the solar spectrum — proved that light of two definite orders of refrangibility is absent from the solar beam. Fig. 22 shows how Fig. 22. the light, after passing through the prism, had become divided into three parts, with spaces between them along which no light travelled. It is quite obvious that the existence of these gaps can be recognised without allowing a spectrum to be formed on a screen, as in Newton's experiment — simply by placing the eye as shown in fig. 22. It was in this way that Wollaston observed the two gaps. It may be remarked in passing ' that this mode of viewing the spectrum bears the same relation to Newton's plan that observation of the Sun with the naked eye bears to observation of the Sun's image received upon a screen. Dr. Wollaston did not pursue the inquiry further. Nor need we greatly wonder at this, if we rightly con- 102 THE SUN. sider the matter. We now know, indeed, that in the two dark spaces on the spectrum of Wollaston there lay the germ of the most wonderful discoveries man has yet made. We know that had he persisted in the inquiry his name would have been associated through all time, as that of Fraunhofer will undoubtedly be, with the very language of the new analysis. But it must be admitted that Wollaston had little reason for expecting any very remarkable results from the study of a peculiarity which seemed quite as likely to depend on the nature of the glass of which the prism was made as upon any inherent property of solar light. And even supposing that the gaps were due to some peculiarity of solar light, who could suspect that that peculiarity, when traced to its source, would be so full of meaning as to reveal to us the very constitution of the solar orb ? It is to the unwearying patience with which Fraun- hofer — like so many others of his countrymen — was willing to work day after day at what seemed a most unpromising subject, that the world owes the first complete recognition of the characteristic peculiarities of the solar spectrum. Wollaston had observed the spectrum directly, with the unaided eye. Fraunhofer improved on this plan by employing a telescope.* * It is well to notice that all the modes of viewing the Sun are avail- able for viewing the solar spectrum, A prismatic spectrum is simply a series of images of a luminous object formed by rays of different refrangibilities. Where the luminous object is a line of light — as where light is received through ~ fine slit — the solar spectrum is in reality made up of an infinite number of lines of light at right angles to its length. It is because light of certain definite orders of refrangibility is ANALYSING SUNLIGHT. 103 Dr. "Wollaston had seen but two gaps in the solar spectrum ; Fraunhofer, in 1814, saw and mapped no less than 576 lines. The positions of the chief lines, seen by Fraunhofer are indicated in fig. 23, and as reference is continually made to the lettered lines, it is well that the student should carefully study their sequence and position. A is a well-marked line close to the limits of visi- bility at the red end of the spectrum. B is a well- defined red line of sensible breadth.* Between a and n is a band of several lines called a. c is a dark and Fro. ->3 very well-marked line. Between B and c Fraunhofer counted nine fine lines ; between C and d about thirty. r> consists of two strong lines close together. Between r> and e Fraunhofer counted eighty-four lines. E is a band of several lines, the middle line of the set being stronger than the rest. At b are three strong lines, wanting that images of the line are wanting at certain definite parts of the spectrum, — that, in other words, dark lines are seen. The clear recognition of this fact will prevent much misapprehension. * It will be understood that I am here describing the lines as seen by Fraunhofer, and as they would appear with similar spectroscopic power to that employed by him. With greater power many single lines are resolved into several, and many new lines make their appearance. For example, D is described above as consisting of two strong lines with an extremely small interval. "With a powerful spectroscope nu- merous lines are seen between these two strong lines. 104 THE SUN - the two farthest from e being close together. Between E and b Fraunhofer counted twenty -four lines, and between b and p more than fifty. F, G, and H are strong lines. Between F and G, and between G and H, Fraunhofer counted 185 and 190 lines respectively; and he found many lines also between H and I — the violet end of the spectrum. Fig. 23 shows the colours of those parts of the spectrum in which the several lines occur. The reader will do well to bear in mind the position of the several lines, as thus, by an easily remembered relation, he will find himself enabled to interpret readily the accounts of spectroscopic researches, whether into astronomical or chemical subjects of inquiry. Let him remember, then, that a, b, and C are in the red portion of the spectrum ; d in the orange-yellow ; e in the yellowish-green ; p in the greenish-blue ; G in the indigo ; and H in the violet. Fraunhofer's contributions to the science of spectro- scopic analysis did not conclude, however, with the recognition and mapping of these lines. Having first convinced himself that the same lines were seen in the solar spectrum, of whatever substance the prism was formed, he proceeded to study the spectra formed by light from other sources. He first examined solar light received indirectly by reflection or otherwise — as from the clouds, the sky, the Moon or planets, and so on ; and he found in the spectrum of such light the same lines which he had seen in the spectrum of direct solar light. He ANALYSING SUNLIGHT. 105 studied .the spectrum of the Sun when that orb is near the horizon, and he found that under such circum- stances the violet end of the spectrum disappears and a number of lines make their appearance in the re- mainder of the spectrum. Fraunhofer found that the spectra of the fixed stars exhibit dark lines resembling those in. the Sun; but none of the stars whose light he examined had a spec- trum exactly the same as the Sun's. Some lines of the solar spectrum he found wanting in star spectra, while other lines were not seen with the same relative dis- tinctness as in the spectrum of the Sun. On the other hand, he found several new lines in star spectra. No two stars appeared to have the same spectrum. An important conclusion followed, as Fraunhofer pointed out, from this observation. If the dark lines in the solar spectrum were caused by an absorptive action exercised by our own atmosphere, it would follow that the same lines ought to be seen in the spectra of the fixed stars. The contrary being the case, Fraunhofer held it to be a demonstrated fact, that the dark lines are due to some property inherent in the light itself which the Sun and the fixed stars severally emit. One more observation of Fraunhofer's, and I pass on to later researches. He found that when the flame of a candle or lamp is the source of light, the spectrum is not crossed by dark lines, but a bright double line is seen in the exact place occupied by the double dark line d af the solar spectrum. To prevent misconception, it is necessary, however, to mention that light from an IC6 THE SUN. incandescent substance — and the flame of a candle is such light — exhibits a continuous spectrum. The double bright line seen by Fraunhofer was due to the presence of the almost ubiquitous element, sodium, in the flame, as will be explained further on. If we carefully weigh the results obtained by Fraun- hofer, it will appear that spectroscopic analysis owes very much to his researches. It may be questioned, indeed, whether, but for his patience and persever- ance, the attention of scientific men might not have been turned away, at least for many years, from a subject of inquiry which seemed when he began his labours to be rather curious than important. Let us next inquire into the spectra given by differ- ent terrestrial sources of light. An incandescent solid or fluid — or, to speak more correctly,* a solid or fluid glowing with intensity of heat- — gives a continuous spectrum. But the nature of the spectrum varies with the heat of the source. If a piece of metal, for instance, be gradually heated till it is at a white heat, only the red part of the spectrum will at first be visible ; then the orange part will show, * The term incandescent is not properly applicable to any source of light which is not actually white. Many spectroscopists indeed go farther, and say that no luminous body ought to be described as incan- descent unless its spectrum extends without dark lines or gaps of any sort from the extreme limit of visibility at the red end to the extreme limit of visibility at the violet end of the solar spectrum. Without insisting on this limitation, it certainly does seem well to point out that the term incandescent is not properly applicable to solids or fluids glowing with light belonging to the red end of the spectrum, nor to vapours glowing with light of a well-marked colour. ANAL TSING S UNL1 GHT. 1 7 then the yellow, and so on, until at length the whole range of the spectrum will be seen, from the extreme red to the extreme violet. But the question will at once suggest itself at this point, — "What are the limits of the spectrum ? When we were considering the solar spectrum, we might have inferred that the limits of visibility are the true limits of the space to which the solar action extends. But now that we have seen a spectrum growing with the growing heat of the source of light, we are naturally led to inquire whether there are limits to this growth. And, again, the spectrum of a gradually heated body begins at the red end so far as our vision is concerned ; but is that its true beginning ? To both these questions an answer has been given. It occurred to several physicists during the latter half of the last century to inquire whether the heat which undoubtedly accompanies the light forming the spec- trum corresponds in reality with the light, in so far that where the light is strongest the heat is strongest, and vice versa, while where light fails totally there heat also fails. They found that no such correspondence exists. So far as the visible spectrum is concerned, the greatest heat is not received where the spectrum is brightest, but at the red end. Sir W. Herschel, however, at the beginning of the present century, found that the maximum of heat comes from beyond the red end of the spectrum ; while, yet farther beyond the red end, heat continues to be- received for a distance whose extreme limit has not yet been determined. 108 THE SUN. Heat, however, is not received beyond the violet end of the spectrum, nor even completely up to the limits of visibility in that direction. Yet we now know that in this direction, also, the limits of visibility are not to be regarded as the limits of the solar action. Besides the heat and light which are transmitted through the prism, there is another form of action whose effects are as distinct from heat and light as heat and light are from each other. We know that the solar rays, besides illuminating and heating substances on which they fall, produce changes in the appearance and constitution of many substances. To take a familiar example : the solar rays falling on the skin not only warm it, and so affect the sense of touch ; not only illuminate it, and so affect the sense of sight ; but tan it, — an effect which is not directly cognisable by any sense that we possess,* though in- * It need hardly be said, perhaps, that the reference here to the senses is not introduced as bearing in any way on the subject of spec- troscopic analysis, but as affording a distinction of «■ popular kind be- tween the three forms of solar action. It is worthy of notice, however, that we have a sense by which the action of the longer light-waves corresponding to the red end and the parts beyond the red end of the spectrum is recognised by us, and another sense enabling us to recognise the action of the medium waves corresponding to the yellow part of the spectrum and in gradually diminishing degree the waves corresponding to parts up to the red end on one side and the violet end on the other side : but we have no sense enabling us to recognise directly the action of the shorter waves corresponding to parts of the spectrum beyond the violet end. Is it not conceivable that some creatures, even among terrestrial beings, may possess a sense enabling them to recognise the action of these short waves, and that such a sense may give them powers as distinct from the powers we possess in virtue of the senses of touch and of sight, as the sense of sight is distinct from the sense of touch? A man born blind may not be more incapable of conceiving the nature of the sense ANALYSING SUNLIGHT. 1 09 directly sensible both to sense and touch. The real nature of this action is not altogether understood (which may be remarked also respecting heat and light), but the observed result is a modification of the chemical condition of the substances acted upon. Now this particular mode of action — actinic action, as it has been called* — is not exerted most powerfully where the spectrum is brightest, but near the violet extremity — between the lines G and h. Nor is this mode of action limited to the visible spectrum; but precisely as heat falls beyond the red extremity of the Fig. 24. spectrum, so actinic effect is produced beyond the violet extremity. In fig. 24 the relation between the several kinds of action is exhibited by the curves marked H, L, and cA respectively, the height of these curves vertically above of sight and of the powers it confers -upon those who possess it, than those who have all the five senses are of the powers which may be actually possessed by creatures having organs suited to appreciate the action of the shorter light-waves. * From a Greek word signifying a ray. Surely the term is ill- chosen. 110 THE SUN. the spectrum indicating the relative intensity of the heat, light, and chemical activity of the portion of the spectrum immediately below. Thus, beyond the red end, the heat-curve is seen to acquire its greatest height, indicating that the maximum heating effect is exerted there. The light-curve reaches its greatest height above the part of the spectrum between D and E, — in the yellow portion, that is, of the spectrum. The heat-curve reaches the base-line close by G, while the light-curve extends somewhat beyond H. At E the actinic curve rises above the base-line, reaching its greatest height above G and H, and thence passing down to the base-line far to the right of the violet end. It appears, then, that from the extreme heat end of the spectrum to near A there is heat only ; from near A to about E we have heat and light combined ; from E to G, all three forms of action — light, heat, and actinism — are present.* From about G to a little beyond h, light and actinism (though very little of the former) are exerted, while from a little beyond H to the extreme actinic end of the spectrum there is acti- nism alone. It may be well, however, to warn the reader against the error sometimes made, of assuming that where two forms of action are present they are separable (or can be conceived to be separated) from each other. For example, it would be incorrect to say that near D a small proportion of heat rays, and a larger proportion * It is worthy of notice how small the amount of all three forms of action becomes near the f line of the spectrum. ANALYSING SUNLIGHT. Ill of light rays, fall on the spectrum. The correct state- ment is that those rays (or rather light-waves) which reach this part of the spectrum are capable of exciting heat to a certain extent, and light to a somewhat greater extent. We see, then, that in addition to what spectroscopic analysis has already taught us, we must add this interesting fact, that the three forms of action which sun-light is capable of exerting are associated with different parts of the spectrum — heat with the red end and parts beyond ; chemical action with the violet end and parts beyond that ; and, finally, light more parti- cularly with the yellow part of the spectrum, though of course, as the very term visible spectrum implies, more or less light is received from all parts of the coloured spectrum. And the intimate real association which exists between the three forms of action is shown by nothing more distinctly than by this, that when a solid or fluid body is gradually raised to a white heat, all the forms of action are generated : — first, heat ; then heat and light ; then heat, and light, and actinism — as the for- mation of the continuous spectrum of such bodies serves abundantly to prove. Only one solid substance, the earth erbia, gives a non- continuous spectrum when heated. So that it has come to be regarded, as a characteristic peculiarity of incandescent solids and fluids, that they present a rain- bow-tinted streak of light crossed by no dark lines or gaps. 112 THE SUN. With glowing vapours the case is altogether different. Although there are exceptions to the rule, it may be stated as a general characteristic of the spectra of such vapours that they consist of coloured lines or bands. Sir David Brewster, Sir John Herschel, and Talbot, were among the first who examined such spectra. In 1822 Sir John Herschel called attention to the im- portance of the study of the lines and bands seen in the spectra of the vapours of different elements. ' The pure earths,' he said, ' when violently heated, yield from their surfaces lights of extraordinary splendour, ■which when examined by prismatic analysis are found to possess the peculiar definite rays in excess which characterise the tints of the flames coloured by them ; so that there can be no doubt that these tints arise from the molecules of the colouring matter reduced to vapour, and held in a state of violent ignition.' It would be interesting to trace the history of those laborious researches by which men of science — Herschel, Brewster, Tyndall, the Millers, Huggins, Gladstone, Frankland, Pliicker, Hittorf, and many other — have determined the spectra of gases and vapours by various methods and under various conditions. But as in this treatise I feel bound to deal only with those parts of the history of spectroscopic analysis which are associated with the study of the Sun, or which serve to elucidate solar physics, I must here content myself with indicating results without describing the processes by which they were obtained, or assigning to each of the eminent men above-named, and to their fellow- ANALYSING SUNLIGHT. 113 workers their exact share in the noble series of labours referred to. At an early stage of the inquiry, though at the time the phenomenon was not correctly interpreted, a means was found of obtaining the spectra of the vapours of elements which cannot be vaporised by ordinary methods. It was noticed that the electric spark has a spectrum consisting of bright lines. But, unlike all the sources of light whose spectra have hitherto been considered, the electric spark yields a variable spectrum. "When the electric discharge takes place between conductors of the same nature, and through any given and unchanged medium, the same spectrum is always seen ; but when either condition is departed from, a different spectrum is obtained. It was presently recognised that the spectrum ob- tained from the electric spark is twofold in its nature. It includes the spectrum of the gas or vapour through which the discharge takes place, and also the spectrum of the vaporised substance of the conductors. Here, then, was a ready means of determining the spectra of the metallic and other elements * not easily volatilised in other ways, as also of such gases as nitrcn gen and oxygen; while conditions of pressure, tem- perature and the like, could be introduced, which would be wholly impracticable if the method of vapor- ising elements in the flame of an ordinary lamp had alone been available. * It seems unnecessary to speak of the spectra of the vapours of such, and such elements, since in reality it is only as vapours that iron, sodium, and the rest have characteristic spectra at all. I 114 THE SUN. In this way a large number of elements bad their spectra assigned to them, while in several instances physicists began to recognise peculiarities in the spectrum of the same element when examined under different conditions. But an important series of researches yet remains to be considered. Hitherto we have dealt with the actual spectrum of luminous objects; we have now to inquire into the effects produced on these spectra, when the light which forms them is allowed to pass through absorbing media. We owe chiefly to Sir David Brew- ster the initiation of this branch of research, — though in this, as in so many other departments of spectro- scopic inquiry, we find a host of distinguished phy- sicists joining in the work. Brewster found that when ordinary solar light is transmitted through the thick vapour of nitrous gas, a number of new dark lines are seen, parallel to the Fraunhofer lines, and congregated in a remarkable degree towards the violet end of the spectrum. He further proved that these lines were seen whatever the source of light might be. Professors Miller and Daniel made further researches into the effects of vapours in causing dark lines to appear in the spectrum of solar or white light. Some of Professor Miller's results are worth quoting, because they show how closely a physicist may approach a great discovery without actually effecting it. f First,' he says, ' colourless gases in no case give additional lines, or lines differing from those of Fraunhofer. Secondly : the mere presence of colour is not a secu- ANALYSING SUNLIGHT. 1 15 rity that new lines will be produced ; for instance, of two vapours undistinguishable by the eye, one, bromine, gives a great number of new lines, while the other, chloride of tungsten, exhibits none. Thirdly : the posi- tion of the new lines has no connection with the colour of the gas ; with green perchloride of manganese, the new lines abound in the green of the spectrum ; with red nitrous acid they increase in number and density as we approach the spectrum's blue extremity.' Some of these results, rightly understood, contain the germ of the great discovery afterwards effected by Kirchhoff; since in some of the cases actually experimented on by Professor Miller, the absence of new lines meant simply that the absorption lines, corresponding to the element he was dealing with, were coincident with some of the Fraunhofer lines. Others, however, approached the solution of the great problem even more nearly, since they actually touched on the principle of the reversal of the spectral lines, which affords the explanation of the coincidences detected but unnoticed by Professor Miller. ' None of these distinguished men,' says Pro- fessor Tyndall, 'betrayed the least knowledge of the connection between the bright bands of the metals and the dark lines of the solar spectrum. The man who came nearest to the philosophy of the subject was Angstrom. In a paper translated from Poggendorff's " Annalen " by myself, and published in the "Philoso- phical Magazine " for 1855, he indicates that the rays which a body receives are precisely those which it can emit when rendered luminous. In another place he 1 2 Il6 THE SUN. speaks of one of his spectra giving the general im- pression of reversal of the solar spectrum. Foucault, Stokes, Thomson, and Stewart, have all been very- close to the discovery ; and for my own part the ex- amination of the radiation and absorption of heat by gases and vapours would have led me in 1859 to the law on which all Kirchhoff's speculations are founded, had not an accident withdrawn me from the investigation.' At the very moment, however, when the great secret was about to be revealed, an eminent physicist wrote thus : ' In quitting the mere phenomena of luminous spectra, and rising to the inquiry as to their causes, we enter a more arduous course. The phenomena defying, as we have seen, all attempts hitherto to reduce them within empirical laws, no complete ex- planation or theory of them is possible. All that theory can be expected to do is this — it may explain how dark lines of any sort may arise within the spectrum.' But theory succeeded in doing very much more than was thus anticipated from her. It had been noticed by Fraunhofef that the two orange-coloured lines close together which form the spectrum of the glowing vapour of sodium, coincide exactly in position with two dark lines in the orange- coloured part of the solar spectrum — the double D line in fig. 23. Kirchhoff, having a spectroscope of great dispersive power,* determined to inquire whether this * Its prismatic battery contained four flint-glass prisms, not fixed or clamped in any -way, but standing freely on four little pedestals. The ANALYSING SUNLIGHT. 117 coincidence was exact. ' In order to test in the most direct manner possible,' he says, ' the frequently as- serted fact of the coincidence of the sodium lines with the lines D, I obtained a tolerably bright solar spectrum, and brought a flame coloured by sodium vapour in front of the slit. I then saw the dark lines D change into bright ones. The flame of a Bunsen's lamp threw the bright sodium lirfts upon the solar spectrum with un- expected brilliancy. In order to find out the extent to which the intensity of the solar spectrum could be increased without impairing the distinctness of the sodium lines, I allowed the full sunlight to shine through the sodium flame, and, to my astonishment, I saw that the dark lines D .appeared with an extraor- dinary degree of clearness.' Let the full force of this result be recognised before we proceed to consider the method by which Kirchhoff certified himself as to its exactness. He had shining in through the slit two kinds of light — sunlight and the light of the sodium flame. The sunlight alone would have given the ordinary solar spectrum with the r> lines of a certain degree of darkness. The sodium flame alone would have given two bright lines just where the dark D lines of the solar spectrum appear. What he expected was, naturally, that the bright lines whole arrangement seems singularly ineffective by comparison with the spectroscopes now in use ; for example, if we compare Kirchhoff' s battery with the battery of prisms in the fine spectroscope made by Browning for Mr. Gassiot, the former seems almost ridiculous in its imperfectness. 'A cough or a sneeze,' it has been well remarked, 'would set that whole battery in disarray, with which, nevertheless, Kirchhoff solved the secret of the solar spectrum.' Il8 THE SUN. of the sodium spectrum would at least partially reduce the darkness of the corresponding D lines, even if they did not altogether cause these dark lines to disappear, or to be replaced by bright lines. But these lines actually appeared darker. It was precisely as though an experimenter were to cast a beam of light exactly upon a shadow, and to see the shadow actually inten- sified instead of the reverse. * Kirchhoff proceeded to test this astonishing result. ' I exchanged the sunlight for the Drummond's or oxy-hydrogen lime-light, which, like that of all incan- descent solid or liquid bodies, gives a spectrum con- taining no dark lines. When this light was allowed to fall through a suitable flame coloured by common salt, dark lines were seen in the spectrum in the position of the sodium lines. The same phenomenon was observed if, instead of the incandescent lime, a platinum wire was used, which, being heated in a flame, was brought to a temperature near its melting point by passing an electric current through it.' These experiments were, if possible, even more striking than the former ; for now Kirchhoff had two lights which seemed to produce darkness. The electric light alone covered with its continuous spectrum the place where the sodium lines appear in the solar spec- trum ; the sodium light alone lit up this very part of the spectrum. When the two lights were both shining one would have expected the result to be an increased brightness in this part ; instead of which there actually resulted relative darkness. ANALYSING SUNLIGHT. 1 19 The observed fact is in itself important : its inter- pretation involves one of the most important facts ever discovered by man. It is well to distinguish between the two. The observed fact is that the sodium flame, which emits rays of a certain order of refrangibility — that is, rays which, if passed through a prism, will follow a certain path — has the power of absorbing rays of pre- cisely the same order. The interpretation of the fact is founded on the existence of a law which is thus worded by Professor Koscoe — ' Every substance which emits at a given temperature certain kinds of light, must possess the power at that same temperature of absorbing the same kinds of light.' * * This is merely a corollary from a more general law, according to which the same relation holds between the powers which substances possess of emitting and absorbing heat-waves as well as light-wares and actinic-wares. The law called the theory of exchanges was first enunciated for heat by Prevost of Geneva, and has since been established for heat and light by the researches of Prevostaye, Dessains, Stewart, and Kirchhoff. So far as the application of the theory to light is con- cerned, it must be admitted that there are still many difficulties in the way of its complete acceptance. These difficulties somewhat importantly affect our conclusions where we are considering the application of the theory in 'mode and measure to spectroscopic researches, though, so far as the general results here chiefly considered are concerned, they need not greatly trouble us. I may refer my reader to Dr. Stewart's Elementary Treatise on Heat for a very interesting u examination of the subject and the demonstration of the fundamental principles involved in the law. But there is one point in the course of Dr. Stewart's reasoning which involves an error in an optical subject of considerable importance. It is necessary for the demonstration that the course of rays (light-rays or heat-rays) not fall- ing quite perpendicularly, or rather not strictly parallel to each other, should be considered, and Dr. Stewart removes the difficulty for small beams of light or heat, by considering the size of the source of light. He says, ' Just as a line is in reality always part of the boundary of a 120 THE SUN. Kirchhoff experimented on other elements. He found that a flame coloured by potassium causes dark lines to appear on the continuous spectrum of the lime- light, and that these lines appear precisely where bright lines are seen when the spectrum of that coloured flame is viewed alone. The same was proved by Kirchhoff and Bunsen for the lines of lithium, calcium, strontium, and barium ; while it has been shown to hold for other elements by Dr. Miller and others. But now that the general law was established, im- portant results respecting solar physics were established along with it. Since Kirchhoff had proved that when the electric light shines through a sodium flame, the sodium solid, so a ray is always in reality part of the boundary of a beam or pencil of light. We may satisfy ourselves that this is the case in nature by considering the light which reaches the eye from a star or other ob- ject apparently very small ; this would seem to be the nearest approach to a geometrical line of light, whereas since a star has a certain real, though very minute, angular diameter, the light from it is in reality a converging pencil, although no doubt the angle of convergence is very small.' On account of the importance of the optical considerations in question, I may be permitted to correct what is undoubtedly an erro- neous statement. The difficulty must be got over by considering the size of the object on which light or heat rays fall, not by considering the size of the source of light. There is no such thing in nature as a converging pencil of light proceeding directly from a luminous object. All real pencils, whether of light or of heat, are originally diverging ; they diverge from every point of the self-luminous or heat-giving body, and the angle of their divergence is real — however minute it may be — so long as the object which receives light or heat has real dimensions, however minute. This consideration does not affect the conclusions deduced by Dr. Stewart, since undoubtedly the diverging pencils of light or heat, from a source of considerable dimensions, form a con- verging beam, when we consider them with reference to a smaller object on which they fall, ANALYSING SUNLIGHT. 121 lines appear as dark lines across the continuous spec- trum of the electric light, what was more obvious than the conclusion that sunlight also, which shows these same dark lines, must, before reaching us, have passed through the vapour of sodium ? Either then, in our own atmosphere, or in the atmosphere of the Sun, this familiar metal sodium, the basis of such commonplace substances as salt and soda, must exist in quantities sufficient to cause the observed absorption-lines. But more than this. Those countless other lines which cross the solar spectrum must each indicate some process of absorption exerted by vapours in our atmosphere or his. Since the presence of sodium has thus been demonstrated, why may not the presence of other elements be in like manner rendered apparent ? Kirchhoff did not long delay the inquiry thus sug- gested. He compared the spectrum of iron (or, to use the accepted mode of expression, the spectrum of the luminous vapour of iron) with the solar spectrum. Now, the spectrum of iron as known to Kirchhoff contained some sixty bright lines.* There was a great difference, then, between this spectrum and that of sodium. Yet, to his astonishment, Kirchhoff found that precisely as the two sodium lines agree with the D lines of the sodium spectrum, so every one of the iron lines had its counterpart in the solar spectrum. f Line for line, and not only so, but strong * Many more hare since been discovered ; in fact, there are now more than 450 recognised iron lines. f It is hardly necessary to note, perhaps, that Kirchhoff could not 122 THE SUN. line for strong line, and faint line for faint line, every line of the iron spectrum appears as a dark line in the spectrum of the Sun ! Kirchhoff in all probability never questioned for a moment the conclusiveness of this relation. He must have felt its significance intuitively. It was probably, therefore, only to satisfy others that he presented a strong argument from the theory of probabilities in favour of the view that the observed relation implied a real association between the two sets of lines. Taking as ^ the chance that a bright line in the iron spectrum would seem to have a counterpart in the richly ' lined ' solar spectrum if accident were alone in question, he calculates that the chance of the observed relation (leaving altogether out of question the relative strength of the lines) is less than 1,000,000,000,000,000,000* ' Hence,' remarks Kirchhoff, ' this coincidence must establish this result in the same way as the former, because he could not cause sunlight to shine through the glowing vapour of iron. He employed a method quite as satisfactory however, causing the light from iron vaporised by the electric spark to form a spectrum side by side with the solar spectrum, the solar light being admitted directly to the battery of prisms ; that of the iron being admitted after reflection through a small prism near the slit. * To this I may add that the chance of the observed relation, now that 450 lines of iron are recognised, is less than a fraction whose numerator is unity, and whose denominator consists of no less than 136 figures (the first four being 2907). This chance is not very unequal to that which I have shown to correspond to the probability that one of the less marked peculiarities of stellar arrangement detected by me while constructing my large star-atlas, is due to mere chance-distribution. ANALYSING SUNLIGHT. 123 be produced by some cause, and a cause can be assigned which affords a perfect explanation of the phenomenon. The observed phenomenon may be explained by the supposition that the rays of light which form the solar spectrum have passed through the vapour of iron, and have thus suffered the absorption which the vapour of iron must exert. As this is the only assignable cause of the coincidence, the supposition appears to be a necessary one. These iron vapours might be contained either in the atmosphere of the Sun or in that of the Earth. But it is not easy to understand how our atmosphere can contain such a quantity of iron vapour as would produce the very distinct absorption-lines which we see in the solar spectrum, and this supposi- tion is rendered still less probable by the fact that these lines do not appreciably alter when the Sun approaches the horizon. It does not, on the other hand, seem at all unlikely, owing to the high temperature which we must suppose the Sun's atmosphere to possess, that such vapours should be present in it. Hence the observations of the solar spectrum appear to me to prove the presence of iron vapour in the solar atmosphere with as great a degree of certainty as we can attain in any question of natural science.' Thus cautiously did Kirchhoff proceed in establish- ing the great principle on which spectroscopic re- searches into solar physics were to depend. Nothing can perhaps be required to strengthen the confidence of the reader in the justice of this principle. Yet it may be added that the effect of our own atmosphere on the 124 THE SUN. solar spectrum is now thoroughly recognised and found to correspond with the density of the layers through which the Sun's rays penetrate at different hours of the day. The reader may further be reminded of what has been already said respecting the evidence afforded by the character of stellar spectra. All those spectra would undoubtedly be similar to each other and to the solar spectrum if our atmosphere contained the source of all the spectral dark lines. It is to be noted also that while the coincidences in the case of iron serve to confirm any evidence we may derive from other coin- cidences, they are also confirmed by such evidence ; because as one after another the terrestrial elements are found to have spectral lines corresponding with the solar dark lines, the general fact becomes more and more firmly established that the Sun is constituted of those elements with which we are familiar. Kirchhoff found that calcium, magnesium, and chro- mium exist in the solar atmosphere. The presence of nickel, also, and cobalt was indicated by the agreement of the most conspicuous of their lines with solar dark lines. All the lines of these metals, however, could not be recognised, nor were the coincidences in the case of cobalt sufficient to satisfy Kirchhoff. ' I con- sider myself entitled,' he says, ' to conclude that nickel is visible in the solar atmosphere ; but I do not yet ex- press an opinion as to the presence of cobalt. Barium, copper, and zinc,' he proceeds, ' appear to be present in the solar atmosphere, but only in small quantities ; the brightest of the lines of these metals correspond to ANALYSING SUNLIGHT. 1 25 distinct lines in the solar spectrum, but the weaker lines are not noticeable. The remaining metals which I have examined, viz. gold, silver, mercury, aluminium, cadmium, tin, lead, antimony, arsenic, strontium, and lithium, are, according to my observations, not visible in the solar atmosphere.' I have thus far quoted Kirchhoff, not because the account of his investigations exhibits the actual state of our knowledge at the present time, but because they are so associated with the discovery of the great prin- ciple on which spectroscopic analysis depends as to have an interest wholly distinct from that — great as it is — which they derive from their intrinsic importance. It would occupy much more space than is here at my disposal to exhibit the progress of research by which our knowledge of the solar spectrum has reached its present position. For the full history of the subject I would refer my readers to Dr. Schellen's admirable ' Spectralanalyse,' * and to the excellent treatise on spectroscopic analysis by Professor Roscoe. So far as my purpose in this place is concerned, it is only neces- sary for me to sum up the results of the search for coincidences between solar dark lines and the bright lines of terrestrial elements — in other words, to exhibit the elements which exist, so far as is yet known, in the * A translation of the second edition of this work by the daughters of Mr. Lassell, F.R.A.S., and edited by Dr. Hnggins, the great English master of the subject, published by Messrs. Longmans, contains many important additions. It is a work of extreme value, which no one should 'be without who wishes thoroughly to master the subject of spectrum analysis. 126 THE SUN. substance of the great central luminary of our system. The following table, drawn up by M. Angstrom, exhibits the number of lines belonging to the several elements enumerated, which have been found to cor- respond with dark lines of the solar spectrum : — Hydrogen . 4 Manganese 57 Sodium 9 Chromium 18 Barium 11 Cobalt . 19 Calcium . 75 Nickel 33 Magnesium . 4 + (3?) Zinc 2 Aluminium 2? Copper . 7 Iron . 450 Titanium . 200 It will be noticed that the solar spectrum shows no traces of the existence of the nobler metals, gold and silver, nor of the heavy metals, platinum, lead, and mercury. On the other hand, it is significant that the lines of nitrogen and oxygen are absent, though these gases can scarcely be supposed to be actually wanting. We must remember, in forming an opinion as to the absence of these elements (as also of such elements as carbon, boron, silicon, and sulphur), that while the presence of certain lines in the solar system may prove abundantly that the terrestrial element which has corresponding bright lines, exists in the Sun's substance, it by no means follows with equal certainty that because all the lines of an element are wanting in the solar spectrum, therefore the Sun does not contain those elements. This will appear when we consider the various circumstances which may cause an element really existing in the Sun's substance to afford no trace of its presence. In the first place, the vapour of ANALYSING SUNLIGHT. 127 that element may he of a density causing it to lie always at a very low level, and therefore perhaps altogether beneath that level whence proceeds the white light of the Sun — that is, the light which gives the continuous spectrum across which the dark lines lie. Or again, the element may exist in the Sun's substance at such a temperature, or at such a pressure, as to produce — not well defined absorption lines, but — only broad faint bands, which no optical means we possess can render sensible as such. Or, again, the element may be in a condition enabling it to radiate as much light as it absorbs, or else very little more or very little less — so that it either wholly obliterates all signs of its existence, whether in the form of dark lines or bright lines, or else gives lines so little brighter or darker than the surrounding parts of the spectrum that we can detect no trace of their ex- istence. In these, and in yet other ways, elements may really exist (or rather undoubtedly do exist in the Sun) of whose presence we can obtain no trace whatever. In other chapters of this work the evidence afforded by the spectroscope respecting the condition of various parts or appendages of the Sun, as the spots, faculte, pores, prominences, corona, and so on, will be con- sidered at length, as also the various theories to which such evidence has given rise. I conclude this chapter by enunciating the general rules of spectral analysis, and by explaining the rationale of certain recent applications of the spectroscope which have deservedly 128 THE SUN. attracted great interest, but are not perhaps very generally understood. The general principles of spectroscopic analysis are as follows : — 1. An incandescent solid or liquid gives a con- tinuous spectrum. 2. A glowing vapour gives a spectrum of bright lines, each vapour having its own set of bright lines, so that from the appearance of a bright line spectrum we can infer the nature of the vapour or vapours whose light forms the spectrum. 3. An incandescent solid or liquid shining through absorbent vapours gives a rainbow-tinted spectrum crossed by dark lines, these dark lines having the same position as the bright lines belonging to the spectra of the vapours. 4. Light reflected from any opaque body gives the same spectrum as it would have given before reflec- tion. 5. But if the opaque body be surrounded by vapours, the dark lines corresponding to these vapours appear in the spectrum with a distinctness proportioned to the extent to which the light has penetrated these vapours before being reflected. 6. If the reflecting body is itself luminous, the spec- trum belonging to it is superadded to the spectrum belonging to the reflected light. 7. Glowing vapours surrounding an incandescent body will cause bright lines or dark lines to appear in the spectrum according as they are at a higher or lower ANALYSING SUNLIGHT. 129 temperature than the body ; if they are at the same temperature, they will emit just so much light as to compensate for that which they absorb, in which case there will remain no trace of their presence. 8. Theelectric spark presents a bright-line spectrum, compounded of the spectra belonging to those vapours between which and of those through which the dis- charge takes place. According to the nature of these vapours, and of the discharge itself, the relative inten- sity of the component parts of the spectrum will vary. It will be seen, as we proceed, that all these prin- ciples bear more or less directly on the application of spectroscopic analysis to the interpretation of solar phenomena. In all branches of spectroscopic research certain dif- ficulties have to be specially dealt with, while certain circumstances avail to help the observer. We are now to consider, — first, the means which are available to the astronomer for the special purpose of advancing our knowledge of solar physics ; and, secondly, the peculiar difficulties which he has to overcome. In the experiment illustrated in fig. 21 we see the action of a single prism on the solar light. But the time has long since passed when the action of a single prism could teach us anything new respecting the Sun. The observer has to work nowadays with a battery of prisms. A portion of light which has been already dis- persed by one prism is received on another, and is thus yet more dispersed; a portion of this doubly dispersed light falls again on another prism and emerges yet more IT j 30 THE SUN. dispersed ; a portion of this trebly dispersed light falls on a fourth prism, and so on— the number of prisms actually employed depending on the special branch of inquiry which is being pursued, for some require more dispersive power than is necessary for others. Space compels me to limit my description specially to the action of the prismatic battery, and I therefore neither describe nor illustrate the means adopted for causin"- a suitably-shaped beam of solar light to fall on Fig. 25. the first face of the first prism. 1 therefore simply state the fact that, in the study of the solar spectrum, a beam whose cross section is as S s' (fig. 25), the shape of the slit through which light is admitted, falls, as shown in fig. 25, upon the first prism of the battery, or prism 1 in the figure. If a screen were placed to intercept this beam anywhere between s s' and prism 1, there would be seen upon the screen a bright bar of light shaped like S s'. Prism 1 disperses this beam in the manner already described, and the beam falls thus dispersed on ANALYSING SUNLIGHT. 131 prism 2. If a screen were placed to intercept the beam anywhere between prism 1 and prism 2, a short solar spectrum would be seen on it, the violet end lying towards the bases of the prisms ; and, assuming the battery of prisms to lie on a horizontal plane, the length of the spectrum — that is, its extension measured from red to violet — would be horizontal. Then this beam passes to 3 ; and if a screen were placed between 2 and 3 a somewhat longer spectrum would be seen. Between 3 and 4 the spectrum would be still longer. And, lastly, a screen placed beyond the last prism, as a b, would show the solar spectrum much longer and pro- p6rtionately fainter than in Newton's experiment. And although no such screen is used by spectroscopists, who receive the emergent rays into a telescope with which they examine the spectrum, it will be convenient for our purpose to refer at present to a spectrum supposed to be received on a screen, as in fig. 25. Now, an. a b 1 have shown a violet image of the slit v, an indigo one 1, and so on, to a red image at E. But there are an infinite number of images ranged from the extreme violet end to the extreme red end. In places, however, no images are formed. The spaces thus left without light are the dark lines. It is not difficult, then, to see on what conditions the visibility of the dark lines will necessarily depend. If we could have a slit which was a true mathematical line, every dark space would be present in the screen, even though the dispersive power were small. But, as a matter of fact, the slit has a definite breadth, however x 2 132 THE SUN. narrow we may make it. Now, suppose that arc (fig. 26) represents a small part of the solar spectrum as shown on the screen in fig. 25, hut that the true nature of this part of the solar spectrum * is shown in the narrow hand a b c, so that in reality sunlight has no rays whose refrangibility corresponds to the band at b. Suppose, further, that the aperture of the slit is equal in width to this band b. Then the light corre- sponding to the extreme limits of the light in a b c, will form the two images of the slit shown at B ; these images will meet, and no absolutely black line will be Tig. 26. I Pig. 27. A B iB' C seen. There will, however, be a dusky band, darkest down the middle, and twice as broad as the true band b. But now consider the effects of an increase of dis- persive power. Say we double the length of the spec- trum, or rather of the particular part shown in figs. 26 and 27. Then the parts a b c of the real solar spectrum will all be doubled in length, and in the observed solar spectrum the light corresponding to the extreme limits of the band b will produce the two images of the slit * The true nature of the solar spectrum may be regarded as that corresponding to the imaginary case of a slit, which should be a true mathematical line, and of a battery of prisms which should cause no optical faults -whatever, insomuch that the image of the slit for rays of any order of refrangibility would also be a mathematical line. ANALYSING SUNLIGHT. 1 33 shown at B and b', but these will he no wider than be- fore, and will be separated by a really black band, half as wide as b. This band will be bordered by a penum- bral fringe whose boundaries are indicated by the dotted lines, the whole breadth from dotted line to dotted line being half as great again as that of the band b. The reader will, therefore, at once see the importance of increasing the dispersive power of our battery of prisms ; since in this way very fine lines which might otherwise escape detection can be rendered visible. Also, it is obvious that two lines very close together would be shown as one with a certain amount of dis- persive power, while with more dispersive power they would be clearly separated. Yet once more, a line in the soJar spectrum which seemed to coincide, without being really coincident, with the bright line of some metallic spectrum (brought into comparison with the Sun's in the manner referred to above) might, by in- crease of dispersive power, be removed appreciably from its supposed counterpart. So far, then, as the direct examination of the solar spectrum is concerned, we must aim specially at the means of increasing dispersion. It will be seen, also, further on, that in two highly important applications of the ana'ysis — viz. to the examination of the promi- nences, and to the recognition of motions of approach or recess due to solar cyclones — great dispersion is the chief point to be secured by the spectroscopist. But fig. 24 shows that certain difficulties have to be encountered in securing great dispersion. Only the 13+ THE SUN. part of the spectrum near G has been formed (in the case illustrated in that figure) by rays which have gone sym- metrically through the battery of prisms. The rays forming the part near v have gone through the middle of prism 1, and come out near the base of prism 4 ; while those forming the part K passing also through the middle of prism 1, come out near the vertex of prism 4. Now, optical considerations render it essential, or at least ex- tremely important as far as the clear definition of the lines is concerned, that each part of the spectrum should be formed by rays which have gone symmetrically round such a battery as is shown in fig. 24.* Furthermore, when the dispersion is very great only a portion of the spectrum will be formed at all (some at either extre- mity falling outside the last prism — beyond apex and base) when the prisms occupy a fixed position. Then, again, when the light has been bent round a nearly * The point to be secured is that the rays forming any part of the spectrum under examination should pass into and out of each prism at equal angles (as the light forming the part g of the spectrum in fig. 25 does). It may be shown that in this case those special rays pass with least possible deviation through each prism, so that the condition is generally called that of minimum deviation. But minimum deviation per se has no advantages, and, as a matter of fact, the real condition secured by this arrangement is that the primary and secon- dary foci of emergent pencils are as nearly as possible coincident ; so that, though the image of the slit formed by those special rays is not formed by absolute points of light, it is formed by circles (technically called ' circles of least confusion ') having the smallest possible diameter. In a paper in the Monthly Notices of the Astronomical Society, vol. xxx., I have shown that the circle of least confusion has in this case a defi- nite, though exceedingly minute diameter, even in the case of a single prism. The mathematical expression for the radius of this circle is given in that paper, but is somewhat too complex to be repeated with advantage in these pages. ANAL YS1NG ' S UNLIOHT. 1 35 Complete circle of prisms, as in fig. 28, the emergent light e e' will be intercepted by the first prism of the battery; and this circumstance limits the dispersion which can be given in this manner. Let us consider how these difficulties have been or may be encountered. We owe to Mr. Browning * the invention of a most ingenious plan by which, whatever part of the solar spectrum is studied, the battery of prisms will be Fig. 28. properly adjusted. In his automatic spectroscope he attaches to each prism a slotted bar, as shown in fig. 29. All the slots pass over a central pivot, and the prisms are attached at their angles as shown. Hence they must necessarily be at all times symmetrioally placed ; so that if 1 be fixed and motion be communi- cated to 4, then 2 and 3 will move, each in its proper degree, and all four will preserve their proj er relative positions. In Browning's, spectroscope there are six * Rutherfurd, the eminent American astronomer, had earlier devised and used an instrument on a somewhat similar.plan ; but Mr. Browning's new contrivance was independently devised, and in one important respect ■was an improvement on Rutherfurd's. 136 THE SUN. such prisms, and the light emerging from the sixth passes just clear of 1. I have suggested a modification, by which only a Fig. 29. corner of the first prism is fixed, as at fig. 30, and this prism is automatically adjusted like the rest, — a fixed slot s s' square to the path of the incident light guid- ing the motion of this prism according to the required Ptr. so. law. In like manner I have suggested that the view- ing telescope should be guided on the same principle of automatic motion.* * This arrangement must be regarded as only one out of several modifications of which Mr. Browning's ingenious plan admits. It is doubtless this modification which alone satisfies the conditions aimed at ; and this circumstance renders it all the more evident that it should be regarded as implicitly contained in Mr. Browning's plan. Kothing is easier than to devise modifications, and even (as in this instance) im- provements on the plaDS of others. But no separate merit can fairly be ANALYSING SUNLIGHT. 137 So far as clearness of definition and the satisfactory study of the whole length of the spectrum are concerned, this arrangement leaves nothing to be desired. But for a yet greater increase of the spectrum's length more is needed. We have reached the limits of dispersion which one circular battery can give ; but it would be desirable to obtain a yet greater dispersion. One way in which this can be done is by the use of what are called direct-vision prisms. In these, two flint-glass prisms (f f) are combined, as shown in fig. 31, with three crown-glass prisms. The prisms C Fig. 31. cause deviation and dispersion in one direction ; the prisms F cause equal deviation and more dispersion in the contrary direction. Hence results a balance of claimed under such circumstances. In this special instance, for example, I should not have turned my thoughts towards the problem of securing the conditions of minimum deviation for all rays, but for the interest I took in Mr. Browning's plan. So soon as he had read the paper de- scribing his plan, before theEoyal Astronomical Society, I felt that his method involved the complete solution of the difficulty. He now con- structs automatic spectroscopes, in which the modifications I have sug- gested have been introduced; and with excellent results. Indeed, complete success depends only on the exactness with which the me- chanical and optical relations are fulfilled ; and rigid accuracy is a characteristic feature of all Mr. Browning's work. Let it be understood clearly, that if I have any merit in the matter at all, it consists in pointing out two unnoticed good qualities of Mr. Browning's own plan. Modified of not, the plan is altogether his. 138 . THE SUX. dispersion without deviation ; and if we add ?uch a prism to any battery of prisms, we get all the advant- age of the dispersion without increasing the deviation which had been our difficulty. Yet there is one important disadvantage in direct- vision prisms, more especially when they are employed in researches requiring very neat and exact definition : it is, of course, wholly impossible to employ any method for securing minimum deviation. A plan by which the dispersion in a battery of prisms may be , doubled, falls next to be considered. Supposing A and B (fig. 32) to be two of the ordinary "Fig. 32. triangular prisms and c a right-angled prism half the size of the others, then the rays which fall on C D are reflected back again through the battery. I need not here describe the contrivance by which the emergent pencil can be viewed in such a case by a telescope in which there is a totally reflecting prism sending the light out at right angles to the axis of the tube. According to another arrangement, in place of such a prism as c, one is employed which by two total reflec- tions raises the rays to a higher level, so that (the ANALYSING SUNLIGHT. 139 ■prisms of the battery being twice as high as usual) the light returns by a separate course. I have devised a plan by which a much greater dispersion than has ever yet been gained may be com- bined with a perfectly true automatic adjustment for all parts of the spectrum. In this plan Mr. Browning's automatic method is extended to a second battery, while the plan for returning the rays, illustrated in fig. 33, is employed in such sort as to double the dis- persive power of the double battery. Fig. 33 shows Fig. 33. the arrangement of the two batteries. A B is the light incident on the first battery, and the course of the light can be traced by the triple set of lines through the double set of prisms, the>dotted return lines show- ing the course of the return rays to emergence at C c'. It will be seen that the large intermediate prism D E may be regarded as belonging to both batteries. There is no loss of light in passing from one battery to the 140 THE SUN. other, since the reflection at d d' is total. The close double lines show the direction of the slotted bars, E r being a long slotted bar kept square to the rays leaving the face v>' e by means of the equality of g f and G E. (It pivots round E and slots at r.)* The figure shows the arrangement of a battery having an effective dis- persing power equal to that of nineteen equilateral prisms of heavy flint-glass.f Now there are optical considerations relating to the method in which the slit admits light, which need not here be specially dealt with. But it is important to notice that a spectroscope may do one of two things. Either it may form the spectrum corresponding to light reaching the slit from a considerable range (as for instance the whole space occupied by the sun, and perhaps the illuminated sky for a width of two or * An instrument on this plan has been construp ted by Mr. Browning, to whom I submitted the proposal last year. The mechanical arrange- ments had, as I expected, to be slightly modified, the bar e f being re- placed by a radial bar carrying the last prism of the second battery. I was perhaps a little surprised to find that this modification — which I had always recognised as a. pis alter — was regarded by those who de- scribed the instrument's successful appearance (at a soiree given by the Royal Society) as making the plan Mr. Browning's. I leave others to judge whether this view, or a view analogous to that expressed in the note on pp. 136, 137, is the more just. t The double battery is represented in about the position correspond- ing to minimum deviation for the line g in the indigo ; and the automatic motion can be carried on till the visible extremity of the spectrum towards the violet is reached. Of course, in examining the red rays, the batteries open out, and there is no limit (except the length of the slots) to motion in that direction, so that the red extremity of the spectrum can be over- passed if need be. In the Monthly Notices of the Royal Astronomical Society for May 1871, 1 show how the plan may be employed to ob- tain a dispersion nearly twice as great. ANALYSING SUNLIGHT. 141 three degrees) or else it may give the spectrum cor- responding only to a narrow line of light. In one ca&e the spectrum is produced by the spreading out as it were of multiplied images of the illuminated slit, the illumination of the slit being derived from a wide area of light. In the other the spectrum is formed of mul- tiplied images of the line of light under examination. A spectroscope serving the former purpose may be called (as Professor Young, of America, suggests) an Fie. 34." integrating spectroscope ; one of the latter kind an analysing spectroscope. What follows relates to the work of the analysing spectroscope. It must be remembered that what the analysing spectroscope really does is to give a range of pictures of whatever luminous object or part of an object would be visible through the slit if the spectroscope were removed. Now, supposing such a portion of the solar photo- sphere is observed as is shown in the space * *', fig. 34 (s being the solar disc), the images of tlis portion will 142 THE SUN. give us the spectrum k v, showing the dark lines due to the absence of certain images, as explained above ; and no other portion of the disc produces any effect whatever. It is very essential to remember this. We are in fact analysing under such circumstances the part s s' of the disc, and no other part. If a spot or a facula be crossed by s s', then the spectrum we get is no longer that of a uniformly, or almost uniformly, bright part of the solar disc. If s s' (fig. 35) represent an enlarged view of the spot and the Fig. 35. space included by the slit, then this last, seen sepa- rately, will be as S s' ; and the spectrum will consist of a number of images of s s' ranged side by side, so as to form a strip, as R v in fig. 34. Hence at the top and bottom of this compound spectrum there will be two narrow solar spectra corresponding to the parts s P and s' p' ; next to these will be two narrow spectra of the penumbral parts P u and p' TJ' ; and about the middle there will be a narrow spectrum corresponding to the umbral part u v', all these spectra forming one com- pound spectrum, whose red end is towards the left (assuming the dispersion to be as in the case illustrated ANALYSING _ SUNLIGHT. 143 in fig. 34) and its violet end towards the right. The ; nature of the penumbral and umbral spectra will be stated further on. It is by comparing these spectra with the adjacent solar spectra that the spectroscopist is enabled to form an opinion as to the nature of the spots, and to make inferences as to the general physical constitution of the Sun. Similar remarks apply to the case where a portion of facula, or pores, or mottlings, or of any other features of the solar disc, falls within the space s s'. All such peculiarities tend to produce peculiarities in the result- ing compound spectrum ; since the image of the por- tion .s / is repeated along the whole length of the spectrum R v after the fashion already described. But, having considered these comparatively simple cases, let us deal with the subject which has of late attracted so much attention — the visibility of the spec- trum of the prominences when the Sun is not eclipsed. Further on the exact nature of the prominence-spec- trum will be considered ; but in this place I note only respecting it that it consists of bright lines. Now, suppose that p p' is a prominence, S s' the edge of the Sun, and ss' the space included by the slit. Then p' s' , as in the former cases, produces a solar spectrum (which, however, commonly presents certain peculiarities when belonging to the edge of the Sun's disc); the part p p' includes a portion of the prominence, and gives a prominence-spectrum which we may suppose to be represented by the bright lines at C and f and near D. But it will also give a solar 144 THE SUN. spectrum, for the light of our own illuminated air comes from the space included within the slit * / ; and as our air is illuminated by solar light, it produces (ac- cording to rule 4, page 128) a solar spectrum. Also the part s p' will give a solar spectrum due to the illuminated air. Now, the prominence p p' is abso- lutely obliterated from view by the illuminated air, which extends all round (and over, be it remembered) the place of the Sun. Since then, if we looked at the space s s alone, with whatever telescopic power, and with whatever contrivances for reducing the glare of light, the portion p p' of the prominence P p' would be wholly invisible to us, why, it may be asked, should the lines c, F, and the one near D — which are in truth but coloured images of the part p p' — be visible, although the spectrum of the illuminated air falling within s p' is spread over these lines precisely as the illuminated air is itself spread over the prominence? The answer is easy. The whole of the light of the illuminated air within the small space s p' is spread over the large space shaded with cross lines in fig. 36, and is reduced in intrinsic brightness in corresponding proportion. On the other hand, the light of the pro- minence-matter within pp' is spread only over the three lines shown in the figure (and a few fainter ones), ANALYSING SUNLIGHT. 145 and is therefore proportionately but very little reduced. Hence, if we only have enough dispersive power, we can make sure of rendering the prominence-lines visi- ble, for we get the same luminosity for them whatever the length of the spectrum, the only effect of an in- crease of length being to throw the bright lines farther apart ; whereas the atmospheric spectrum which forms the background will obviously be so much the fainter as we spread its light over a longer range. By this plan we get a certain number of images of a \* s portion of a prominence — a mere strip, so to speak ; and we can get any number of such portions, and in any direction as compared with the Sun's limb. For example, if s s' (fig. 37) be the Sun's limb, p p' a pro- minence, we can get from such a strip as s s' the spec- trum e v. And obviously since the length of the bright lines tells us the length of the part p p' in figs. 36 and 37, we can, by combining a number of such parallel strips as s s', learn what is the true shape of the pro- minence P p'. L 146 THE SUN. But the plan can be applied to show the whole of a prominence. For let us suppose that in place of a narrow strip as s s', in figs. 36 and 37, we have a space such as is shown in fig. 38, through which, but for the intense brightness of the illuminated air, the prominence p p' would be visible. Then the part s s' of the Sun will produce a solar spectrum — altogether impure, of course, on account of the great width of S s', and brighter than the solar spectrum produced by / p' in the case illustrated by fig. 36 in precisely the propor- tion that S s' in fig. 38 is greater than s' p' in fig. 36. All the remainder of the space, including the promi- nence P p', will give an impure solar spectrum due to Fig. 38. the illuminated air, and very much brighter than in the cases illustrated in figs. 36 and 37, because so much more of this light is admitted through the open slit. Three coloured images will be formed of the promi- nence (other fainter ones need not be considered), one red at C, one orange near D, the other greenish- blue near r. These images will be as bright (neglect- ing variations in the intrinsic brilliancy of the promi- nence) as the corresponding lines in the cases illus- trated by figs. 36 and 37 ; but they will of course not be so well seen, since the background, as I have said, will be very much brighter than in those cases. They ANALYSING SUNLIGHT. lifi can be made as conspicuous only by an increase of dispersive power ; hence the importance of construct- ing prismatic batteries of great dispersive power. In connection with this portion of my subject it is necessary to remark that the bright lines seen in the prominence-spectrum are not uniformly wide throughout, but commonly are wider close to the Sun's limb. This circumstance will be referred to more at length further on ; but it is proper to state in this place, that this increase of width is held to in- dicate an increase of pressure, because the researches of Plucker, Hittorf, Huggins, and Frankland have shown that the spectral lines of hydrogen grow wider as the pressure at which the gas exists is increased. And now, lastly, it remains that I should explain what has been justly regarded as the most wonderful of all applications of the powers of spectroscopic analysis — the measurement of the velocity of recess or ap- proach of stars, or other self-luminous objects moving with very great rapidity. Reverting to fig. 21, the reader will see that the violet rays are most affected by their passage through the jjrism, the red rays least. Now, it has been demon- strated by the careful mathematical analysis * of the motions of light-waves that this difference of refrangi- bility is due to the different velocities with which the * I refer here to the investigations of Cauchy, and Baden Powell, and others (see specially Cauchy's Mimoire sur la Dispersion), not, of course, to the proof that when the differences of velocity are admitted, differ- ences of refrangibility are accounted for. The latter may be regarded in fact as self-evident L 2 148 THE SVN. longer light-waves forming red light, and the shorter light-waves forming violet light, travel (respectively) through material media.* The shorter waves travel more -slowly than the long ones, and the difference is the greater according to the density (or approach to opacity) of the medium. So that, in fine, the part of the spectrum formed by light of any order depends on the wave-length of that light ; and if under any cir- cumstances the wave-length could he altered, then the light of that order would no longer occupy the same portion of the spectrum, but would pass nearer to the violet end if the waves were shortened, and nearer to the red end if they were lengthened. Now, so far as we know, it never happens that light- waves of a certain length are really modified. Pre- cisely as waves of a certain breadth propagated along a canal are not found to change their breadth as they proceed, or, again, precisely as a sound of a certain tone does not change in tone as it travels onwards, so light- waves of a certain length or order do not as they travel through ether, or through material media, become changed into light-waves of some other order, t * In the ether of space they travel of course with appreciably equal velocities ; otherwise the satellites of Jupiter, after emerging from eclipse, would show the same changes of colour that we see in a metal heated from a red to a white heat. t I have sometimes been inclined to suspect, however, that under certain circumstances of excessive agitation within the substance of the source of light, the wave-length might be altered, precisely as waves travelling along a canal might be modified in length by the action of the cause which gave them birth. When we know that the c line of the prominences has been observed to be tranquil, while the f line has been broken, the idea is certainly suggested that those molecular mo- ANALYSING SUNLIGHT. 149 But there is a circumstance which may cause the light-waves to appear to change in length. Supposing the source of light is approaching or receding at a very- rapid rate — at a rate which bears an appreciable propor- tion to that of light — then the length of the light-waves must needs appear modified — shortened when the source of light is approaching, lengthened when it is receding. The same will also hold if the observer be carried very rapidly towards or from the source of light. To see that this is so, it is only necessary to consider that more light-waves must necessarily reach the observer in a given time when the source of light is approach- ing, than when it is at rest (with respect to him), and fewer when it is receding. They must then in one case succeed each other more rapidly, and so seem to be. separated by shorter intervals, while in the other they must succeed each other more slowly, and so seem to be separated by longer intervals.* tions within the substance of the hydrogen of the prominences, which produce that part of the light corresponding to the E line, may by some violent action be so far modified that the observed disturbance of the wave-length corresponding to that particular line may be brought about. It seems difficult to understand how, under any other circum- stances, one line of the hydrogen should be undistorted, while the other is, to use Professor Young's description, ' absolutely shattered.' * The principle on which this brief but sufficient explanation depends admits of several illustrations. I do not know of any which more clearly exhibits the true nature of the principle than one which I em- ployed in the first matter I ever wrote for publication, a paper on ' The Colours of the Double Stars,' which appeared in the Comhill for De- cember, 1863. I quote the portion referred to : — ' Let the reader imagine himself on the bank of a canal, observing a series of waves uniformly propagated along the stream. ... A very simple method will suffice to determine the breadth of the waves with any required degree of 150 THE SUN. Now, Doppler, who first called attention to this cir- cumstance, supposed that an alteration of colour would accuracy. Let the observer, fixing his eye on a certain wave, walk any measured distance (say 100 yards) at the same rate as the wave is mov- ing. Suppose he accomplishes this distance in 65 seconds. He knows then that the velocity of transmission of the waves is 100 yards in 65 seconds. Let him now, standing still for 65 seconds, count the number of crests that pass him in that time. Suppose 360 pass him. Then, from his first observation, he knows that the first which passed him has travelled 100 yards from him. Within that distance all the 360 waves are uniformly distributed. Thus the breadth of each is l-360th part of 100 yards, or ten inches. This result is perfectly reliable if, during his second observation, his position on the bank has been unchanged. But let us imagine that he has made this observation from a truck — on rails by the canal's edge — and that, unnoticed by him, the truck has glided uniformly along the rails. First, suppose that this motion has taken place in a direction contrary to that of the waves, and that while he is counting the passing crests the truck glides a distance of 20 yards. . It is evident that when the last wave passes him, the first is 120 yards, instead of 100, from him. Thus the 360 waves are distributed over 120 yards, and the true breadth of each is l-360th part of 120 yards, or 12 inches. If, on the other hand, the truck had moved over 20 yards in the same direction as the waves, it is equally obvious that the 360 waves would be distributed over only 80 yards, and the breadth of each would be only 8 inches. Similarly, at whatever rate the truck moves, it is evident that the observer can no longer depend on the result of his observations. If it moves in a direction opposite to that in which the waves travel, they appear narrower ; if it travel with them they appear broader than they really are. Indeed, it is not difficult to conceive the truck to move in the same direction and at the same rate as the waves travel, in which case (if we could suppose the observer to remain uncon- scious of that motion) all undulation would appear to him to have ceased, and the water to have a waved but unmoving surface.' This account illustrates in a very direct (and I think distinct) man- ner the effect of the approach or recess of a source of light. We see that the effect depends on the ratio which the velocity of approach or re- cess bears to ths velocity with which the waves travel. It will be seen at once that equal velocities of approach and recess produce equal but not corresponding effects. For, by the supposed velocity of 20 yards in 65 seconds, the length of the waves is increased or diminished by two inches, but the increase is only in the ratio of 6 to 5, while the de- crease is in the greater ratio of 5 to i. Corresponding to this, we have ANALYSING SUNLIGHT. 151 result ; and this would, in fact, be the case (though the alteration would, under any conceivable conditions, be wholly inappreciable) if the source of light emitted rays of a certain order only. But in the case of such a source of light as a star — or the Sun — no change of colour could be produced, because though — to take the case of approach — the red light would be shifted towards the orange, while a portion of the violet would disappear, yet heat rays from beyond the red end would become visible as red rays through being shortened, and so the spectrum would be complete as before. A similar result would follow in the case of recession. But the presence of dark lines in a spectrum gives the observer a far more effective means than mere change of colour would supply of determining the approach or recession of a source of light. If some recognised dark line in the spectrum of a star — say, for instance, the F line — is found not to agree exactly in position with the corresponding line in the spectrum of the fact that the approach of a star at a given rate produces a greater relative effect on wave-lengths of any order than the recess of the star at the same rate would produce. Of course, in the interesting case of stellar approach or recess, the problem is somewhat complicated by the Earth's own motion, which does not take place in the direction of the star, nor necessarily in the same plane. The successful measurement of the velocity with which Sirius is receding from us is a problem of such interest that I may be permitted to note a very small contribution of mine to the work, in the determination of the formula for eliminating the effects of the Earth's motion (v) viz. — Earth's motion towards star=i) cos A sin (I — I') where I and V are the respective longitudes of the star and the Earth, and A is the star's latitude. The contribution has no particular value, but, such as it is, it chanced that I made it. 152 THE SUN. a fixed source of light — as a hydrogen flame, for instance — then the difference of position must be ascribed to a motion of recess or approach on the part of the star, and the rate of such motion may be determined by noticing the amount by which the line is displaced. Now, this method admits of being applied under exceptionally favourable conditions to the examination of solar cyclonic motions — if only these motions are sufficiently rapid to fall within the province of this special mode of research. For we have in the lines appertaining to parts of the Sun which are relatively at rest the means of determining very surely, and Fig. 39- measuring somewhat exactly, the displacement due to such motions as we are considering. Let us take as an instance the case represented in fig. 39. Here s /, as before, represents the portion of the prominence P v' which is under examination. Only a small part of the spectrum is shown, that, namely, near the r line ; and the bright line of the prominence which under normal conditions coincides with the dark line F of the solar spectrum is seen to be displaced towards the violet. It thus appears that, owing to a motion of approach affecting the portion of the prominence included within the narrow space s s' , the light-waves producing the f line seem shortened. ANALYSING SUNLIGHT. 153 The general fact of a motion of approach is thus ascer- tained. But the rate of approach can also be measured ; for we know the length of the light-waves correspond- ing to the line f, Van der Willingen and Angstrom having independently determined the wave-length corresponding to the principal lines in the solar spec- trum. So that if we measured in any way the dis- tance of the bright F line of the prominence from the dark f line of the solar spectrum, we should be able to calculate the amount of the apparent change. A better plan, however, is available. For the bright solar spectrum (fig. 39), as also the atmospheric spec- trum above, is crossed by other dark lines besides the F line, and these enable us to see at once how far the bright line has shifted. Suppose I, for example, to be another dark line of the solar spectrum, and that the bright prominence-line has moved half-way from its proper place towards I, then we know that its wave- length is changed to a value midway between the wave-length corresponding to the lines F and L* The change of value thus indicated gives us at once the rate of the motion of approach which affects the portion of the prominence-matter corresponding to the space s / •)■ — because, though the wave-length corresponding * This is true for such small displacements as are here considered. For greater differences of refrangibility, no such simple proportions exist, partly because the actual change of wave-length (for given differ- ences of refrangibility) diminishes towards the violet end, and partly on account of the irrationality of dispersion for all known media, when the spectra they give is compared with what Angstrom has called the Twrmal solar spectrum. t The general rules on which the calculation proceeds are sufficiently 154 THE SUN. to the line I will not be indicated in the tables of Angstrom or of Van der "Willingen (which only include the principal lines), yet it is readily determinable, and indeed may be regarded as a known quantity. And, in a similar way, if the line is shifted towards the red end, the velocity of recession of the prominence-matter can readily be determined. But as a general rule the whole line would not be shifted bodily as in fig. 39 ; since indeed this would imply that the whole of that portion of the prominence which is seen within the space s s' was travelling bodily towards the observer ; whereas obviously such a motion could very seldom be expected to occur. Ordi- narily, then, we may expect to find a configuration of the bright line indicating varieties of motion. The same holds also in the case of portions of the solar photosphere, or spots, lying near the edge of the Sun's disc (so that ordinary cyclonic motions within them may be capable of being recognised by the method we are considering), or in the case of more central portions of the Sun's disc where ascending and descending motions are taking place, resulting in motions of re- cess or approach with reference to the terrestrial observer. In all such cases we may expect to find peculiarities in the affected lines, corresponding to simple. Suppose the wave-length corresponding to the line I to be 485-89 millionths of a millimeter, that corresponding to the line f being 486'39 such millionths. Then, since the prominence r-line appears half-way between r and I, the wave-length has been reduced to 486-19 millionths of a millimeter, or diminished by 0'2 such mil- lionths. Hence the velocity of approach of the prominence matter is js%s tIls of tQe velocity of light, or some 80 miles per second. ANALYSING SUNLIGHT. 155 varieties in the motion or rates of motion of the parts examined. I give a few examples, illustrating the way in which such peculiarities are to be interpreted. I consider, for convenience, motions taking place in that coloured envelope (whence the solar prominences seem to spring) which has been called the sierra or chromatosphere : — Suppose s s' to represent the portion of the Sun and chromatosphere under examination, s s' being the Sun's limb, c c' the (invisible) outline of the chromatosphere. Now, if the f line appeared as in 1., we should con- clude that the hydrogen in the part of the sierra under Fig. 40. examination was quiescent near the Sun's surface, as far as motions of approach or recess are concerned (though it might be moving very rapidly in a direction square to the line of sight), but that at some distance from the Sun's surface it was moving very rapidly towards the eye, the rate of motion increasing with the vertical height above the Sun. If, on the other hand, the spectrum appeared as at 11. (fig. 40), we should come to a similar conclusion, substituting only a motion of recession for one of approach. If the spectrum appeared as at in. we should conclude that to a certain level above the Sun's limb there was a gradually 156 THE SUN. increasing motion of approach, but that at and above that level there was a motion of recession tolerably- uniform in rate to a considerable height. The case would resemble those instances in our own atmosphere where an upper air current blows in a different direction than the air nearer the sea-level. If, lastly, the spectrum appeared as at iv., we should conclude that to a considerable height above the Sun's surface there was no motion of recess or approach ; but that in higher regions of the chromatosphere there were masses (within the long range of chromatospheric matter really included in the direction of the visual line) moving both from and towards the eye at an enormously rapid rate. The greater or less width of different parts of the bright line would indicate the greater or less pressure at which the hydrogen existed at the corresponding levels during the time of observation. Hence, a bulb on any part of the bright line would indicate a corre- sponding layer of relatively compressed hydrogen, while a marked narrowing would indicate a layer of hydrogen existing for the time at relatively low pressure. Similar considerations apply to the spectroscopic analysis of solar spots, or of faculous regions of the Sun's surface, or generally of any regions where dis- turbances may produce solar atmospheric currents of approach or recess. Combining the observed shifting of portions of a spectral line with its observed thickness, and also with its relative brightness or darkness (as indicative of greater or less temperature), we have a means of studying those conditions of motion, pressure, ANALYSING SUNLIGHT. i$7 and temperature, respecting which the telescope alone can give us no information whatever. It is wonderful, indeed, to consider that that analysis of the dark lines of the solar spectrum which seemed half a century ago so unmeaning, those speculations of Doppler which but a quarter of a century ago were rejected by many as wholly fanciful, and those inquiries as to the almost evanescent wave-lengths of light which from the days of Newton downwards had been ridi- culed as a complete waste of time and thought, should have resulted, under the labours of Bunsen and Kirch- o hoff, of Plucker and Huggins, and, finally, of Angstrom and Van der Willingen, in a means of dealing with problems so recondite and seemingly so hopeless. By an observation not occupying many seconds any clear-sighted observer, armed by our opticians with adequate spectroscopic power, can measure the swift- ness of the solar windstorm, can gauge the pressure of the solar atmosphere, and can estimate the relative temperature of spot and faculse, of photosphere and chromatosphere, and, lastly, of the higher regions to which eruptions cast those masses of glowing vapour which form the solar prominences. 158 THE SUN. CHAPTER IV. STUDY OF THE SUN'S SURFACE. We may regard the discovery of the spots on the Sun as the commencement of that long series of telescopic researches to which we owe our present knowledge of the solar orb. It is highly probable, indeed, that spots on the Sun had been seen, and even watched for long intervals, when as yet astronomers were not aided by the powers of the telescope. But there is no reason to believe that the nature of the spots so seen, or even the fact that they are true solar phenomena, had ever been suspected by astronomers.* Whatever opinion we * Kepler supposed that in lines 441 and 454 of Virgil's first Georgia, the solar spots were referred to. For ' if any one,' he reasons, ' should refuse to see anything else than an allusion to our clouds in the "words ' Hie ubi nascentem maculis variaverit ortum,' I shall oppose to the interpretation this other verse : ' Sin maculae incipient rutilo immiscerier igni.' But the latter verse is quite as applicable to clouds as the former. As regards the occasional recognition of spots by the ancients, however, there seems less room for doubt. We learn from Father Mailla that the Chinese recorded the appearance of spots on the Sun in the year 321 a.d., and Acosta tells us that the natives of Peru told the Spanish invaders that the Sun's face had in former times been marked with spots. In the year 807, a large spot was seen on the Sun for eight STUDY OF THE SUN'S SURFACE. 159 may form of ancient records of solar obscurations, we must turn to the telescopic discovery of the spots for the real commencement of astronomical researches into the Sun's physical condition. I do not propose to enter here into the discussion which has been raised respecting the astronomer to whom the credit of having first seen the solar spots is to be assigned. This discussion has been pursued by grave authorities with an earnestness which would really seem to imply that they have regarded the mat- ter as of serious importance. Let us simply recog- nise the fact that the credit of the discovery is not worth contending about,* and proceed to consider successive days, and was supposed by those who were little familiar with the laws of planetary motion to be the planet Mercury. Arago is of opinion, also, that the transits of Mercury said to have been wit- nessed by Averrhoes, Scaliger, and Kepler himself (May 28, 1607) were only observations of Sun spots. It is worthy of notice that the ancients could very well have observed Sun spots, and have even traced the progress of these spots across the solar disc, had they employed the method which Gassenius adopted in observing the transit of Mercury in 1631. He admitted the Sun's rays into a darkened chamber through a small aperture in a shutter, and thus obtained an inverted image of the Sun, on which, when the transit had begun, he could perceive the disc of Mercury. Although no spots are ever seen which, even in the nucleus, are so dark as Mercury, yet many (or rather all the noteworthy spots) present a much more conspicuous appearance than Mercury in transit. Fabricius, indeed, as we shall presently see, did actually apply this method. * It has been justly remarked by an eminent astronomer of our own time, that the discovery of the spots was a necessary sequel of the invention of the telescope; and whether Galileo, or Fabricius, or Scheiner, or Harriot, first set eyes on these objects, is a matter which can in no way increase the reputation of any one of these astronomers. To discuss the question of priority seems therefore to be simply a waste of time. 160 THE SUN. what the first observers actually saw — a matter of more moment. From a work by Fabricius,* we learn that in the commencement of the year 1611, while observing the Sun just after sunrise with a telescope of inconsider- able power, he noticed a black spot upon its disc which he supposed to be a terrestrial cloud. He found, how- ever, that the object, whatever it was, belonged to the Sun. As the Sun rose he had to discontinue his observations, for he possessed no means of mitigating the brilliancy of the Sun's light. ' My father and I,' he says, ' passed the rest of the day and the whole night in great impatience, trying to think what this spot might be. " If it is in the Sun," I said, " we shall no doubt see it again ; if it is not, its motion will have carried it away from the Sun's disc, and so we shall be unable to see it." On the next morning, however, to my delight, I saw the spot again. But it was not in the same place — a peculiarity which in- creased our perplexity. We determined to obtain an image of the Sun on a sheet of paper by permitting his rays to pass through a small hole in a darkened chamber, and in this way we saw the spot quite clearly in the form of an elongated cloud. For three days we were prevented by bad weather from continuing our observations ; but at the end of that period we again saw the spot, which had crossed obliquely towards the western side of the Sun's disc. Another smaller one * Entitled Be Macnlis in Sole observatis, &c, and published at Wittemterg in 1611. STUDY OF THE SUNS SURFACE. 161 had made its appearance near the eastern edge, and in a few days this second spot reached the middle of the disc. Lastly, a third spot appeared. The three spots vanished in the order of their appearance. I was hopeful that they would be seen again, but yet per- plexed by doubts and fears ; however, ten days after- wards the first re-appeared on the eastern side of the disc. I knew then that it had revolved completely round (the Sun), and since then I have convinced myself that this is really the case.' Fabricius studied the import of his observations, and came to the conclu- sion that the spots are probably upon the body of the Sun itself. ' We invite the students of science,' he says, ' to profit by our description ; they will doubtless conclude that the Sun has a motion of rotation, as Jordanus Bruno has asserted, and, more lately, Kepler. Indeed, I do not know what we could make of these spots on any other supposition.' Galileo, at Florence, and Father Scheiner, a German Jesuit, besides independently discovering the spots, investigated the laws which regulate the motion of these objects. Scheiner was at first disposed to regard the spots as due to the existence of planets travelling round the Sun close to its surface, and indeed for a while these imagined planets were admitted as true members of the solar family, under the title of the Borbonian stars.* But Galileo having pointed out * They are referred to under this name in Burton's Anatomy of Me- lancholy (a very short time after their discovery), in that singularly interesting chapter which he entitles, quaintly enough, a Digression of M 1 62 THE SUN. that the motions of the spots do not correspond to this hypothesis, hut point clearly to the conclusion that the Sun rotates on its axis in about a month, Scheiner re-examined his hypothesis, and presently admitted that Galileo was in the right. He then made a long and elaborate series of observations in order to deter- Ayre. The following passage from this chapter is sufficiently apropos, and will, I doubt not, interest the curious reader : — ' In the meantime, the world is tossed in a blanket amongst them ; they hoyse the earth up and down like a ball, make it stand and goe at their pleasures. One saith the sun stands ; another, he moTes ; a third comes in, taking them all at rebound; and, lest there should any paradox be wanting, he findes certain spots and cloudes in the sun, by the help of glasses, which multiply (saith Keplerus) a thing seen a thousand times bigger in piano, and make it come 32 times neerer to the eye of the beholder: but see the demonstration of this glass in Tarde, by means of which, the sun must turn round upon his own center, or they about the sun. Fabritius puts onely three, and those in the sun: Apelles, 15, and those without the sun, floating like the Cyanean isles in the Euxine sea. Tarde the Frenchman hath observed 33, and those neither spots nor clouds, as Galileus (Epist. ad Veteernm) supposeth, but planets concentrick with the sun, and not far from him, with regular motions. Christopher Schemer [Scheiner], a German Suisser Jesuit, Ursica Eosa [Qy., in his Rosa Ursina~], divides them in maculas et facvlas, and will have them to be fixed in solis superficie, and to absolve their periodicall and regular motion in 27 or 28 dayes; holding withall the rotation of the sun upon his center ; and are all so confident, that they have made skemes and tables of their motions. The Hollander, in his dissertatiuncula cum Apelle, censures all; and thus they disagree amongst themselves, old and new, irreeoncileable in their opinions ; thus Aristarchus, thus Hipparchus, thus Ptolomams, thus Albateginus, thus Alfraganus, thus Tycho, thus Eamerus, thus Boeslinus, thus Fracastorius, thus Copernicus and his adherents, thus Clavius and Maginus, &c, with their followers, vary and determine of these celestiall orbs and bodies ; and so, whilest these men contend about the sun and moon, like the philosophers in Lucian, it is to be feared the sun and moon will hide themselves, and be as much offended as shee was with those, and send another message to Jupiter, by some new fangled Icaromenippus, to make an end of all those curious con- troversies, and scatter them abroad.' STUDY OF THE SUITS SURFACE. 163 mine the true period of rotation and the actual position of the solar axis of rotation. In his 'Rosa Ursina' (a most monstrous volume)* he published the results of his labours. He assigned to the Sun a rotation period of between twenty-six and twenty-seven days. He also stated that the plane of the Sun's equator is inclined between 6° and 8° to the plane of the ecliptic, — a very creditable result, considering his means of observation, since the best modern measures assign 7^° as the value of this angle, Scheiner, Galileo, and Hevelius would seem to have independently recognised the fact that the solar spots are not of uniform brightness, but commonly sur- rounded by a fringe less dark than the central part. The last-named astronomer also recognised the exist- ence of certain bright streaks in the neighbourhood of the spots. He called these the faculse. * Yet not meriting the disparaging comments of Delambre, who says, ' There are few books so diffuse and so void of facts. It contains 784 pages ; there is not matter in it for fifty.' The prolixity, howeTer, belongs to the age in which Scheiner lived, and is by no means peculiar to him. By a similar mode of judging, we should be entitled to hold in derision the works of Kepler. Lalande thus writes respecting Scheiner. ' Quoi qu'il en puisse etre de celui a qui le hasard a fait voir les taches pour la premiere fois, il est Bur que personne ne les observa aussi bien et n'en donna la theorie d'une maniere aussi com- plete que Scheiner. Son ouvrage a 774 pages sur cette matiere, et eela suffit pour faire voir avee quelle assiduite il s'en oecupa, et combien il donna d'etendue a ses recherches. Hevelius le cite avec le plus grand eloge: " Incomparabilis et omnigense eruditionis .... ut in hac materia omnibus palmam quasi prseripuisse dici posset.'" So much it has seemed fitting to say respecting a most laborious and ingenious inves- tigator, whose valuable researches into solar physics have not received the credit which they deserve. 11 2 164 THE SUN. A long series of observations of Sun-spots now began, and many hypotheses of more or less ingenuity were put forward to account for the phenomena which they present. For some time, indeed, the possibility of their existence was earnestly denied by the students of Aristotelian philosophy. It is impossible, they gravely urged, that the Eye of the Universe should suffer from ophthalmia ; and it is related that when Scheiner communicated his discovery of the solar spots to the provincial of his order, the latter, who was an earnest Aristotelian, answered, ' I have read Aris- totle's writings from beginning to end many times, and I can assure you I have nowhere found in them anything similar to what you mention ; go, therefore, my son ; tranquillise yourself; be assured that what you take for spots in the Sun are the faults of your glasses or your eyes.' Despite the defenders of Aristotle's infallibility, however, the progress of solar research went on. Galileo continued his labours, until, from viewing the Sun so often without the dark glasses now commonly employed, he lost his eyesight. Scheiner, Hevelius, and other observers, added largely to the store of known facts ; and gradually the observation of solar spots began to be recognised as a regular part of the astronomer's work. I do not propose, however, to give a detailed ac- count of the observations made in those earlier years of telescopic observation of the Sun. Indeed, to give a full history of those observations, and to extend the STUDY OF THE SUN'S SURFACE. 165 same fulness of narrative to later researches, would require twenty or thirty such volumes as the present. I shall content myself with selecting certain illus- trative instances of solar observation by the earlier astronomers, choosing those observations specially which tend to introduce the results more completely educed by recent researches. Cassini writes thus of a spot observed by him in August 1671.* 'It is now about twenty years since, that astronomers have not seen any considerable spots on the Sun, though before that time, since the inven- tion of the telescope, they have from time to time observed them. The Sun appeared all that while with an entire brightness, and I saw him so on the ninth of the month of August. But on the eleventh of the same, about six a clock at night, being furnisht only with a three-foot glass, I remarked in the Sun's disque two spots very dark, distant from his apparent centre about the third part of his semi-diameter. . . . The first of these spots, being look'd upon with a telescope of seventeen foot long, appeared of a somewhat oval figure ; the other was oblong and a little curved, like the Hebrew letter Jod ; and both together were sur- rounded by a corolla or coronet made up of little dark points ' (the penumbra) ' which conformed itself to the figure of the spots, considered as they were joined together. . . . The twelfth of August, 1671, I ob- * New Observations of Spots in the Sun ; made at the Boyal Academy of Paris on August 11, 12, and 13, 1671; and EaglisKt out of the French. -Phil. Trans, vol. vi. p. 2251. 166 YH.E SUX. served them from the time of sun-rising, and perceived that they were nearer his centre. . . . The first was composed of two others almost round and conjoyn'd. The second represented the shape of a scorpion. The third was round ' (this is the first intimation we have of the triplicity of the group), ' and they were all three environed with a coronet, which was composed, as said above, of abundance of little obscure pricks. This coronet appeared to be clearer than the rest of the Sun when looked upon with the short glass, and darker when seen with the long. Without it there were other points, but very black ones, viz. five near the round spot on the south side, and anoth'er near the scorpion's tail on the north side. At eight a clock and forty- eight minuts, the figure of the scorpion was seen divided into several pieces, as if his tail and arms had been cut off. The northern point remained no more, there remaining none but those seen on the south side ; and the length of the enclosure of all the spots, com- prehended between the extremities, was of one minut and fifteen seconds, and the breadth of thirty seconds.' On August 12 Cassini found no great change had taken place, but the black points outside the spot were now spread in a straight row. On the 13th ' the edge of the coronet was turned to a point on the south side.' The spot had indeed changed in that strange way which all observers of the Sun must be familiar with, the following extremity of the penumbra drawn out to a point which was so curled round as to be directed towards the .preceding end of the. spot. STUDY OF THE SUN'S SURFACE. 167 Cassini afterwards traced the progress of this spot to the Sun's limb. He remarks, ' The apparent velocity nigh the centre was such that if it had continued the same, the spots would have arrived almost in four days to the limb of the disque ; but in the hypothesis ' — that they are either attached to the Sun's surface, or not far from it — ' this apparent velocity was to lessen accord- ing as the spots should remove from the centre; as hath come to pass in effect. The diminution in the length of the misty crown' (a strange name for the penumbra) ' was in a manner proportionat to the diminution of the apparent velocity ; since that when this crown was in the midle, and in a scituation wherein its true figure could be best seen, it appeared oblong and of the form of an human ear, its greatest diameter respecting east and west ; but being nigh the limb, this same diameter seemed to shorten ; and having appeared greatest in its first scituation, it appeared least in this, because it was almost in a circle that passed through the centre of the Sun, whose equal arches are by so much the more oblique by how much they approach more to the limb of his disque, and consequently appear less according to the rules of opticks ; meantime the diameter that was turned from north to south apparently kept the same bigness it had near the centre, because it was in a circle almost parallel to the horizon of the Sun, which formed the representation of its limb, and whose equal arches ' (for the same optical reasons) ' do not appear contracted.' It will be seen by this reasoning — which, 1 68 TEE SUN. being interpreted, signifies that fore-shortening caused the spot to appear longest in a direction square to a line from the Sun's centre — that Cassini had on this occasion come very near to the discovery afterwards made by Dr. Wilson, that the nucleus of a spot is at a lower level than the solar photos- phere. For had he but noticed an excess of breadth in the penumbra nearest to the Sun's limb, the same just reasoning which he applied to the figure of the spot would have enabled him to pronounce at once respecting the relative level of the nucleus, the penumbra, and the photosphere. He gives a picture of the spot close by the Sun's limb, in which the penumbra is of equal width on the side next the centre and on that next the limb. Most probably no such peculiarity as Dr. "Wilson detected existed in the case of this particular spot. In fact, so far as my own experience of the aspect of Sun-spots is con- cerned, spots such as the one observed by Cassini seldom exhibit the peculiarity in question in a manner which would enable an observer to theorise safely respecting the level at which nucleus, penumbra, and photosphere actually lie. In November 1769, Dr. Wilson, of Glasgow, began the careful study of a large spot (visible to the naked eye). The results which he deduced from its changes of appearance are of great interest and importance. When he first examined the spot (November 22) it was situated not very far from the western edge of the Sun's disc. On the next day he found that the spot STUDY OF THE SUN'S SURFACE. 169 had changed in appearance. ' The penumhra, which on the previous day was equally broad on all sides of the nucleus,* was now very much contracted on the side which lay towards the centre of the disc, while the other parts retained nearly their former dimensions. On the 24th he again observed the spot. The distance from the limb was now only twenty-four seconds, and the ' contracted side of the penumbra had entirely vanished. The breadth of the nucleus on the same side also appeared to be more suddenly impaired than it ought to have been by the motion of the Sun across the disc' Dr. Wilson showed that these results correspond with those which would follow if the spot was a vast cavernous opening, having the nucleus at the bottom, and the penumbra forming its sloping sides. It only remained to be seen whether a corresponding succession of changes occurred when the spot re-appeared on the eastern edge, and thence passed across the solar disc. This actually happened. ' On December 11, the spot appeared on the opposite side of the disc. It was then distant about a minute and a half from the edge. The side of the penumbra next the edge, which formerly vanished, was now wholly visible, while that turned towards the centre of the disc appeared to be wanting. On December 12 it came into view, and he saw it distinctly, though narrower than the other side. He did not see the spot again until December 17, when it * From Professor Grant's abstract of the original narrative, in his excellent History of Physical Astronomy. 1 70 THE SUN. had passed the centre of the disc, and the penumbra now appeared to surround the nucleus equally on all sides. In fig. 41 the upper row of spots represents the succession of changes actually presented by this spot, while the lower shows what would occur as a spot traversed the Sun's disc, if the spot were simply a surface stain with a penumbral fringe. It will be seen Fig. 41. that the difference between the appearances depicted in the two rows is of a sufficiently marked character. It will also be noticed that even without any definite explanation of the peculiarities shown in the upper row, the mind at once recognises the fact that we have here to do with a cavity or depression. Let us examine, however, the line of reasoning by which Dr. Wilson demonstrated this. STUDY OF THE SUN'S SURFACE. 171 Let A, b, c (fig. 42) * be supposed to represent per- spective views of a saucer-shaped depression on the surface of a sphere — the depressions being all of like dimensions (the sloping sides are assumed in these three views to be transparent). Then, it is obvious that to an eye supposed to view them from above, the relative breadth of the black base and of the shaded sides would be indicated by the breadth of the dark and shaded spaces carried vertically upwards from the Fig. 42. spots. Now, suppose the sphere to be rotated on a horizontal axis so that the spots are brought (their edges sliding as it were along the dotted lines) down to the positions a', b', o'. Then the relative breadths of the penumbra and nucleus will be indicated by the * This figure in essentials closely resembles a figure in Fr. Secchi's recently published work on the Sun. It was drawn by me, however, and employed in illustrating lectures, nearly a year before Secchi's book appeared. 172 THE SUN. distances separating these dotted lines, and must needs therefore be such as are shown in the figure. The shape of the depression would also be as shown. We see, then, that to an eye watching a depression of this sort as it rotated from the position a' to the position c', changes of shape would be shown which correspond exactly with those recognised by Dr. Wilson. This is the proper place to point out, however, Fig. 43. that Dr. Wilson's observations were insufficient to demonstrate that a spot is a region which lies as a whole beneath the solar surface. They show only that the nucleus of the spot he observed lay at a lower level than the penumbra. It is evident from fig. 43 that appearances precisely corresponding to those ob- served by Wilson would be seen if spots were caused by a double layer of clouds in the solar atmosphere, the lower opaque, the upper semi-transparent and ex- STUDY OF THE SUN'S SURFACE. 173 tending on all sides beyond the limits covered by the lower. Such an interpretation has indeed been put forward in recent times by KirchhofF, and is still main- tained, I believe, by Sporer. "Wilson's views, respecting the physical interpreta- tion of the occurrence of these solar depressions need not detain us here. They have not been regarded as so successful as his geometrical analysis of the observed phenomena. It is only just to add that he himself did not attach equal weight to them ; for in answer to objec- tions urged by Lalande to his theory that the spots are depressions, Wilson wrote thus in 1783 : 'Whether their first production and subsequent numberless changes depend upon the eructation of elastic vapours from below, or upon eddies or whirlpools commencing at the surface, or upon the dissolving of this luminous matter in the solar atmosphere, as clouds are melted and again given out by our air, or, if the reader pleases, upon the annihilation and reproduction of parts of this resplen- dent covering, is left for theory to guess at.' Passing also over the theories of Bode, which dif- fered in no important respect from those of Wilson, let us turn now to the observations and theories of the greatest observational astronomer the world has ever known — Sir William Herschel. I propose to treat at considerable length what he has advanced upon the subject of the Sun, partly because of the great value of his work, but partly also to re-claim for him many discoveries which have been assigned to later observers. What these are I need not in every in- 174 THE SUN. stance particularise ; but it seems to me essential that the original observation of facts of so much interest and importance, only determinable by long and patient scrutiny of the Sun, should be assigned to their proper place. Let me begin by quoting the fine passage in which Sir William Herschel speaks of the central luminary of our system. ' Among the celestial bodies,' he says, { the Sun is certainly the first which should attract our notice. It is a fountain of light that illuminates the world ! it is the cause of that heat which maintains the productive power of nature, and makes the Earth a fit habitation for man ! it is the central body of the planetary system ; and what renders a knowledge of its nature still more interesting to us, is that the numberless stars which compose the universe, appear by the strictest analogy to be similar bodies. Their innate light is so intense, that it reaches the eye of the observer from the re- motest region of space, and forcibly claims his notice.' Next let us hear his summary of the theories which have been put forward respecting the physical constitu- tion of the Sun. I may note in passing that the open- ing remarks are as applicable in the present day as when Herschel wrote them : — 'I should not wonder,' says the great astronomer, ' if we were induced to think that nothing remained to complete our know- ledge ; and yet it will not be difficult to show that we are still very ignorant, at least with regard to the inter- nal constitution of the Sun. The various conjectures STUDY OF THE SUN'S SURFACE. 175 which have been formed on this subject are evident marks of the uncertainty under which we have hitherto laboured. The dark spots in the Sun, for instance, have been supposed to be solid bodies revolving very- near its surface. They have been conjectured to be the smoke of volcanoes, or the scum floating upon an ocean of fluid matter. They have also been taken for clouds. They were explained to be opaque masses, swimming in the fluid matter of the Sun ; dipping down occasionally. It has been supposed that a fiery liquid surrounded the Sun, and that, by its ebbing and flowing, the highest parts of it were occasionally un- covered, and appeared under the shape of dark spots ; and that by the return of this fiery liquid they were acrain covered, and in that manner successively assumed - different phases. The Sun itself has been called a globe of fire, though perhaps metaphorically. The waste it would undergo by a gradual consumption, on the supposition of its being ignited, has been ingeniously calculated. And in the same point of view its immense power of heating the bodies of such comets as draw very near to it has been assigned.' ' In supporting,' he proceeds, ' the ideas I shall propose in this paper with regard to the physical constitution of the Sun, I have availed myself of the labours of all these astrono- mers, but have been induced thereto only by my own actual observation of the solar phenomena, which, besides verifying those particulars that had been already observed, gave me such views of the solar led to the foundation of a very rational 176 THE SUN. system. For having the advantage of former observa- tions, my latest reviews of the body of the Sun were immediately directed to the most essential points ; and the work was by this means facilitated and contracted into a pretty narrow compass.* ' In the year 1779,' he begins, 'there was a spot on the Sun which was large enough to be seen with the naked eye. By a view of it with a 7 -feet reflector, charged with a very high power, it appeared to be divided into two parts. The largest of the two on April 19 measured 1'8"*06 in diameter, which is equal in length to more than 31,000 miles. Both together must certainly have exceeded 50,000. The idea of its being occasioned by a volcanic explosion, violently driving away a fiery fluid, which on its return would gradually fill up the vacancy, and thus restore the Sun, in that place, to its former splendour, ought to be rejec- ted on many accounts. To mention only one, the great extent of the spot is very unfavourable to that supposi- tion. Iudeed, a much less violent and pernicious cause may be assigned, to account for all the appearances of the spot. ' The Earth is surrounded by an atmosphere com- posed of various elastic fluids. The Sun, also, has its atmosphere, and if some of the fluids which enter into its composition should be of a shining brilliancy while others are merely transparent, any temporary cause which may remove the lucid fluid will permit us to see the body of the Sun through the transparent ones. If an observer were placed on the Moon, he would see the A PORTION OFTHE SUNS DISC as seen m May 1870, by M 1 : Browning. FRAS, ■ Broy*n/nq-With Reflector. STUDY OF THE SUNS SURFACE. 177 solid body of our Earth, only in those places where the transparent fluids of the atmosphere would permit him. In others the opaque vapours would reflect the light of the Sun without permitting his view to penetrate to the surface of our globe. He would probably also find that our planet had occasionally some shining fluids in its atmosphere ; as, not unlikely, some of our northern lights might attract his notice, if they happened in the unenlightened part of the Earth, and were seen by him in his long dark night. Nay, we have pretty good reason to believe, that probably all the planets emit light in some degree ; for the illumination which re- mains on the Moon in a total eclipse cannot be entirely ascribed to the light which may reach it by the refrac- tion of the Earth's atmosphere.* . . In the instance of one large spot on the Sun, I concluded, from appear- ances, that I viewed the real solid body of the Sun itself, of which we rarely see more than its shining atmosphere.' 'In the year 1783,' he proceeds, ' I observed a fine large spot, and followed it up to the edge of the Sun's limb. Here I took notice that the spot was plainly depressed below the surface of the Sun ; and that it had very broad shelving sides. I also suspected some * There is, however, an important flaw in the reasoning on which Sir "William Herschel hases this opinion. He takes the horizontal refrac- tion due to a ray of solar light so entering the Earth's atmosphere as to reach the surface of the Earth tangentially, without noticing that the same amount of refraction will affect the ray as it passes out of the terrestrial atmosphere. This error is repeated in Ferguson's Astronomy, and has since been often quoted without correction. N 178 THE SUN. part at least of the shelving sides to be elevated above the surface of the Sun ; and observed that, contrary to what usually happens, the margin of that side of the spot which was farthest from the limb was the broadest.' It will be noticed that in the picture of the Sun pre- sented in Plate I., Mr. Browning delineates in the case of one spot of the group a precisely analogous appearance. Sir William Herschel's explanation of these peculiar appearances need not be quoted, as it has been disposed of by recent researches. ' In the year 1791,' he proceeds, * I examined a large spot in the Sun, and found it evidently de- pressed below the level of the surface ; about the third part was a broad margin or plane of considerable extent, less bright than the Sun, and also lower than its surface. This plane seemed to rise, with shelving sides, up to the place where it joined the level of the surface.'* ' How very ill,' proceeds Herschel, ' would this ob- * Herschel reasons that there could have been no deception in this appearance, because the Sun looked convex, whereas he had noticed that on those occasions when the Moon's mountains and valleys were apparently reversed, the Moon herself always looked concave, the illusion disappearing when the mind was directed to the fact of the Moon being in truth convex. It may be questioned, however, whether this reasoning can fairly be applied to a self-luminous body. The peculiarity affecting the apparent concavity or convexity of the lunar mountains or craters, depends entirely on the ideas present in the mind at the moment of observation, respecting the direction in which the source of illumination lies— precisely as in the analogous experiment with a seal or coin, discussed in Brewster's Natural Magic. Such con- siderations cannot affect our views respecting a self-luminous body. STUDY OF THE SUN'S SURFACE. 179 servation agree with the ideas of solid bodies bobbing up and down in a fiery liquid — with the smoke of volcanoes, or scum upon an ocean ; and how easily is it explained upon our foregoing theory. The removal of the shining atmosphere, which permits us to see the Sun, must naturally be attended with a gradual dimi- nution on its borders ; an instance of a similar kind we have daily before us, when through the opening of a cloud we see the sky, which generally is attended by a surrounding, haziness of some short extent, and seldom transits from a perfect clearness to its greatest obscurity.' On August 26, 1792, Herschel examined the Sun with several powers, from 90 to 500. ' It appears evidently,' he remarks, ' that the black spots are the opaque ground or body of the Sun; and that the luminous part is an atmosphere, which, being inter- rupted or broken, gives us a transient glimpse of the Sun itself.' He presently suggests that possibly even where there are no spots, the real surface of the Sun may now and then be perceived — ' as we see the shape of the wick of a candle through its flame, or the con- tents of a furnace in the midst of the brightest glare of it ; but this, I should suppose, will only happen where the lucid matter of the Sun is not very accumulated.' A few days later he studied some well-marked faculaB. In the neighbourhood of a dark spot pretty near the edge, ' I saw,' he says, ' a great number of elevated bright places, making various figures. I shall call them facula, with Hevelius ; but without assign- M 2 i«o ■ THE SUN. ing to this term any other meaning than what it will hereafter appear ought to be given to it. I see these f'acula? extended on the preceding side, over about one-sixth part of the Sun ; but so far from resembling torches, they appear to me like the shrivelled elevations upon a dried apple, extended in length; and most of them are joined together, making waves, or waving lines. By some good views in the afternoon, I find Fig. 44. The Sun's corrugated surface. — Secchi. that the rest of the surface of the Sun does not contain any facula?, except a few on the following and equa- torial part of the Sun. Towards the north and south I see no faculffi ; there is all over the Sun a great unevenness in the surface, which has the appearance of a mixture of small points of an unequal light; but STUDY OF THE SUN'S SURFACE, I8l they are evidently an unevenness or roughness of high and low parts.' The accompanying views (figs. 44 and 45) of por- tions of the Sun's surface, as delineated by Secchi with the fine telescope of the Roman observatory, corre- spond exactly with this description. Note also the drawing by Chacornac (fig. 55) farther on. After a week's observations of the faculse, Herschel thus reasons about them: — 'The facuhe being eleva- Fra. 45. The Sun's surface, showing facula. — Secchi. tions, very satisfactorily explains the reason why they disappear towards the middle of the Sun, and reappear on the other margin; for, about the place where' we lose them they begin to be edgeways to our view ; and if between the faculae should lie dark spots, they will most frequently break out in the middle of the Sun, I»2 THE srx. because they are no longer covered by the side views of these faculae.'* On September 22, 1792, Herschel observed few faculas on the Sun and few spots, but the whole disc very much marked with roughness, like an orange, and some of the lowest parts of the inequalities blackish. On the following day he thus associates this roughness with the faculae : — ' The faculae are ridges of elevation above the rough surface.' f On February 23, 1794, Herschel noticed an appear- ance which is very well illustrated in Mr. Browning's drawing of the Sun, Plate I. ' One of the black spots on the preceding margin, which was greatly below the margin of the Sun, had, next to it, a protuberant lump of shining matter, a little brighter than the rest of the Sun. About all the spots,' he adds, ' the shining matter seems to have been disturbed ; and is even lumpy and zig-zagged in an irregular manner. I call the spots black, not that they are entirely so, but merely to distinguish them ; for there is not one of them to-day which is not partly or entirely covered with whitish and unequally bright nebulosity or cloudi- ness. This in many of them comes near to an ex- * Perhaps the most convincing proof we have of the fact that faculae are elevations, is that supplied by Mr. Dawes, who actually saw a facnla projecting beyond the edge of the Sun's disc. f The observation next, following the above in Herschel's paper (Phil. Trans, vol. lxxxv.) is worth citing, though, the fact involved is now well-known to all observers of the Sun. It runs thus : — ' Feb. 23, 1794. By an experiment I have just now tried, I find it confirmed that the Sun cannot be so distinctly viewed with a small aperture and faint darkening glasses as with a large aperture and stronger ones ; this latter is the method I always use.' STUDY OF THE SUN'S SURFACE. 183 tinction of the spot, and in others, seems to bring on a subdivision.' On September 28, 1794, Sir William Herschel ob- served a spot similar in its general characteristics to that on the observation of which Wilson had based his hypothesis. ' There is a dark spot in the Sun on the following side. It is certainly depressed below the shining atmosphere, and has shelving sides of shining matter which rise up higher than the general surface, and are brightest at the top. The preceding shelving- side is rendered almost invisible by the over- hanging of the preceding elevations ; while the fol- lowing is very well exposed ; the spot being appa- rently such in figure as denotes a circular form viewed in an oblique direction. Near the following margin are many bright elevations close to visible depressions. The depressed parts are less bright than the common surface. The penumbra, as it is called, about this spot is a considerable plane, of less brightness than the common surface, and seems to be as much depressed below that surface as the spot is below the plane. Hence, if the brightness of the Sun is occasioned by the lucid atmosphere, the intensity of the brightness must be less where it is depressed ; for light being trans- parent, must be the more intense the more it is deep.' Having thus described the most striking of his first series of observations, Herschel now proceeds to enun- ciate his theory respecting the solar constitution. He remarks, in the first place, that it cannot be doubted but that the Sun has a very extensive atmo- 184 THE SUN. sphere; ' and that this atmosphere,' he proceeds, ' consists of various elastic fluids, that are more or less lucid and transparent, and of which the lucid one is that which furnishes us with light, seems also to be fully established by all the phenomena of its spots, of the faculse, and of the lucid surface itself. There is no kind of variety in these appearances but what may be accounted for with the greatest facility, from the continued agitation which we may easily conceive must take place in the regions of such elastic fluids.' After dwelling on certain illustrations drawn from the clouds in our own atmosphere, which Herschel (strangely enough) regards as ' probably decompositions of some of the elastic fluids of the atmosphere itself,' Herschel points out that the analogy of our own atmo- sphere will not be less to his purpose to whatever cause the clouds may owe their origin. ' The lucid clouds of the Sun, so to call them, plainly exist, because we see them ; the manner of their being generated may remain an hypothesis, and mine, till a better can be proposed, may stand good ; but whether it does or not, the consequences I am going to draw from what has been said will not be affected by it.'* * It is a peculiarity of Sir William Herschel's reasoning, that it is nearly always divided very definitely into two portions, 'which yet many who study his writings are apt to confound. We find certain con- clusions on which Sir William Herschel insists, and certain hypotheses which he simply enunciates. Eespecting these last, he has perhaps as often been in the wrong as in the right, and it indicates his surprising acumen, that so even a proportion should be observed in the case of mere hypotheses. Eespecting the former, I cannot recall one instance in which he has been proved to have been in error. Owing to his singular clearness of mental vision, and also in part to the extreme lucidity of STUDY OF THE SUN'S SURFACE. 185 Herschel then states that he regards the spots as regions where the atmosphere is free from lucid clouds, the faculae as regions where such clouds are more numerous than elsewhere. The penumbra being gene- rally depressed about half-way between the level of the nucleus and that of the photosphere, must of course be fainter than other parts. ' No spot favourable for taking measures having lately been on the Sun,' he adds, ' I can only judge from former appearances that the regions in which the luminous solar clouds are formed, adding thereto the elevation of the faculae, cannot be less than 1,843, nor much more than 2,765 miles in depth. It is true that in our atmosphere the extent of the clouds is limited to a very narrow com- pass ; but we ought rather to compare the solar ones to the luminous decompositions which take place in our aurora borealis, or luminous arches, which extend much farther than the cloudy regions. The density of the luminous solar clouds, though very great, may not be exceedingly more so than that of our aurora borealis. For if we consider what would be the bril- liancy of a space two or three thousand miles deep filled with such coruscations as we see now and then in our atmosphere, their apparent intensity, when viewed at the distance of the Sun, might not be much inferior to that of the lucid solar fluid.' his descriptions, one is very apt to forget, when he is describing mere hypotheses, that he is not discussing established conclusions ; and to this probably is due the fact that some of his warmest admirers do him the injustice of insisting as earnestly on views which he put forward simply as hypotheses, as though they had been enunciated by him as legitimate deductions from observed facts. 100 THE SUN. From the luminous atmosphere of the Sun, Herschel proceeds to the opaque body, which he surmises to be of great solidity, on account of the power it exerts upon the planets. From the phenomena of those dark spots which have been repeatedly seen in the same place, ' and otherwise denote inequalities in their level,' he suggests that the Sun's surface ' is diversified with mountains and valleys.' Then follows that remarkable passage which every student of astronomy knows by heart ; but which yet (even though we may not accept — as I confess I do not — the opinions suggested in it) will well bear re- petition : — ' The Sun, viewed in this light, appears to be nothing else than a very eminent, large, and lucid planet, evi- dently the first, or, in strictness of speaking, the only primary one of our system, all others being truly secondary to it. Its similarity to the other globes of the solar system with regard to its solidity, its atmo- sphere, and its diversified surface ; the rotation upon its axis, and the fall of heavy bodies, lead us on to suppose that it is most probably also inhabited, like the rest of the planets, by beings whose organs are adapted to the peculiar circumstances of that vast globe. Whatever fanciful poets might say in making the Sun the abode of blessed spirits, or angry moralists devise in pointing it out as a fit place for the punish- ment of the wicked, it does not appear that they had any other foundations than mere opinion and vague surmise; but now I think myself authorised, upon STUDY OF THE SUNS SURFACE. 187 astronomical principles, to propose the Sun as an in- habitable world, and am persuaded that the foregoing observations, with the conclusions I have drawn from them, are fully sufficient to answer every objection that may be made against it.' Herschel proceeds to consider the objection founded on the great heat which here at a distance of so many millions of miles we receive from the Sun, and the tremendous nature of the heat which consequently (one would suppose) must affect the imagined inhabi- tants of the Sun. Our admiration for the greatest astronomer of modern times must not cause us to lose sight of the fact that the reasoning at this stage of the inquiry is founded on inexact notions of the nature and laws of heat — though not such as in his day could have been unfavourably commented upon by most physicists. He remarks that the Sun's rays are ' the cause of the production of heat by uniting with the matter of fire which is contained in the substances that are heated.' He then instances the snow-covered summits of lofty mountains, and the cold experienced by aeronauts ; and he concludes, ' that we have only to admit that on the Sun itself, the elastic fluids composing its atmo- sphere, and the matter on its surface, are of such a nature as not to be capable of any excessive affection from its own rays, which seems indeed to be proved by their copious emission.'* * After noting other possible objections, Sir William Herschel — who did not disdain at times to be as imaginative and fanciful in theorising as he was exact and scrupulous in observing — proceeds to consider the possibility that the inhabitants of the Moon and of the satellites of IBS THE SUN. In a later paper (communicated to the Royal Society in 1801) Sir William Herschel records the results of further observations. He draws special at- tention to certain characteristic features of the Sun's surface. These are, first, corrugations, which he re- gards as elevations and depressions causing the mottled appearance of the Sun ; secondly, nodules, or smaller elevations in the corrugations themselves over which they are distributed as bright spots ; punctulations, or dark spaces between the nodules'; and pores, or' darker- coloured places in the punctulations.' He also enters into many particulars as to the behaviour of spots, pores, corrugations, nodules, and so on. To this valuable paper, as to the other from which I have quoted, I would invite the attention of every reader Jupiter, Saturn, and Uranus, regard the primary orbs round which they travel as mere attractive centres, to keep together their orbits, to direct their revolution round the Sun, and to supply them with reflected light in the absence of direct illumination. ' Ought we not,' he asks, ' to condemn their ignorance, as proceeding from want of attention and proper reflection ? It is very true that the Earth and those other planets that have satellites about them, perform all the offices that have been named for the inhabitants of these little globes ; but to. us, who live upon one of these planets, their reasonings cannot but appear very defective, when we see what a magnificent dwelling-place the Earth affords to numberless intelligent beings. These considerations ought to make the inhabitants of the planets wiser than we have supposed those of their satellites to be. We surely ought not, like them, to say, ' The Sun (that immense globe, whose body would much more than fill the whole orbit of the Moon) is merely an attractive centre to us. From experience we can affirm that the performance of the most salutary offices to inferior planets is not inconsistent with the dignity of superior purposes ; and in consequence of such analogical reasonings, assisted by telescopic views, which plainly favour the same opinion, we need not hesitate to admit that the Sun is richly stored with inhabitants.' STUDY OF THE SUN'S SURFACE. 189 interested in solar physics. Here, I shall only quote two observations bearing on the periodicity of the dis- turbances which affect the Sun's surface. We have seen that in 1671 Cassini had for a long time noticed the absence of Sun-spots. But on July 5, 1795, Sir William Herschel remarked that the Sun presented an appearance far more remarkable, and such, he re- marks, as differed wholly ' from what he had ever seen before. There was not a single opening in the whole disc ; there were no ridges or nodules, and no corruga- tions.' On December 9, 1798, he noticed that a similar state of things prevailed. We may sum up as follows the views of Sir William Herschel as to the general constitution of the solar globe and surface : — He supposed the Sun to be an opaque globe surrounded by a luminous envelope. He considered that this envelope is neither fluid nor gaseous, but consists rather of luminous clouds floating in a transparent atmosphere. Beneath this layer or envelope of luminous clouds he conceived that there floats in the same atmosphere a layer of opaque clouds, rendered luminous on the outside by the light which they receive from the outer layer. These opaque clouds protect, according to this theory, the solid and rela- tively unilluminated nucleus of the Sun. When open- ings are formed in the same region in both layers of clouds, we see the body of the Sun as a dark spot. If the apertures are equally large, the spot will be uniformly dark ; but if, as more commonly happens, the outer aperture is the greater, the dark nucleus of 190 THE SUN. the spot will seem to be surrounded by a dusky border. If the upper layer alone is perforated, a dusky spot without any dark central portion makes its appearance. Herschel supposed that those spots in which both layers are broken through, are caused by an uprush of some highly elastic gas breaking its way through the lower layer, and then, after expansion, removing the upper self-luminous clouds. We shall see that while all the facts observed by Herschel have been confirmed, and while his reasoning, so far as it relates to observed facts, has been abun- dantly justified, some of his hypotheses have been disproved by recent observations. I -pass on next to the researches of Sir John Herschel, recorded in that store-house of valuable facts, the ' Results of Astronomical Observations at the South Cape.' Sir John Herschel's observations led him to pay particular attention to a feature of the solar surface which had been first noted by Galileo. The spots are confined to two definite zones, extending about 35°* on each side of the equator ; an intermediate zone to a distance of some 8° on either side of the solar equator being ordinarily free from spots. Fig. 46 serves to indicate the regions where spots occur, and also (where * If we may trust an observation of La Hire's (which, however, Mr. Carrington, than whom no higher authority can be cited, is dis- posed to reject), a spot has been seen as far as 70° from the Sun's equator. In 1846, Dr. Peters, of Altona, saw a spot 50° 55' from the equator, while Carrington and Capocci have each seen spots about 45° from that circle. STUDY OF THE SUN'S SURFACE. 191 Fig. 46. the darkest zones are shown) those regions in which spots occur most frequently and attain the greatest dimen- sions. The Sun is so placed in the four views as to show the way in which the spot- belts are actually presented — I. early in December; II. early in March ; III. early in June ; and IV. early in September. The actual dates are those indicated under the several figures. I shall have to discuss, further on, those researches of Carrington and others into the laws of the Sun's rota- tion and the position of the solar equator* on which these dates and the presentations shown in fig. 46 are founded. It was to the explanation of this peculiarity that Sir John Herschel directed his chief attention. He remarks that the very existence of these zones ' at once refers the cause of spots to fluid * In some -works on astronomy pic- tures are admitted in which the effects of the Sun's inclination are altogether exaggerated.- 192 THE SUN. circulations, modified, if not produced by the Sun's rotation, by reasoning of the very same kind whereby we connect our own system of trade and anti-trade winds with the Earth's rotation. Having given any exciting cause for the circulation of atmospheric fluids from the poles to the equator and back again, or vice versa, the effect of rotation will necessarily be to modify those currents as our trade-winds and mon- soons are modified, and to dispose all those metero- logical phenomena (on a great scale) which accompany them as their visible manifestations, in zones parallel to the equator with a calm equatorial zone interposed.' Thus far, be it observed, Sir John Herschel is dealing with observed facts, and pointing to almost inevitable conclusions. He passes on (following in this respect, as in so many others, the procedure of his father) to hypothetical considerations, in dealing with the ques- tion — Whether any cause of atmospheric circulation can be found in the economy of the Sun, ' so far as we know and can comprehend it ? ' He is thus led to the inquiry whether a transparent atmosphere extends beyond the luminous surface of the Sun. He mentions the deficiency of light at the borders of the visible disc of the Sun, remarking that this feature is so obvious that he is surprised it should ever have been controverted. He mentions in corro- boration ' the extraordinary phenomenon of the rose- coloured solar clouds witnessed during the total eclipse of July 8, 1842, which must have floated in, and been sustained by, an exterior transparent atmo- STUDY OF THE SUN'S SURFACE. 193 sphere.' * And he suggests that this atmosphere must extend to some distance beyond the visible disc, because the darkening of the solar disc is not limited to the immediate neighbourhood of the edge, but ex- tends some distance within the disc.f Assuming the existence of such an atmosphere, the rotation of the Sun would cause the outer surface of the atmosphere to take up an oblately spheroidal figure, the least axis of which would correspond with the polar axis of the Sun. ' Consequently, the equatorial portions of this envelope must be of a thickness different from that of the polar, density for density, so that a different obstacle must be thereby opposed to the escape of heat from the equatorial and the polar regions of the Sun. The former therefore ought, according to this reasoning, to be habitually maintained at a different temperature from the latter.' ' The spots,' adds Sir John Herschel, ' would come, on this view of the subject, to be assimilated to those regions on the Earth's surface in which, for the moment, hurricanes * When Sir John Herschel thus wrote, less thoughtful astronomers were questioning whether these prominences belong to the Sun, whether they may not be lunar images, or phenomena of the Earth's atmo- sphere, or finally, whether they have any existence at all. t This argument, however, is not strictly sound. The extension of the darkening over the disc indicates shallowness rather than depth. This is easily seen, if we consider that were the atmosphere indefinitely deep, the luminosity of the disc would be uniform. In the actual case, of course, the atmosphere is not uniformly dense ; but still the reason- ing is analogous, and the extension of the darkening over the disc implies shallowness rather than the reverse ; though not the same degree of shallowness as would follow in the case of an atmosphere of uniform density. O 194 THE SUN. and tornadoes prevail — the upper stratum being tem- porarily carried downwards, displacing by its impetus the two strata of luminous matter beneath (which may be conceived as forming an habitually tranquil limit between the opposite upper and under currents), the upper of course to a greater extent than the lower, and thus wholly or partially denuding the opaque surface of the Sun below. Such processes cannot be unaccom- panied by vorticose motions, which, left to themselves, die away by degrees and dissipate — with this peculiarity, that their lower portions come to rest more speedily than their upper, by reason of the greater resistance below, as well as the remoteness from the point of action, which lies in a higher region, so that their centre (as seen in our water-spouts, which are nothing but small tornadoes) appears to retreat upwards. Now, this agrees perfectly with what is observed during the obliteration of the solar spots, which appear as if filled in by the collapse of their sides, the penumbra closing in upon the spot, and disappearing after it.' With all deference to one who is as high an authority in meteorological and thermological questions (which are both involved in this matter) as in astronomical matters, I must venture to point out what appears to me a flaw in the reasoning by which an excess of heat is assigned to the solar equator. If we assume that the depth of the solar atmosphere is really greater at the solar equator, then we cannot but admit as an in- evitable sequel that this atmosphere, checking as it does (and as the theory itself requires), the radiation STUDY OF THE SUN'S SURFACE. 195 of heat from the solar equatorial regions, to a greater extent than that from the polar regions, would cause the former regions as observed by us to' appear deficient in heat-rays (and presumably in light-rays also). The deficiency must certainly become observable if the amount of heat retained in this way were adequate to produce the effects described by Sir John Herschel. I am not endeavouring, be it understood, to negative the general conclusion to which Herschel has been led, that a difference of condition really prevails between the equatorial and polar regions to a degree sufficing to account for the spot zone as a zone of solar cyclones. Very probably this difference of temperature subsists, and almost certainly, whatever the cause may be, the spot zone is a zone of solar tornadoes. But the imagined action of a deep atmospheric layer over the solar equator seems incompatible with the observed appearance of the solar disc. We come next to a most important series of obser- vations directed, by Schwabe of Dessau, to the deter- mination of the laws according to which the number and size of the solar spots may vary from time to time. Such at least was the character which Schwabe's re- searches eventually assumed. At first they were directed to less important objects, though from the beginning the observation of the Sun on every day when its face can be seen formed a part of Schwabe's plan.* * ' I went out like Saul,' he afterwards said, ' to seek my father's asses, and lo ! I found a kingdom.' o 2 196 THE SUN. His observations began in the year 1826, during which many spots were visible from time to time upon the Sun's face ; indeed, on the 277 days during which the weather permitted Schwabe to observe the Sun, there were but twenty-two on which no spbts could be seen. In the next year there were even more spots, and only two days when none were seen. In the next two years the Sun's face was not on any day seen without spots. In 1830 only one day occurred on which no spots could be seen ; in 1831, only three. But in 1832 there were no less than forty-nine days (out of 270 on which observations were made) during which no spots were seen. In 1833 there were 139 such days out of 267 ; in 1834, 120 out of 273 ; in 1835, 18 out of 244 ; and then followed four years during which not a single observing day occurred on which no spots were visible. Schwabe recognised even at this early stage of the inquiry (only twelve years having as yet elapsed since he began his researches) that a certain periodicity marks the occurrence of Sun-spots, or, rather, the re- currence of years rich and poor in such phenomena. It is true that as he had noted but one full period, it might seem that he had absolutely no evidence on which to ground such a view. But in the observation of periodic variations, there are other features besides the periodic return of maxima or minima to guide the experienced observer. The progression towards and from the maximum or minimum is as instructive when carefully watched as the recurrence of. many maxima. STUDY OF THE SUN'S SURFACE. 197 The observer requires only to assure himself that this progression possesses certain characteristics, to feel assured that he is dealing with no accidental relations; but with true periodic changes. These characteristics are, chiefly, a steady (not uniform) progression from maximum to minimum, and vice versa, a rapidity of change midway between maximum and minimum as com- pared with the rate of change near either of these stages, and a certain uniformity of character in the progression towards each stage (though the progression from mini- mum to maximum may not resemble in character the return towards the minimum). These charac- teristics Schwabe noted, and he felt satisfied that the numerical relations of the solar spots vary in a truly periodic manner. But he felt that further observations were necessary to convince the scientific world — always slow to recog- nise new truths, or to accept results not rendered palpable, so to speak, by an accumulation of evidence. He laboured on therefore for twenty more years, tra- cing the gradual increase and diminution of spots in frequency and in their general dimensions, until he had completed the observation of no less than three complete oscillations from maximum frequency through mini- mum back to maximum again. It began to be felt lhat Schwabe was establishing his case, and accordingly the scientific societies of Europe (somewhat tardily, it must be admitted) accepted the result which Schwabe had demonstrated, — viz., that the solar spots increase and diminish in frequency and size in a period of about 198 THE SUN. ten years. The following remarks, addressed by the Rev. Mr. Main, then President of the Eoyal Astrono- mical Society, in awarding to Schwabe the Society's Gold Medal for 1857, justly express the importance of Schwabe's researches : — ' What the Council wish most emphatically to express is their admiration of the indomitable zeal and untiring energy which Herr Schwabe has displayed in bringing that research to a successful issue. Twelve years he spent to satisfy himself; six more years to satisfy, and still thirteen more to convince mankind. For thirty years never has the Sun exhibited his disc above the horizon of Dessau without being confronted by Schwabe's imperturbable telescope, and that ap- pears to have happened on an average about 300 days a year. So that supposing he observed but once a day, he has made 9,000 observations, in the course of which he discovered 4,700 groups. This is, I believe, an instance of devoted persistence (if the word were not equivocal, I should say pertinacity) unsurpassed in the annals of astronomy. The energy of one man has revealed a phenomenon which had eluded even the suspicion of astronomers for 200 years.' But even after these thirty-one years of labour, Schwabe continued his observations. With the excep- tion of a few weeks during which he was unwell, he has watched the Sun as pertinaciously as ever until the present time. The results of his labours (so far as the question of the periodicity of spot-frequency is con- cerned) are included in the table on p. 200. STUDY OF THE SUN'S SURFACE. 199 The careful study of Schwabe's results (combined with such scattered records as the observations of former astronomers supply) has led Professor Wolf, of Zurich, to the conclusion that a period of 11 "11 years (or the ninth part of a century) is indicated, rather than a ten-yearly period. He also recognises the existence of minor periods. ' He finds,' says Sir John Herschel, ' that a perceptibly greater degree of apparent activity prevails annually, on the average, in the months of September to January, than in the other months of the year ; and, again, by projecting all the results in a continuous curve, he finds a series of small undulations succeeding each other at an average interval of 7 '65 months, or 0*637 of a year. Now, the periodic time of Venus (225 a ), reduced to a fraction of a year, is O6I6,— a coincidence certainly near enough to warrant some considerable suspicion of a physical connection.' A long period, estimated at about fifty-six years, has also been suspected. The most cursory examination of the numbers in the table given in the next page, suffices to indicate the peculiarity that the progression from minimum to maximum is more rapid than the progression from maximum to minimum. In other words, if we regard the periodic changes of spot-frequency in the light of a series of waves — the maxima corresponding to the crests of the waves and the minima to the ' troughs ' — the front slope of each wave is more abrupt than the rear slope. One of the most remarkable of the results following 200 THE SUN. Year Days of observation Days -without spots New groups 1826 277 22 118 1827 273 2 161 1828 282 225 1829 244 199 1830 217 1 190 1831 239 3 149 1832 270 49 84 1833 267 139 33 1834 273 120 51 1835 244 18 173 1836 200 272 1837 168 333 1838 202 282 1839 205 162 1840 263 3 152 1841 283 15 102 1842 307 64 68 1843 312 149 34 1844 321 111 52 1845 332 29 114 1846 314 1 157 1847 276 257 1848 278 330 1849 285 238 1850 308 2 186 1851 308 141 1852 337 2 125 1853 299 4 91 1854 334 65 67 1855 313 146 28 1856 321 193 34 1857 324 52 98 1858 335 202 1859 343 205 1860 332 211 1861 322 204 1862 317 3 160 1863 330 2 124 1864 325 4 130 1865 307 26 93 1866 349 76 45 1867 312 195 25 1868 301 12 101 1869* 196 224 * This line is from the records of the Kew Observatory. STUDY OF THE SUN'S SURFACE. 201 Sohwabe's noble discovery was the recognition of an association between the Sun-spot period and magnetic disturbances on the Earth. In every part of the Earth the magnetic needle has at any given epoch a certain definite position, about which, under normal conditions, it would oscillate during the day. Both as regards inclination and direction with respect to the compass-points (called the magnetic declination), this position may be regarded as determi- nate, at least for every fixed observatory ; and, further, the intensity of the needle's directive power, that is, the energy with which, if slightly disturbed, it seeks to recover its position of rest, may • also be regarded as determinate. From year to year all these magnetic elements undergo change ; but with these changes we are not here concerned.* Changes of a much more * I may note here my belief that the recognition of the laws affect- ing the secular variation of the Earth's magnetism would be simplified if attention were primarily directed to the magnetic lines determined by the inclination of the needle, instead of to those which depend on the intensity of the directive action. Eully admitting the weight of General Sabine's arguments as to the importance of the intensity, and as to its being the more essentially magnetic element, as it were, (north and south, or vertical and horizontal, having no direct relation to the Earth's magnetic forces,) I yet cannot but regard the inclination as affording the more trustworthy means of determining the geographical features (so to speak) of terrestrial magnetism. As the matter is of some importance, I will exhibit the reasons on which I found this opinion, leaving others to judge whether they are or are not valid. The intensity, as well as the inclination and declination (which together form one magnetic feature, — the position of rest), is doubtless affected by local circumstances ; so that, superadded, as it were, on the normal intensity, there is an intensity depending on local conditions ; and so in like manner, as to the inclination and declination. Now, the question arises, which of these two features is likely to be most significantly affected by these doubtless slight peculiarities ? In determining, for 202 THE SUN. minute character, and the changes affecting these changes, are what we have at present to deal with. Each day the needle oscillates gently about its position of rest, the oscillation corresponding to a very slight tendency on the part of that end of the needle which lies nearest to the Sun to direct itself towards his place. The daily oscillation is itself variable in a systematic manner, not only with the progress of the year, but with that of the lunar month. The daily example, the position of the magnetic equator and poles, with reference to intensity or inclination, would the error due to some small increment or decrement of intensity cause a greater or less divergence from the true position than a correspondingly small increment or decrement of inclination ? The answer is obvious. The intensity-equator is the line of minimum intensity, and the intensity poles are points of maximum intensity. Near a minimum or a maximum, quantities change very slowly, and thus a very minute increment or decrement would largely shift the estimated place of minimum or maximum intensity. But a corresponding increment or decrement of inclination would have no such effect, because the inclination changes as quickly near the inclina- tion-equator and poles as on any inclination-latitude. The case may fairly be compared to the determination of the geo- graphical equator and poles. Undoubtedly gravity is a far more essentially terrestrial element than the elevation of the pole star, or of the true pole of the heavens ; and also, undoubtedly, the Earth's equator is the region where gravity is least, while the poles are regions where gravity is greatest. Yet these reasons are not considered sufficient to induce us to take the force of gravity as the most satisfactory indication of latitude, or to lead us to mark down as the true equator of the Earth that line along which careful observation shows that gravity has its minimum value. We know, in fact, that, however excellent the observations might be, the deduced line would differ very importantly from the true equator. A similar objection might be urged on like grounds against the stress laid on the position of the line of no declination ; since from the very nature of this line minute local peculiarities must cause enormous irregularities, and (when coupled with secular variations) the most rapid and remarkable changes of figure. STUDY OF THE SUN'S SURFACE. 203 oscillation also varies at times in a sudden and irregular manner. The needle has been exhibiting for several weeks the most perfect uniformity of oscillation. Day after day the careful observation of the needle's pro- gress has revealed a steady swaying to and fro, such as may be seen in the masts of a stately ship at anchor on the scarce-heaving breast of ocean. Suddenly a change is noted ; irregular movements become per- ceptible which are totally distinct from the regular periodic oscillations. A magnetic storm is in progress, and its progress does not affect only the place of observation, but widely-extended regions of the Earth ; and in some well-authenticated instances, the magnetic vibrations thrill in one moment through the whole frame of the Earth. Lamont, of Munich, was the first to announce that these magnetic disturbances attain a maximum of frequency in periods of about ten years. This was in 1850. Two years later General Sabine and (in- dependently) Professors Wolf and Gautier noted the coincidence of this period with that of the solar spots. Of course, mere coincidence in duration was not the sole circumstance on which they based this conclusion'. It was the coincidence of maximum of spot-frequency with maximum of magnetic disturbance, and of mini- mum with minimum, which enabled them to assert the true correspondence of the two periods. Wolf subsequently proved that the period of mag- netic disturbances has the length of 11 '11 years, which he had assigned as the correct value of the solar spot period. 204 THE SUN. A relation so strange might well excite grave doubts. Coincidences have so often misled men of science, and indeed it is so certain in the very nature of things that misleading coincidences must occur, that physicists were justified in requiring further evidence. Such evidence fortunately was not long wanting. Independently of the continuance of observation and the close cor- respondence which has been observed during the past score of years between Sun-spots and magnetic phe- nomena, an occurrence of a very interesting nature served in 1859 to place beyond all possibility of ques- tion the influence which solar action exerts upon the Earth's magnetism. I have so often described the occurrence in my own words that I think it well in the present instance to give the words of the two telescopists by whom it was independently observed, especially as the details of the observation have re- cently been called in question. On September 1, 1859, Mr. Carrington was engaged in making his customary observation of the forms and positions of the solar spots. ' I had secured diagrams,' he says, ' of all the groups and detached spots, and was engaged at the time in counting from a chrono- meter and recording the contacts of the spots with the cross-wires used in the observation, when within the area of the great north group (the size of which had previously excited general remark) two patches of intensely bright and white light broke out ' in the middle of the group. ' My first impression was,' he adds, ' that by some chance a ray of light had pene- STUDY OF THE SUN'S SURFACE. 205 trated a hole in the screen attached to the object glass, by which the general image is thrown into shade, for the brilliancy was fully equal to that of direct sun- light ; but by at once interrupting the current obser- vation, and causing the image to move by turning the right ascension handle, I saw I was an unprepared witness of a very different affair. I thereupon noted down the time by the chronometer, and seeing the out- break to be very rapidly on the increase, and being somewhat flurried by the surprise, I hastily ran to call some one to witness the exhibition with me, and on returning within 60 seconds, was mortified to find that it was already much changed and en- feebled. Very shortly afterwards the last trace was gone, and although I maintained a strict watch for nearly an hour no recurrence took place. The spots had travelled considerably from their first position, and vanished as two rapidly fading dots of white light. The instant of the first outburst was not fifteen seconds different from llh. 18m. Green- wich mean time, and llh. 23m. was taken for the time of disappearance. In this interval of five minutes, the two spots traversed a space of about 35,000 miles. It was impossible on first witnessing an appearance so similar to a sudden conflagration, not to expect a con- siderable result in the way of alteration of the details of the group in which it occurred ; and I was certainly surprised, on referring to the sketch which I had care- fully and satisfactorily (and I may add, fortunately) finished before the occurrence, at finding myself un- 206 THE SUN. able to recognise any change whatever as having taken place. The impression left upon me is that the phe- nomenon took place at an elevation considerably above the general surface of the Sun, and accordingly altogether above and over the great group on which it was seen projected. Both in figure and position the patches of light seemed entirely independent of the configuration of the great spot, and of its parts, whether nucleus or umbra.' Mr. Hodgson's account (written before he had ' ex- changed any information ' with Mr. Carrington) runs as follows : — ' While observing a group of spots on September 1, I was suddenly surprised at the appear- ance of a very brilliant star of light, much brighter than the Sun's surface, most dazzling to the protected eye, illuminating the upper edges of the adjacent spots and streaks, not unlike in effect the edging of the clouds at sunset; the rays extended in all directions ; and the centre might be compared to the dazzling bril- liancy of the bright, star Alpha Lyrse when seen in a large telescope with low power. It lasted for some five minutes, and disappeared instantaneously, about llh. 25m. a.m.' It seems probable that whereas two spots were seen by Mr. Carrington, who observed the solar image projected on a screen, these were blended, owing to their extreme brilliancy, into the semblance of a single spot, when observed in the telescope itself by Mr. Hodgson. At the moment when the Sun was thus disturbed, the magnetic instruments at Kew exhibited those STUDY OF THE SUN'S SURFACE. 207 signs which indicate the occurrence of a magnetic storm. ' It was found,' says Dr. Balfour Stewart, ' that a magnetic disturbance had broken out at the very moment when this singular appearance had been observed.' But this was not all. A magnetic storm never rages without accompanying signs of disturbance. Auroras in both hemispheres, and the interruption of magnetic communication all over the Earth, are the signs of a great magnetic storm. Both these evidences of great disturbance were afforded during the hours which followed the solar outbreak witnessed by Car- rington and Hodgson. Vivid auroras were seen not only in both hemispheres, but in latitudes where auroras are very seldom witnessed. Even in Cuba the sky was illuminated by the auroral radiance. Strong earth-currents were observed along telegraphic lines, and these currents continually changed their direction, while all the time the magnetic needles in fixed ob- servatories were kept markedly on one side of their normal position. ' By degrees,' says Sir John Hers- chel,* ' accounts began to pour in of great auroras seen on the nights of those days, not only in these latitudes but at Borne; in the "West Indies; on the tropics within 18° of the equator (where they hardly ever * I have been careful, it will be noticed, to quote the words of many authorities in dealing with this matter. The account I gave in my Other Worlds than Ours has been spoken of as though it had been based on imagination ; so that I have thought it well to re-exatnine the records and statements from which the account, was really taken (though by the aid of memory instead of transcription). Although several years had elapsed since I read those statements, I do net find that my account required correction in a single essential particular. 208 THE SUN. appear) ; nay, what is still more striking, in South America and in Australia, — where, at Melbourne, on the night of September 2, the greatest aurora ever seen there made its appearance. These auroras were accompanied with unusually great electro-magnetic disturbances in every part of the world. In many places the telegraph wires struck work. They had too many private messages of their own to convey. At AYashington and Philadelphia, in America, the telegraphic signal-men received severe electric shocks. At a station in Norway, the telegraphic apparatus was set fire to ; and at Boston, in North America, a flame of fire followed the pen of Bain's electric tele- graph (which writes down the message upon chemically- prepared paper).' It is demonstrated then that some association exists between the disturbance of the solar photosphere and the phenomena of terrestrial magnetism. What the nature of the association may be is not so clearly ap- parent. We have seen that the solar spot period has been supposed to be associated by Wolf with the mo- tions of tbe planets, and we shall presently see that the phenomena of spots, — as their change of form and of size, the regions in which they appear, and so on, — have been conceived to depend on the planetary motions. If these views be correct, we should have to account for laws of association between the planetary motions, terres- trial magnetism, and solar spots. But, which of these three orders of phenomena should be regarded as the cause of the others ; or whether the association be of the STUDY OF THE SUN'S SURFACE. 209 nature of cause and effect, and not rather due to some as yet unknown common cause of the two latter classes of phenomena ; or whether, finally, the planets' motions, without heing the direct cause of the other phenomena, yet indirectly bring them about, remains to be de- termined.* Let us next turn to Mr. Carrington's researches, intimately associated with Schwabe's, and among the most important of all the contributions which have been made in recent times to our acquaintance with solar physics. The object to \riiich Mr. Carrington specially devoted his, energies was the endeavour to detect regularity in the distribution of the spots, the determi- nation of the true period of rotation of the body of the * I would point out, however, that the consideration of the asso- ciation here discussed led General Sabine, Professor Challis, and Dr. Stewart to inquire whether the coloured prominences of the Sun may not be due to solar auroras. Since we now know that the prominences are not of this nature, may we not transfer the suggestion to the solar corona and the zodiacal light? May not these be the true solar auroras? The latter, at any rate, gives the same spectrum as the aurora, and the former gives a spectrum closely analogous. If, as General Sabine suggests, an auroral outburst in the Sun ' may perhaps be responded to simultaneously by the different planets, so that the whole solar system would seem to thrill almost like a living being under the magnetic excitement,' it seems at least probable that the solar auroras would extend to distances enormously exceeding those assigned them when the prominences were taken to be such auroras. And as we have abundant evidence that terrestrial auroras occur where the atmosphere has an inconceivable rarity, it would seem possible that under the enormous action of the Sun, even the quasi-vacuum of the interplanetary spaces might be traversed by electrical discharges. The meteoric systems occupying more or less densely the whole of these spaces, would seem to afford the requisite channel for this electric action. 210 THE SUN. Sun," and the detection of ' systematic movements or currents of the surface, if such existed in a definable manner.' In these researches he perfectly succeeded. It would be desirable if space permitted to describe Carrington's method of observation, and the exact and systematic processes by which he deduced his results. I must be content, however, so far as these points are concerned, to refer the curious reader to Carrington's voluminous and masterly treatise, in which also will be found an interesting account of the results obtained by former observers. Here I have room only for the conclusions to which he was led«by his series of obser- vations, which commenced in the year 1853 and ended in the year 1861. He discovered, in the first place, that the discrepan- cies between the values formerly deduced for the Sun's rotation, arise from real differences in the velo- cities with which the spots move in different solar latitudes. Near the equator a spot moves at a rate indicating a more rapid rotation than in higher latitudes. Further, even among spots in the same latitude, proper motions may be recognized. These latter motions are to be regarded, however, as abnormal, and simply rendering unreliable such observations as are made on but a few spots. The peculiarities affecting the motions of spots in different latitudes have been reduced by Mr. Carrington into a formula.* The following table * This formula is as follows : — Let | he the angle through which a part of the Sun in latitude \ rotates in one day. Then i = 14° 25'- 165' sin ' \. STUDY OF THE SUN'S SURFACE. 211 gives the observed rates of rotation for different lati- tudes (the formula being based on these values) : — deg. 50 N. Lat. Sun's rotation-period. d. h. m. 27 10 41 Kotation per day 787 30 „ 26 9 46 824 20 „ 15 „ 25 25 17 9 8 10 840 851 10 „ 25 3 29 859 • 5 „ Equator 5 S. Lat * 25 24 24 2 23 42 11 18 863 867 865 10 „ 25 5 35 856 15 „ 25 13 31 845 20 „ 25 17 52 839 30 „ 26 12 50 814 45 „ 28 11 759 Sporer gives the formula | = 16°-8475 - 3°-3812 (sin \ + 41° 13'). * It is remarkable that in all southern latitudes the observed daily- mean rotation is less than in the corresponding northern latitudes. It is doubtful whether we have in this relation any indication of the true cause of the observed variations in the rate of rotation, or merely a peculiarity which would have disappeared in a long series of observa- tions. In favour of the former view, we have the consideration that the determination for each southern as well as for each northern lati- tude was independently effected, so that the coincidence of the results ndicates the existence of some real cause. If Sir John Herschel is right in considering that the more rapid rotation near the solar equator implies the action of external matter in maintaining the rotation of the photosphere, it may be suggested that the northern surface of the Sun being directed somewhat more fully towards that region whither the Sun's proper motion is carrying him (see the concluding chapter of this work, and the illustrative cuts), would probably be more exposed to the in- fluence of this external action — ' the frictional impulse of circulating planetary matter in process of subsidence into, and absorption by, the central body' — much as our northern hemisphere is saluted with a large number of meteoric missiles from June to December, when the northern hemisphere is in advance, then from December to June, when this hemisphere is towards the more sheltered side of the Earth. V 2 212 THE SUN. I shall have occasion to dwell further on upon the significance of the varying rotation-period deduced for different solar latitudes. This result of Mr. Carrington's labours cannot but be regarded as one of the most im- portant contributions recently made to our knowledge of solar physics. Sporer has re-examined the whole subject, taking into account later observations, and in particular those which have been made by Fr. Secchi. The following table includes the general elements of the solar rota- tion as deduced by Carrington and Sporer, and reduced by Secchi to the year 1869 : — Longitude of node of solar equator j 73° 57' Inclination of solar equatur 7 Jo Diurnal rotation | 14 18 Rotation-period i 25 d ' 38 Sporer 74° 37' 6 57 14 2664 2.5 J - 2340 (No weight can be attached to the last two decimal figures in Sporer's value of the diurnal rotation and the rotation period.) In fig. 46 the varying presentation of the Sun towards the Earth on account of the inclina- tion of his equator to the Earth's orbit is exhibited as exactly as possible. On or about December 6,* the Earth crosses the plane of the Sun's equator (passing * The date for any year can always be determined from the almanac. It is only necessary to note when the Sun's longitude is 180° + the longitude of node of Sun's equator (say 180 + 74°, or 254°). In like manner, the Earth is again in the plane of the solar equator when the Sun's longitude is ahout 74°. The solar longitude is given for each dav in Hannay's Almanac, in the last column of the first page for each month. STUDY OF THE SUN'S SURFACE. 213 southwards), and then the Sun is presented as at 1, fig. 46. Three months later, the Earth reaches her greatest distance south of the solar equator, so that on or about March 6 the Sun is presented as at ti. On about June 5 the Earth again crosses the plane of the solar equator, this time passing northwards, and the Sun is presented as at in. Lastly, on about Sep- tember 5, the Earth reaches her greatest distance north of the solar equator, and the Sun is presented as at iv. The observations of Sohwabe and Carrington have been continued, under improved conditions, by De La Rue, Stewart, and Loewy. The powers of photography, under the able superintendence of Mr. De La Rue, have been applied to secure records of the aspect of the solar disc on every clear day. But independently of the valuable series of records thus obtained, the three physicists above-named have undertaken a careful scru- tiny, not only of the solar photographs taken at Cran- ford and afterwards at Kew, but of the observations made by former students of the solar surfaee. Accumu- lating a vast mass of records, they have applied pro- cesses of statistical research to educe any information respecting the Sun, which, though in reality contained in observations already made, may not lie at or even near the surface.* In the three papers by De La Rue, Stewart, and * It is a promising sign of progress when students of science are thus willing to discuss the labours of others as well as their own, undeterred by the fear of being called mere theorists. 21+ THE SUN. Loewy, entitled ' Researches on Solar Physics,' we find an extension and elaboration of the modes of research employed by Wilson, Schwabe, and Wolf. Where Wilson inquired into the behaviour of individual spots, our three allies discuss the peculiarities presented by hundreds of spots. Where Schwabe discussed the number of new groups of spots, they consider the area of the spotted portion of the Sun's surface. And, lastly, where Wolf examined the evidence which the nume- rical statistics respecting spots afford in favour of the theory that the planets exert an influence on the solar envelopes, De La Hue and his colleagues inquire into the behaviour of individual spots as they approach or cross the region towards which the several planets lie ; they examine the general distribution of spots as re- spects proximity to the equator under certain circum- stances of planetary position; and by discovering other like peculiarities they obtain evidence altogether dis- tinct in character from that adduced by professor Wolf. Taking first the question whether the umbrae of spots are below the level of the photosphere, the inquirers examine 605 observed cases of spots having measurable penumbra, and they find that while in seveDty-five in- stances the penumbra is equal on both sides (referring to Dr. Wilson's mode of dealing with the question,) in 456 instances the penumbra was widest on the side nearest the limb, while in seventy-four the penumbra was narrowest on that side. Hence the percentage of favourable instances is 75-37 ; of unfavourable cases, STUDY OF THE SUN'S SURFACE. 215 12*23 ; and of neutral cases, 12"40.* This seems to place the existence of a real depression beyond question ; while, at the same time, it demonstrates the truth of what Carrington has said respecting Wilson's obser- vations, — that ' there is more variety in the appearances than "Wilson confesses to, and there are marked depar- tures from his description of form, which is rather one specific type out of several which might be adduced, and will be familiar to every one when photography has furnished us with forms on which all, whether observers or not, may rely.' I would invite the special attention of the reader at this point to Plate I., where Mr. Browning has delineated a case in which two spots close by each other exhibit altogether dif- ferent characteristics, one agreeing with Dr. Wilson's description, the other presenting an opposite peculiarity. I was much struck with the great variety observable in this respect, when I was drawing the picture which illustrates this chapter (Plate II.), though the low power I employed did not permit me to see the minuter details of the spots. f * In the Researches the neutral instances are not taken into account, and thus the percentage of favourable cases becomes 86'04, and that of unfavourable cases 13 - 96. But the rejection of neutral cases is not in accordance with the accepted rules for dealing with such matters. I can discover no reason for making an exception in this particular case. f The telescope I employed was a very small one, about 2^ inches in aperture. I used an ordinary erecting eye-piece, having a power of about 26. Thus seen, the Sun appeared in the middle of a large field, ' and I could scarcely have believed that I should have been able to recognise the features I had seen with larger telescopes and higher powers. I had never before examined the Sun with so low a power (for four years before the day on which the design for Plate II. was drawn I 2l6 TEE SUN. Xext, investigating the relative position of spots and their accompaning faculae, our inquirers find that out of 1,137 spots, 584 have their faculae either wholly or mostly on the left — that is, behind them, as respects the motion of solar rotation, 508 have the faculae nearly equally on both sides, while only forty-five have their faculse mostly on the right. ' The most natural ex- planation of this would be,' they say, ' that the faculae of a spot have been uplifted from the very area occupied by the spot, and have fallen behind to the left from being thrown up into a region of greater velocity of rotation.' Thirdly, they attack the following question : — ' Is a spot, including both umbra and penumbra, a pheno- menon which takes place beneath the level of the Sun's photosphere or above it ? ' They note in evidence on this matter, that there are many instances in which ' a bridge of luminous matter of the same apparent luminosity as the surrounding photosphere, and unac- companied by any penumbra, appears to cross over the umbra or centre of a spot.' Detached portions of luminous matter are also seen at times to move across a spot without producing any permanent alteration. ' On these accounts,' say the inquirers, ' we are disposed to think that a spot, including both umbra and had had few opportunities for observation), and it was with a sense of considerable pleasure that I found the familiar features coming clearly into view as I scrutinised the tiny image more and more searchingly. I found myself able to comprehend better than ever before how Galileo with his small telescope and low power (30) had been able to detect so many features of the Sun's surface. STUDY OF THE SUN'S SURFACE. 2IJ penumbra, is a phenomenon which takes place beneath the level of the brighter part of the Sun's photos- phere.' Summing up the results of this portion of their researches, they express their belief that, — 1. The umbra of a spot is nearer the Sun's centre than the penumbra, or, in other words, it is at a lower level. 2. Solar faculte, and probably also the whole pho- tosphere, consist of solid or liquid bodies of greater or less magnitude, either slowly sinking or suspended in equilibrium in a gaseous medium. 3. A spot including both umbra and penumbra is a phenomenon which takes place beneath the level of the Sun's photosphere. As respects the sequent series of researches by which Messrs. De La Rue, Stewart, and Loewy have endeavoured to estimate the influence of the planets upon the solar spots, it is to be remarked that the evidence adduced seems as yet not wholly decisive. They believe that it has been rendered probable that Venus exerts a special influence on the solar spots, and that the conjunctions of the planets also affect importantly the condition of the solar photosphere. There is room, in my judgment, for some doubt as to the justice of either conclusion. It should not be forgotten that the planetary system presents so many periodic relations as to render it almost certain that any observed periodic changes in the Sun's condition may be associated statistically with some period of planetary motion — sidereal, synodical, nodical, or other- 2l8 THE SUN. wise.* There is a remark towards the close of Carrington's volume on the solar spots, which bears very significantly on this subject. After exhibiting the relation between the phenomena of the solar spots (as tabulated by Professor Wolf) and Jupiter's varia- tions of distance, he says, that 'from the year 1770, there is a very fair agreement between maxima of frequency and maxima of Jupiter's radius vector, and between minima and minima ; ' . . . but ' in the two periods which precede that date there appears to be a total disagreement.' ' It is important,' he then adds, ' to see before us an instance in which eight consecutive cases of general, but imperfect agreement, between the variations of two physical phenomena, are shown to be insufficient to base any conclusion upon, at the same time that they powerfully stimulate further inquiry with the view of ascertaining whether the discrepancy may admit of future explanation.' I would by no means be understood to imply, how- ever, that I regard the conclusions ' of Messrs. De La Rue, Stewart, and Loewy, respecting the influences exerted by the planets on solar phenomena, as inadmis- sible. On the contrary, I regard them as, on the whole, the most probable yet advanced. Based as they are on observed facts, and on statistical relations, they are worthy of the most attentive consideration. * I was much struck with this fact when perusing a valuable contri- bution by Professor Kirkwood to the subject of planetary influences on solar phenomena. His long experience in dealing with such matters enables him to exhibit relation after relation, each showing a remark- ably close agreement, as respects period, with periodic solar phenomena. STUDY OF THE SUN'S SURFACE. 219 They do not seem to me, however, to be by any means demonstrated, nor are they so regarded (it is proper to add) by their propounders. It remains only that 1 should indicate in a general, and necessarily brief manner, those features of the Sun's surface which recent observations have revealed to us. Let me begin with the famous ' willow-leaves.' It was announced by Mr. Nasmyth in 1862 that the pores seen in the solar photosphere are ' polygonal in- terstices ' (I quote Sir John Herschel's account) ' be- tween certain luminous objects of an exceedingly definite shape and general uniformity of size, whose form (at least as seen in projection in the central portions of the discs) is that of the oblong leaves of a willow-tree. These cover the whole disc of the Sun (excepting the space occupied by the spots) in countless millions, and lie crossing each other in every imaginable direction.' The appearance of the Sun, according to this view, is exhibited in figs. 47 and 48, both of which are from drawings by Mr. Nasmyth. This announcement led to a controversy which still remains undecided. Mr. Dawes asserted his belief that no such interlacing as Mr. Nasmyth described is ever observable among the small bright spots which lie scattered over the general ground of the photosphere ; that these spots can in no sense be said to resemble willow-leaves, though they present every variety of figure and size ; and, lastly, that they had been long known to solar observers, and are, in fact, no other than the nodules of Sir William Herschel. ' The only 220 THE SVK. situation,' he wrote, ' in which I have usually noticed them to assume anything like the shape of willow- Fig. 47. Sun-spot observed by Nasmyth, showing three bridges composed of solar willow-leaves. (Nasmyth.) leaves, is in the immediate vicinity of considerable spots, on their penumbrae, and frequently projecting beyond it for a small distance on to the umbra, — an appearance with respect to which, in April, 1852, I used the following expressions : — " The interior edge of the penumbra frequently appears extremely jagged ; the bright ridges on its surface, which are directed nearly towards the centre of the spot, being seen pro- jected to irregular distances on to the cloudy stratum (or umbra), and looking much like a piece of coarse thatching with straw, the edge of which has been left untrimmed." After nearly twelve years of careful STUDY OF THE SUN'S SURFACE. 221 Fig. 48.'— A large spot-group, showing the solar willow-leaves. 222 THE SUN. observation of the same phenomena, I do not think that I could improve upon this description.' The appearance here described will be recognised in Mr. Fig. 49. (J Sun-spots, showing penumbral rills. (Secchi.) Browning's drawing (Plate I.) ; it is also observable in figs. 49 and 50. Other astronomers now joined in the discussion. Messrs. Stone and Dunkin, of Greenwich, asserted that with the fine equatorial of that observatory, luminous spots shaped like rice-grains could be seen, and STUDY OF THE SUN'S SURFACE. 223 that these spots, by overlapping, produce the dark pores.. Fr. Secchi described the appearance of the luminous spots as resembling strokes made with a camel's-hair pencil. He described them, in fact, in Fig. 50. Illustrating the markings of the penumbrae of spots. (Capocci.) terms closely according with those employed by Mr. Dawes. Dr. Huggins speaks of these luminous objects (or granules, as he calls them) as certainly not interlacing on the general surface of the photosphere. Fi°\ 51 represents a view of a portion of the Sun's disc as seen by him. It will be noticed that if this view is held at a considerable distance from the eye the general aspect corresponds closely with the mottled appearance presented by the Sun in telescopes of moderate power.* Other observers agree with Mr. Nasmyth, who adheres, I believe, to his original opinion. As regards * In drawing Plate II. with the low power and small aperture described at p. 215, I was struck with the perfect distinctness of this mottling all over the disc. 224 THE SUN. the assertion that Sir William Herschel had long since recognised these objects, it is of course to be rejected or accepted according as Mr. Nasmyth's opinion respecting Fig. 51. A portion of the solar surface, showing granules. (Hmggins.) the real nature of the objects is confirmed or confuted. If there really is the interlacing he speaks of on the general surface of the Sun, then undoubtedly he is the STUDY OF THE SUN'S SURFACE. 225 discoverer of the phenomenon, for Sir William Her- schel nowhere speaks of any such appearance (to my recollection, at least). Sir John Herschel, too, deals with Nasmyth's observation as undoubtedly new and also as of unmistakable interest and importance.* * Let me here quote Sir John Herschel's account of Nasmyth's willow- leaf theory, partly on account of its liveliness and graphic clearness, partly for the sake of his reflections on the subject : — ' These leave s, or scales, are not arranged,' he says, ' in any order (as those of a butter- fly's wings are), but lie crossing one another in all directions, like what are called spills in the game of spillikins ; except at the borders of a spot, where they point for the most part inwards, towards the middle of the spot, presenting much the sort of appearance that the small leaves of some water-plants or sea-weeds do at the edge of a deep hole of clear water. The exceedingly definite shape of these objects ; their exact similarity one to another ; and the way in which the y lie across and athwart each other (except where they form a sort of bridge across a spot, in which case they seem to affect a common direction, that, namely, of the bridge itself) — all these characters seem quite repugnant to the notion of their being of a vaporous, a cloudy, or a fluid nature. Nothing remains but to consider them as separate and independent sheets, flakes, or scales, having some sort of solidity. And thesp flakes, be they what they may, and whatever may be said about the dashing of meteoric stones into the Sun's atmosphere, etc., are evidently the immediate sources of the solar light and heat, by whatever mechanism or whatever processes they may be enabled to develope, and as it were elaborate, these elements from the bosom of the non-luminous fluid in which they appear to float. Looked at in this point of view, we caDnot refuse to regard them as organisms of some peculiar and amazing kind ; and though it would be too daring to speak of such organization as partaking of the nature of life, yet we do know that vital action is competent to develope at once heat, and light, and electricity. These wonderful objects have been seen by others as well as by Mr. Nasmyth, so that there is no room to doubt of their reality. To be seen at all, however, even with the highest magnifying powers our telescopes will bear when applied to the Sun, they can hardly be less than a thousand miles in length, and two or three hundred in breadth.' So that if these things are solar inhabitants whose fiery constitution enables them to illuminate, warm, and electricise the whole solar system, they are not wanting in that evidence of might whk-h gigantic size affords. Truly, Milton's picture of him who on tho fires of Q 226 THE SUN. Before leaving the subject of the willow-leaves, I must quote one of the latest observations bearing on this subject. On April 13, 1869, Secchi noticed that over the whole of a large spot and its neighbourhood there were multitudes of leaves, and that a bridge across the spot was formed of elongated leaves. The leaves in the neighbourhood of the spot were oval, the greater diameter about three times the less — thus, -^^ ' like the leaves of the olive and certain salices? He says, "What are these things ? There are veils of the most intricate structure. The spot has a third nucleus in which are the leaves seen in the drawing (fig. 52), all Fig. 52. arranged in a radiating manner, precisely like a crys- tallization of sal-ammoniac seen by means of the solar microscope — attached to a stalk {lingua). It is clear that the rest of the nucleus is due to the rarefaction of such leaves. There cannot be a shadow of doubt on this matter.' On April 14, the spot in which Secchi had observed these singular objects presented the appearance Hell ' lay floating many a rood,' seems tame and commonplace compared ■with Herschel's conception of thes» floating monsters, the least covering a greater space than the British Islands. STUDY OF THE SUN'S SURFACE. 227 depicted in fig. 53. 'It was in a marvellous condition says Secchi, ' full of bridges, arcs, stalks, and leaves, like the great spot of 1866.'* The discovery by Dawes that within the umbra, or Fig. S3. Sun-spot observed on April 14, 1869, by Secchi. what was formerly called the nucleus of every fully- developed spot, there exists a darker region, or (so far * Memoria III. sngli spettri prismatici de' corpi celest . Secchi assigns the following dimensions to this remarkable group :— Total diameter in length 2' 37" •5 Diameter of the two principal nuclei 0' 23" •78 Breadth of the spot , 0' 42' •03 Breadth of the nucleus . 0' 23' ■70 Breadth of the bridge . 0" I' •50 «2 228 THE SUN. as telescopic research has yet gone) a true nucleus, not only opens a new field for speculation, but renders the interpretation of the phenomena before recognised altogether more difficult. By using a solar eye-piece of his own invention, in which the field of view was so contracted as to exclude even the light from the penumbra; of large spots, he detected in the stratum which had before been regarded as black, ' a mottled appearance — the degree of darkness being by no means uniform, and suggesting the idea that the surface is far from level, the lighter parts being probably the most elevated, and feebly reflecting the light received from the self-luminous strata above it. . . In all spots which are tolerably symmetrical, and large enough to admit of accurate scrutiny, this umbra will be found to be perforated near its centre by a perfectly black hole, which is to be regarded as the true nucleus.' From the researches of Fr. Secchi it would ap- pear as though the umbral portion of large spots were formed of luminous matter which is undergoing a continual process of dissipation towards its interior edge. He compares the process to the gradual dissi- pation of cumulus clouds under the heat of a summer Sun, and regards the umbra as of the nature of a veil of clouds ; the nucleus as a region where an intenser heat has caused these clouds to melt away. Within spots such as these he has recognised the presence of coloured matter, which he compares to the coloured envelope whence the prominences spring ; and it was partly on account of his having noted such appearances STUDY OF THE SUN'S SURFACE. 229 that he pronounced, so far back as 1860, his conviction that an envelope of this coloured matter — the chromato- sphere of the next chapter — surrounds the whole globe of the Sun. These appearances have been noticed also by other observers. Mr. Lockyer has recognised the gradual disappearance of portions of luminous matter ^as if by subsidence in some semi-transparent medium) in the umbrae of large spots ; while the presence of various tints of red within the spots has been noticed also by Schwabe, Capocci, Schmidt, and other ob- servers. The processes of formation, enlargement, and dis- appearance of spots are well worthy of study ; and although no regular law has been detected in their succession, we can yet recognise certain distinctive features ordinarily belonging to each stage of develop- ment. The formation of a spot is usually preceded by the appearance of faculas. Then a dark point makes its appearance which increases in size, the penumbral fringe being presently recognised around it, and the distinction between the umbra and the penumbra being well defined. The same clearness of definition con- tinues ordinarily until and after the spot has reached its greatest development. But when the spot is about to diminish, there is a change in this respect. The edges seem less sharp, and an appearance is presented as though there floated over them a luminous cloud- veil, brighter in some places than others, and not unfrequently attaining a brightness which seems to exceed even that of the facula?. At certain parts of 230 THE SUN. the spot's circumference, this bright matter projects, hiding the whole width of the penumbra and forming a sort of cape or promontory, with sharply serrated edges, singularly well defined against the dark back- ground of the umbra. It is usually in this manner that the formation of a bridge begins, two promontories on opposite sides of a spot, or even on the same side, joining their extremities, so as to form either a bridge of light across the umbra, or a curved streak having both its extremities on one side of the spot. But indeed no strict law or sequence has yet been assigned to these processes of change. In a large spot the wildest and most fantastic variations will take place, and often when the spot seems approaching the stage of disappearance, it will seem to renew its existence, as though fresh forces were at work in disturbing the region it belongs to. Some of the processes of change which take place in large spots are very well exemplified in fig. 54, where the drawings 1 to 4 show the successive changes of appearance presented by the great spot of 186.5, from October 7, when it was on the Sun's eastern limb, until October 16, when it had passed the central part of the disc. These drawings were made by the Rev. Mr. Howlett, one of our most enthusiastic solar observers, and specially skilled in the delineation of Sun spots. Were not his accuracy beyond all question, it would be worth while to note the fact that on October 16, Chacornac made a drawing of the same spot, corre- sponding so closely with 4, fig. 54, that one might well STUDY OF THE SUN'S SURFACE. 231 Fig. 54. is mmw A> fjB£ '4 The great Sun-spot of 1865, from Oct. 7 to Oct. 16. (Howlett.) suppose the two drawings were tracings from one 232 THE SUN. picture.* The reader will be prepared to examine with so much the greater confidence the interesting picture of a spot with the surrounding faculse (fig. 55), drawn by M. Chacornac. He will recognise also the close Fig. 55. . *\;~~t***- . .f.\3Wfij ,' wi;/ . :\ ClfMt^ '.•'■';.-' V / '/• ,./:>. to '. * jr v IfvJyi f ) ..; u> .: .. -';-:>" <* '' 1 ^ ■' ~^' WlfrM) 8* * ~*- ' ':' - >S y^gjf. lit #®$ « //.) .f&£ ■:""'*> '---' ■- - ; ' ■&.--, ? . -.-k >"--' . -''. ''">' -. z * fi ^—"' n '> vfv /.< Sri £ ■>> ,"..' I" tjfc c^>- Facula near a Sun-spot. {Chacornac.) correspondence between the appearance of the region around this spot and Sir William Herschel's descrip- tion of the faculae (p. 180). * Unfortunately such extreme accuracy of delineation is not ordinarily to be met with among solar observers. STUDY OF THE SUN'S SURFACE. 233 We owe to Mr. Dawes the detection in certain -pots of a rotatory motion, as though these regions were the scene of some tremendous solar tornado. A spot of enormous dimensions was observed by him to have rotated through half a complete circuit in the course of about six days. Other spots have exhibited an even more rapid motion, and the spectroscopic Fig. 56. A spot presenting the appearance of cyclonic motion. (Secchi.) observations made by Mr. Lockyer on parts of the Sun near the limb (where such cyclonic motions would necessarily involve a rapid motion towards and from the eye) seem to place beyond question the existence of solar tornadoes having a velocity of 40 or 50, in some cases even 120 miles per second. So that we need no longer regard (with Sporer) the whirling 234 THE SUN. appearance noticed in the accompanying drawing by Fr. Secchi (fig. 56) as due to optical illusion. The immense dimensions of some spots well deserve thoughtful consideration. When we remember that the least spot which could be perceived with the most powerful telescope must have an area of at least 50,000 miles, it will be understood how enormous these spots must be which have been distinctly visible to the unaided eye.* But we have trustworthy measurements to refer to in this matter. Pastorff, in 1828, measured a spot whose umbra had an extent four times greater than the Earth's surface. In August, 1859, a spot was measured by Newall which had a diameter of 58,000 miles — that is, exceeding more than seven times the diameter of our Earth. But spots of even greater dimensions have been observed. In June, 1843, a spot was visible, which, according to Schwabe's measurements, had a length of no less than 74,816 miles. On March 15, 1858, observers of the great eclipse had the good fortune to witness the passage of the Moon over a spot which had a breadth of 107,520 miles. It was in the same year that the largest spot of any whose records have been handed down to lis, was visible upon the solar disc. It had a breadth of more than 143,500 miles; so that across it no less than eighteen globes as large as our Earth might have been placed side by side. At a very moderate computation of the depth of this solar cavity, it may * Of the groups shown in Plate II. three were visible to the naked eye, the largest of them (near the centre) being quite conspicuous. STUDY OF THE SUN'S SURFACE. 235 be assumed that the mass of 100 earths such as ours would barely have sufficed to fill it to the level of the solar photosphere. During the past two years many spots and groups of enormous extent have been noticed. Those shown in Plate II. may be cited as instances ; but others fully as large have lately been observed. The rapidity with which some spots have changed in figure, or even wholly disappeared, would be wholly incredible were it not that astronomers of the highest repute for accuracy have supplied the records of such changes. Dr. Wollaston says: — f I once saw with a 12-inch reflector a spot which burst in pieces as I was looking at it. I could not expect such an event, and therefore cannot be certain of the exact particu- lars ; but the appearance, as it struck me at the time, was like that of a piece of ice when dashed on a frozen pond, which breaks in pieces and slides on the surface in various' directions.'* Biela also notes that spots disappear sometimes almost in a single moment. The converse of such a change has been witnessed by Krone, who observed a spot of no inconsiderable dimensions which sprang into existence in less than a minute of time. On one occasion a momentary dis- traction caused Sir William Herschel to turn away his * Of course this description refers only to the appearance which . the spots ordinarily present, of being real bodies rather than openings. What "Wollaston has described as the breaking up of a spot into pieces must in reality be looked upon in all probability as the sudden change of a single whirlstorm into a number of smaller ones. 236 THE SUN. eyes from a group of spots he was observing. When he looked again the group had vanished ! In this place I shall not enter into the discussion of the nature of the spots — a matter, indeed, on which at present it is very difficult to form an opinion. It has been my purpose in this chapter to consider rather the evidence which has been adduced respecting the solar surface than the physical theories put forward in explanation of that evidence. Two points remain, however, to be briefly touched upon. The first is the evidence supplied by the spectro- scope respecting the spots. It is evident that by bringing a spot under the slit of the spectroscope in the way described at the close of the preceding chapter (see fig. 35) it becomes possible to institute a direct comparison between the spectrum of the umbra, penumbra, and surrounding faculse. If any lines belonging to the ordinary solar spectrum dis- appear in these regions, or if new lines make their appearance, we can at once become cognisant of the fact, because we see the spectra of these regions simultaneously. In like manner we can determine whether any change takes place in the character and appearance of any solar line — for instance, whether it is wider or narrower in the spectra of certain regions, or whether it changes into a bright line. Now, fig. 57 will illustrate the peculiarities which make their ap- pearance when a spot is brought in this way under examination. Here the length of the spectrum (only STUDY OF THE SUITS SURFACE 23; a small portion of which is shown in the figure) is 1 horizontal, so that the vertical lines are the dark lines of the spectrum. The horizontal lines indicate the regions of the spectrum corresponding to those parts of a spot where a general absorption takes place. It 1 will be seen that where this general absorption is suffi- cient only to produce a degradation of brilliancy, all the lines in this part of the spectrum are visible. The Fig. S7. G. Ml. in 1 Sec. ill 1 Sec. G. Ml. I 1; Towards tte ye Illustrating the changes in certain lines in the spectra of Sun-spots. F line (belonging to hydrogen) is, however, peculiarly affected across the whole region of the spot. At the upper and lower extremity we see it of its normal width, while over all the remaining breadth of the spectrum, except two small portions, it is much broader and has shaded edges. In one place it is bent ; along another part of its length a narrow line of light is seen to be almost centrally placed upon it ; and lastly in two places it appears bright and irregularly shaped. 238 THE SUN. Now, the facts here noticed (first observed I believe by Mr. Lockyer, and confirmed by Dr. Huggins, Fr. Secchi, and Professor Young) are thus (probably) to be interpreted. Beginning with the top of the line F in fio\ 57, we have first the normal black line show- ing that in the part of the Sun included within the " uppermost part of the slit, the hydrogen is in its ordinary condition as respects temperature. It is less heated than the matter "whence the main portion of the solar light is radiated, and so absorbs that portion of the light which it has itself the power of radiating. Next we come to a bright hydrogen line of the normal width on a shaded background. The corresponding part of the Sun (that is the part next below the former within the slit) is, then, either somewhat reduced in temperature, or else partially covered by generally ab- sorbing matter, over which there is a layer of hydrogen at the normal pressure, but more heated than the radiat- ing region below. Hence in this place the hydrogen radiates more light than it absorbs, and the line f is rendered relatively bright. Next is a region where the hydrogen line is still bright but very much wider. Over the corresponding portion of the Sun, therefore, the hydrogen not only exists at a higher temperature but at a greater pressure. Then we come to the widened dark line, indicating that over the corre- sponding portion of the Sun there is hydrogen at a relatively low temperature and at an abnormally high pressure. The bend towards the red end of the spec- trum indicates that the corresponding portion of the STUDY OF THE SUN'S SURFACE. 239 hydrogen envelope is moving from the eye,* or, in other words, that there is in this part of the Sun a downrush of hydrogen. Where we see a relatively bright line superposed on the relatively dark one, we learn that above the compressed hydrogen at a relatively low temperature there is a layer (or tongue, or prominence) of more heated hydrogen. While, lastly, where we see the pointed dark line close to the bottom of the figure, we learn that above hydrogen as heated as the general radiating substance of the Sun, there is the usual layer of hydrogen at lower temperature, very shallow where the line is pointed, but deepening within a short distance to its normal condition. Along a narrow strip, then, crossing the width of a solar spot, all these varieties of condition are thus recognisable. Nor is hydrogen the only element whose lines exhibit such peculiarities. The lines of sodium, magnesium, barium, and other elements, have been observed to exhibit similar indications of violent action, rapid motion, and remarkable changes of pressure. But perhaps the most striking of all the phenomena revealed by the spectroscope is the occurrence, in the spectra, of large spots, of lines and bands corresponding to those due to the presence of aqueous vapour in our own atmosphere. Fr. Secchi not only testifies to this, but * The vertical dotted lines 1, 2, 3, on either side of F indicate how far the line F should be shifted to indicate a velocity of 8, 16, and 24 geographical miles respectively from or towards the eye. The decimal figures between the vertical lines numbered 1, 1, indicate the length of the light-waves (in parts of a millimetre), corresponding to the part of the spectrum where the Imp. f is. 240 THE SUN. he describes experiments by which he convinced himself that these lines really belong to the spots, and not to our own atmosphere. He found that these ' water-lines ' were not visible when the instrument was directed in clear weather to unspotted parts of the solar disc ; but that as the instrument was shifted, the approach of a spot was clearly indicated by the appear- ance and gradually intensifying of these lines. When the sky was covered with thin clouds he saw the same lines towards whatever part of the Sun the instrument was directed ; but they always appeared strongest in the spectrum of a spot.* The second point which I wish to notice, in conclu- sion, is the evidence of the polariscope respecting the general condition of the solar photosphere. I do not feel justified in giving space to an account of the principles on which polariscopic analysis depends, be- cause, to say the truth, the polariscope has thrown but little light on the subject of solar physics. I therefore merely state that light under certain conditions of emission, reflection, and refraction, acquires a peculiar property called polarisation, by which its capability for subsequent reflection or refraction is materially modified. "We have here to deal with emission ; and the special law which concerns us is this, that light emitted from an incandescent solid or liquid at a * The reader is referred for fuller details than there is here space for 1o Dr. Schellen's work Die Spectralanalyse, already referred to. The English Edition lately published, under the able supervision of Dr. Huggins, is specially worthy of very careful study in all matters relat- ing to the spectral analysis of the Sun. STUDY OF THE SUN'S SURFACE. 241 very oblique angle is partially polarised in such sort that when incident on a plane at right angles with the angle of emanation, the polarised portion does not, like the rest, undergo reflection. Now, the light from near the edge of the Sun's disc shows no signs of having this particular quality. Hence, Arago and others have concluded that the solar photosphere cannot be formed of incandescent solid or liquid substance, but must neces- sarily be gaseous. We shall have occasion further on to consider the bearing of this evidence on the views we are to form respecting the Sun's physical constitu- tion. It is necessary to note, however, that Sir John Herschel has called in question Arago's conclusion; and, without asserting that the solar photosphere must necessarily be either solid or liquid, he has shown that the evidence of the polariscope is more than question- able, since the Sun can by no means be regarded as a smooth, uniform globe. Its surface is, in all probability!, so rough and uneven that the light received from near the edge may come for the most part from surfaces nearly at right angles to the visual line.* Here I conclude my survey of the solar surface. I have presented but such portions of the vast mass of material really available as seemed most instructive * The case may be compared with that of the Moon. If the Moon were a smooth, uniform globe, she ought, when full, to seem much darker near the edge than near the centre of her disc. That she does not is due to the inequalities of her surface ; and Dr. Zollner has been able to show from the observed luminosity of the Moon at different times that the probable average inclination of the lunar mountains is about 56 degrees. 242 THE SUIf. and to bear most pertinently on the views we are to form respecting the Sun's physical condition. Ten or twelve such volumes as the present would be needed to contain even an abstract of the observations which have been made on the great central luminary of our system. To the student of solar physics I can- not too earnestly recommend the careful study of all such observations as he can obtain access to ; but it has been necessary for my present purpose to give a more general view of the subject. This chapter must be regarded as bearing the same relation to the ponderous volumes in which our Carringtons and Herschels, our Schwabes and Sporers and Secchis, have recorded their observations, as Plate II. bears to Plate I., or, generally, to such large-scale and elabo- rate views as are afforded by powerful telescopes. We must now pass on to other matters well worthy of attention — to matters that are perhaps even more inte- resting and instructive than the facts which astrono- mers have discovered in their survey of the solar surface. THE PROMINENCES AND THE SIERRA. 243 CHAPTER V. THE PROMINENCES AND THE SIERRA. The coloured prominences which have recently attracted so large a share of the attention of solar physicists were first fully recognised during the total solar eclipse of 1842. It is probable, however, that they were seen more than a century before that date, though their real nature was not suspected. During the total solar eclipse of May 2, 1733, Vassenius, at Gottenburg, observed several red clouds floating, as he supposed, in the atmosphere. One of them seemed larger than the rest, and appeared to be composed of three masses placed one above the other, and com- pletely detached from the Moon's limb. ' These spots seemed,' he writes, ' composed in each instance of three smaller parts or cloudy patches of unequal length, having a certain degree of obliquity to the Moon's periphery. Having directed the attention of my com- panion, who had the eyes of a lynx, to the phenomenon, I drew a sketch of its aspect. But while he, not being accustomed to the use of the telescope, was unable to find the Moon, I again, with great delight, B 2 244 THE SUN. perceived the same spot, or, if you choose, rather the invariable cloud occupying its former situation in the atmosphere near the Moon's periphery.' "We need not be surprised that Vassenius assigns the spots without scruple to the Moon's atmosphere, since it was thought by many in his time that the Moon has an atmosphere of appreciable extent. Yet it was unfortunate for science that the prominences (for we can scarcely doubt that the appearances seen by Vassenius were really prominences) should have been thus explained away as relatively unimportant phenomena, since otherwise observers during succeeding eclipses would probably have searched for similar objects, and we might thus have possessed a long series of observations tending to indicate the laws according to which these objects make their appearance. For more than a century eclipse passed after eclipse, and no observer recognised these flames of coloured light, which have seemed to the observers of recent eclipses so striking and obvious. Ferrer, indeed, in 1806, and Van Swinden, in 1820, noticed faint traces of some peculiar coloured appendages ; but their ob- servations were not satisfactory, nor was any attention drawn to the subject. During the great eclipse of 1842, however, a number of first-rate observers were distributed along the line of total obscuration. Airy, Arago, and the younger Struve; Littrow, Baily, Santini, Valz, and Biela — a host, in fine, of the most skilful astronomers in Europe — watched the eclipse with careful scrutiny. All of them THE PROMINENCES AND THE SIERRA. 245 recognised with surprise the presence of rose-coloured prominences round the disc of the eclipsed Sun. The Astronomer- Royal saw three prominences at the summit of the disc ; Arago, Struve, and Schid- lowski saw two near the lowest point of the disc; Schumacher, of Vienna, saw three — two below, and one above. It will be instructive to consider the account given by the first observers of these interesting objects. The Astronomer-Royal compared them to the In- clined teeth in a circular saw, and estimated their height at about one minute of arc. Schumacher compared the protuberances to ice- bergs, and the pictures which illustrate his paper re- present them as much more closely resembling ice- bergs than any protuberances seen in recent times. We may not unfairly conclude that Schumacher's drawings are somewhat idealised. Baily compared the prominences to Alpine peaks coloured by a setting Sun. He noticed that one was bifurcated almost to its base. M. Mauvais employs a similar comparison. He had seen a reddish point soon after the Sun was totally obscured. ' When fifty-six seconds had passed after the commencement of totality,' he writes, ' this reddish point transformed itself into two protuberances, resembling two adjacent mountains, and well defined. Their colour was not uniform, streaks of a deeper red marking their flanks. I can- not describe them better than by comparing them to distant Alpine peaks, illuminated by the rays of the 246 THE SUN. setting Sun. One minute and ten seconds from the time of total obscuration a third mountain became visible to the left of the other two. In colour it re- sembled the others. Beside it were some smaller peaks, all of them well defined.' Mauvais noticed that the other two protuberances grew higher while the third was making its appearance. Near the end of the eclipse they were no less than two minutes of arc in height. Biela, Schumacher, and others recognised a border of rose-coloured light surrounding a part of the Moon's limb at a lower level than that attained by the pro- minences. It is worthy of note that this pheno- menon had been noticed earlier than the prominences themselves ; for during the total eclipse of 1706, Captain Stannyan remarked that a blood-coloured streak of Kght appeared, before the Sun's limb emerged from behind the Moon. In 1715, also, Halley noticed that two or three seconds before the emersion, the Moon's limb appeared to be tinged with a dusky but strong, red light, forming a long and narrow streak ; and during the same eclipse, Louville saw what he describes as an arc of deep red colour along the edge of the Moon's disc. The latter astronomer was careful to assure himself that the appearance was no illusion, and to this end he brought the red arc into the middle of the telescopic field of view, when he found that the red colour remained unchanged. Don Ulloa, in 1778, and Ferrer, in 1806, had noticed a similar pheno- menon. THE PROMINENCES AND THE SIERRA. 24/ When the various accounts of the eclipse of 1842 came before the astronomical world, several theories were propounded in explanation of the red prominences. The theory that they are mountains in the Sun was for a while in favour; but Arago pointed out that some of them were too considerably inclined to the perpendicular to be so regarded. Others supposed them to be clouds in the solar atmosphere ; while others, again, suspected them to be enormous flames. As ordinarily happens in such cases, there were not want- ing those who denied that the coloured prominences had any real existence whatever. M. Faye, for ex- ample, asserted his belief that they are purely optical illusions— 'mirages, perhaps, produced near the Moon's surface.' The eclipse of 1851 removed these doubts for the most part, though it is to be noted in passing that despite the evidence obtained then, and yet again in 1860, there were some who continued, even until the great Indian eclipse of 1868, to deny that the coloured prominences and the rose-tinted arcs seen at a lower level could really be regarded as solar appendages. During the total eclipse of 1851 many observers of great skill made drawings of the very remarkable pro- minences which were visible on that occasion. These pictures exhibited a sufficiently satisfactory agreement to convince the observers that they had all witnessed the same phenomena ; though the discrepancies between the pictures afford instructive evidence of the difficulty of delineating with exactness the details presented 248 THE SUN. during eclipses. The following six pictures represent in order the work of the Astronomer-Royal, Mr. Dawes, Mr. Hind, Mr. Lassell, Mr. Gray, and Mr. Stephenson. Mr. Airy thus writes respecting the prominences : — ' The form of the prominences was most remarkable. That which I have marked a (fig. 58) reminded me of Fig. 58. Fig. 59. Fig. 60. Fig. 61. Fig. 62. Fig. 63. The coloured prominences seen during the eclipse of 1851. a boomerang. Its colour for at least two-thirds of its breadth — from the convexity towards the concavity- — was full lake-red ; the remainder was nearly white. The most brilliant part of it was the swell farthest from the Moon's limb ; this was distinctly seen by myself and my friends with the naked eye. I did not measure its height ; but judging generally by its proportion to the Moon's diameter, it must have been three minutes of arc. This estimation perhaps belongs to a later THE PROMINENCES AND THE SIERRA. 249 period of the eclipse. The prominence b was a pale white semicircle based on the Moon's limb. That marked c was a red detached cloud or balloon of nearly circular form, separated from the Moon's limb by a space differing in no way from the rest of the corona of nearly its own breadth. That marked d was a small tri- angular, or conical, red mountain, perhaps a little white in the interior. These were the appearances seen instantly after the formation of the totality. I em- ployed myself in an attempt to delineate roughly the appearances on the western limb, and I took a hasty view of the country; I then remarked the Moon a second time. I believe (but I did not carefully re- mark) that the prominences a b c had increased in height ; but d had now disappeared, and a new one, e, had risen up. It was impossible to see this change without feeling the conviction that the prominences belonged to the Sun, and not to the Moon. I again looked round, when I saw a scene of unexpected beauty. I went to my telescope with a hope that I might be able to make the polarization observations, when I saw that the sierra, or rugged line of projec- tions shown at /, had arisen. The sierra was more brilliant than the other prominences, and its colour was nearly scarlet. The other prominences had per- haps increased in height, but no additional new ones had arisen. The appearance of the sierra nearly in the place where I had expected the appearance of the Sun warned me not now to attempt any other physical observations. In a short time the white Sun 250 THE SUN. burst forth, and the corona and every prominence vanished.' Mr. Hind's narrative is as follows : — ' On first view- ing the Sun without the dark glass after the com- mencement of totality, three rose-coloured prominences immediately caught my eye, and others were seen a few seconds later. The largest and most remark- able of them (a in Mr. Airy's drawing) was straight through two-thirds of its length, but curved like a sabre near the extremity, the concave edge being towards the horizon. The edges were of a full rose- colour, the central parts paler, though still pink. Twenty seconds, or thereabouts, after the disappearance of the Sun, I estimated its length at forty-five seconds of arc, and on attentively watching it towards the end of totality I saw it materially lengthened— probably to two minutes — the Moon having apparently left more and more of it visible as she travelled across the Sun. It was always curved, and I did not remark any change of form, nor the slightest motion during the time the Sun was hidden. I saw this extraordinary prominence four seconds after the end of totality, but at this time it appeared detached from the Sun's limb, the strong white light of the corona intervening between the limb and the base of the prominence. About ten degrees south of the above object I saw during the totality a detached triangular spot of the same rose-colour, sus- pended, as it were, in the light of the corona, which gradually receded from the Moon's dark limb, as she moved onwards, and was therefore clearly connected THE PROMINENCES AND THE SIERRA. 25 1 With the Sun. Its form and position with respect to the large prominence continued exactly the same so long as I observed it. On the south limb of the Moon appeared a long range of rose-coloured flames, which seemed to be affected with a tremulous motion, though not to any great extent. The bright rose-red of the tops of these projections, gradually faded towards their bases, and along the Moon's limb appeared a bright narrow line of a deep violet tint ; not far from the western extremity of this long range of red flames was an isolated prominence, about forty seconds in altitude, and another of similar size and form at an angle of 145° from the north towards the east.' I may add Mr. Dawes' account of the great promi- nence marked a in Mr. Airy's picture (fig. 58). ' A red protuberance of vivid brightness and very deep tint arose to a height perhaps 1J when first seen, and increased in length to 2' or more, as the Moon's progress revealed it more completely. In shape it somewhat resembled a Turkish scimitar, the northern edge being convex, and the southern concave. Towards the apex it bent suddenly to the south, — or upwards, as seen in the telescope. Its northern edge was well defined, and of a deeper colour than the rest, especially towards its base. I should call it a rich carmine. The southern edge was less distinctly defined, and decidedly paler. It gave me the impression of a somewhat conical protuberance, partly hidden on its southern side by some intervening substance of a soft or flocculent character. The apex of this protuberance 252 THE SUN. was paler than the base, and of a purplish tinge, and it certainly had a flickering motion. Its base was, from first to last, sharply bounded by the edge of the Moon. To my great astonishment, this marvellous object con- tinued visible for about Jive seconds, as nearly as I could judge, after the Sun began to reappear, which took place many degrees to the south of the situation it occupied on the Moon's circumference. It then rapidly faded away, but it did not vanish instantaneously . From its extraordinary size, curious form, deep colour, and vivid brightness, this protuberance absorbed much of my attention ; and I am therefore unable to state precisely what changes occurred in the other phenomena towards the end of the total obscura- tion.'* Such are a few of the records of the appearance presented by the prominences during the eclipse of 1851. It would have been easy to have filled forty or fifty pages with the narratives of the different obser- vers, many of them skilful and well-practised astrono- * The evidence of Mr. Dawes is very valuable on account of his exceptional powers of vision. No astronomer save M. Goldschmidt has surpassed him in this respect. It may therefore be regarded as fortunate that he addressed his sole attention to one prominence ; since some of the facts he detected are such as no later observations could have more satisfactorily established. Such, for instance, is his observation of the flickering motion of the upper part of the prominence. He was too well accustomed to recognise the apparent motions produced by our own atmosphere to be deceived into inferring real motion where none existed. His observation of the visibility of the prominence for several seconds after the Sun's reappearance confirms Mr. Hind's, and the fact is one of extreme importance, as tending to afford a measure of the luminosity of the larger prominences. THE PROMINENCES AND THE SIERRA. 253 mers. All agreed as to the principal details, and, as will be seen by figs. 58-63, there was a very satis- factory agreement in the pictures taken by different observers. It would appear that no doubt could any longer remain that the prominences were solar appen- dages of some sort. They had been visibly traversed by the Moon, according to the unexceptionable evidence of such astronomers as Airy, Hind, Dawes, and others. They had continued visible when our atmosphere had already begun to be lighted up by the direct rays of the returning Sun. At stations far apart they had presented the same appearance. It would seem there- fore that nothing was wanting to establish their real relation to the solar orb, and that no question should any longer have existed as to the fact that the promi- nences are true solar appendages, since the proofs were so complete that they belonged neither to the Moon nor to our own atmosphere, and, further, that they were not mere optical illusions. Yet, in the face of all this evidence, some astronomers were still found who maintained that the observations were insufficient to establish the existence of coloured objects so enormous as these must be if they really were solar appendages. It has always happened that in the ranks of the scientific army some have been found who refuse to credit the marvels which obser- vation is continually revealing on every hand. Des- pite all the known wonders of the universe, the mere circumstance that the sole available interpretation of observed facts involves some surprising conclusion, is 254 the sun. held by such men to be a sufficient reason for rejecting the observations of the most trustworthy astronomers.* So it was in this instance. For nine long years astronomy was compelled to wait before she could be allowed to take possession of her well-won new territory. The amazing fact had been proved beyond all possi- bility of reasonable question that the great globe of the Sun is surrounded by a deep layer of coloured matters; while from portions of this vast envelope * One is almost ready to despair of the cause of scientific progress — to despair at least that that progress will ever he so rapid as it might readily become — when one finds that each new result must he estab- lished over and over again before it is admitted by a large proportion of the scientific world. It may be remarked, indeed, that the progress of science has been at least as seriously checked by undue caution as by undue boldness. It would seem almost as though some students of science were continually in dread lest the work of our observers should become too productive. The value of scientific observation seems to be enhanced in their eyes precisely in proportion as its fruits are insignificant. In all ages there have been those who would thus unwisely restrain the progress of legitimate inquiry. ' "We must not admit that Jupiter has moons,' they said of old ; ' the Evil One may have sent these appearances to deceive us. Let us wait for further observation.' ' The Sun cannot have spots,' they reasoned again, ' for the Eye of the Universe cannot suffer from ophthalmia. These things are illusions ; let us wait for more satisfactory observations.' ' The idea that the Sun-spots wax and wane in a definite period is too fanciful for acceptance; and still more absurd is the conception that terres- trial magnetism can have any relations whatever with the progress of solar disturbance. "We must wait for fresh researches.' ' "Who can believe that flames, or clouds, or mountains, many times exceeding the Earth in magnitude, exist upon or close by the Sun ? These things must needs be illusions ; at any rate, fresh observations are required before such marvels can be admitted.' And as this has happened with facts now accepted, so it is happening, and so (it is feared) it will always happen, as respects many other facts which have been in truth demon- stated, but the demonstration of which does not chance to lie exactly on the surface. THE PROMINENCES AND THE SIERRA. 255 enormous protuberances start out, their height so vast that ten globes such as our Earth might be piled one upon the other on the Sun's surface without attaining to the summit of the highest prominences. But this great fact was not to take its place in our treatises of astronomy until, although twice proved already, it had been proved once again at least. Accordingly, in 1860, when a total eclipse was to be visible in Spain, preparations were made for finally resolving the problem of the prominences. A host of skilful observers devoted their powers to demonstrating what had already been abundantly demonstrated. It happened fortunately, however, that amongst the astronomers who took part in observing this important eclipse, there were some few who duly recognized the importance of the occasion, and who, therefore, leaving fruitless labours to others, applied themselves to solv- ing important questions respecting the coloured promi- nences. Their results I now propose to describe ; but, in the first place, I will quote the account which Gold- schmidt, one of the most skilful telescopists of modern times, gave of the prominences visible on this occasion. Some of the facts recorded by him are of extreme interest and importance, especially as respects the colour of the prominences, since M. Groldschmidt's practice as a painter gave him exceptional experience in this respect. M. Goldschmidt employed a telescope of four inches in aperture, magnifying about forty times. About half a minute before totality he could distinguish ' little 256 THE SUN. grey clouds, isolated in part, and floating outside the solar disc at some distance from the edges. One of these isolated clouds of a rounded form, and another of an elongated form which touched the exterior edge of the Sun, were observed to be of a grey colour on the ground of the sky, which was a little brighter. An iustant afterwards the pyramidical cloud became more clear, and then rose-coloured.' ' I had thus been present,' adds Goldschmidt, ' at the formation of a protuberance ' — a remark which has been somewhat misunderstood through being quoted apart from the context. Clearly, Goldschmidt did not mean that under his eyes a prominence had started into existence ; but that he had been able to recognise the gradual process by which the prominence became visible with the diminishing sunlight. ' Several smaller promi- nences,' he proceeds, ' were seen in the neighbourhood of this one, resembling globules of mother-of-pearl, but of an irregular form. These likewise became of a rose colour immediately afterwards, but quickly disappeared. The most imposing as well as complicated of the pro- minences — which I will call the chandelier (fig. 64) was grand beyond description. It rose up from the limb, appearing like slender tongues of fire, and of a rose colour ; its edges were purple and transparent, allowing the interior of the prominence to be seen ; in fact, I could see distinctly that this prominence was hollow. Shortly before the end of the totality, I saw escape from the extremities of these rose-coloured and transparent sheaves of light, a slight display in the shape of a fan, THE PROMINENCES AND THE SIERRA. 257 which gave to the protuberance a real resemblance to a chandelier. Its base, which at the commencement of the totality was noticed very decidedly on the black limb of the Moon, became slightly less attached, and the whole took an appearance more ethereal and vapourish ; however, I did not lose sight of it for an instant. The jets of light which came from the ex- tremities disappeared with the appearance of the first rays of the Sun ; but it was not so with the pro- tuberance itself, for, an instant before the end of the Ite, 64. totality, I saw several small prominences appear lying close to each other on the right of its base, and form- ing a square, which is the character of toothed promi- nences ; two others of the same height were seen on the left side of its base when the Sun had already appeared. The north horn of the solar crescent touched the last of these prominences four minutes and forty seconds after the reappearance of the Sun. The intense light caused me to abandon this interesting observation, for I was not at the time using a coloured glass ; however, I am certain that the ' chandelier ' and the little prominences at its base had not disappeared 258 THE SUN. up to that moment. Although I am convinced that the protuberances belong to the Sun, nevertheless I ought to remark that at the last moment I was sur- prised to see the direction of the chandelier referred to the centre of the Moon rather than to the centre of the Sun ' (in other words, the chandelier was somewhat inclined). * The height of the prominence was estimated as about three minutes and a half at the commence- ment of the totality, and four minutes at the end. A second protuberance (fig. 65) appeared on the apparent Fie. 65. right of this, at a distance of about thirty-five degrees, being about 3^ minutes in height and nearly of the form of the symbol of the planet Saturn ( 1? ) ; this prominence I have called the hook, A third, to the right of the two preceding and at a distance equal to that of the two others, assumed a form of which it is difficult to give an idea ; however, I will call it the tooth. About eleven degrees to the right of the second protuberance, I noticed a fourth, small and in the form of a square ; between this and the third there was situated a rose-coloured cloud, the shape of which was elongated and bent, inclined at an angle of forty-five THE PROMINENCES AND THE SIERRA. 259 degrees towards the left limb of the Moon. This cloud was entirely detached, floating on the corona like a red cloud at sunset. Its centre was elevated above the limb of the Moon about one-half the altitude of the other prominences, or about two minutes. A fifth pro- tuberance also appeared at the beginning in the south- east, and was of increased size in the middle of the totality. I ought to remark that all the protuberances which I noticed had a tendency in their form to describe a curve, the concavity of which was turned from the side of the west.'' But the principal interest of the eclipse of 1860 attaches to the photographic work of Mr. De La Rue and Fr. Secchi. To secure a photograph of the Sun itself is a problem of a wholly different character from that which these two astronomers had set themselves ; for owing to the intense brilliancy of the solar light, the exposure neces- sary to secure a photograph of the Sun is of the briefest. For a minute fraction of a second the image of the Sun must fall on the prepared plate ; but for no longer interval, or a mere blurred patch would reward the photographer. But to secure a picture of the solar prominences, an exposure of appreciable length is required. Nor was it one of the least difficulties of De La Rue and Secchi that the length of time actually required was unknown to them. All their work was tentative ; and there was every reason to fear that success was impossible on account of the colour of the prominences. A red or orange light has commonly no S 2 260 THE SUN. actinic power whatever ; insomuch that the 'dark room ' of the photographer is, in a photographic sense, nearly as dark when its walls are of orange-coloured glass as though they were absolutely opaque. So that if the light of the prominences really were pure red, it was hopeless to endeavour to obtain photographs of these objects.* Despite the difficulties which the problem presented, and the disheartening anticipations which their ex- perience as photographers justified them in forming, the two physicists I have named boldly entered on their task. They adopted different methods. Mr. De La Rue employed the Kew heliograph, and the small image formed at the focus of the object-glass was enlarged before being received upon the plate. Secchi preferred to receive on the plate the image formed by the object-glass of his telescope. This image was about an inch in diameter. ' The result,' says Secchi, ' proved that both systems are excellent, each having its special advantages. In • It is worthy of notice, and affords a fresh proof (if proof were ■wanted) of the fact that observations may involve important results altogether apart from their direct significance, that the successful photo- graphing of the prominences afforded all but complete proof of that which was afterwards demonstrated by the spectroscope— the fact, namely, that the prominences consist of glowing vapour. Secchi or De La Rue might quite confidently have asserted that when the promi- nences came to be examined with the spectroscope, their spectra would show a band or bands near the^blue end of the spectrum, separated bv wide dark gaps from certain bands in the red and orange part. The greater part of the light of the prominences corresponds to these latter bands, but that part by which the photographs were taken corre- sponds to the former. THE PROMINENCES AND THE SIERRA. 26 r the enlarged image one can distinguish more details, but the direct image gives a greater extension to the corona.' The two observers were situated at different stations : — Mr. De La Eue at Riva Bellosa, near the Atlantic ; Fr. Secchi at the Desierto de las Palmas, near the Mediterranean. Thus an interval of about six minutes elapsed before the Moon's shadow passed from one Fig. 66. From photographs of the Sun during the total solar eclipse of June, 1860. {De La Rue.) station to the other, and an opportunity was afforded of determining whether the prominences change rapidly in figure. Besides this, there was a slight difference in the apparent course traversed by the Moon's disc in crossing the Sun's; for Secchi and De La Rue were at different distances — and, as a matter of fact, on opposite sides of the path of the centre of the Moon's shadow, so that De La Rue's series of photographs 262 THE SUN. shows more of the prominences on the superior part of the Sun's limb, while Secchi's series shows more of the prominences on the inferior portion. It will be unnecessary — so closely do the two series resemble each other in all essential respects — to exhibit both ; but further on there will be found a copy of one of Secchi's pictures (fig. 82, p. 327), which the reader may compare with figs. 66 and 67, copied from Mr. De La Rue's photographs. It will be noticed that fig. 66 represents the earliest phase. The Moon is advancing from right to left, and has just hidden the last fine thread of direct sunlight on the left. Thus we see the full height of the pro- minences on the left, while no prominences are seen at all on the right of the Sun. At the upper and lower part of the Sun's limb the prominences are partly concealed, and necessarily remain so throughout the eclipse. In fig. 67 the Moon has obliterated a large proportion of the prominences on the left, while it has in turn revealed a number of small prominences, a long range, or sierra, and a lofty and massive projection, on the upper right-hand quadrant. Fr. Secchi ob- served the prominences directly, and with great care, while his assistants managed the photography. Amongst the phenomena he noticed, I may mention the circum- stance that the strange prominence seen in both figures on the upper left-hand quadrant, possessed a helicoidal * * It would be well if this word 'helicoidal ' were always employed in this sense, in preference to the word 'spiral,' which might be conveniently restricted to curves lying in a plane, I am not insisting on either term THE PROMINENCES AND THE SIERRA. 263 structure. In the magnified picture from Mr. De La Rue's photographs, this structure can be clearly recognised.* Fr. Secchi "thus summed up the result of his observations : — 1. The prominences are not mere optical illusions ; they are real phenomena appertaining to the Sun. Our observations having been made at two places separated a hundred leagues from each other, it is impossible to suppose that shapes so well defined and so exactly identical could be produced by a phenomenon resembling mirage. 2. The prominences are collections of luminous matter of great brilliancy and possessing a remarkable photographic activity. This activity is so great that many of the prominences which are visible in the negatives could not be seen directly, even with power- ful instruments, perhaps because they emitted only chemical rays and few or no luminous rays.f 3. There are masses of prominence matter suspended as more correct, but only on the convenience of a recognised term to express spiral curves not lying in one plane. * Secchi notices the apparent encroachment of this prominence upon the Moon's limb — a peculiarity which he ascribes to the fact that the Moon was moving away from the prominence while the plate was under exposure. Doubtless, this circumstance produced its effect; but the phenomenon is chiefly due, according to the experimental researches of Dr. Curtis, to a process of chemical encroachment taking place during the development of the plate. t The spectroscopic observations made during the American eclipse tend to throw doubt on this conclusion, which, however, is in accordance with the observations made by Mr. De La Eue, and also, be it noticed, with laboratory experiments on the spectrum of hydrogen. 264 THE SUN. and isolated like clouds in the air. If their form is variable the variations take place so slowly that it is impossible to recognise their effect during an interval of ten minutes.* 4. Besides the prominences, a zone exists of the same material, enveloping the whole of the Sun's globe, f The prominences spring from this envelope ; they are masses which raise themselves above the general level, and even at times detach themselves from it. Some among them resemble smoke from chimneys or from the craters of volcanoes, which, when arrived at a certain elevation, yields to a current of air, and extends horizontally. 5. The number of prominences is incalculable. When observing the Sun directly, its globe appeared to be encircled with flames ; there were so many that it seemed hopeless to attempt to count them. 6. The height of the prominences is very great, especially if we notice that account must be taken of the portion concealed by the Moon. Thus estimated, the largest protuberance visible in 1860 was certainly not less than three minutes in height, which corre- sponds to about ten times the diameter of the Earth ; the others had a height of from one to two minutes. * Later observations show that this opinion must be modified, and that though many of the large prominences remain unchanged in figure for a considerable interval, yet others change very rapidly. t Grant, Swan, and Von Littrow had already recognised this ; and Leverrier, from observations made during the same eclipse, had come to the same conclusion. The matter is alluded to further on. (See note in pp. 291, 292.) THE PROMINENCES AND THE SIERRA. 265 We may consider that the prominences were finally placed in their true place in the solar scheme by the observations made in 1860. Doubts still continued to be expressed by a few ; but the majority were satisfied ; and henceforth the coloured prominences were very generally regarded as solar appendages. It may be well to consider briefly the interest of this result before proceeding to inquire into the researches of the last few years, which even surpass in importance those already described. In the first place, it must be remembered that though the prominences are seen all round the cir- cumference of the solar disc they do not really form a circle. They are the foreshortened projections of objects which may lie — and many of which must lie — thousands of miles from that circle on the Sun which at the moment forms the apparent boundary of his disc. We know, in fact, that certain prominences are as high as three minutes — that is, extend to some 80,000 miles from the Sun's surface. Now, supposing a b c to represent a part of the Sun's circumference, and a b c three prominences of this height, an observer, viewing the Sun from a point at a great distance away towards the right or left, would only see the extreme tips of the prominences a and e, while he would see the full height of the prominence b. But in order that these two prominences should be thus in appearance sunk below the solar limb ; the line a c would need to be about 500,000 miles in length. So that if there were any prominence of so great a height as a, b, or e 266 THE SUN. along any part of the arc A B c it would appear to rise above the Sun's level. Nor need we modify this con- clusion on account of the layer of red matter which has been shown by the observations already described to surround the solar globe, since undoubtedly the highest prominences extend considerably more than three minutes (even) above the upper surface of this red layer. But now, having considered a side view, let us con- ceive the case of a view from above. Suppose a', V ' , and c' (fig. 68) to represent the same three Fig. 68. Illustrating the distribution of large prominences over the Sun's surface. prominences brought round so as to be viewed from above. Then the actual boundary of the Sun thus viewed would be represented by the circle s b s', such that whereas a' c' represents 500,000 miles, s s' repre- sents 850,000 miles. In order, therefore, that a pro- minence of the full height of 80,000 miles seen from a THE PROMINENCES AND THE SIERRA. 267 distant point lying towards the right or left may be visible above the level of the red envelope, it is obvious that it must lie somewhere on the part A d f c, so far as the hemisphere shown in fig. 68 is concerned, or on a similar portion of the other hemisphere. Now, the curved surface A f bears to the whole surface of the hemisphere sds'c the same proportion that a' c' bears to s s'. Hence the extent of surface on which a promi- nence 80,000 miles high must be placed in order that it should be visible from a distant point is about- fifty eighty-fifths, or ten-seventeenths of the whole surface of the Sun. Hence it would follow that if ten such prominences could at any time be counted by the observer, then only some seventeen exist in all pro- bability at that time over the whole surface of the Sun. Furthermore, it is easily calculated in the same way that for a prominence really as high as those which seem three minutes high to appear so high as two minutes, it must lie on a zone of the Sun including nearly one- third part of his whole surface ;* insomuch that if even * The mode of calculation is exceedingly simple. Thus, adding twice three minutes to the Sun's diameter we get about thirty-eight minutes (which may be regarded as including the red layer as well as three minutes for the prominences). Now, for a prominence really three minutes high to appear at least two minutes high (I use minutes here as a convenient expression both for real and apparent height) its summit must obviously lie not further than VI x 37 minutes from a great circle of an imaginary sphere thirty-eight minutes in diameter, this great circle being that whose plane is perpendicular to the line of sight from the observer. Thus the whole zone on which its summit may lie will constitute rather more than twelve thirty-eighths of the surface of that imaginary sphere, and obviously the whole zone on which its base may lie will constitute the same proportion — that is, nearly one-third — of the surface of the Sun itself. 268 THE SUN. so many as ten prominences of this height could at any time be counted (which cannot but be regarded as a very exceptional circumstance) then only about thirty such prominences exist in all probability at that time over the whole of the Sun's surface. In such a case, also, the total number of prominences actually visible above the level of the red layer should {by the result of the preceding paragraph) be ten-sevepteenths of thirty, or about eighteen, — unless, besides the thirty prominences of the height mentioned, there were other inferior ones, in which case more than eighteen would, of course, be visible. It follows obviously from the comparatively small number of very high prominences ordinarily seen either during eclipses or by the methods presently to be de- scribed, that even if the astronomer could roam at his will around the Sun in search of lofty prominences, instead of being limited, as he is in reality, to a single view of the Sun as a disc, he would seldom (if ever) be able to discover many of these amazing objects. Al- though, taking eclipse with eclipse, the prominences cannot be regarded as exceptional, since every total eclipse yet observed has revealed one or more promi- nences of an amazing altitude, yet as regards the Sun's The general rule would be as follows : — The zone on which a promi- nence — which if really on the limb would appear m minutes high- — must lie, in order that it may not appear less than m minutes high, corers a space of _ i __ i/(m — m') (d + ni+ n>' „ d + 2m where d is the Sun's apparent diameter in minutes and S the Sun's surface. THE PROMINENCES AND THE SIERRA. 269 surface they are few and far between. So few, indeed, must they be, as to render it extremely probable that not one of those yet seen has been so exactly on the real limb of the Sun as to be seen in its full propor- tions ; nor would it be very hazardous to assign five or six minutes as a height to which the highest promi- nences, when most favourably seen, may be expected to attain.* So that we may regard 130,000 miles as no exaggerated estimate of the height of these wonder- ful objects. If fig. 69 represents a flame four minutes in height, then the four interior (or so-called Fig. 69. Illustrating the vast scale of the larger prominences. terrestrial) planets would be represented on the same scale by the four small discs shown on the convolu- tions — Mercury uppermost, then Venus, the Earth and Moon, and Mars ; while even the giant bulk of Jupi- ter would bear no greater proportion to the enormous solar prominence than is shown by the dimensions of the largest disc in fig. 69. Let us proceed, however, to those more recent * Since this was written I received the account of Respighi's obser- vations. In one instance he has seen a prominence no less than six minutes, or about 160,000 miles in height. 270 THE SUN. researches by which the actual constitution of the pro- minences, and even in some sort the part they play in the economy of the Sun's globe, have been ascertained by astronomers and physicists. Direct observation had been employed during the eclipses of 1842, 1851, 1855, and 1860. Photographic records had been obtained during the last of these eclipses. But a method of research more powerful than telescopic observation or photography was now to be applied to the solar prominences. These objects, whether regarded as self-luminous or as shining by reflecting certain portions of the solar light, were clearly well suited for the application of the spectro- scope. That powerful mode of analysis which will detect the elements present, even to the minutest con- ceivable extent, in coloured terrestrial flames, seemed to promise results of the highest interest and impor- tance whenever it should be applied to these coloured solar prominences. Therefore, Colonel Tennant (to whom astronomy owes a debt of gratitude on this account) early called the attention of the scientific world to the importance of studying the great eclipse of August 18, 1868, not only with the telescope and by means of photographic appliances, but under the searching powers of the most wonderful method of research yet invented by man. Accordingly, two well-furnished expeditions set forth from England, supplied, through the Hberality of the Indian Government and of the learned societies of this THE PROMINENCES AND THE SIERRA. 271 country, with the means of satisfactorily pursuing the important investigations which had been suggested by Messrs. Huggins, De La Rue, and other experienced astronomers and physicists. In the first place it will be well to consider the photographic and other records of this eclipse. I have selected the six photographs taken by Colonel Tennant, at Guntoor, as not only affording the most complete illustration of this particular eclipse, but as supplying the best illustration of eclipse photographs yet obtained. The length of time during which the Indian eclipse lasted gave Major Tennant the means of taking a large number of records, with a longer ex- posure than was possible during the American eclipse ; and a value belongs to one series taken at a given station with the same instrument and by the same process, which cannot be assigned to a larger series combining the labours of different observers. It is on this account that I have preferred to present in Plate III. the photographs of the eclipse of 1868, rather than those obtained during the eclipse of 1869. The instrument employed by Colonel Tennant was a Browning- With reflector of nine inches clear aper- ture. ( It was determined,' said Colonel Tennant, ' to adopt the Newtonian form, and to place the sensitive plates at the side of the tube in order to find room, which would be very difficult at the end of the tube ; the small mirror thus rendered necessary was made unusually large in order that it might reflect all the 272 THE SUN. rays from a surface of some diameter, and it was hoped that by this means, and by giving a sufficient exposure, a picture of the corona might be secured ; but light clouds so decreased the actinic power of the light as to defeat this, though still we have traces of the corona.' Colonel Tennant remarks, also, that although, owing to the lowness of the latitude, an entirely new design was required, while ' from the necessities of its construction the instrument could not be tried before being sent out,' yet the photographic arrangements were very good and convenient, the instrument very firm and steady, while the force required to direct the telescope was very small. The weather had been remarkably clear for several days before the eclipse, and a magnificent view of the phenomenon would have been obtained had August 18 resembled its precursors. But unfortunately on the morning of the eclipse the eastern sky was clouded over with cumulo-stratus clouds, which although not sufficiently dense to interfere with vision, were very annoying accompaniments to the processes of solar photography. It had been arranged that during the six minutes of totality six plates should be exposed. Notwithstanding the unfavourable condition of the air, this arrangement was carried out; and it was fortunate, as it turned out, that no longer time was given for the exposure of the successive negatives, since otherwise they would in all probability have been spoiled. Sergeant Phillips and two Sappers, specially trained for the purpose at Mr. De La Rue's observatory, ECLIPSE OF AUCT 1«™ 1868- Photographed l>y Major TenriJUHfc . THE PROMINENCES AND THE SIERRA. 273 at Cranford, superintended the subordinate arrange- ments. Colonel Tennant thus describes the six photographs (see Plate III.) :— Photograph 1. The exposure was as short as possi- ble — probably less than a second. In this photograph some glare has fogged the plate a little, but the follow- ing things are noteworthy : — First. The enormous horn on the north side, whose height, as derived from the Moon's diameter (about thirty-four minutes), is three minutes eighteen seconds, corresponding at the distance of the sun to 88,900 miles. Secondly. The ridge of light extending round the upper limb of the Moon, so bright as to be markedly seen through the thickest of the fog, though the glorious horn is paled. Near the horn this light is clearly broken into beads, as may be seen in Mr. James's picture of the horn, and is occasion- . ally double. Thirdly. At its southern end this ridge is terminated by a protuberance of singularly complicated structure. Photograph 2. Taken almost exactly one minute after, and exposed for five seconds. ' North of the great horn we see a peculiar and faint figure, shaped (generally) like a bow, but with interruptions and nuclei of light, and having an arrow-like spike in the middle of its base. Next we have the great horn, which has here been over-exposed. The Moon's limb has advanced as far as the point where the two lower branches seen in No. 1 meet, leaving only a trace of the bifurcation, and the upper part of the horn is of T 274 TSE SU] Y- course larger than in No. 1, as fainter parts have come into view. When this photograph is well illuminated, traces of the spiral structure, so apparent in the rest, are seeu ; but it would be difficult to recognise it without the other pictures. Passing on, the Moon's limb is on the edge of the faintly luminous stratum I have described in No. 1, which is, of course, now far more marked ; and as we approach the south we see the prominences mentioned under No. 1 as being in the faint stratum, and then reach the upper part of the former terminal prominence, where the long exposure has now blotted out all traces of structure.'* Photograph 3. Exposed one minute six seconds later, and for ten seconds. ' It is evidently less affected by light than No. 1, and was probably taken through a light cloud. Here the great horn again claims attention ; its spiral structure is now clearly made out The outline of the Moon is wanting till we reach the south terminal protuberance of No. 1, which is now nearly hidden ; and in its neighbourhood we again see the flare of which I spoke in describing No. 2.' Photograph 4. Exposed fifty-two seconds later, and for five seconds. ' Nothing is seen but the great horn, but that picture is of singular beauty. The continually varying intensity of the light and its markedly spiral structure are the material points.' Photograph 5. Exposed one minute three seconds * ' The neighbourhood of both these prominences is marked by light flaring into the corona in irregularly-curved lines,' adds Colonel Tennant, ■ and this corresponds to the part of the corona -which has been described by Captain Branfill and others as most bright.' This remark will be ound important when we are upon the subject of the corona. THE PROMINENCES AND THE SIERRA. 275 later, and for a short interval only. ' The great horn is still conspicuous on the east side of the disc ; and the outline of the Moon is marked round three-quar- ters of the circumference. The top of a prominence' on the north-west side of the limb is seen.' Photograph 6. This plate was exposed but for a short time, like the first and fifth, and one' and a half minutes after the last. ' It is a picture as late as it was thought prudent to take it during the absence of the Sun. On the south-east side, where the corona was brightest, are traces of it; and the whole limb from north to south-west shows a trace of corona, and a series of beautiful protuberances, only one of which, however, is large. In the original it appears inflated. To me it resembles exactly a toy balloon in the shape of an animal. There is a mark as of an open mouth ; the eye and the neck are marked, and the tail stands out stiffly behind. I mention these details because they complete the appearance of inflation by the re- semblance they cause to what one has seen in that state. I would especially draw attention to the appear- ance as of a strong current of air (so to speak) blowing from north to south, and bending over and even detaching and carrying away the tops of prominences.' It is well worthy of notice, also, that the horn still continues partly visible. It cannot, indeed, have been completely hidden even at the moment when the Sun was about to reappear.* * I think this account would be incomplete were I not to quote the generous remarks with which Colonel Tennant closes this section of his T 2 276 THE SUN. At other places along the track of the eclipse similar appearances were perceived ; nor have we any The eclipsed Sun, August, 1860; photographed at Aden. report. 'Throughout,' he says, ' I have spoken of myself as sharing in the photography. I certainly exposed and developed a few plates, and generally superintended all that went on, and so far I was the superin- tendent of this department ; but the trouble of endeavouring to break in new and unskilled hands, and of carrying out the details, fell on Sergeant Phillips, to whose steadiness and determination to succeed the result in this matter is mainly due. He was, moreover, most useful THE PROMINENCES AND THE SIERRA. 277 distinct evidence pointing to a change of figure during the progress of the Moon's shadow from Aden, in the west, to Mantawaloe Kekee, the most easterly station where the phenomena of the eclipse were witnessed by competent observers. The accompanying picture (fig. 70) represents the eclipse as photographed at Aden about forty minutes before the shadow reached Gun- toor. There is little in this picture to establish the occurrence of a change of figure in the ' great horn ' prominence. In comparing it with Colonel Tennant's views it must be remembered (in justice to the German photographers) that the atmospheric conditions were much less favourable at Aden, where the eclipse oc curred very shortly after sunrise. In the next chapter a picture (fig. 84) of the eclipsed Sun as seen at the extreme easterly station, Mantawaloe Kekee, exhibits a tolerably close general accordance with Colonel Tennant's views. Other pictures and descriptions need not here be referred to, because they differ in no essential respects from those relating to previous eclipses. Had the work done during this eclipse been limited to what I have hitherto described, the results, however interesting, would not have advanced in any important respects our knowledge of solar physics. We have now, however, to consider far more important results. At nearly all the stations preparations had been in other ways, for I could always rely on his care, and was thus enabled to avoid constant exposure to the Sun. Indirectly he has been the means therefore of facilitating my other work.' 278 THE SUN. made for observing the coloured prominences and the corona with the spectroscope. All the spectroscopic observers were successful in determining the general character of the prominence spectrum. I select the graphic and lively narrative of Captain Herschel, who commanded the expedition sent out by the Royal Society, and observed the eclipse at Jamkhandi. * The week preceding the event,' he writes, ' had quite pre- pared me for disappointment. There seems to be an annual cloudy and rainy season at Jamkhandi, which lasts about a fortnight, and was said to be somewhat more marked than usual this year. The morning broke, however, as usual— clear, but the driving mon- soon-clouds soon showed the kind of sky we were to expect. About a quarter of a minute before totality a third cloud obscured the Sun.' He had directed the spectroscope towards that part of the Sun which would be the last to disappear, and was watching the solar spectrum as it grew gradually narrower and narrower. It will be understood from what has been said towards the close of Chapter III. that the moment this spec- trum disappeared the spectrum of the prominence matter (or failing the presence of such matter, the spectrum of the corona) would start into view. ' You may conceive,' he says, addressing Dr. Huggins, ' my state of nervous tension at this moment. Whatever the corona was competent to show must in a few seconds have been revealed ; unless, indeed, it should happen that a prominence or sierra should be situated at that particular spot, in which case the double THE PROMINENCES AND THE SIERRA. 279 spectrum would be presented. But the solar spectrum faded out while it had still appreciable width, and I knew a cloud was the cause.' He turned to the finder of the telescope, removed the dark glass, and waited for the passage of the cloud. As the telescope was mounted so as to follow the motion of the Sun by- means of clockwork, there was simply nothing to be done. ' I was,' he writes, ' in that fever of philosophi- cal impatience, which recognises the futility of irrita- tion even while it chafes under the knowledge of fleeting seconds. How long I waited I cannot say — perhaps half a minute. I can well recall the kind of frenzied temptation to turn screws and look somewhere else, checked by the calm ticking of the clock telling of a firm hold in the right place, cloud or no cloud. And soon the cloud hurried over, following the Moon's direction, and therefore revealing first the upper edge of the disc, with its radiating, and — as I fancied — scintillating corona ; and then the lower. Instantly I marked a prominence (of whose identity there will be no question) near the needle point in the finder. Those few seconds of unveiling were practically all that I saw of the eclipse as a spectacle. With the exception of a hurried glance into the finder at a later period to watch for another break, I was the whole time engaged at the spectroscope. I have not the remotest idea (from actual experience) of the external phenomena which were presented to the thousands of upturned faces of those whose voices I heard outside. I might easily have lifted the curtain and looked out while the clouds 280 THE SUN. were obstructing. That I did not do so is only to be explained by the absence of mind as regarded all else, produced by the concentration of my attention on the problem before me.' Resuming the narrative of his observations, Lieut. Herschel proceeds : — ' A rapid turn of the declination-screw covered the prominence with the needle point, and in another instant I was at the spectroscope. A single glance, and the problem was solved. Three vivid lines — red, orange, and blue ! I think I was a little excited about this time, for I shouted quite unnecessarily to my recorder, " Red ! green ! yellow ! " quite conscious of the fact that I meant orange and blue.' The problem was indeed in a general sense solved. Among all the ideas that had been put forward respect- ing the actual constitution of the prominences one, and one only, now remained available. Had the prominences been self-luminous solar mountains or clouds (properly so called) a continuous spectrum would have been seen ; had they been bodies which shine by reflecting solar light the rainbow-tinted streak, crossed by dark lines, which forms the solar spectrum would have made its appearance. The sole remaining theory was that the prominences consist of glowing gaseous matter. But it remained now to determine what the nature of this matter might be. To this work Captain Herschel turned his attention, and to this work also all the observers who, like him, had succeeded in solving the general problem, devoted themselves during the remaining minutes of totality. The results they THE PROMINENCES AND THE SIERRA. 28 1 obtained may be thus summed up : — Herschel noted three lines — a red one, which he regarded as possibly c, the red line of hydrogen ; an orange one, which he regarded as almost certainly d, the orange (double line) of sodium ; and a blue one, which he thought might possibly be F, the blue-green line of hydrogen. Major Tennant saw five lines, which he associated with the lines c, D, b, f, (probably) and a. M. Janssen saw six — red, yellow, green (two), blue, and violet, and esta- blished the coincidence of the red and blue lines with the lines c and f. M. Rayet saw no less than nine lines, five of which he recognised as brighter than the rest ; these he associated with the lines b, d, e, b, and f. But we need not consider the evidence on which these various determinations rest, since a new and far more effective mode of observation was to place beyond all possibility of question the true position of these several lines, and of others as yet undetected. M. Janssen had been struck during the progress of the eclipse by the exceeding brilliancy of some of the bright lines which formed the prominence spectrum. ' Immediatement aprSs la totality,' he writes, ' deux magnifiques protuberances ont apparu: l'une d'elles, de plus de trois minutes de hauteur, brillait d'une splendeur qu'il est difficile d'imaginer.' As he looked, the idea seized him that these lines might be seen when the Sun is not eclipsed. He cried out, he tells us, ' Je reverrai ces lignes-la ! ' The principle by which he hoped to effect this will be understood by the reader who has carefully studied the 282 THE STJN. latter portion of Chapter III. Briefly re-stated it is this : The light of the prominences when dispersed by the spectroscope forms a few lines ; that of the illumi- nated terrestrial atmosphere is spread out into the rainbow-tinted solar spectrum. Therefore, if we only use adequate dispersive power, we can cause the pro- minence-lines to show conspicuously on the background of dispersed atmospheric light. Clouds which gathered over the face of the Sun soon after totality prevented M. Janssen that day from pursuing this idea. But on the morrow he applied it with complete success. ' I have experienced,' he said, ' to-day a continuous eclipse.' "We have seen how it becomes possible by this method not only to recognise the spectrum of a pro- minence during full daylight, but by a series of sec- tional views, so to speak, to determine the figure of a prominence. To this work Janssen applied himself, and he was presently able to forward to Europe news of his complete success in this new branch of research. The news of this important discovery was nearly two months in reaching Europe, and a few days before it arrived Mr. Lockyer had independently obtained a similar result. The history of his work and of the way in which it was suggested is not without interest. In May, 1866, Dr. Huggins had examined the spectrum of the star which blazed out suddenly in that year in the constellation Corona. He found that this star — now known as T Corona?* — gave a spectrum which * In Mr. Roacoe'a valuable work on spectroscopic analysis, this star THE PROMINENCES AND THE SIERRA. 283 differed altogether from any he had previously ex- amined. There was the rainbow-tinted streak crossed by dark lines which ordinarily forms the spectrum of a star ; but over this, and obviously corresponding to a large proportion of the star's light, there were bright lines. These bright lines corresponded in position with the lines belonging to hydrogen ; so that Dr. Huggins was able to pronounce that the great increase in the star's light was due to an outburst of glowing hydrogen. Afterwards he found that other stars, and notably the middle star in the conspicuous W group of Cassiopeia, show bright lines, superposed on the rainbow-tinted background, though relatively far less bright than those seen in the spectrum of T Corona?. In these cases, also, the lines were those of glowing hydrogen. Here, then, was a perfectly apt illustration of the principle dwelt on in the concluding part of Chapter III. Here were the lines of a certain element rendered separately visible as bright lines, though the total amount of light received from the glowing hydrogen was not necessarily (in the case at least of the star in Cassiopeia) equivalent, or nearly so, to the re- maining portion of the star's light. The concentra- tion of the light into three definite lines enabled it to is always referred to as t Coronae. The star t Coronse, however, is a well-known fifth-magnitude star towards the north of the constellation. The real variable (ordinarily a tenth-magnitude star) lies to the south of all the conspicuous stars of Corona. It is called T in accordance with the rule by which the variables successively discovered in a constel- lation are named from the last letters of the alphabet, beginning with E. 284 THE SUN. prevail, as far as intrinsic brightness was concerned, over the diffused light, absolutely greater in amount, which formed the rainbow-tinted streak. "We cannot wonder, therefore, that the idea should at once have been suggested that if any portions of the Sun — as, for example, the coloured prominences — con- sist of glowing gas, the spectrum of such portions might be recognisable even amidst the spectrum of the far more intense light (so far as absolute brightness is concerned) of the solar photosphere. Mr. Lockyer was preparing about this time to undertake a careful scrutiny of the photosphere with more powerful spectroscopic appliances than had yet been employed. Amongst the expectations which he formed at that time, there is a reference to the problem presented by the prominences. It may be interesting to give at full length the concludiug words of the paper he addressed to the Royal Society : — ' Seeing that spectrum analysis has already been applied to the stars with such success, it is not too much to think that an attentive and detailed spectroscopic examination of the Sun's surface may bring us much knowledge bearing on the physical constitution of that luminary. For instance, if the theory of absorption be true (!) we may suppose that in a deep spot rays might be absorbed which would escape absorption in the higher strata of the atmosphere ; hence, also, the darkness of a line may depend somewhat on the depth of the absorbing atmosphere. May not also some of the variable lines visible in the solar spectrum be due THE PROMINENCES AND THE SIERRA. 285 to absorption in the region of the spots ? and 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, although they escape all other methods of observation at other times ? and if so may we not learn something from this of the recent outburst of the star in Corona ? ' It does not appear quite clearly from this . passage whether Mr. Lockyer expected (as De La Rue had before) to find traces of the prominences, when examin- ing the Sun's surface, or whether it fell in with his plans to examine outside the Sun's disc. But the latter is the more probable explanation. The grant he asked for from the Royal Society was allowed him, and the construction of a suitable spectroscope was entrusted to Mr. Browning (after a delay consequent on the death of Mr. Cooke). In the meantime Dr. Huggins applied his spectroscope unsuccessfully to the search, and Fr. Secchi, who had thought of examining the Sun's edge, gave up the attempt on hearing of Mr. Lockyer's failure to detect anything remarkable there.* Whether, supposing no further discoveries had been made, Mr. Lockyer would have succeeded in recog- nising the bright lines of the prominences, is one of those questions which never can be answered. For * ' Era gia gran tempo che arevamo intenzione di esplorare 1'orlo del sole eon lo spettroecopio, ma 1' assicurazione data da molti abili osserva- tori e dal Sig. Lockyer stesso, ehe nulla vedevasi di piu all' orlo del disco che al centre ce ne avea distolto.' — Memoria III. sugli xpettri prismatici de' eorpi celesti. 286 THE SUN. my own part, I cannot but think that he would assuredly have succeeded in the long run, even if the Indian expedition had been a failure. My reason for believing this is that Mr. Browning had so successfully mastered the optical conditions of the problem as to ensure complete success on the observer's part, when- ever the examination of the Sun's limb should be thoroughly undertaken. However, owing to one cause and another, there ensued considerable delay before Mr. Lockyer attempted to make the required observa- tions; and in the meantime news came to England that the spectrum of the prominences included three conspicuous bright lines. This placed it out of Mr. Lockyer's power to discover the lines ; but it was still left for him to show that they can be seen when the Sun is not eclipsed. Two months after the eclipse he announced that he had so seen them ; and the story runs that some five minutes or so before the announce- ment of Janssen's prior success had been received by the President of the Imperial Academy at Paris, a communication from Mr. Lockyer to a similar effect had been read to that learned body.* * M. Faye, while admitting Mr. Lockyer's claim to the independent recognition of the visibility of the lines without eclipse, expresses a feeling of regret that the mere distance between India and Europe should have prevented France — as represented by M. Janssen — from enjoying the full credit of the discovery. This, surely, is not the just mode of viewing the matter. How M. Janssen's credit can be diminished because Mr. Lockyer independently effected the same observation it would be hard indeed to say. But this is not all. It seems to me that nothing can be more mischievous to science than the promulgation of the idea that mere priority in making and announcing an observation is to afford the chief measure of the credit to be assigned THE PROMINENCES AND THE SIERRA. 287 Soon after, Secchi successfully applied the same method, as did Dr. Huggins, whose spectroscope was, however, not altogether well adapted to the work, having been specially designed for stellar researches. It was a peculiarity of the new mode of observing the prominence-spectrum that it enabled the observer to determine at his leisure, and in a much more satis- factory manner than had been possible during the eclipse of 1868, the true position of the prominence- lines, their characteristics as respects shape (the sig- nificance of which feature has been already referred to, p. 146), and also the existence of lines which had escaped observation while the eclipse was in progress. It is obvious that the spectrum of the Sun's limb seen at the same time (or that of the illuminated terrestrial atmosphere) affords the most satisfactory means of determining the position of the bright prominence -lines ; for there in the solar spectrum are those very lines — to the observer. If this were so, the wisest (though most selfish) course which the students of science could follow would be to conceal all original ideas as to modes of observation or processes of research until they could apply those ideas on their own account. The true lover of science cannot do this. He feels bound to publish any such ideas at once, let who will adopt and utilise them (and let who will obtain credit for the result). There may be cleverness in a more diplomatic course, but there is no true love of science. I may add that as regards the question of priority in this matter more importance might be attached to it were it not well known that after Dr. Huggins's observations of T Coronse, the principle on which the new mode of observation rests was recognised by all who understand spectroscopic analysis. Like the discovery of Sun-spots, therefore, this recognition of the prominence-lines in full daylight was an inevitablt sequel of the construction of an adequately dispersive spectroscopic battery. 288 THE SUN. the dark lines c and d and f, and so on — with which the eclipse observers had associated the bright lines. It is only necessary to see whether those dark lines coincide in position with the bright ones (see figs. 36, 37, and 38, in which, however, but a few dark lines are shown) in order to tell whether the bright promi- nence-lines are or are not due to the presence of those particular elements to which the solar dark lines belong. Janssen found in this way that the orange line of the prominences does not correspond, as had been thought, with the D or sodium (double) line of the solar spectrum. He found, however, that the red and blue prominence- Fig. 71. lines do actually coincide with the C and r lines of hydrogen. So that these enormous objects, extending in some instances to a height of more than 80,000 miles above the Sun's surface, consist in part, at least, of the glowing vapour of hydrogen. But it was possible to recognise other lines besides these three by the new method, when an instrument of adequate dispersive power was employed. The accompanying picture (fig. 72), for example, exhibits the spectrum of a prominence, and of the adjacent portion of the Sun's limb, as shown by Mr. Lockyer's THE PROMINENCES AND THE SIERRA. 289 spectroscope. It will be seen that the double line of sodium as well as three lines of the magnesium spec- trum are shown. It is only necessary to suppose that M. Rayet saw the two close double lines as single ones in order to account exactly for the nine lines seen by him.* * The following is Fr. Secchi's description of the principal lines seen in the prominence-spectrum under ordinary conditions : — ' The line c of hydrogen,' he says, ' is the most easily seen of all. It sometimes reaches the enormous height of three minutes, indicating the presence of such colossal prominences as are seen during eclipses. On the limb of the Sun generally the height of this line is very irregular, and on the average attains to from ten to fifteen seconds. This line also extends in a well-marked manner within the limb, overlapping the disc by ten seconds and more. Yet further on the disc there is a region where the line cannot be seen, being neither bright nor dark, but of the same tint exactly as the neighbouring part of the spectrum, which is thus in this part of its length continuous and uniformly bright. With a slit placed parallel to the limb the space throughout which the line disappears is often considerably extended. Outside the disc the line is much brighter near its base than at the summit, and the line is dilated at the base, and seems to terminate in a point where its light fades off, until, as I have already said, it becomes of the same brightness as the neighbouring part of the spectrum before becoming a dark line. Out- side the Sun the line is bounded by two dark lines, which appear at first sight to be the effect of contrast, but may probably have another and a real cause. The bright line is Often formed of knots and separate pieces, which are evidently so many fragments of different prominences, placed one beyond the other and unequally bright. If the aperture of the slit be enlarged as much as is possible without rendering the light unbearable, the bright line of the rose-coloured fringe is seen to be marked by irregularities which are due to the roughness of texture {scabrosita) of the prominences themselves.' After mentioning that he has been able to apply Janssen's method of determining the shape of the prominences by taking line-sections Secchi remarks that he has often tried to determine whether there i~ any fixed law as regards the direction in which the prominences are bent, but has hitherto not succeeded in tracing any. ' The line in the yellow, near the sodium lines, is about twice and a half as far from the nearest of these as these are from each other. This U 290 THE SUN. Mr. Lockyer was presently able to confirm the views of Grant, Secchi, and others, who had, as already mentioned, enunciated the theory that the Sun is wholly surrounded by a layer or envelope of the line is sensibly the prolongation of a bright line in the solar spectrum. In height and brightness it corresponds closely to the c-line, but I have noticed that it will not bear high magnifying power so well. While the f and c lines remain brilliant under such powers, this line becomes weaker, so that only the practised eye can detect it. The line ends in a point, and it often extends itself brilliantly upon the disc of the Sun. Outside the disc it is not bordered by dark lines ; on the contrary, with high powers it becomes diffused, and does not stand out sharply like the c-line. ' The F-line is in general not so high as the c-line, and grows faint at the extremity, where it takes the form of a lance. Outside the disc it is accompanied by a narrow dark zone on the more refrangible side. Sometimes I have seen it prolonged beyond the edge of the solar disc, as in the case of the c-line ; at other times a very fine black thread shows itself on the more refrangible side. This seems to show that the F-line is not due to hydrogen alone.' (This conclusion, however, has been negatived.) ' The third line of hydrogen near s I have seen as a bright line, but it is necessary to reduce the dispersive power of the prismatic battery in order to obtain sufficient light.' Secchi is of opinion that where there are faculae on or close to the limb, there prominences exist ; but he is unable to say with equal con- fidence that wherever prominences exist there are also faculse. He was surprised to find on one occasion, when several lines besides the above-mentioned were visible, that of the three lines forming the group b of magnesium one and one only was visible, — a second line holding a position midway between the other two lines of the same group. He remarks that Eayet had observed during the eclipse of August, 1868, only two lines of this group, and that these two doubtless corresponded with those seen by himself, of which, as we have seen, one does not accord with either of the two remaining magnesium lines. He was so surprised at this peculiarity that he searched diligently for two hours to detect traces of the other lines of magnesium, but could find none whatever. He considers the objections which have been urged against this observation as ' inconclusive — to use no more severe ex- pression — since it is unreasonable to suggest that he would have been careless when observing so remarkable a phenomenon.' THE PROMINENCES AND THE SIERRA. 29 1 coloured matter whence as it would seem the promi- nences, spring.* To whatever part of the Sun's edge he directed his spectroscope the bright lines belonging to the prominence-spectrum could be seen, though reach- ing but to a short distance from the edge of the Sun, save where there is a prominence. Unaware that this layer had already been discovered, Mr. Lockyer devised a name for it, entitling it the chromosphere. The term sierra is in better agreement with the now accepted name of the coloured flames; but for some purposes the word cltromatosphere may be conveniently employed.f Whether this envelope is to be regarded as a true solar atmosphere, in the sense in which that term is usually understood, may be seriously questioned. As seen in the telescope, it presents a well-defined and very uneven limit, giving the idea rather of a region * Professor Grant wrote, in 1858, 'The zone of a deep red colour observed at Toulon toward the part of the Moon's limb where the Sun was about to emerge clearly indicates the accumulation of nebulous matter in the lower regions of the solar atmosphere, as well as the con- densation of the circumambient fluid of which the latter is composed towards the surface of the Sun, arising from the pressure of the super- incumbent strata.' Leverrier wrote in I860, ' The existence of a bed of rose-coloured matter partially transparent, covering the whole sur- face of the Sun, is a fact established by the observations made during the time of totality in the eclipse of this year.' Seechi was equally convinced of the existence of this rose-coloured region. ' The observa- tion of eclipses,' he wrote, 'furnishes indisputable evidence that the Sun is really surrounded by a layer of this red matter, of which we com- monly see no more than the most elevated points.' t It need hardly be remarked that the word ' chromosphere ' is in- correctly formed, and can never hold a permanent place in scientific vocabularies. It is as incorrect as phography would be for photography, chromic for chromatic, or chronatic for chronic. v 2 292 THE SUN. of flames or clouds than of an envelope of the nature of an atmosphere. That the substance producing the coloured light of the chromatosphere is gaseous admits indeed of no question; but so also the prominences are gaseous. Yet we do not regard these as of the nature of an atmosphere. Now, if the surface of the Sun be covered at all times with small prominences, bearing somewhat the same relation to the gigantic ' horns ' and ' boomerangs ' seen during eclipses that the bushes covering certain forest regions bear to the trees, then there can be no doubt that the chromatosphere could not rightly be regarded as an atmosphere. We have no evidence that it even extends downwards as far as the visible limits of the photosphere ; since undoubt- edly if a distance of forty or fifty, nay, even of two or three hundred miles, separated the lower limit of the chromatosphere from the photosphere, no telescopes we possess ■ could suffice (when supplied with suitable spectroscopic appliances) to reveal any trace of this space. A width of two hundred miles at the Sun's distance subtends an arc of less than half a second; and telescopists who know the difficulty of separating a double star whose components lie so close as this will readily understand that a corresponding arc upon the Sun would be altogether unrecognizable. I am very far, therefore, from accepting with confi- dence the view that the chromatosphere is the true solar atmosphere. Rather, I believe that it consists of matter, gaseous, no doubt, but suspended in the true solar atmosphere. I think that not only is this THE PROMINENCES AND THE SIERRA. 293 view confirmed by the appearance of the chromatos- , phere as depicted in Plate VI., further on, but that the general resemblance as regards structure and appear- ance between the sierra and the prominences, suffices to render it at least highly probable that the former can no more be described as the solar atmosphere than the latter. But even more striking is the evidence deduced from Plate V. For here we see in the first figure a prominence which has been obviously erupted. We see it as it springs upward, expanding with the diminishing atmospheric pressure. And we see in the second figure how the erupted matter has slowly begun to sink, with expansion, indicating low pressure in the upper regions, but with a figure indicating a real re- sistance. It is then this invisible atmosphere, through which the upper portion of the prominence has begun to subside, that would seem to constitute the true solar atmosphere, while the sierra should be regarded as consisting of a multitude of smaller prominences, either freshly erupted or the remains of former erup- tions, floating in the lower regions of the solar atmos- phere, much as the second prominence of Plate V. is floating (for a brief while) in the upper regions. The inferences which have been deduced respecting the pressure exerted by the solar atmosphere at and near the level of the photosphere remain unchanged, how- ever, whatever view we adopt. Fig. 72 illustrates the evidence on this point. Here 1 is a part of the spec- trum of the Sun's limb, while 2 represents the line F in the spectrum of the chromatosphere. The widening 294 THE SUN. of this line close by the Sun's limb may be regarded as unquestionably indicating an increase of pressure, because the researches of Pliicker, Hittorf, Huggins, and Lockyer have demonstrated that the F-line of Pig. 72. 20 ?3 T 3J2,1 |h '■'ill I 1 II 1 *: l. : : ' ■ : 1 1 :. I 1 2 - 1 HyS Illustrating the widening of the F-line of hydrogen near the base of the chroniatosphere. hydrogen does actually increase in this way in width when the pressure at which the hydrogen subsists is increased. Temperature, also, has an effect on the hydrogen lines ; nor is it quite easy to separate the effects due to pressure from those due to temperature.* * In a paper by Dr. ZSllner (Uber die Temperatur und physische Beschqffenheit der Sonne), an abstract of which, by the present writer, appeared in the English Mechanic for September, 1 870, it is suggested that means might be devised for distinguishing between the effects of pressure and temperature by causing a discharge of gas to take place at the moment when the induction spark is passed through the hydro- gen. It is certainly a problem of the •utmost importance thus to dis- THE PROMINENCES AND THE S1EKSA. 295 But, on the whole, it seems probable that pressure is chiefly in question, while it may be regarded as absolutely certain that temperature alone is insufficient to account for the observed change, l^ow, whether we regard the glowing hydrogen of the chromatosphere as forming a true constituent of the solar atmosphere or as bearing a somewhat similar relation to that atmo- sphere that the aqueous vapour in our own air bears to the permanent constituents, it is yet certain that the pressure of the hydrogen near the solar photosphere is a measure of the atmospheric pressure there. And from the observed width of the F-line near the Sun's limb (see fig. 72) it has been estimated by Wiillner that ' the pressure on the base of the chromatosphere, or at the surface of the photosphere, is below the pressure of the Earth's atmosphere.' * He has even assigned the tinguish between the two effects which play the chief part in solar phenomena. The disappearance of one of the magnesium lines at a certain height above the spectrum of the limb (see fig. 71) in which the lines b belong to magnesium, is significant of a very low degree of pressure at the level where the shortest line ceases to be visible. * It must not be forgotten, however, that the width of the hydrogen- line where it actually reaches the spectrum of the limb is not known. The observed width on which Wiillner founded his researches may be that corresponding to a height of 50, 100, or even 200 miles above the photosphere ; and within these 50, or 100, or 200 miles an increase of pressure may take place by which the actual density of the atmo- sphere close by the photosphere may be enormously increased. There may be an atmosphere including the vapours of iron, sodium, mag- nesium, &c. (of all the elements, in fine, whose dark lines appear in the solar spectrum), extending, say, 100 miles above the photosphere ; and yet no instruments we possess could suffice to reveal any trace of its existence, unless the dark lines in the solar spectrum be thought to demonstrate the fact that such an atmosphere actually does exist. The 29 6 THE SUN. limits of pressure at the level of the solar photosphere as lying certainly ' between 50 and 500 millimetres (or between 2 inches and 20) of a mercurial barometer at the Earth's surface.' It is within the sierra and certain of the prominences that spectroscopes of high dispersive power exhibit those signs of cyclonic motions taking place in the solar atmosphere, the recognition of which has seemed so surprising and inexplicable. The study of the last part of Chapter III. will have shown the reader that if very rapid motions are taking place, either due to the swift rush of the glowing hydrogen through the solar atmosphere or to the effects of cyclonic motions in the atmosphere itself, by which the glowing hydrogen is borne along, the spectroscope — if only its dispersive power be sufficient — -cannot fail to give unmistakable fact that on one occasion Mr. Lockyer has seen hundreds of the Fraunhofer lines (as bright lines) in the spectrum of the chromatosphere renders this far from improbable. The arguments on the strength of which it has been assumed that the absorption to which the dark lines are due takes place below the visible photosphere, appear, to say the least, far from demonstrative. [1 have preferred to leave this note precisely as it appeared in the first edition, save for the correction of one clerical error. What I have here suggested as probable was established by an observation made by Prof. Young, of America, during the eclipse of December 1870. The slit of the spectroscope being so placed as to include the part of the Sun's edge which was last covered by the Moon, included, necessarily, the lowest layers of the Bolar atmosphere. So long as the Sun's direct light illuminated the spectroscopic field only, a few bright lines belong- ing to the sierra could be seen. But so soon as the last fine sickle of the Sun's disc was concealed, hundreds of bright lines made their appearance, and continued visible for a few seconds, — in fact until the Moon had covered the region thus shown to form a true atmosphere, exceedingly complex, and formed in large part of the glowing vapours of our familiar gases.] THE PROMINENCES AND THE SIERRA. 297 evidence on the point. The problem is altogether simpler and easier indeed than that which Dr. Huggins had to deal with when he undertook to measure the velocity with which Sirius is winging its flight through space. For there in the solar spectrum are the very lines of hydrogen with which the chromatospheric lines Illustrating the spectroscopic indications of solar cyclonic action. are to be compared. If the spectroscopic dispersion suffice, the least experienced observer can tell as cer- tainly that a solar storm is in progress as the terrestrial observer can tell by the motions of cirrus clouds that the upper regions of our own atmosphere are dis- turbed. The accompanying illustration (fig. 73), for instance, shows how the F-line in the chromatosphere- spectrum is at times swayed (as it were) from coinci- dence with the dark F-line of the ordinary solar spectrum. At 1 we see the line deflected towards the violet, showing that the portion of the chromatosphere under examination was moving rapidly towards the observer? at 2 we see a deflection both towards the 298 THE SUN. red and towards the violet, indicating that in the same field of view (that is, in the portion of the chromato- sphere included within the slit) there were masses moving towards as well as from the eye ; while, lastly, at 3 we see a deflection towards the red, indicating a rapid motion from the eye. During some observa- tions such as these, Mr. Lockyer has had evidence of motions at the almost inconceivable velocity of 120 miles per second. I cannot, however, accept his con- clusions as to the distribution of the motions over the Sun's surface, because he seems to take very little account of what certainly is the fact, that the extension of the portion of chromatosphere under examination is very great in the direction of the line of sight. As- suming an apparent depth of only ten seconds (which is within the usually observed limits), a tangent-line to the Sun's surface passes through a range of upwards of 60,000 miles ; and to speak of the clear recognition of a solar cyclonic storm only 1,500 miles in diameter (as described by Mr. Lockyer) when the visual ray passes through a depth forty times as great, seems to me wholly inadmissible. We have no evidence that the portions of the chromatosphere giving the three displacements shown in fig. 73 may not have lain 20,000, 30,000, or even 40,000 miles apart, in the direction of the line of view ; * in other words, that * One may notice in Borne papers on this subject a mode of explain- ing the observed phenomena which seems to imply that the spectroscope exhibits to us the condition of a mere slice of the solar envelopes. The effect of the Sun's solidity in giving an enormous extension to the visual lines through the chromatosphere seems wholly neglected even THE PROMINENCES AND THE SIERRA- 2 go, the observed solar storms, though undoubtedly raging with amazing fierceness, were necessarily cyclonic in character. This remark does not apply with equal force to the evidence deduced from the appearance of the prominence F-line depicted in fig. 74,* because we Fig. 74. if w ufni n Illustrating the spectroscopic indications of rapid motions in tall coloured prominences. cannot consider it likely that two prominences of the enormous height indicated by the extension of the shattered F lines from the spectrum of the limb would lie along the same visual line at the moment of observation. Professor Young, of America, has, however, noted where a reference to the breadth of the region really included within the slit would appear to invite a reference to the other form of extension. * In this figure the dots below the spectra indicate the amount of displacement on either side of the normal position of the F-line, corre- sponding to a velocity of 8, 16, and 28 German geographical miles per second (a German geographical mile being equal in length to a fifteenth part of a degree of the Earth's equator— i.e., to about 4| English miles). 3°0 THE SUN. a remarkable circumstance which seems in some sense to throw doubt on the inferences which have been deduced from the displacement of the F-line. He observed on one occasion that while the F-line of hydrogen in a prominence was absolutely shattered, the C-line of the same element presented its ordinary appearance. It seems absolutely essential to the in- terpretation hitherto placed on the displacement of one line of hydrogen that the other line should exhibit a similar displacement, differing only to a slight degree as regards extent. If the observation should be con- firmed that the C and F lines of hydrogen in the same prominence-spectrum behave differently, a most per- plexing problem will be presented not only to astrono- mers, but to physicists. We have now, however, to turn to a yet more in- teresting and valuable series of researches, by which astronomers have been able to observe the exact figure and changes of figure of the prominences. The prin- ciple on which these researches, first successfully pur- sued by Dr. Huggins, have been based, have been already explained towards the close of Chapter III. The visibility of the prominence-lines depends on the dispersive power of the spectroscope ; and, clearly, the narrower the opening of the slit the more effective will this dispersion be. The lines of the prominences are thus, indeed, narrowed, but they do not diminish in brightness; whereas the spectrum of the illuminated atmosphere diminishes in brightness in precise propor- tion to the narrowness of the slit. But if we widen THE PROMINENCES AND THE SIERRA. 301 the slit the forms of the prominences may be seen, if only the brightness of the atmospheric spectrum is not too greatly increased. In order, therefore, that this feat may be achieved, we must have a spectroscope of great dispersive power. Now, I have said that Dr. Huggins's spectroscope was not particularly well suited for the kind of ob- servation we are considering. It had not, in fact, sufficient dispersive power ; so that when the idea occurred to him of seeking for the prominences with an open slit, he had no great hope of succeeding. As a matter of fact, he failed. The increased light blotted out the prominences altogether. But he was not so to be foiled. As he had shown astronomical spectroscopists the way to success in the beginning, he now — with inadequate means — showed his pupils (if one may so speak) how to study the prominences to new purpose. Supplementing the powers of his spectroscope by the use of coloured glass, he was able to solve the great problem which was the true end of all the observations hitherto made. Others were aiming at the solution of the problem. One had suggested the use of coloured media admitting only such rays as the prominences emit. Another was trying I know not what device of rotating or vibrating slits, — And one had aimed an arrow fair, But sent it slackly from the string; And one had pierced an outer ring And one an inner here and there ; But last the master Bowman, he Had eleft the mark, 302 THE SUN. Fig. 75 represents the picture — rough, but instruc- tive — of the first solar prominence ever seen when the Sun was not eclipsed. Fig. 75. The first prominence seen by means of the spectroscope. (Huygins.) So soon as the open-slit method had been suggested by Huggins many other observers adopted it. Mr. Lockyer, availing himself of the great dispersive power of the instrument Mr. Browning had made for him,found that he could dispense with the use of coloured glasses. The prominences were rendered distinctly visible with the open slit alone ; and he could readily watch the changing figures of these mysterious objects. The accompanying drawings (figs. 76 and 77) exhibit two views of a wild and fantastic group of prominences, drawn by Mr. Lockyer, the second only ten minutes after the first was completed. Dr. Zollner, the eminent German photometrician, applied the same method in a systematic manner. Some of the pictures he has published are singularly interesting. THE PROMINENCES AND THE SIERRA. 303 It is to be noticed, in the first place, that Dj\ Zollner observed the same protuberance in three different J\g. 76. A group of solar prominences. — March 14, 1869, llh. om. (Loc/eyer.) colours, corresponding to the three lines of its spectrum. He found a material difference between the red and the blue image on the one hand and the yellow on the other. The latter is very intense only in close proximity to the Sun's limb,* and corresponds there to * It will be observed that Zollner's results as regards the coloured images correspond with those obtained by Secchi in observing the coloured lines. 304 THE SUN. the other images ; but the more delicate details dis- appear at a greater distance. Zollner suggests that this may be explained on the hypothesis that ' the rays which give rise to the yellow image emanate from a Fig. 77. The same group ten minutes later. gas having a greater specific gravity than hydrogen, and therefore existing at a lower level.' The most interesting of Zollner's drawings are ex- hibited in Plates IV. and V. It must be conceived by the reader that these views are only the red images. Zollner saw perfectly similar blue images, and nearly THE PROMINENCES AND THE SIERRA. $05 similar orange-yellow images. The combination of the three produces the complete image as seen, during eclipse, with the telescope. Respecting the several protuberances shown in Plate IV., he makes the following remarks: — 'In fig. 1 we see anintensely luminous peak-shaped mass rising from the border of the Sun, above which is spread a crowd- like formation of less intensity. To the same type be- long the protuberances of figs. 4 and 9. In fig. 4 it i< remarkable that the surprisingly beautiful cumulus shape of the cloud was separated from the peak by a considerable distance. The cloud was exceedingly delicate, and even its finest details could be recognised. The separate cumuli of which it was composed appeared almost like dull luminous points.' Fig. 2 was one of the most remarkable formations. ' 1 hardly believed my eyes,' says Zollner, ' when I noticed in it the tongue-like motion of a flame. This motion was slower, however, compared with the size of the flame, than that of high towering flames at great conflagrations. The time required by such a wave in passing from the base to the apex was a out two or three seconds. I endeavoured on the following days to verify this observation by the discovery of similar formations, but I have not succeeded, in spite of my arduous and continued search. I therefore request that this statement may be considered as one requiring further proof.' In fiss. 3, 6, and 10 we have remarkable illustrations of the great rapidity with which the protuberances 306 THE SUN. change both in form and brilliancy. In these figures the different shapes are represented which the same protu- berance assumed after the intervals of time indicated by the hours and minutes (Leipsic time) named under- neath them. The heights range from 30 to 120 seconds, and when it is remembered that a second of arc at the Sun's distance corresponds to a distance of no less than 450 miles, it will be seen on how enormous a scale these prominences are formed. Zollner, even at this early stage of his inquiries, was led to arrange the prominences into two distinct classes —the cloud-like and the eruptive. Respecting objects of the former class, he remarks that they remind him of the different forms of our clouds and fogs. ' The cumulus type is completely developed in some of the figures. Other formations remind one of masses of clouds and fogs floating closely over lowlands and seas, whose upper parts are driven and torn by currents of air, and which present the ever-varying forms so well known, when viewed from the tops of high mountains.' An important observation was made by Zollner on June 27, 1869'. ' On this day,' he says — ' the first clear day after a long spell of cloudy weather — I observed the bright protuberance-lines, without, however, being able at that time to make a complete observation of these formations. As soon as I approached the slit of the spectroscope to a certain position in the Sun's limb, where the protuberance-lines appeared particu- larly long and bright, brilliant linear flashes passed through the whole length of the dull spectrum over the THE PROMINENCES AND THE SIERRA. 307 limb of the Sun, about three or four minutes distant from the latter. These flashes passed over the whole of the spectrum in the field of view, and became so intense at a certain point of the Sun's limb as to pro- duce the impression of a series of electrical discharges rapidly succeeding one another and passing through the whole of the spectrum in straight lines. Mr, Vogel, who afterwards for a short time took part in these observations, found the same phenomenon at a different portion of the Sun's limb, where protuberance - lines also appeared. ' The phenomenon can be ex- plained,' adds Zollner, 'by the hypothesis that small, intensely incandescent bodies, moving near the surface of the Sun, emit rays of all degrees of refrangibility, and produce flashes of a thread-like spectrum as their image passes before the slit of the spectroscope.'* Plate V. illustrates a later paper of Zollner's, and the most cursory study will serve to show that he ob- served the prominence, there exhibited, under excep- tionally favourable conditions. Nothing can be more * We must not, however, forget the possibility that these objects may have been insects in the air at no very considerable distance from the observer. Capt. Herschel was for some time misled by a similar phenomenon, but, after long watching, one of the bright objects suddenly stopped and flew off in a new direction, and he then recognised the fact that he had been intently observing with the telescope, and carefully analysing with the spectroscope, a flight of locusts ! These objects were much more conspicuous than those seen by Zollner. however ; and the observation made by Mr. W. S. Gilman, jun., during the American eclipse, that in the heart of a large prominence (g of fig. 78, p. 312) there were exceedingly bright red points, seems to confirm the justice of Zollner's conclusion. X 2 308 TEE SUN. striking than the aspect of the prominence shown in these two drawings. It is obvious that Zollner had here been regarding the action of solar eruptive forces casting forth enormous masses of glowing hydrogen gas.* The evidence on this point has already been referred to. It seems impossible to conceive that any evidence could be more convincing. But we owe to Professor Respighi the most syste- matic and successful attempts yet made to deal with the solar prominences. He has in fact demonstrated the possibility of applying to the prominences syste- matic observations comparable with those applied by Schwabe and Carrington to the spots. On each of the days, 7, 12-15, 17, 18, 21, 24, 26, 29, and 31 of January, 1869, he took views round the solar limb, until he had completed the circuit, ranging the observed prominences in a straight line (the prominences being presented, for convenience, on a scale twice that of the chromatosphere's horizontal extension). The result is * Zollner remarks that the prevalence of one or other of the charac- teristic forms into which prominences may be divided — viz., the cloud form and the eruptive form — appears to depend partly on their local distribution over the solar surface, partly on the time of observation ; insomuch that at a. given time one form, at another the other, may be the prevailing type. 'That the cloud-formed prominences present so lively a resemblance to terrestrial cloud and smoke, is easily explained," he adds, ' when we recollect that the forms of our clouds must be supposed to depend, not on the vesicles of water suspended in them, but (essentially) only on the mode in which the air masses, variously warmed and moved, expand. The water-vesicles in terrestrial clouds form but the material by which such differences of temperature and motiun are rendered visible to us. In the clouds of the protuberances this visibility is brought about by the glow of the luminous hydrogen SPECTROSCOPIC OBSERVATIONS OF THI _ . . . ii ^\ ~_ . . - 18707. Jan* Scale ot* pronu'netices twi SUNS BORDER. BY PROF? RESPICHI ? &S& SE TlSr. E I-NE M TliAivtKKvL —■ » >iml -1 tfiadr of rAromoSpkere, THE PROMINENCES AND THE SIERRA. 309 shown in Plate VI. We have only to suppose each horizontal row of coloured prominences bent round a disc representing the Sun, and the prominences reduced to one-half their present dimensions, in order to have a complete picture of the Sun's condition on any of the days illustrated in the plate. It cannot be but that the application of Respighi's system throughout a long series of years will result in revealing laws in the number and magnitude of prominences at different times and in different solar latitudes. Such laws, once clearly recognized, will probably serve to indicate relations, as yet wholly un thought of, between the prominences, the phenomena of the photosphere, the magnetic relations of the planetary scheme, and many other periodic phenomena. Leaving the reader to study at his leisure the singularly interesting features shown in Plate VI., I will briefly indicate certain general conclusions to which Professor Respighi's researches have led him. He considers that the prominences, at least as regards their origin, bear no analogy whatever to terrestrial clouds, but are strictly phenomena of eruption.* He has noticed that where there are faculas there are usually prominences, but he considers that the promi- nences are certainly not identical with the faculas. Over Sun-spots there are low jets, but no high prominences. In the circumpolar regions great prominences are never * This view is not altogether opposed to Dr. Zollner's, who regards the two types into which he divides prominences as different forms of the same objects, and, therefore (necessarily), the cloud form as the , sequel of the eruptive. 310 THE SUN. recognized, and the prominences actually seen, besides being small, are few in number, and last but a short time. At the solar equator, also, the prominences are less frequent, less active, and less developed than in higher solar latitudes. He noticed some prominences exceeding three minutes, or ten terrestrial diameters, in altitude ; and one prominence observed by him had an elevation of no less than twenty terrestrial diameters, or about 160,000 miles. He found that the formation of a prominence is usually preceded by the appearance of a rectilinear jet, either vertical or oblique, and very- bright and well defined. This jet rising to a great height, is seen to bend back again, falling upon the Sun like the jets of our fountains, and presently the sinking matter is seen to assume the shape of gigantic trees more or less rich in branches and foliage. Gradually the whole sinks down upon the Sun, some- times forming isolated clouds before reaching the solar surface. It is in the upper portions of such promi- nences that the most remarkable and rapid transforma- tions are witnessed ; but a great difference is observed in the rate with which prominences change in figure. Their duration, too, is very variable. Some develope and disappear in a few minutes, while others remain visible for many successive days. He considers that the sharply-defined bases of the eruptive jets prove that the eruption takes place through some compact sub- stance forming a species of solar crust.* He agrees * Thia corresponds to the Trennungsehicht of Zollner's theory, who, however, does not agree with Respighi in regarding some form of electric action as the probable cause of these eruptions* THE PROMINENCES AND THE SIERRA. 31 1 with Zollner in considering that the enormous velocity with which these gaseous masses rush through the solar atmosphere implies that the latter is of exceeding tenuity. His conclusion that repulsive forces must balance the solar gravity, is not shared, however, by Zollner, who prefers to regard the forces at work in producing the prominences as strictly analogous to those which produce eruptive action on our Earth.* A few pages further on later evidence on this point will be considered. During the total solar eclipse of August, 1869, many interesting observations were made. But so much space has already been occupied by the subject of this chapter that I must deal very briefly with the observations on the prominences and sierra. In the next chapter I shall have occasion to quote at some length those narratives of the American observers which relate to the corona. The accompanying draw- ing (fig. 78) exhibits the prominences photographed by the American astronomers, the Moon's disc being reduced so as to permit all the prominences to be visible at once. The line v e indicates the course of the Moon's centre across the Sun, A B being a declination- * It seems conceivable that the phenomena of geysers may have their counterpart in these solar prominences. It is true that a gaseous, not a liquid, mass is erupted ; so that there is here nothing comparable to the rapid change of a column of liquid. into vapour which causes the out- bursts of geysers. But the temperature and pressure at which disso- ciation occurs may bear the same relation to the outbursts of these gaseous masses that the temperature and pressure at which water boils bear, in BuDsen's theory of geysers, to the occurrence of geyser erup- tions. [Prof. Young's account of the explosion he witnessed on Sep- tember 7, 1871, p. 320, seems to accord well with the view here expressed. 3 i2 THE SUN. circle, and c d a declination-parallel. The most suc- cessful photographing party was that headed by Dr. Mayer, at Burlington. But the two photographs by Fin. 78. ; 'llie iscnpse of August 7, 186t>. i'rom Photographs by the American astronomers. Dr. Curtis possess a special value on account of their delicacy and the number of details they exhibit. He has shown that the sharply-defined outlines of promi- nences, and even of the coronal glare, seen in most photographs of eclipses, result from excessive develop- ment. He proves satisfactorily, also, that the encroach- ment of the prominence-bases on the black disc of the Moon is due to a similar cause. THE PROMINENCES AND THE S1EHRA. 313 In Dr. Curtis's contribution to Rear- Admiral Sand's elaborate report of the eclipse the curious reader will find a very interesting investigation of the aspect of the prominence shown between E and C in fig. 78. He refers all its peculiarities of appearance to the action of a cyclonic storm in the upper regions of the solar atmosphere at this place. The following account by Mr. G. H. Knight, who studied the prominences with a telescope magnifying Fig. 79. 120 times, is extracted from the ' Scientific American ' of September 11. But fig. 79 is copied from part of a very fine drawing, kindly sent to me by Mr.; 3H THE SUK. Knight.* It is the best drawing of prominences as visible during eclipse that I have yet seen. The part of the Sun's edge included in the figure is that lying between G and H in fig. 78. ' We have only two precious minutes,' writes Mr. Knight in his narrative of the eclipse, 'and leaving our new acquaintances — Mercury, the sombre woods, the leaden sky, the inky river — to other observers, we direct our 120-magnifier to the red specks, some six or seven in number, now plainly discernible around the Moon's margin. ' These appearances, when brought within the field of the telescope, show a surprising individuality, and all, by shape, suggest violent disturbance, whose motions are, however, of course invisible by reason of the im- mense distance, and can be ascertained, if at all, only by a record of impressions of successive observers stationed along the track of the swiftly-gliding shadow. ' The tube is directed to a point, near the Moon's nadir (uppermost or inverted in the instrument), * .Respecting this drawing Mr. Knight makes the following striking remarks : — ' Of course I am dissatisfied with my effort. Who ever was satisfied in attempting to depict celestial phenomena ? Who, for example, ever yet pourtrayed a comet? Still I believe my rude attempt more true to life than any photographs I have seen. One marked peculiarity of such appearances — the delicate blended haze of light — which to some observers extends much further into 6pace than to others, is ignored by the photograph, which is an inveterate Rem- brandt in its rejection of middle tints. Then, again, the protuberances and the quivering fringe of tire, of which they are the more salient portions merely, are of various tints of gold, copper, and pink, con- fessedly difficult colours for actinic action, but which the wonderful camera of the human eye takes in perfectly. Besides which, I had the advantage of a more powerful instrument than any other observer I have heard of.'- THE PROMINENCES AND THE SIERRA. 315 occupied by the brightest of these lights (G of fig. 78). The apparition seems to radiate from some point hidden behind the Moon's disc, beyond which it emerges in brilliant silver, copper, and ruby-colored coruscations, the copper tints predominating, and terminates in a circular arc like a half-set Sun. The impression con- veyed to an observer is of a vast explosion from a centre some twenty thousand miles below the edge of the Sun's disc, and extending therefrom about fifty thousand miles in every direction. 'About fifty degrees of the Moon's circumference from this prominence we observe a second and wholly differ-" ent one (F of fig. 78) which bears a grotesque resem- blance to a stag's antlers or to the strands of a raveled rope tossed about by a whirlwind. The shape and coruscations of this apparition suggest electrical action {fancy an electric spark 500 miles thick !) or the defla- gration of some liquid metal. Its color is crimson ; its height about twenty thousand miles. ' Still another and totally different emanation is seen (H of fig. 78). It wears the semblance of a horse's tail, or, more nearly, of a puff of smoke drifting north- ward, and illuminated by the rosy hues of sunset. ' At this stage of observation some one jogged the instrument, and before it could be adjusted to another group, a glint of sunlight from the disc's right margin blinded our unaccustomed retinas and flooded the land- scape with returning day. At the same instant, looking upward, we beheld the Moon's black shadow, sharply defined as a wall in the air, sweep majestically away 316 THE SJJN. from right to left before our very eyes — and the total eclipse of 1869 had become a thing of the past ! '* The spectroscopic study of the coloured prominences, since 1870, has revealed some facts of extreme interest. In the first place, Father Secchi has made a series of observations by which the researches of Zollner and Respighi are confirmed and supplemented. The follow- ing are the chief points in Secchi's latest papers which it seems necessary to notice in this place. I have italicized the facts to which I desire to invite special attention.! He notes that the chromatosphere presents four aspects, — (i) smooth, with a defined outline; (2) smooth, with no definite outline; (3) fringed with filaments ; and (4) irregularly fringed with small flames. He divides the prominences into three orders — heaps, jets, and plumes. The heaped prominences need no special description here ; they will be recognized in Zollner and Respighi's views. The jets are the pro- minences to which Respighi's description at p. 309 is specially applicable. Secchi notes that their luminosity * ' With our present meagre array of facts,' notes Mr. Knight, ' hypo- theses are premature. The Sun's heat seems too intense for many of the terrestrial phenomena of chemical action. A cause may, however, exist in meteorolites which, falling with inconceivable velocity and possessing a high spheroidal repulsion, may carry with them into the Sun's seething cauldron a comparatively cold body of disturbing ele- ments and give rise to the mechanical and other perturbations whose manifestations have been noted. t Fr. Secchi's paper was received with enthusiasm by the Academy of Sciences, at Paris, and the whole memoir, though exceeding the usual space, was printed in extenso in the Comptes Rendus. THE PROMINENCES AND THE SIERRA, p 7 is intense, insomuch that they can be seen through the light clouds into which the sierra breaks up. Their spectrum indicates the presence of many elements besides hydrogen. When they have reached a certain height they cease to grow, and become transformed into ex- ceedingly bright masses, which eventually separate into fiery clouds. The jet prominences last but a short time, rarely an hour, frequently but a few minutes, and they are only to be seen in the neighbourhood of the spots. Wherever there are ,jet prominences there also there are faculce. The plume prominences are dis- tinguished from the jets in not being characterised by any signs of an eruptive origin. They often extend to an enormous height, last larger than the jets, though subject to rapid changes of figure, and, lastly, tla-y are distributed indifferently over the Sun's surface. ' It loould seem,' says Secchi, ' that in jets a part of the photosphere is lifted up, whereas in the case of plumes only the chromatosphere is disturbed." 1 On September 7, 1871, Prof. Young, of America, observed the most remarkable outburst from the Sun ever yet witnessed by man. His observation is sin- gularly interesting in itself; but it acquires a fresh interest from the light it throws on observations already described, and more particularly on those made by Kespighi and Secchi. ' On the 7th of September last,' he writes in October, 1871, 'between half-past twelve and two p.m. there occurred an outburst of solar energy remarkable for its sudden violence. J list at noon the writer had been 3'8 THE SUN. examining with the telespectroscope * an enormous protuberance or hydrogen closed on the eastern limb of the Sun. ' It had remained with very little change since the preceding noon — a long, low, quiet-looking cloud, not very dense or brilliant, nor in any way remarkable except for its size. It was made up mostly of filaments nearly horizontal, and floated above the chromatosphere with its lower surface at a height of some 15,000 miles, but was connected with it, as is usually the case, by three or four vertical columns brighter and more active than the rest. Lockyer compares such masses to a banyan grove. In length it measured 3' 45", and in elevation about 2' to its upper surface — that is, since at the Sun's distance \" equals 450 miles nearly, it was about 100,000 miles long by 54,000 high. ' At 12.30, when I was called away for a few minutes, there was no indication of what was about to happen, except that one of the connecting stems at the southern extremity of the cloud had grown considerably brighter, and was curiously bent to one side ; and near the base of another at the northern end a little brilliant lump had developed itself, shaped much like a summer thunder-head. Fig. 80 represents the prominence at this time, a being the little " thunder-head." f ' What was my surprise, then, on returning in less * 'This is the name given by Schellento the combination of astrono- mical telescope and spectroscope. f 'The sketches do not pretend to accuracy of detail, except the fourth ; the three rolls in that are nearly exact. THE PROMINENCES AND THE SIERRA. 319 than half an hour (at 12.55), to find that in the mean- time the whole thing had been literally blown to shreds by some inconceivable up-rush from beneath. In place Fig. 80. of the quiet cloud I had left, the air, if I may use the expression, was filled with flying debris — a mass of detached vertical fusiform filaments, each from 10" to 30" long, by 2" or 3" wide, brighter and closer together where the pillars had formerly stood, and rapidly ascending. ( When I first looked some of them had already reached a height of nearly 4' ( 100,000 miles), and while I watched them they rose with a motion almost perceptible to the eye, until in ten minutes (1.5) the uppermost were more than 200,000 miles above the solar surface. This was ascertained by careful measurement ; the mean of three closely-accordant determinations gave 7' 49" as the extreme altitude attained, and I am particular in the statement because, so far as I know, chromatos- pheric matter (red hydrogen in this case) has never 320 TUB SUK. before been observed at an altitude The velocity of ascent also, 166 miles per second, is considerably greater than anything hitherto recorded. A general idea of its appearance when the filaments attained their greatest elevation may be obtained from fig. 81. ' As the filaments rose they gradually faded away like a dissolving cloud, and at 1.15 only a few filmy Pig. 81. wisps, with some brighter streamers low down near the chromatosphere, remained to mark the place. ' But in the meanwhile the little " thunder-head " before alliuled to had grown and developed wonderfully THE PROMINENCES AND THE SIERRA. 321 into a mass of rolling and e>per-changlng flame, -to speak according to appearances. First it was crowded down, as it were, along the solar surface ; later it rose. Fig. 82. almost pyramidally 50,000 miles in height; then its summit was drawn out into long filaments and threads which were most curiously rolled backwards and down- Fig. 83. wards, like the volutes of an Ionic capital ; and finally it faded away, and by 2.30 had vanished like the other. Figs. 82 and 83 show it in its full development ; the former having been sketched at 1.40, and the latter J. 55. T 322 THE SUN. ' The whole phenomenon suggested most forcibly the idea of an explosion under the great prominence, acting mainly upwards, but also in all directions out- wards, and then after an interval followed by a cor- responding in-rush ; and it seems far from impossible that the mysterious coronal streamers, if they turn out to be truly solar, as now seems likely, may find their origin and explanation in such events. ' The same afternoon a portion of the chromatosphere on the opposite (western) limb of the Sun was for several hours in a state of unusual brilliance and ex- citement, and showed in the spectrum more than 120 bright lines whose position was determined and cata- logued, — all that I had ever seen before, and some 15 or 20 besides.' * It remains only to mention that Professor Young, of America, has succeeded in obtaining a photograph of a prominence when the Sun has been, shining in full splendour. Although the result is, he tells us, not remarkable as a presentation of a solar prominence, yet as indicating the possibility of applying photography to record the condition of the chromatosphere and prominences from day to day, from month to month, * Boston Journal of Chemistry. — Prof. Young has published a list of 103 of these chromatospheric bright lines, whereof 5 have been detected by Eayet, 1 6 by Lockyer, 3 by Lockyer and Janssen, 5 by Loekyer and Rayet, and the remaining 74 (besides the 17 above referred to) by Young. The elements whose existence has been recognised in the chromatosphere, are hydrogen, iron, sodium, magnesium, barium, calcium, chromium (?), nickel, and titanium, the presence of the last being a discovery of Prof. Young's. THE PROMINENCES AND THE SIERRA. 323 and from year to year, it is full of promise. The time seems not far off when we shall be as familiar with the laws according to which these mysterious objects appear, develope, and disappear within the solar atmospheric envelope as we have already become with the general laws affecting the behaviour of Sun- spots. T 2 324 THE SUN. CHAPTER VI. THE COBONA AND ZODIACAL LIGHT. The coloured prominences, as we have seen, are phenomena which have been recognized only in recent times. We have now to deal with a phenomenon which has been known to astronomers for a much longer period, hut has proved more difficult of interpretation, and remains even at the present day not clearly under- stood. It may fairly be believed that during the earliest total solar eclipses observed by mankind, the corona, or crown of glory, which surrounds the black disc of the Moon, must have attracted attention. Yet the records of the recognition of this phenomenon are by no means so distinct as might have been expected. We do not find, indeed, in ancient works professedly treating of the phenomena of nature, any reference to the im- posing appearance presented by the solar corona. The earliest allusion to the phenomenon is held by Professor Grant to be that passage in the Life of ' Apollonius ' where Philostratus, speaking of the signs and wonders which occurred before the death of Domitian, says, ' In the heavens there appeared a prodigy of this nature : THE CORONA AND ZODIACAL LIGHT. 325 a certain corona, resembling the Iris, surrounded the orb of the Sun, and obscured his light.' Taken alone, this passage would certainly not seem intended to describe the phenomena of a total eclipse.; but as Philostratus afterwards remarks that the darkness was so great as to resemble night, we may assume with some confidence that a total solar eclipse had oc- curred. Plutarch more distinctly describes the appearance actually presented by the corona when he endeavours to explain why the darkness during a total eclipse is not so great as that of night. ' Even though the Moon,' he says, ' should hide at any time the whole of the Sun, still the eclipse is deficient in duration as well as amplitude, for a peculiar effulgence is seen around the circumference which does not allow a deep and very intense shadow.' I do not propose to record in full the observations which have been made upon the corona. To do so would occupy, indeed, much more space than can here be spared. Referring the reader who wishes for a more complete account of the earlier observations to Professor Grant's admirable ' History of the Physical Sciences,' I shall consider here those observations alone which tend to throw light on the nature of the : corona. Some of the earlier observers of total solar eclipses would seem to have been misled by the great brightness of the corona close by the Sun, and to have supposed that a ring of direct sunlight had remained uncovered. 326 THE SUN. We shall see presently that modern observers have also been struck with the brightness of the light close by the Sun; and it seems obvious that this bright light is to be regarded as wholly distinct from the light of the chromatosphere, the redness of which is too marked to escape recognition. Remembering that before the invention of the telescope the corona was the only marked phenomenon to which observers were able to direct their attention, a certain weight attaches to their comments on the brightness near the Moon's disc. Clavius having expressed his belief that the eclipse of 1567 was annular, Kepler was led to investigate the subject, and he proved that that eclipse must needs have been total. In 1605 he witnessed a total eclipse at Naples, and found, in the features it presented, the explanation of the remarks of Clavius. ' The whole body of the Sun,' he says, 'was completely covered for a short time, but around it there shone a brilliant light.' We might suppose that he referred to the chromatosphere, because he says that the light was ' of a reddish hue ; ' but as he adds that it was ' of uniform breadth, and occupied a considerable part of the heavens,' there can be no doubt that he is speaking of the corona. Dr. Wyberd gives a remarkable account of the appearance of the corona during the total eclipse of March 29, 1652. 'When the Sun was reduced to a narrow crescent of light,' he says, ' the Moon all at once threw herself within the margin of the solar THE CORONA AND ZODIACAL LIGHT. 327 disc with such agility that she seemed to revolve like an upper millstone, affording a pleasant spectacle of rotary motion. In reality, however, the Sun was totally eclipsed, and the appearance was due to a corona of light round the Moon, arising from some unknown cause. It had a uniform breadth of half a digit, or a third of a digit at least ; * it emitted a bright and radiating light, and appeared concentric with the Sun and Moon' when the middle of totality was reached. It need scarcely be remarked that the agility with which the Moon seemed to throw herself within the margin of the Sun's disc was merely apparent. It has been noticed by many observers that the total obscura- tion of the Sun seems to occur suddenly, the Moon covering the last sickle of sunlight, apparently at a leap. Irradiation is no doubt in question. The sickle of light, even when in reality it is indefinitely fine, appears, through the effects of irradiation, to have a definite breadth ; so that the Moon seems to traverse a definite distance in obliterating' what is in truth the finest possible curve of light. But beyond this there is a circumstance which cannot but give an appearance of somewhat agile motion to the eclipsing disc of the Moon. Up to the very moment when totality is about to begin, the air between the ob- server and the Moon is illuminated by direct sunlight. This is easily seen by a consideration of fig. 84, in * A digit signifies the twelfth part of the solar diameter. The term is nearly obsolete. 3^8 THE SUN. which A b C D represents the portion of the Moon's shadow in the atmosphere, E the place of an observer towards whom this shadow is swiftly advancing. Then, until the shadow actually reaches e (at which moment totality begins) a line drawn from e to d will pass to the left of the line A T> m — as in the direction e v> f\ so. that, since the line A D m is necessarily directed towards the Moon's limb, the directly illuminated air (bounded, of course, by a r>) extends between the observer and the Moon. Hence, the Moon's disc Seems lighted up by this atmospheric glare until the Pig. 84. Illustrating the condition of the Earth's atmosphere a few minutes hefore totality in a solar eclipse. very moment when totality begins ; and as the curtain of glare is drawn suddenly away towards the edge where the last sickle of the Sun's direct light is vanish- ing, all the circumstances tend to give an appearance of agile motion in that direction. As respects the rotatory motion which seemed to ac- company this leap forward on the Moon's part, we can very well understand it as referring to the comple- tion of the corona, which must, needs take place by a sweeping round of the bounding rays to close in upon each other opposite the point where the last part of the THE CORONA AND ZODIACAL LIGHT. 329 Sun's disc disappears. It will be well to notice, however, as we proceed, whether we have convincing evidence of an apparent motion of the coronal beams after totality has begun. The evidence on this point cannot but have an important bearing on the views we are to form respecting the corona. Dr. Wyberd saw a corona of very limited extent — indeed only half a digit wide — corresponding to a height not half so great as that of many prominences which have been observed during recent eclipses. Our next observation refers, however, to a much more favourable view of the corona. In May 1706 Plantade and Capies saw a bright ring of white light surrounding the eclipsed Sun, and extending to a distance equal to about one^tenth part of the Moon's apparent diameter. Outside this bright ring a fainter light could be recognised, which extended no less than four degrees from the eclipsed Sun, fading off insensibly, until its light was lost in the obscure background of the sky. In 1724 Maraldi noted a circumstance of some importance. At the beginning of the total eclipse which was observed in France in that year he per- ceived that the corona was wider on the side towards which the Moon was advancing than on the opposite side. At the close of totality the widest part of the corona was on the opposite side. As this would exactly correspond to what would be observed if the corona lies beyond the Moon, and so is traversed by the: Moon precisely as the Sun himself is, it will be well for us to 330 THE SUN. notice as we proceed whether Maraldi's observation has been confirmed or disproved, or how the evidence stands with respect to it. In 1733 a total eclipse occurred in Sweden, which was observed in a manner reflecting great credit on the astronomers of that country. The Royal Society • of Sweden invited all who could spare the time to assist, as far as their ability permitted, in recording the phenomena presented during total obscuration. Ac- cordingly there are few eclipses - perhaps there is not one — to the phenomena of which so many independent witnesses give testimony. ' At Catherinesholm the pastor of Forshem noticed that a ring of light which appeared round the disc of the Moon was of a reddish colour — an observation confirmed by Vallerius, another pastor, M T ho noticed, however, that at a considerable distance from the Sun the ring appeared of a greenish hue. The pastor of Smoland states that " during the total obscuration the edge of the Moon's disc resembled gilded brass, and that the faint ring around it emitted rays in an upward as well as in a downward direction, similar to those seen beneath the Sun when a shower of rain is impending." The mathematical lecturer in the Academy of Charlesstadt, M. Edstrom, observed these rays with special attention, and remarks, respect- ing them, that " they plainly maintained the same posi- tion, until they vanished along with the ring upon the reappearance of the Sun." On the other hand, the ring, as seen at Lincopia, seemed to have no rays. Professor Grant remarks that " from the descriptions THE CORONA AND ZODIACAL LIGHT. 331 given by several observers, it would seem that at the commencement of the total obscuration the ring ap- peared brighter and broader at the part of the Moon's limb where the Sun had disappeared, but that towards the close of the obscuration it was more conspicuous in both these respects at the part where the Sun was about to emerge." ' * It is observable, therefore, of this well-watched eclipse that it confirms Maraldi's observation, while the variety of appearance presented by the corona at different stations would point to the conclusion that if the corona is not a phenomenon of our own atmosphere, its light must for the most part be of a very delicate nature, insomuch that seemingly unimportant differ r ences in the circumstances under which it is viewed suffice to modify its aspect to a very noteworthy extent. We shall see presently that during recent eclipses similar evidence has been afforded of the extreme faintness of a large portion of the coronal light. During the eclipse of 1766 the corona exhibited four remarkable expansions, separated from each other by nearly equal intervals. In the account given by Don Antonio d'Ulloa of the appearance presented by the total eclipse of 1778, we again find a reference to the appearance of rotatory motion in the corona. ' Five or six seconds after the commencement of the total obscuration,' he writes, ' a brilliant luminous circle was seen surround- ing the Moon, which became vivid as the centre of * -From an article in the CornhiU for October 1870 (by the present writer). 332 THE SUN. that body continued to approach the centre of the Sun. About the middle of the eclipse its breadth was equal to one-sixth of the Moon's diameter. There appeared issuing from it a great number of rays of unequal length, which could be discerned to a distance equal to the lunar diameter. It seemed to be endued with a rapid rotatory motion, which caused it to resemble a firework turning around its centre. The colour of the light was not uniform throughout the whole breadth of the ring. Towards the margin of the lunar disc it appeared of a reddish hue ; then it changed to a pale yellow, and from the middle to the outer border the yellow gradually became fainter, until at length it- seemed almost quite white.' The next eclipse during which new features of im- portance were noticed, was that of 1842. We have seen how carefully the prominences were observed during that eclipse, and how many eminent astronomers were engaged in noting the phenomena presented during the total obscuration of the Sun. The corona was also carefully studied. Indeed, one is inclined almost to regard it as a misfortune that the same astronomers who tell us about the appearance of the prominences, are those from whom we derive our in- formation respecting the corona. One cannot but feel that the accurate observation of both phenomena was more than could be expected even from the most skilful astronomers, and that a division of labour might have been advisable. The apparent motion of the corona was noticed by THE CORONA AND ZODIACAL LIGHT. 333 several observers. Some of those stationed at Mont- pellier thought that the corona had a rotatory motion. Francis Baily compared the appearance of the corona to the flickering light of a gas illumination. Otto Struve, also, was much struck by the violent agitation to which, as it seemed to him, the light of the ring was subjected. The apparent extent of. the corona as seen by different observers varied in a somewhat surprising manner. Otto Struve, observing at Lipesk, found the breadth of the corona equal to the Moon's apparent diameter; while M. Petit, observing at Montpellier, assigned to the corona a breadth scarcely exceeding one-fourth of this amount. Baily 's estimate lay be- tween these values. Nor were the observers in agree- ment as to the general appearance of the corona. Otto Struve observed several luminous expansions, some of them extending fully four degrees from the Moon's limb. Signor Picozzi, observing at Milan, saw two jets of light occupying a position which corresponded very nearly with that of the ecliptic. Several observers in France noticed a similar peculi- arity. Minor rays, also, were distinctly recognised by Mauvais at Perpignan, and by Baily at Pavia. The last-named astronomer remarks, indeed, that the diverging rays were sufficiently marked to deprive the corona of the appearance of a ring. But Mr. Airy, observing the corona from the Superga, could scarcely recognise any radiation whatever; and he remarks, that ' although a slight radiation might have been perceptible, it was not sufficiently intense . to affect in 334 THE SUN. a sensible degree the annular structure by which the luminous appearance was plainly distinguished.' If we consider the accounts which the observers give of the brilliancy of the corona, we shall find that these peculiarities are in accordance with the theory that atmospheric conditions were alone in question. At the Superga the brightness of the corona seemed no greater than that of the Moon ; but at Pavia, where Baily was stationed, the splendour of the corona was much greater. 'I had imagined,' this observer writes, ' that the corona, as to its brilliant or luminous appearance, would not be greater than that faint cre- puscular light which sometimes takes place in a summer evening, and that it would encircle the Moon like a ring. I was therefore somewhat surprised and astonished at the splendid scene which now so suddenly burst upon my view.' We have seen that at Lipesk the corona seemed to extend much farther from the Sun than elsewhere, and accordingly we find that its brilliancy was also far greater. The light was so bright, Struve writes, that the naked eye could scarcely endure it. Many could not believe, indeed, that the eclipse was total, so strongly did the corona's light resemble direct sunlight. But perhaps the most interesting and important observations made in 1842 are those which refer to the structure of the coronal light. It was noticed at Montpellier that the light of the corona was not uni- form, nor merely marked with radiations, but that, in places, interlacing lines of light could be seen. Arago, THE CORONA AND ZODIACAL LIGHT. 335 at Perpignan, recognised this peculiarity with the naked eye. He saw, ' a little to the left of a diameter passing through the highest point of the Moon's limb, a luminous spot composed of jets entwined in each other, and in appearance resembling a hank of thread in disorder.' The accompanying picture (fig. 85) represents the Fig. 85. The Eclipse of 1812. general phenomena seen during the eclipse of 1842 ; but it must be remembered, that such illustrations cannot be regarded as accurately representing details, because they are usually drawn after the eclipse is over, and represent merely what the observer remembers. Until the whole duration of a total eclipse is devoted by a skilful observer and draughtsman to the delineation of the corona, we cannot expect to have really trust- worthy views. 33& THE SUN. During the eclipse of 1851 no observations were made which tended to throw new light on the nature of the corona. It is worthy of notice, however, that the Astronomer Royal found the aspect of the corona different from what he had noticed in 1842. ' The corona,' he says, ' was far broader than that which I saw in 1842. Roughly speaking, the breadth was little less than the Moon's diameter, but its outline was very irregular. I did not notice any beams projecting from it which deserved notice as much more conspicuous than the others ; but the whole was beamy, radiated in structure, and terminated — though very indefinitely — in a way which reminded me of the ornament frequently placed round a mariner's compass. Its colour was white, or resembling that of Venus. I saw no flicker- ing or unsteadiness of light. It was not separated from the Moon by any interval, nor had it any annular structure. It looked like a radiated luminous cloud behind the Moon. These observations, combined with what had been before noticed, seem to demonstrate that the aspect of the corona is variable according to the circumstances under which it is viewed. It does not seem to be established that the rotatory and flickering motions suspected by other observers were only optical illusions, though the observation of the steadiness of the corona, by such an observer as the Astronomer Royal, goes far to negative observations of motion by less experienced astronomers. In the case of a phenomenon like the THE CORONA AND ZODIACAL LIGHT. 337 corona, it is easier to imagine movement in the ring of light than to become convinced of its fixedness. The eclipse of 1858, visible in Brazil, is chiefly re- markable on account of the strange drawing made by the French astronomer Liais (fig. 86). During the late eclipse (Dec. 1872), the accuracy of this singular picture was confirmed. The eclipse of 1860 is remarkable as the first in Fig. 86. The Eclipse of 1S68. (Liais.) which the powers of photography were employed to aid in the resolution of the problems presented by the corona. It will be seen, on a reference to figs. 66 and 67, that Mr. De La Rue succeeded in obtaining traces of the corona. Those seen in Father Secchi's photo- graphs are somewhat more distinct, the method he employed giving a smaller and more fully illuminated z 338 THE SUN. image. In a third of Fr. Secchi's photographs (fig. 87) the corona is yet more distinctly shown. The extension of the corona in fig 87 is regarded by Fr. Secchi as corresponding to the solar equator, whose position is very nearly indicated by the cross-wire Pra. 87. Tne Eclipse of I860, i'ruin a Photograph by Secchi. shown in the figure. ' La couronne,' he says, ' est tres irreguliere, mais on peut remarquer qu'elle pre- sente une etendue plus considerable a droite et a. gauche que dans les autres directions, c'esta-dire qu'elle est plus developpee dans le plan de l'equateur (solaire) que suivant la ligne des poles.' But, as Fr. Secchi himself points out further on, the figure indicates THE CORONA AND ZODIACAL LIGHT. 339 rather an extension opposite four points lying between the equator and poles, than an extension at the equator. In fact, fig. 87 presents a very striking resemblance to Mr. Whipple's picture further on (fig. 90). It is worthy of notice, too, that not only does the outline of the corona present this quadrilateral aspect, but in the bright parts close to the Moon's limb there are four corresponding regions of greatest brilliancy. In the second of Mr. De La Rue's photographs the brightest portions seem similarly disposed.* Some of the direct observations fttade in 1860 serve also to throw important light on the nature of the corona. To the Astronomer Royal the corona presented much the same aspect as in 1851. Bruhns of Leipsic states that when the last rays of the Sun disappeared, the corona shone out with a white light of such brilliancy that the protuberances were almost * One cannot wholly agree with Father Seechi's remark, that Mr. De La Rue's photographs afford no evidence of the peculiar quadrilateral expansion of the corona corresponding to the position of the zone of spots. It is true, however, .that when Fr. Secehi ' published his results astronomers did not conceal their doubts.' If the expansion of the corona in four directions be regarded as an ordinary phenomenon (and we have seen how often it has been noticed), some very perplexing questions would be presented as to the cause of the peculiarity. A rectangular figure, like that shown in fig. 87, would correspond to a cylindrical real figure ; but it would also correspond to other figures of three dimensions. It is indeed possible that there may be no corona at all opposite the solar poles, the light we see there being merely a fore- shortened view (on this supposition) of the great extension over the spot belts. In this case the true figure of the corona would resemble that due to the rotation of two hyperbolas, having the same axes, around one of these axes. z 2 34° THE SUN. obliterated. He adds, 'the black -looking Moon was surrounded by a clear light of unequal breadth. It was considerably wider below than above, varying from nearly one-half to a quarter of a degree, and its gene- ral appearance gave me the idea that the Moon was ec- centrically placed within it. Its general outline was circular, but on the eastern side a long ray shone out to a distance of about a degree (that is, twice the Moon's apparent diameter) ; it was of a tapering figure. During the ten seconds that my attention was directed to it, neither the direction nor the length of the ray varied ; its light was considerably feebler than that of the corona, which was of a glowing white, and seemed to coruscate or twinkle. My assistant, M. Auerbach, noticed, in the south-western part of the corona, a curved ray about a tenth of a degree in length.' Father Secchi found that the corona could be seen with the naked eye for about forty seconds after the reappear- ance of the Sun, ' the solar light shining like an electric lamp projecting tremulous shadows.' Mr. De La Rue states that ' several minutes before totality the whole contour of the Moon could be distinctly seen : when totality had commenced, the Moon's disc appeared of a deep brown in the centre of the corona, which was extremely bright near the Moon's limb, and appeared of a silvery white, softening off with a very irregular outline, and sending forth some long streams. It ex- tended generally to about from seven to eight tenths of the Moon's diameter.' This description corresponds THE CORONA AND ZODIACAL LIGHT. 34 1 very satisfactorily with the appearance represented in the accompanying picture (fig. 88) by Feilitsch. During the eclipse of April 1865 it was noticed that the corona continued visible for thirty-six seconds after the appearance of the first rays of direct sunlight. The eclipse of March 1867 was only annular, yet it presented a feature well worth careful consideration, Fro. 88 The Corona during the Eclipse of 1860. (Feilitsch.) and that at a station considerably removed from the line of central eclipse. O. Struve and Wagner, using the great equatorial of the Pulkowa observatory, noticed that when the eclipse reached its greatest phase, the outline of that part of the^Moon's disc which was outside the Sun's, could be distinctly seen. Schwabe, who observed the same phenomenon at Dessau, noticed that this part of the Moon's disc was 342 THE SUN. rendered visible by its superior blackness ; a circum- stance which proves that the light of the corona must be considerably stronger than the illuminated atmosphere near .the Moon's place,* even when a portion of the Sun's direct light is falling upon the air there. The total eclipse of August 1867 attracted very much less attention than those which occurred during; the same month of the two following years. Yet the observations made by Herr Grosch, of the Santiago observatory, Chili, in conjunction with Vice-Director Verg-aza and Lieut. Vidal, are full of significance in connection with the main object of this chapter, which is the determination of the true theory of the corona. At the moment when totality began, ' there appeared,' says Herr Grosch,f ' around the Moon a reddish glimmering light, similar to that of the aurora, and almost simultaneously with this (I mean very shortly after it) the corona. This reddish glimmer, which surrounded the Moon with a border of the breadth of at most five minutes, was not sharply bounded in any part, but was extremely diffused, and less distinct in the neighbourhood of the poles. I can decidedly confirm this at least as regards the Sun's north point, but not so much so of the south point, as that part was less observed by me, but was more particularly attended to by Lieutenant Vidal, * By the atmosphere near the Moon's place, I mean that part of the atmosphere which lies nearly in the same direction as the Moon. Of course it is, in reality, very far removed from the Moon's true place. t I quote (from the Student for March 1869) Mr. Lynn's translation of Herr Grosch's narrative. THE CORONA AND ZODIACAL LIGHT. 343 who could not afterwards give any more positive infor- mation concerning this phenomenon. . . . To speak now of the corona: — its extent was considerably longer in the direction of the Sun's equator ; and in considering its nature, we must, I believe, look upon it as decidedly unconnected with him.* Whereas, in the direction of its poles, its apparent height exceeded that of the Moon by Only a third of her diameter; in the direction at right angles to this, its extent amounted to four- fifths of that diameter. Its light was white, brighter on the Moon's limb, and becoming gradually fainter on the other side. This white light was not in the least radiated itself, but it had the appearance of rays penetrating through it ; or rather as if rays ran over it, especially in the direction of east and west, forming symmetrical pencils diverging out- wards and passing far beyond the boundary of the white light. These rays had a more bluish appearance, and might best be compared to those produced by a great electro-magnetic light. Their similarity to these indeed was so striking that under other circumstances I should have taken them for such, shining at a great distance. The view of the corona here described is that seen with the naked eye. I employed but a very short time upon it, only as much, in fact, as was necessary to obtain a mere momentary view of the general appearance of the totality. And now, in * Herr Grosoh obviously does not mean that the corona is not, in his opinion, a solar appendage, but that there is no continuous connection between the Sun and the corona; that it is not, in fact, of the nature of a solar atmosphere. 344 THE SUN. conclusion, I would just mention another phenomenon, ■which seemed to me too important to pass unnoticed. It showed itself exactly at the north point of the Sun. In the white light of the corona, close upon the Moon's limb, there appeared several dark curves. They were symmetrically arched towards the east and west, sharply drawn, and resembling in tint lines drawn with a lead pencil upon white paper. They gave the impression as if they proceeded from one point, which point was on the other side of the Moon ; not, however, on the limb, but somewhat nearer the centre of the Sun. Beginning at the distance of one minute, they could be traced up to about nine minutes from the Moon's limb. Throughout the duration of the eclipse they underwent no alteration whatever, remaining con- stant both in form and colour until the disappearance of the corona. Lieutenant Vidal (agreeing with Signor Vergaza) speaks of a similar appearance, ex- actly at the south point, in the form of a fan or sheaf-formed tuft of light ; but lie says nothing of cha- racteristic dark curves, such as I saw at the north point. There is, however, no cause to doubt the accuracy of the observed facts. These dark curves indicate a strong magnetic polar force of the Sun, so that an appearance of the kind iu question might be seen at the south as well as at the north point ; but perhaps being less developed at the former than at the latter, it was less perceptible.' It is impossible to read this account without feeling how much might be learned from a systematic survey THE CORONA AND ZODIACAL LIGHT. 345 of the corona during any considerable total eclipse. Here were three observers only, who each gave to the Fig. 89. The Corona during the total Eclipse of August 1868, as seen at Mantawalok-Kekee. 346 THE SUN. corona but a fugitive attention, and yet their accounts agree in pointing to the existence of appearances which merited a thorough study during the whole continuance of totality, and that not by two or three observers only, but by ten or twelve at least, each appointed to the investigation of some special feature. The study of the corona was one of the subjects to which the attention of the observers sent out to view the great eclipse of August 1868 was specially directed. More seems to have been hoped from the application of spectroscopic and polariscopic analysis than from direct observation ; and, accordingly, the best accounts we have of the general aspect of the corona are those derived from observers not belonging to the expe- ditionary parties. We owe to the professors of the college of Manilla, who observed the eclipse at Mantawalok, the accompanying very remarkable draw- ing of the corona. It is important to notice that, unlike most pictures of the corona, this one can be trusted. Owing to the want of success which had attended all attempts to photograph the corona, the professors were led to think of an ingenious plan for obtaining exact drawings. They prepared beforehand several sheets of paper, which were introduced one after another into a dark chamber, so that the image of the eclipsed Sun fell upon them, and the features of the corona were rapidly sketched out on each. The corona is described as of a somewhat triangular aspect ; yet it is to be noticed that there are four rays of the longer sort, so that the corona in this case, as in so THE CORONA AND ZODIACAL LIGHT. 347. many others, exhibits a general approach to the trape- zoidal figure. The slightly curved streak of white light crossing the longest of the coronal beams is a very remarkable feature. This streak is described as of an intensely white and uniform light. It appeared (or was at least first noticed) some two minutes after the beginning of the totality, and remained visible until the Sun began to reappear. The polariscopic observations made, during this eclipse, on the light of the corona were not successful. The observers agreed, indeed, that the light of the corona was polarised in a plane through the Sun's centre ; a circumstance which, if confirmed, would go far to prove that the corona shines by reflecting the Sun's light ; but the Astronomer Royal, who has carefully examined their accounts, considers that little depen- dence can be placed on the evidence. I may as well add that, during the eclipse of 1869, the American observers obtained a different result, and that, in the opinion of those best competent to judge, the question of the polarisation of the corona's light in a plane through the Sun's centre remains still in abeyance. Colonel Tennant examined the light of the corona* with the same spectroscope which, as we saw in the last chapter, has given information of such interest respecting the coloured prominences. He saw a faint continuous spectrum. ' Thinking that want of light prevented my seeing the bright lines which I had fully expected to see on the lower strata of the corona, I opened the jaws of the slit.' He still failed to 348 THE SUN. recognise any signs of bright lines. ' What I saio,' he writes (the italics are his) ' was undoubtedly a con- tinuous spectrum, and I saw no lines. There may have been dark lines, of course, but with so faint a spectrum, and the jaws of the slit wide apart, they might escape notice.'* Before discussing this result, I proceed to mention other evidence bearing on the same point. During the eclipse of August 1869 several of the American observers renewed the attempt to determine the exact nature of the corona-spectrum. Their re- sults were not accordant. Professor Pickering obtained a faint continuous spectrum crossed by three bright lines. Professor Harkness recognized only one bright line on a continuous background. After observing the spectrum of the prominences, he asked Professor Eastman, who was directing the telescope, to bring the corona into the field. A bright part of the corona was thus brought under examination, but no spectrum ap- peared. ' I asked him to try another place. Still nothing was visible ; and, raising my head from the in- strument for the first time since the commencement of the totality, I remarked, " Can't see any spectrum ; don't believe we will get any." " Oh ! yes, we will," * It is strange that, notwithstanding the very plain account given by Colonel Tennant, it should continue to be asserted that, according to his observations, the corona gave a solar spectrum ; that is, a spectrum crossed by the Fraunhofer lines. The American observers were so misled by this assertion as to search specially for the dark line6 which Colonel Tennant was supposed to have seen. The obvious meaning of his narrative is, that he saw a continuous spectrum, without either dark lines or bright lines. THE CORONA AND ZODIACAL LIGHT. 349 said he. At that instant it struck me that perhaps the slit was too narrow; so I opened it a little, and then again placed my eye at the instrument. In the mean- time Eastman had put the needle at a very bright part of the corona, and I at once saw a continuous spectrum, about as bright as that given by the full Moon on a clear night. Remembering that the observers in India, in August 1868, had said that the corona gives a con- tinuous spectrum with absorption lines ' (a mistaken idea, as mentioned in the last, note), ' I looked very carefully for them ; but, to my great surprise, I could see none, and I am perfectly satisfied that none were visible in my instrument. On the contrary, I saw an absolutely continuous spectrum crossed by a single bright line, whose position was recorded.' This line was in the green, and, if actually in the place assigned by Harkness, would correspond to a line belonging to the spectrum of copper. But as he makes the line co- incident with one of the prominence-lines, it seems cer- tain that it can be po other than a line of iron, close by the E lines, which has been seen by several ob- servers in the spectrum of the prominences.* Professor * This iron line appears also as a bright line in the spectrum of the aurora, according to the best observations hitherto made. As I write, I receive from Mr. Browning an account of his observations on the aurora of October 25, with one of his miniature spectroscopes. He saw a bright lite near e, and another not far from B. Mr. Birmingham, of Tuam, with a similar instrument, saw the usual bright line in the green, one not far from it to the left, very faint, and one of medium brightness near F. Professor Wenlock notes four lines in the yellow-green part of the spectrum, and one somewhat more refrangible than the f line. All these accounts are reconcilable when we remember the extreme faintness of the auroral light, and the fact that no exact determination by the 35° THE SUK. Young paid particular attention to the spectroscopic observation of the corona. He also had been misled by erroneous accounts respecting the Indian observa- tions, and so expected to see a faint solar spectrum. He found, on the contrary, that the light of the corona gave a faint continuous spectrum* crossed by one bright line. He suspected, also, that there were two other lines. He regarded it as certain, from the close agreement of the bright line (and of the two suspected lines) with Professor Winlock's picture of the aurora- spectrum, ' that the corona is simply an electric dis- charge, no doubt varying with great rapidity, as we see in the case of the aurora ; in fact, that the corona is a permanent solar aurora.' Now, although these accounts seem at first sight discordant, they can be brought into agreement, not only with each other, but with Lieutenant-Colonel Tennant's, by a consideration of the circumstances under which they were severally given. Tennant, seeing only a continuous spectrum, opened the slit somewhat widely: 'the jaws of the slit were wide method of coincidences has ever yet been attempted. I had written further to the effect that, ' even when the auroral light is only ruddy to the eye, no red lines are seen, so that we may conclude that the excess of red is due to a peculiarity in the light of mixed refrangihility form- ing the continuous spectrum,' when I learned that, on the evening of October 24, Mr. J. R. Capron, with one of Browning's small direct- vision spectroscopes (adapted to star observation), had succeeded in observing a line in the red, ' very much like the lithium line, but rather more dusky. It was only well seen in the rosy patches of the aurora but could be faintly traced wherever the rose-tint at all extended.' * He found also that the light of the faint-continuous spectrum was polarized. THE CORONA AND ZODIACAL LIGHT. 351 apart,' he says ; too wide, I imagine, to show the bright lines. For, from what is shown at p. 144, it will be seen that the brightness of the coronal bands or lines could not be increased in this way, though their breadth, and so the total amount of light from them, would be increased in precisely the same proportion as the opening of the slit. But the brightness of. the con- tinuous background would be increased in this same proportion. Hence the Jbright lines which Tennant could not see, on account of their fineness, were changed by opening the slit into broad bands of no greater brightness, and rendered invisible by reason of the increased brightness of the background. An interme- diate amount of opening would in all probability have shown the lines. Now we see that Professor Harkness failed even to see a continuous spectrum when he used a narrow slit ; and the fineness of the lines (not nearly so brilliant as the prominence-lines) caused them to escape his notice precisely as had happened with Ten- nant. But when he opened the slit * a little,' he saw the continuous spectrum and one bright line. Pro- fessor Young saw the same under similar circumstances. Had the slit been opened somewhat more they would not have seen that bright line, but would have failed as Tennant had, and for the same reason. There is no difficulty in explaining why Professor Pickering saw three bright lines, since he was using an integrating not an analysing spectroscope (see p. 141), and a large part of the light forming the spectrum he observed must ECLIPSE OF AUCUST 7T M 1869. Drawn by W. S . Oilman JunT E S , . , Sun . . / = lfaglare " 1 appendages .) : thnr S lare " I think I am right in stating two things with regard to your photographs. In the first place, the details of the luminous appendages surrounding the Sun are well represented. Secondly, there is little or no photo- graphic effect over the centre of the Moon's disc. illumination of the molecules near the Sun than of those near the Earth, which (quite apart from any question as to their possibly greater fre- quency in his neighbourhood) would give an immense predominance to their influence in producing coronal light. In this you are perfectly right.' THE CORONA AND ZODIACAL LIGHT. 407 Judging from the photographs of the Sun himself taken by the Kew heliograph, we may conclude that when these pictures are not overdone, but give the various details of the solar disc, the exposure has been too small to give us the faintest trace of the atmo- spheric glare which surrounds the Sun. Making use of the proportion above stated, I should conclude that a photograph of the solar appendages, not over- done, but presenting like yours the details of these appendages, would not present a perceptible trace of their atmospheric glare.' ' In the next place, the atmospheric glare due to the light from the eclipsed Sun should be thrown inwards over the Moon's disc as much as outwards beyond the true corona ; the light, therefore, which reaches us in a total eclipse from the centre of the Moon's disc, and which may be partly due to earthlight reflected from the Moon, may be safely taken as somewhat exceeding that which can possibly be due to atmospheric glare ; and inasmuch as in your photograph there is very little effect in the centre of the Moon's disc,* I am led to think that very little of the result obtained can be * Some light was certainly received from the direction of the Moon's disc, because when the spectroscope was directed towards the Moon's centre the bright coronal- lines were seen. A singular stress was laid on this circumstance by those who had once advocated the atmospheric glare theory, — as though the result had been unexpected, and confirmed their views. But so far back as March 1 869 I had pointed out that the light from the solar appendages seen during totality muso illuminate the air towards the Moon's place ; and clearly, the recogni- tion of a non-solar spectrum in that direction confirmed my statement that no direct sunlight can reach the air which lies in this direction. 408 THE SUN. due to glare. I have here confined myself strictly to your photographs, but the principle laid down is applicable to all kinds of observation; and I must confess that I cannot at the present moment see why the streamers, if they are caused by the atmosphere, should invariably shoot outwards, and never venture to trespass upon the Moon's disc' It should be added that the greater extension of the corona on the western side (in Mr. Brothers's picture) seems to be in no way connected with the Moon's motion, as was at one time suggested. For the same peculiarity is recognised in his second and fourth photographic plates (the only other good pictures, see p. 400), whereas the extension in the second plate should have been greatest on the eastern side if that were the correct explanation.* * It 'will be noticed that the American photograph {exposed during nearly the whole continuance of totality) shows a greater extension of the bright inner corona on the same side. The best of Lord Lindsay's photographs (the only one published, as already mentioned) shows the inner corona brightest on the eastern side ; and as that photograph was taken near the commencement of totality, some stress was laid on the fact by the advocates of Oudemann's theory (see p. 379). It seemed clear by comparing Lindsay's view with Brothers's that the light of the corona had passed from the eastern to the western side of the corona, as the Moon passed over the Sun's disc from west to east. But, unfortunately for this argument, it has now been proved beyond all possibility of question, that either Lindsay's photograph, or else Brothers's (and Willard's), has been inverted. I have shown in the note on p. 398, that the former view is altogether the more probable, or rather is practically certain ; but for the present purpose it matters nothing which view we take. Whether we invert Lord Lindsay's photograph or Mr. Brothers's, we bring the brightest part of the corona to the. same side in both pho- tographs. So that the eccentricity of the corona was of the same char- acter at the commencement as at the end of the totality — a circumstance THE CORONA AND ZODIACAL LIGHT. 469 I am thus led to consider what I hold to be far the most important result of the eclipse expeditions of December 1870. When the photographic records, the best drawings; the verbal descriptions, and the spectroscopic observa- tions are carefully compared together, two relations are revealed which, whatever their interpretation, are undoubtedly full of meaning. They may be thus ex- pressed : — (1) Where any great gap or rift appears in the outer or radiated part of the corona, there a depression is seen in the inner and brighter portion. (2) Where the inner portion of the corona is de- pressed, there the coloured prominences are wanting and the sierra is very shallow. The former relation is very markedly shown in Lieutenant Brown's drawing, fig. 92 (and he referred to it as a fact he had specially noticed), and the Spanish and. Sicilian photographs (figs. 93, 94) illustrate the relation in a very striking manner. In confirma- tion of the second, I may quote Professor Roscoe's words respecting Mr. Seabroke's spectroscopic views of the prominences, and Professor Watson's drawing of the corona : — ' On comparing the two drawings thus independently made, a most interesting series of coin- cidences presented themselves. Wherever on the solar disc a large group of prominences was seen in Mr. Seabroke's map, there a corresponding bulging out of disposing completely per se of Oudemann's theory (untenable, as \re have seen, for other reasons). 410 THE SUN. the corona was chronicled on Professor Watson's draw- ing ; and at the positions where no prominences pre- sented themselves, there the bright portions of the corona extended to the smallest distances from the Sun's limb. Of course the two relations involve an agreement between the greatest extension of the outer corona and the regions richest in prominences. Mr. Brothers distinctly notes this agreement; for he says, ' there can be no question that there was more coronal light on the west side of the Moon than at the other points ; ' and then he calls attention to the fact ' that the pro- minences are more numerous on the side where the corona is brightest.' To this I may add Father Secchi's description of the western range of promi- nences. He tells us that towards the close of the totality (when the western prominences were dis- closed) the protuberances formed a beautiful small semicircular corona, ' full of the well-known rosy jets, of admirable shape and beauty.' Now, at first sight, one might be led to recognise in these relations some evidence for the atmospheric glare theory — in that modified form of the theory which makes the outer corona due to atmospheric glare pro- duced by the inner corona and the prominences. In- deed Mr. Lockyer adopted even so late as last August the following line of reasoning : — ' The outer corona was observed to have a rosy tinge over the pro- minences, and the spectrum of the prominences was detected many minutes above them, as well as on the THE COMONA AND ZODIACAL LIGHT. 41 1 dark Moon. It could not have got this colour at the Sun, for its intrinsic colour was green, and the red light of the hydrogen supplied at the Sun was aholished altogether — was absorbed — and could only reach the corona at the Sun, so to speak, as dark light.' * Here the reasoning as to light is sound, except as respects the conclusion that the intrinsic colour of the corona is green — a result by no means necessarily to be inferred from the green colour of the bright line in the coronal spectrum.f But a brief consideration of the geometrical relations involved shows that the red- ness of the coronal light over the prominences cannot possibly be explained by the atmospheric glare theory. The part of our atmosphere which lies towards the rose-tinted extensions of the corona is not more illumi- nated by the corresponding rich prominence region than those neighbouring parts of the atmosphere which do not show this rosy light. If the rich prominence region were the source of the red tint, there could be no reason why there should be an extension in that particular direction, and not in others. We have, in fact, in the above reasoning a repetition, but in a modi- fied form, of the misapprehension which characterised the form of the glare theory dealt with in pp. 308, 309. It is only necessary to consider a rich red prominence * From a paper read before the British Association last August. t The light concentrated on a bright line may be far less in quantity than light forming a continuous spectrum, or a spectrum of broad bands. The light of the aurora, for instance, has sometimes shown a green line as the most conspicuous line, even when the colour of the auroral cur- tain has been markedly red. Similarly the zodiacal light has a red tinge, yet the spectrum shows a green line. 412 THE SUN. region as a source of light illuminating our atmosphere, to perceive that the law according to which various parts of the atmosphere would be illuminated, must ut- terly preclude anything like radial extension from the region towards which the eclipsed Sun lies. There would also be a gradual increase of brilliancy (instead of diminution) with distance from the Sun's place. We are bound then to find an explanation of the phenomenon in some solar relations. In fact, we find ourselves brought by a new line to the difficulty already dealt with at p. 378, and considered as almost necessitat- ing the idea of some form of radial repulsion exerted by the Sun. The meteoric theory per se, as pointed out at p. 378, will not avail to explain the straight radial bars and streaks seen in the corona; neither will it serve to interpret the relations we have now to con- sider. Regarding the prominences as phenomena of eruption, the occurrence of great solar eruptions cannot affect meteoric or cometic matter travelling myriads of miles above the regions of outburst. We cannot sup- pose for a moment that a sufficient superiority exists in the light-giving or heat-giving powers of the promi- nence regions to explain so marked a difference. Nor is it easy to suppose that some special form of electrical or magnetical action exerted above the prominence regions is the true explanation of the peculiarity. That electricity is involved in some way in the produc- tion of the coronal light may well be believed ; and further, it is far from improbable (as indeed has already been suggested) that there is some special THE CORONA AND ZODIACAL LIGHT. 413 electrical action in progress above the prominence regions. But to explain a coronal extension by the assumed extension of this electrical action, is to ex- plain one problem by another of greater difficulty. The sole explanation which seems available is so startling, that at first sight many will be disposed to reject it on that account alone. It is this — that a portion of the coronal light is due to the ejection of matter from the Sun, and either chiefly or wholly from those bands of the solar surface which we have thus far termed the spot- zones. Let it be distinctly understood, however, that this theory (in favour of which I, propose to cite further evidence) is not to be regarded as intended to displace the meteoric theory, or even the theory that a solar atmosphere of extreme tenuity exists as far as, or be- yond, the observed extension of the corona. On the contrary, we may be led by the emission theory to attach more weight than heretofore to the theory of a solar atmosphere extending very far from the Sun ; while the meteoric theory, as accounting for some slight part at least of the coronal light, can scarcely but be regarded as demonstrated. The first and most obvious objection to the theory that matter is projected from the Sun to such vast dis- tances as the theory requires, resides in the enormous initial velocities which such ejection would imply. From what has been shown in Chapter II., it will readily be seen that a velocity of projection of about 300 miles per second would be required to carry matter 4H THE SUN. from the Sun's surface even in vacuo to the observed limits of the corona. But as such matter, when first ejected from the Sun, must necessarily be in the state of vapour, and as it has to pass through the Sun's vaporous atmosphere (which close by the photosphere must be of no inconsiderable density), there must be enormous retardative effect, and consequently a much greater initial velocity would be required. We have, however, in the jet prominences indis- putable evidence of velocities of ejection as fully be- yond our powers of conception as the enormous initial velocities just referred to; and I think these prominences supply strong reason for believing that movements of far greater velocity must take place within the solar orb. The jet prominences quite commonly reach to a height exceeding 50,000 or 60,000 miles, and some of them have been 80,000 or 90,000 miles in height. Now to propel a projectile in vacuo to a height of 80,000 miles, an initial velocity of about 140 miles per second would be required. But the erupted matter is not a solid projectile flung through empty space, but glowing hydrogen (and other vaporous elements) flung through a hydrogen atmosphere. Now a cannon ball, though propelled with a velocity of less than a third of a ^second *per 'mile, is yet so retarded by atmospheric resistance, that its range is a mere fraction of that calculated on the supposition that there is no resistance. It will be understood how enormously the initial velo- city of these hydrogen projectiles (so to describe them) must exceed that inferred from the hypothesis of an THE CORONA AND ZODIACAL LIGHT. 415 ordinary projectile travelling in vacuo. So that it cannot but be regarded as probable (though it may be perfectly inexplicable) that the glowing hydrogen of the jet prominences is ejected with a velocity many times exceeding that which would be necessary to carry matter altogether away from the Sun, if he were not sur- rounded by an atmosphere.* Now since other matter is ejected along with the hydrogen, and some of this other matter is certainly denser than hydrogen, under equal circumstances of pressure and temperature, we cannot but believe that such matter undergoes far less retardation than the hydrogen in passing through the solar atmosphere. How much less we cannot say ; but it seems no rash assumption to suppose that the metallic vapours (a fortiori if they condensed into me- tallic globules) would retain a velocity sufficient to carry them even beyond the observed limits of the corona. But if any evidence were really required to prove that the power of the solar eruptions is sufficient for our theory, we need only consider the circumstances of the eruption witnessed by Professor Young on Sep- tember 7, 1871 (see pp. 318-322). For it will be noticed that he saw matter carried from a height of less than 100,000 miles to a height of more than 200,000 miles » We could scarcely expect to detect spectroscopic evidence of these enormous initial velocities — first, because it is probable that the real jets have a relatively minute cross-section ; and, secondly, because by com- municating motion (gradually diminishing with distance) to the sur- rounding photospheric and chromatospheric matter, the outflowing bvdrogen tends to mask (by widening the lines) the spectroscopic evidence of its own motion. 41 6 THE SUN. in ten minutes. Now I have gone through the requi- site calculation for determining in what interval of time this distance would be traversed by a projectile in vacuo, having 200,000 miles as the upper limit of its flight ; and I find that no less than twenty-five minutes fifty-six seconds would be required. Now on the sup- position that there was no retardation, we should have to suppose that when the hydrogen wisps became in- visible (doubtless through cooling), they still continued to move upwards, and that they reached a height of about 350,000 miles before their upward motion ceased. For in this case the observed distance would be tra- versed in the observed time. Even this altogether in- credible supposition forces us to believe in a velocity of ejection of about 255 miles per second. But if we assume (what is altogether more likely) that the hydrogen ceased to move upwards after reaching a height of about 200,000 miles, we must infer that it was projected with a much greater velocity than 255 miles per second. For this velocity would only carry it across the observed range in the observed time when not subject to any retardation within that range. But it was subjected in reality to such an enormous retardation that, instead of passing to a height of 350,000 miles,* it was brought to rest at a height of little more than 200,000 miles. Therefore when at a height of 100,000 miles (or at the beginning of its ob- * The whole reasoning is very much strengthened if we suppose that the real seat of eruption lay some distance from the Sun's limb. — or that the motion of the hydrogen was so seen as to be foreshortened. THE CORONA AND ZODIACAL LIGHT. 417 served course) it must have been travelling much faster than a projectile, in vacuo, making for a height of 350,000 miles. So that even if projected to that height through a vacuum the uprushing hydrogen would have required a much greater initial velocity than 255 miles per second. But so far from the re- tardation being nil in the lower half of the hydro- gen's course, it must have enormously exceeded the retardation in the upper half (since the atmospheric pressure increases not only rapidly, but with con- stantly increasing rapidity, as the Sun's surface is approached). Thus it is not merely probable, but certain, that in the case of this outburst, the ejectional velocity exceeded enormously — exceeded many times — the utterly inconceivable velocity of 255 miles per second. Furthermore, since the velocity due to a projectile in vacuo, flung from the Sun's surface to a height of 200,000 miles amounts to but 212 miles per second, it is not only probable but certain, that in this instance the velocity thus estimated falls enormously short of the real velocity of ejection. Hence, in all jet pro- minences the like must be inferred. It may be con- cluded that the matter of the smallest jet prominence actually recognisable as such must have been ejected with a velocity sufficient, if there were no atmosphere, to carry the matter for ever away from the Sun. Now when we remember what the spectroscope has taught us respecting the constitution of the corona, we cannot but see that the inference deducible from the E E 41 8 THE SUN. preceding reasoning — the inference, namely, that erupted metallic vapours pass away to a far greater distance than the erupted hydrogen — is very much strengthened. It is noteworthy also that the polari- scopic evidence has been regarded by Professor Stokes (the highest English authority on the sub- ject) as seeming to imply the condensation of metallic vapour moving on radial lines from the Sun, into liquid or solid particles of exceeding minuteness.* And here we are brought to consider several matters which bring our present subject into somewhat un- expected relationship with other departments of cosmi- cal research. Space will only permit me to touch very briefly upon them. If matter is thus propelled from the Sun so con- stantly as to account for the permanence of the corona, it can scarcely be doubted that, exceptionally perhaps, or perhaps not uncommonly, matter is projected as far as the Earth's orbit, or even beyond. One would even expect that no inconsiderable proportion of the Sun's substance might be ejected with such velocity as to pass for ever away from his dominion. Now in the case first mentioned, it would be reason- able to anticipate that, as a direct result, the number of aerohtic masses falling on our earth by day would somewhat exceed the number falling at night. For if * He conceives that their dimensions fall short of the wave-lengths of light ; and he suggests magnesium as the element principally in ques- tion. It does not appear, however, that he lays stress (so far as optical considerations are concerned) on the nature of the elementary consti- tution of these metallic clouds. THE CORONA AND ZODIACAL LIGHT. 419 a flight of erupted globules (gathering probably into a single mass as they travelled onwards; came into direct contact with the earth, that Face of the Earth turned at the moment towards the Sun must necessarily be the scene of the encounter. Now, as a matter of fact, it has been observed that more aerolites fall near noon than at any other hour. We have this on the au- thority of Humboldt, a careful student of the subject of meteoric astronomy, and the fact is confirmed by the systematic researches of Alex. Herschel, Heis, and others in our own time. Now as to the second case, the point to be chiefly noticed is this : that v if our Sun sends forth such missiles towards the domains of other suns, analogy should guide us to the conclusion that those other suns — the stars — must also, from time to time, eject metaUic matter from their substance. If this is so, we might expect that some of the meteors which reach our Earth would afford some indications of their stellar origin. For example, their velocity might be in excess of that due to the Sun's attracting influence (see p. 87); in which case no explanation would be so plausible as the very conclusion towards which we are now being forced. But here again, as a matter of fact, the most careful observers, Alexander Herschel in England, Professor Newton in America, Fr. Secchi in Rome, Heis in Germany, tell us of meteoric velocities of 60, 70, 80, even 90 and 100 miles per second. Such velocities, if real (and it is difficult to question many E E 2 420 THE SUK of the observations on which these results have been based), correspond exactly with the results to which we have been just led. Again, the structure of meteors, if carefully ex- amined, might be expected to reveal evidence of their having once existed within the interior of an orb like our Sun. But Mr. Sorby, so far back as 1864, wrote thus respecting the microscopic examination of certain meteors. ' The most remote condition of which we have positive evidence, was that of small detached melted globules, the formation of which cannot be explained in a satisfactory manner, except by sup- posing that their constituents were originally in the state of vapour as they exist in the Sun.' He found evidence that the meteors had been in the state of vapour while under enormous pressure, and ' in moun- tain masses.' Yet again, the chemical structure of meteors might be expected to supply evidence of a stellar origin. Now, the late Professor Graham, after a careful analysis of the Lenarto meteor, reasons thus: — 'It has been found difficult to impregnate malleable iron with more than an equal volume of hydrogen under the pressure of our atmosphere.' Now, 'the meteoric iron (this Lenarto iron is remarkably pure and malleable) gave up almost three times that amount without being fully exhausted. The inference is, that the meteorite had been extruded from a dense atmosphere of hydrogen gas, for which we must look beyond the light cometary matter floating about within the limits of our own THE CORONA AND ZODIACAL LIGHT. 421 system Hydrogen has been recognised in the spectrum analysis of the light of the fixed stars by Messrs. Huggins and Miller. The same gas con- stitutes, according to the researches of Fr. Secchi, the principal element of a numerous class of stars, of which a Lyrae is the type. The iron of Lenarto has no doubt come from such an atmosphere in which hydrogen greatly prevailed. This meteorite may he looked upon as holding imprisoned within it, and bearing to us, the hydrogen of the stars.''* I would further invite the attention of the reader to the account of the behaviour of sun-spots and faculte (see pp. 216, 217) ; to Gilman's account of the aspect of a large prominence (p. 307), and to the reasoning on the striations observed by him (pp. 378, 379) ; and, lastly, to Zollner's account of linear flashes in the spec- trum (p. 307). All these matters bear very pertinently, as it seems to me, on the theory we are now considering; and they acquire fresh force from the fact, that they have not been introduced here for the purpose of sup- porting the theory, but appeared in the first edition of this work, in which the theory was not advocated. Even the perplexing relation considered at p. 218, may find an explanation in the theory now under discussion. For regarding the spots as phenomena of eruption, that is, as beginning with eruption, we might * See further ' The Fuel of the Sun,' by Mr. W. Mattieu Williams, F.C.S., in which the theory of the expulsion of matter from the Sun is ably advocated. Some of his inferences are not in -accordance with strict dynamical principles ; but the book is well worth careful reading. 422 THE SUN. •expect that their occurrence would be associated with the planetary motions of recess or approach. Erup- tions and earthquakes on our own earth, stable as is its substance by comparison with the Sun's, occur more frequently when the moon is in perigee ; and Sir John Herschel has explained the predominance of active volcanic and earthquake regions along shore-lines, as depending on the seemingly insignificant changes due to tidal action. How much more, therefore, might we expect that the solar equilibrium would be disturbed by planetary action, when all that has been revealed respecting the Sun tends to show that the mightiest cenceivable forces are always at work beneath his photosphere, one or other needing only (it may well be) the minutest assistance from without to gain a temporary mastery over its rivals. And if, as recent observations tend to show, the mightiest of the planets sympathises with solar action — if when the Sun is most disturbed, the belts of Jupiter are also subject (as of late, and in 1860) to strange phenomena of change — how readily do we find an explanation of what would otherwise seem so mysterious, when we remember that as Jupiter disturbs the mighty mass of the Sun, so the Sun would reciprocally disturb the mass of the largest of his attendant orbs. The reasoning which has led to the suggestion that electrical processes are at work in producing a portion of the coronal light, as well as that which has seemed to many acute minds to support the theory of an atmosphere of extreme tenuity, extending millions of THE CORONA AND ZODIACAL LIGHT. 423 miles from the Sun, would appear to be confirmed by, and to confirm the arguments for the emission theory. For electrical action could not be excited where the coronal streamers are seen, save by the rush of matter projected radially from the Sun through a medium resembling an atmosphere. Furthermore, the meteoric theory advocated above — a theory quite distinct from that of a meteor-generating emission — need by no means be abandoned, if the emission theory is regarded as established. It will remain for future observations to determine what proportion of the coronal light is due to the several causes here considered; but it can scarcely be doubted that an appreciable proportion of that light is due to each. The study of the corona has thus led us to con- clusions involving relations of cosmical importance. "We have been led to associate the corona with the scheme of meteoric systems, and therefore with the mysterious phenomena presented by comets. We have evidence also associating the solar corona with coronas surrounding the millions on millions of suns scattered throughout space. In studying the pro- cesses which thus seem to be at work, we are led to consider their bearing as well on the past as on the future of the universe of solar systems. A new bond of association is recognised between these uncounted millions of systems. Already light has brought its message from the star depths surrounding us on all sides ; and we have learned to believe that the light- 424 THE SUN. record dates from a few years to thousands of years ago. But meteoric visitants from other suns must have occupied millions of years in each voyage from sun to sun, and many have in all probability traversed myriads of such voyages, before their existence has been brought to a close on our tiny earth. The past thus revealed to us is to all intents infinite in duration. But processes resembling those which led to these infinitely prolonged migrations, are even now be- ginning, as fresh meteoric matter is propelled from our Sun and his fellow suns. These meteoric out- bursts reveal the future to us as no less infinite than the past. We may truly say of time as thus pictured to us, what in Goethe's wild vision the Angel suid of space : — ' End is there none to the universe of God. Lo ! also there is no beginning.' 425 CHAPTER VII. PHYSICAL CONDITION OF THE SUN. \ In the course of the last four chapters a number of facts bearing on the physical condition of the Sun have been dealt with at greater or less length. Considered separately these facts are full of interest and seem to afford somewhat satisfactory information on the points they severally relate to. We feel little difficulty, for instance, in giving a general interpretation to the dark lines of the solar spectrum, regarding them as un- doubtedly due to the existence of the vapours of certain metallic and other elements in the solar atmosphere. We regard with a certain confidence, again, the con- ception that the spots are depressions of greater or less depth, and further that the light received from the umbra of a spot shines through absorbing vapours, some of which exist at a greater pressure and at a lower tem- perature than over the rest of the photosphere. We are able, also, to form certain sufficiently definite opinions respecting the prominences, more particularly as regards the pressure at which their substance exists and the motions to which they are subjected. While, lastly, the 426 THE SUN. corona has been studied with results which cannot but be regarded as trustworthy. But when we attempt to combine these several results, and further to determine what the general condition of that orb may be which presents these several features, we recognise at once that a problem of enormous difficulty lies before us. The more we have learned respecting the Sun, nay the more we have learned respecting those physical laws by which we are to interpret solar phenomena, the more insuperable have our difficulties become. It was easy to theorize when as yet but little was known. It was easy to suppose that the few physical laws we imagined we understood sufficed to account for all the phenomena presented by the solar orb. But as one fact after another has been discovered, the true complexity of the problem has been revealed to us ; and as the physical laws which it is in our power to discuss and experiment on have been more carefully studied, we have begun to recognise how very limited our experience has hitherto been. It is not too much to say that theories respecting the Sun's physical condition which would have been regarded twenty years since as deserving of careful study, have now no worthier standing in science than the idea of Anaxi- mander that ' the Sun is a great vessel filled with fire, at the top of which is an opening through which the fire escapes.' It is easier to consider those facts which have re- vealed to us the enormous difficulty of the problem we are upon, than to present any considerations tending to render our conceptions clearer. PHYSICAL CONDITION OF THE SUN. 427 In the first place, we recognise the fact that in the Sun the elements exist in conditions altogether differ- ent from those we are familiar with. This is true of all orders of elements, from those whose normal condition (as recognised by us) is gaseous, to those which at ordinary or even very high temperatures remain solid or liquid. For instance, metals which we can vola- tilise indeed in small quantities, and by the aid of special contrivances, but which yet we can only expe- riment upon (properly speaking) when they are in the solid or liquid state, are present in the Sun as glowing vapours. But also gases which no amount of pressure or refrigeration we can command will cause even to show signs of approaching the liquid condition, probably exist in certain portions of the solar globe as liquids or even as solids. On the one hand, then, we have an in- conceivably high temperature volatilising our most fixed elements — nay, for aught we know, perhaps dissoci- ating substances which we regard as elements into their true primary constituents ; on the other we have an inconceivably high pressure at a relatively inconsider- able distance beneath the photosphere, reducing our so-called perfect gases into the liquid or solid form.* * I do not know that we have any sufficient evidence that hydrogen, oxygen, nitrogen, and other gases which we call perfect, are really ex- ceptions to the general rule that all substances are capable of assuming ■ — under suitable conditions — the solid, liquid, and gaseous form. It is true that no pressure or refrigeration we can apply causes these gases to exhibit the least trace of those qualities which the imperfect gases ex- hibit at ordinary temperatures and pressures. In other words, the perfect gases, as their title implies, obey perfectly ( 1) the law associating density and pressure ; and (2) the law according to which the relation of specific 428 TEE SUN. In the next place, we have learnt of late to recog- nise on how very doubtful a basis many of the received axioms (almost) of physical science have been placed. No laws of science were perhaps more thoroughly ac- cepted than those which were supposed to distinguish the solid, liquid, and gaseous states from each other. All physicists believed that a definite and well-marked change of condition necessarily accompanies the process by which gas passes into the liquid state, and very few were disposed to doubt that a correspondingly distinct line of demarcation separates the liquid from the solid state. Yet the researches of Dr. Andrews have re- cently shown that under certain conditions carbonic acid gas may be made to pass by absolutely insensible gradations from an undoubtedly gaseous to an un- doubtedly liquid state ; and it is recognised that what has been proved in the case of a single gas is in all probability true of all gases and vapours. It has been also rendered highly probable that under suitably great heats at constant pressure and constant -volume remains constant. But it is known that imperfect gases at ordinary temperatures and pressures seem very closely to obey these laws, though as they approach more and more nearly to the circumstances under which they become liquid (whether through increased pressure, simple refrigeration, or both com- bined) they depart in a marked way from these laws. And it is probable that if the imperfect gases could be experimented on at enormously high temperatures and low pressures, they would be found to obey the above- named laws as perfectly to all appearance as the perfect gases do. All we can assert respecting the perfect gases is that none of the processes of combined pressure and refrigeration hitherto applied — perhaps none we are capable of applying — bring them even so far towards the state under which they would become liquid as the imperfect gases are brought under ordinary pressures and temperatures. PHYSICAL CONDITION OF THE SUN. 429 pressure the passage of a molten metal to the solid form, or conversely the melting of such a metal, may- take place in a gradual manner, so that at a certain stage of the process it shall be impossible to say whether the metal or certain portions of it be liquid or solid. Another physical law had been thought to be tho- roughly established. It had been supposed that the whiteness (or true incandescence) of flame was due to the presence of minute particles of incandescent matter. But Frankland has shown that with increase of pres- sure the faintly luminous light of burning hydrogen may be rendered bright, and that by sufficiently in- creasing the pressure the spectrum of the light becomes continuous. So that what had been supposed the most marked characteristic of incandescent solid and liquid bodies, is thus shown to be a possible characteristic of the light of glowing gas. Thus the whole basis of our reasoning (which had been thought so sound) respecting the actual condition of the solar pho- tosphere, whether as evidenced by direct observa- tion or by analysis with the spectroscope, has been shaken. Yet again we have learned to recognise the fact that the imagined limits to the rarefaction at which gases may subsist, as such, had been placed far too low. Our experimenters have gone indeed very far towards the production of an actual vacuum, without obtaining any evidence that the almost infinitesimal quantity of gas which can alone remain in the so-called 430 THE SUN. vacuum-tubes, behaves otherwise than at appreciable pressures.* It will be seen at once how importantly this bears on the subject of solar physics, since it compels us to reconsider all our ideas respecting the probable limits of atmospheric envelopes ; and the most difficult ques- tions of solar physics are precisely those which depend on this matter.. Unfortunately the observed fact gives us no new means at present of forming a satisfactory opinion as to atmospheric limits. Even if it should be regarded as demonstrating that there is no real limit to atmospheric extension, we should still be no nearer than before to a determination of the atmospheric pressures at the surface of the Sun or of any planet except our own Earth ; and this is the problem which chiefly concerns us here.f * Physicists have carried the rarefaction so far that the electric spark will no longer traverse the tenuous medium ; and some have been led to suppose that when this state of things has been brought about there must be an actual vacuum. No such conclusion can be regarded, how- ever, I will not say as demonstrated, but as probable or even conceivable. All the evidence we have tends to show that an absolute vacuum is as imaginary a conception as the philosopher's stone or the perpetual motion. Take the most stable and dense metal, platinum, and conceive that for a moment in the heart of a mass of such metal there was a vacuous space ; then in an instant that space would be occupied. For, even setting aside the probability that the most solid metals undergo an indefinitely minute but still real dissipation at their surfaces, corresponding to that dissipa- tion which ice undergoes at the lowest temperatures yet experimented on — setting aside, I say, this probability, there yet remains the certainty that in the intimate molecular structure of the platinum a perfectly free com- munication exists between the imagined space within and the space without. The communication is not, indeed, such as suffices for the conveyance of certain forms of matter or of motion ; but that it is none the less real on that account cannot be questioned. t It would, indeed, be pleasant to theorise on this matter, to conclude, PHYSICAL CONDITION OF THE SUN. 431 Still further, in considering the Sun's physical con- dition, we have to discuss the effect of motions wholly surpassing in velocity any that we are familiar with on Earth, and therefore a fortiori any that we can experi- ment upon. Setting aside the fact that matter from without must continually be falling upon the Sun (in whatever condition) with velocities such as have been dealt with in Chapter II., we have the observed fact that movements fairly comparable with these have been recognised in the very substance of the Sun by the aid of spectroscopic analysis. Now, what means have we for determining the probable effect of motions of 100 miles per second taking place even among substances in conditions such as we are familiar with ? We know the effects of certain velocities ; we see the bullet melted as it reaches the target, the meteor vaporised as it speeds through the air. But we have no means whatever of determining what effects -would be produced by veloci- ties enormously exceeding even the inconceivable velocity of many meteors. "When to this we add that the swift motions referred to take place amid depths of vaporised metals, and, for aught we know, over seas and continents of liquefied and solidified gases, we may well shrink from the task of attempting — at least for example, that every planet could have just so much atmosphere as corresponded to the range of its attractive influences ; and consequently to deduce the atmospheric pressure uppn the Sun. But no confidence could be placed in such theories. For, on the implied supposition, quite other forces than attraction would have to be considered. And, further it would be an inevitable consequence of such a state of things that as the planets changed their relative positions, the atmospheric pressure the surface of each would vary. 432 THE SUN. in the present state of our knowledge — to estimate their probable effects. I confess, therefore, that at this stage of my subject I am very far from sharing that confidence which I find some men possess in dealing with problems of solar physics. I shall not pretend to place all the phenomena in that due order in which they appear in the theories hitherto propounded. I can only look on with a sense of bewildered admiration while the pro- fessors of rival theories exhibit the physical habitudes of the Sun as obviously explicable according to con- tradictory hypotheses. I must admit that it seems to me that only a very energetic forgetfulness of a large proportion of the evidence can account for the adoption of these theories. I must content myself, therefore, with an exceedingly brief statement of certain general rela- tions, which are all that I find satisfactorily exhibited by what has as yet been learned respecting the Sun. We have in the Sun a vast agglomeration of the elements we are familiar with on Earth ; and this vast agglomeration is subject to two giant influences, pro- ducing in some sort opposing effects — viz., a tempera- ture far surpassing any we can form any conceptions of, and a pressure (throughout nearly the whole extent of the solar globe) which is perhaps even more dis- proportionate to the phenomena of our experience. Each known element would (beyond all question) be vaporised by the solar temperature at known pres- sures; each would (there can be little question) be solidified by the vast solar pressures, did these occur at PHYSICAL CONDITION OF THE SUN. 433 known temperatures. Now, whether under these cir- cumstances the laws of gaseous diffusion prevail where the elements are gaseous in the solar globe ; whether where liquid matter exists it is in general bounded in a definite manner from the neighbouring gaseous matter ; whether any elements at all are solid, and if so under what conditions their solidity is maintained and the limits of the solid matter defined — all these questions are such as we must answer before we can form a satis- factory view of the solar constitution ; and yet they are questions which we have at present no means of answering. Again, we must learn how far combustion, properly so called, can take place within the Sun's mass, and whether those processes which we recognise as combustion are the only processes of combustion which can actually take place there. For aught that is yet known, the intensity of the forces at work upon and within the Sun may wholly prevent the occurrence of any processes of combustion familiar to ourselves ; while other processes of true combustion altogether unthought of by us may be in continual action. Assuming, however, that some general resemblance exists between the processes at work upon the Sun and those we are acquainted with (the wildest assump- tion possible), we may imagine that the various elements in the solar substance ordinarily exist down to certain definite levels in the gaseous form, at lower levels (definite for each) in the liquid form, at yet lower levels in the solid form. That part of each element which is gaseous must again be divided into two por- F F 43+ THE SUN. tions — that whose light is capable of giving charac- teristic spectra of lines or hands (these spectra being, however, different for portions lying at different depths), and that low er portion whose light is capable of giving a continuous spectrum. Now, here we approach the great difficulty of inter- preting the results of the spectroscopic analysis of the Sun. We have no means of learning whence that part of the light comes which gives the continuous spectrum. When we recognise certain dark lines, we know certainly that the corresponding element exists in the gaseous form at a lower temperature than the substance which gives the continuous spectrum. And so, also, we can interpret the appearance of bright lines. They show beyond question that the corre- sponding element exists in the gaseous form at a higher temperature than the substance which gives the con- tinuous spectrum. But as regards that continuous spectrum itself we can form no such exact opinion. It must be remembered that a substance giving- a COn- tinuous spectrum is not necessarily opaque to light from a substance at higher temperature also giving a continuous spectrum. It is capable of exercising a general absorption, but not necessarily (nor probably, under such conditions as exist in the Sun) of exercis- ing an absorption at once general and complete. Hence we have no means of determining how great a depth of the solar substance is concerned in sending out the light which gives the continuous background of the spectrum. This light may come from the surface PHYSICAL CONDITION 01 THE SUN. 435 layers only — but it may be a shell whose thickness forms no inconsiderable aliquot part of the Sun's diameter.* And the reversal of the lines' of certain elements, although it cannot take place at such exces- sive depths, may yet take place very far below the visible limits of the photosphere. For, as I have already shown, a depth of a few hundred miles would be wholly inappreciable in the most powerful telescope (spectroscopically armed), and so no peculiarities would be recognised as the result of processes taking place within such distances of the solar surface, however diligently the edge of the solar disc might be examined with the spectroscope. And further, as respects the examination of the Sun's edge, on which so much stress has been laid, it is far from unlikely — if, indeed, it is not to be regarded as certain — that the visible edge of the solar disc lies considerably above the true limit of the photosphere. That light at the Sun's edge which seems to belong to the hemisphere of photospheric matter turned towards us comes probably from parts of the photosphere which lie really beyond the borders of that hemisphere, and are simply brought into view through the refractive power of the lower layers of the solar atmosphere. I am not here venturing a mere opinion or conjecture — though I profess no certainty * This is in no way opposed to the evidence adduced by Sir John Herschel from the apparent uniformity of light derived from a glowing and transparent liquid not uniformly deep ; for every part of such a liquid (at least in Sir J. Herschel's experiment) is at appreciably the same temperature. If the lower layers could be heated to a higher temperature, the effect of depth would become apparent. F F 2 436 THE SUN. of conviction in the matter. We have the evidence of facts— observed by Carrington, Secchi, and others — showing that the motions of the spots across the solar disc are really affected, as respects apparent rate, by the refractive power of the solar atmosphere. And it is impossible to doubt that if the apparent place of a spot can be affected in this way, then the solar regions beyond that hemisphere which is turned towards us at the moment, must be brought into view and form the real limits of the solar disc* I am sensible that I am not making definite state- ments as to the Sun's condition, but only stating diffi- culties. The difficulties are real, however, not imagi- nary, and ignoring them can serve no useful purpose. When we turn to the details of the solar orb and its surroundings, we find the evidence slightly more defi- nite ; but still greater difficulties surround us. In the course of the several chapters bearing on these mat- ters, nearly all the known facts which bear directly on the views we are to form respecting tbe Sun's physical constitution have been discussed as they have been described, so that but few words about the several solar features are here called for. * This really amounts to saying that the Earth, if viewed from the solar photosphere, would be visible above the solar horizon (as our Sun is after real sunset and before real sunrise) when the geometrical line to her passed below that horizon. We cannot doubt that the solar atmosphere would exert this refractive effect at least as powerfully as our own ; and if the Earth could be seen in this way from parts of the Sun really turned away from her, then certainly those parts of the Bun must be visible from the Earth ; for a visual ray passes along the same path from whichever of its extremities we suppose it to travel. PHYSICAL CONDITION OF THE SUN. 437 Taking the actual telescopic aspect of the solar spots and their surroundings, a perplexing series of problems is suggested. These problems are indeed so perplex- ing as abundantly to justify the disagreement hitherto found among theorisers. Admitting the spots to be depressions, what is the real disposition of the matter which produces the appearances we call faculae — penum- bra — umbra — nucleus ? The penumbra may belong to a lower layer ; but the general arrangement accord- ing to which the willow-leaves on the penumbra point inwards towards the umbra seems to indicate a real connection between the penumbra and the faculous bordering. This arrangement is indeed sometimes so marked that one is led to imagine that the so-called willow-leaves are filamentous bodies which usually hang in a nearly vertical position and so appear nearly round, but when thrust aside during the formation of a spot hang nearly horizontally, the ends which had been lowest floating like streamers towards the region whence they had been removed. If we could but con- ceal from ourselves a large portion of the evidence we have (or else explain it away) this view might be in- sisted upon with pleasing confidence ; but as a matter of fact it merely serves to indicate the impression pro- duced by certain phenomena, and has at present no value whatever.* * Lest I should here be supposed to be too curtly criticising the views of others, let me hasten to say that the fancy thus summarily rejected is my own, and, so far as I know, as original as it is probably valueless. Yet I have not introduced it without a purpose. There is at least as much evidence in its favour as in favour of many theories which have been very confidently put forward. 43 8 THE SUN. A great difficulty lies in the fact that we have no clear evidence to show whether the Sun-spots are formed by forces acting from without or from within. Here I set on one side the theory that in a spot we see a region where a great heat has dissolved solid or liquid or cloudlike matter forming the photosphere, and that thus the intensely hot, but feebly radiating gaseous nucleus of the Sun (according to this theory) is disclosed. Kirchhoff has fairly disposed of this theory by showing that this intensely hot nucleus would be transparent to the light from the farther side of the Sun, and that therefore no spot could appear unless two openings on opposite sides of the Sun hap- pened to be in the same visual direction.* I refer * Fr. Secchi was the original propounder of this theory (not M. Faye, to whom it is usually ascribed). The theory really does account for many observed features of the solar spots, but it is none the less untenable. Fr. Secchi's answer to Kirchhoff s objection would seem to indicate that he has not recognised the exact force of that objection. He says it is not true that a gaseous nucleus would be perfectly transpa- rent to rays from the further side of the Sun, for we se« that our own atmosphere absorbs light as well as heat. Kirchhoff s argument is that the solar nucleus would be transparent on account of its existing at a higher temperature than the photosphere — according to the theory at least which he deals with. Since the above was written, Fr. Secehi has made the theory the subject of a reclamation, and Faye has answered that he claims only the portion of the theory relating to the motion of matter within the solar orb. On this last subject his reasoning is somewhat remarkable. He considers that the existence of currents within the solar orb, whereby various portions of his gaseous substance are successively brought to the surface, would account for the continued emission of light and heat ; whereas a more stable condition would result in the cessation of radiation in the course of a few days. Surely the precise reverse of this is the case, and the loss of heat would be more rapid in the former case than in the latter. PHYSICAL CONDITION OF THE SUN. 439 now, not to this or similar theories, but to the defi- nite problem, whether the seat of that action which leads to the formation of a spot lies below. or above the level of the photosphere. The spectroscope shows that a spot is a region where certain gases exist at a lower temperature than in other parts of the Sun. But whether this low temperature results from the expansion of compressed gas erupted from the Sun, or from the fact that matter has reached the Sun from outer space, remains as yet undetermined. The evi- dence recently obtained respecting the eruption pro- minences seems, however, to favour the former view. Again, as to the prominences, it seems to be demon- strated that some are mere clouds in the upper region of the solar atmosphere, while others are due to some form of eruption, and only assume the cloud form after the eruption which gave them birth has ceased. But what are the circumstances which give birth to these eruptions, what the nature of the layer (Zollner's ' Trennungschicht') beneath which the eruptive action is prepared, and what the actual depth whence the erupted matter springs, we have very little to show. And lastly, as to the corona and the general rela- tions involved in the access of external matter from the interplanetary and intersidereal spaces to the neighbourhood of the Sun's globe, and the emission of matter from within that globe, we have, I apprehend, small means of forming definite opinions- The con- dition, indeed, of the space which lies immediately around the Sun is still very little understood by us. 44° THE SUN. It may be that in the study of the corona during total eclipses we may find a means of answering the many perplexing questions associated with this matter. It may even be that new appliances may enable us to study the corona when the Sun is not eclipsed, and so to learn whether systematic processes affecting the Sun's economy are at work in the region immediately surrounding him. At present our information on this subject is meagre in the extreme ; and our means for acquiring information are far from promising. Here, as in so many matters related to the physical constitu- tion of the Sun, we must perforce wait until our expe- rimental knowledge and our instrumental means have been very largely increased. 441 CHAPTER VIII. THE SUN OUB FIRE, LIGHT, AND LIFE. Few of the results of modern scientific research are more remarkable than the recognition of the real ex- tent of the influence which the Sun exerts upon the Earth. Of old the Sun's power as ruler over the sea- sons, his action upon vegetation, and other like in- fluences, were recognised in a vague and general way. But men were far from regarding the Sun as the true source of many forms of force which seem almost equally important. Still less were they prepared to trace his influence in nearly every kind of action or mode of motion taking place upon our globe. It is the most striking feature of recent scientific research that it has taught us to see in nearly all terrestrial phenomena the action of a certain proportion of Sun- force. We owe to the greatest astronomer of our time — Sir John Herschel — the first definite enunciation of this great principle. ' The Sun's rays,' he wrote in 1833, ' are the ultimate source of almost every motion which takes place on the surface of the Earth. By its heat are produced all winds, and those disturbances in 442 THE SUN. the electric equilibrium of the atmosphere which give rise to the phenomena of lightning, and probably also to terrestrial action and the aurora. By their vivify- ing action vegetables are enabled to draw support from inorganic matter, and become in their turn the support of animals and man, and the source of those great de- posits of dynamical efficiency which are laid up for human use in our coal strata.* By them the waters of the sea are made to circulate in vapour through the air, and irrigate the land, producing springs and rivers. By them are produced all disturbances of the chemical equilibrium of the elements of nature, which by a series of compositions and decompositions give rise to new products, and originate a transfer of materials. Even the slow degradation of the solid constituents of the surface, in which its chief geological change con- sists, is almost entirely due— on the one hand to the abrasion of wind or rain and the alternation of heat and frost, on the other to the continual beating of sea- waves agitated by winds, the results of solar radiation. Tidal action (itself partly due to the Sun's agency) exercises here a comparatively slight influence. The effect of oceanic currents (mainly originating in that influence), though slight in abrasion, is powerful in diffusing and transporting the matter abraded; and when we consider the immense transfer of matter so pro- duced, the increase of pressure over large spaces in the * This remark is commonly but erroneously attributed to the cele- brated engineer Stephenson. It appears in the first edition (1833) of Hersehel's Outlines of Astronomy. THE SUN OUR FIRE, LIGHT, AND LIFE. 443 bed of the ocean, and the diminution over correspond- ing portions of the land, we are not at a loss to perceive how the elastic force of subterranean fires, thus re- pressed on the one hand and released on the other, may break forth in points where the resistance is barely adequate to their retention, and thus bring the phe- nomena of even volcanic activity under the general law of solar influence.' Since this was written men of science have learned to enounce the complete law of the ' conservation of solar energy,' as applied to the organic and inorganic world. That which was put forward in a general way by Sir John Herschel has been made the subject of special scrutiny. We have learned how to weigh and measure the Sun's action and the force-supplies which we derive from it. Let us take first the supply of heat the Earth de- rives from the Sun. We shall have much to excite our wonder, whether we regard the real vastness 01 the relative minuteness of this supply. From the researches of Sir John Herschel it appears that the direct heat of the Sun, if ' received on a sur- face capable of absorbing it and retaining it, would suf- fice to melt an inch of ice in thickness in 2h. 13m. ; ' and he thence calculates that no less than 26,000 tons of ice would be melted per hour by the heat actually thrown on a square mile exposed at noon under the equator. This amount must be multiplied fifty million times to correspond to the heat actually received by the Earth's globe during a single hour. Pouillet ob- 444 THE SUN. tained results not differing very greatly from these. He calculated that an interval somewhat greater than 2h. 13 m. would be required to melt a layer of ice one inch thick.* Expressing his result according to a somewhat different method, he states, again, that if the Sun's heat were distributed uniformly over the Earth's surface ' it would in one year suffice to liquefy a layer of ice 100 feet thick, or to heat an ocean of fresh water sixty miles deep from the temperature of melting ice to the boiling point.' Yet this enormous annual supply of heat is but the l-2,138,000,000th f part of that which the Sun actually radiates into space in the course of a year. All the planets of the solar system are able to intercept but about the 227-millionth part of the heat actually emitted by the Sun. There is a fine passage in Her- schel's ' Outlines of Astronomy ' which shows how enormous is the amount of heat deduced by increasing in the ratio above indicated the supply of heat actually received from the Sun by the Earth : — ' Supposing a * The actual relation between Pouillet's and Herschel's results may be thus expressed. Sir John Herschel deduced 4339 feet as the thick- ness of ice which the Sun is capable of melting per minute, supposing the ice continually applied to the Sun's surface (and the water pro- duced by its fusion continually carried off). Pouillet deduced 38'7 feet per minute. Sir John Herschel says that 40 feet may be regarded as a probable mean. (It will be noticed that, with characteristic modesty and generosity, he places the mean much nearer to Pouillet's value than to his own.) f It is singular how persistently the number 2,300,000,000, calculated by Mayer, maintains its ground in scientific treatises. This number was correctly deduced from the old value of the Sun's distance. But the above is the true value, according to the best modern estimates of the solar parallax. THE SUN OUR FIRE, LIGHT, AND LIFE. 445 cylinder of ice forty-five miles in diameter to be con- tinually darted into the Sun with the velocity of light, the heat now given off constantly by radiation would then be wholly expended in its liquefaction on the one hand, while on the other the actual temperature at the Sun's surface would undergo no diminution.' The luminosity of the Sun's surface is more readily estimated than the heat, since the intrinsic brilliancy of a self-luminous substance is in no way affected by distance ; and we have only to take into account the effect which our own atmosphere may have in diminish- ing the apparent brightness of the Sun in order to form an accurate estimate of the intrinsic brilliancy of the Sun's light. Comparisons have been instituted directly between the light of the Sun and that of known terrestrial lights. It has been found that the most intense light we can produce appears absolutely black by comparison with the brightness of the solar orb. It has been estimated that the intrinsic brilliancy of the Sun's surface exceeds more than 146 times the bril- liancy of the lime-light, and 32,700 times that of a sperm candle. In order to conceive the real amount of light to which a body close by the Sun — within a foot, say, of the photosphere — would be exposed, we must conceive the amount of light we receive in the full splendour of a summer's day increased in the same proportion that the whole hemisphere of sky exceeds the solar disc — besides, of course, a further addition corre- sponding to the proportion in which the brilliancy of the solar disc, if viewed without the interposition of 446 THE SUN. any atmosphere, would exceed the actual brilliancy observed on a summer day. Of the chemical activity of the solar rays, it is not in our power to speak with so much confidence, since we have not as yet measured the Sun's power in this respect in a way which enables us to pronounce on its real extent. We can compare the intensity of the Sun's chemical action with that of terrestrial lights ; but we have not yet found a means of determining its value as compared with those forces which the chemist more ordinarily employs to produce chemical changes. It is worthy of notice, as respects the last two forms of solar activity, how large a share of the force we derive from the Sun is obtained through their action. This will be apparent when we remember the impor- tant bearing of the processes of vegetation on the wants of the human race. ' Nature,' says Mayer, ' has proposed to herself the task of storing up the light which streams earthward from the Sun — of con- verting the most volatile of all powers into a rigid form, and thus preserving it for her purposes. To this end she has overspread the Earth with organisms, which, living, take into them the solar light, and by the consumption of its energy incessantly generate chemical forces. These organisms are plants. The vegetable world constitutes the reservoir in which the fugitive solar rays are fixed, suitably deposited, and rendered ready for useful application. With this process the existence of the human race is inseparably connected. THE SUN OUR FIRE, LIGHT, AND LIFE. 447 And even if we regard the effect of the Sun's heat as exerted upon the oceans and continents of our globe, we find that a large proportion of that which is eventually utilised by man, in one way or another, is first made subservient to the processes of vegetation. When the Sun's rays are poured down upon the ocean, or on parts of the Earth's surface in which water is abundant, the heat raises into the atmosphere large quantities of aqueous vapour. And this vapour, rising by reason of its extreme lightness, reaches eventually a region where it is condensed into clouds. Again, the heat of the Sun producing various effects, according to the nature of the regions on which it falls, gives rise to those differences of temperature which result in the generation of loinds. By the agency of winds the clouds are transferred from the place of their formation to regions which require to be nourished by copious showers. And thus winds and clouds combine to support vegetation. The winds convey the clouds from place to place, and the clouds themselves, in the expres- sive language of Scripture, ' drop fatness on the earth.' It is worthy of notice, too, that beside the action of the Sun in supporting vegetation at the present time, if was the same form of action exerted in long- past ages which resulted in storing up for our use those vast supplies of energy which are contained within our coal-mines. In other words what may be called our force-principal is as fully due to the Sun's action (direct or indirect) in promoting vegetation as 448 THE SUN. that force-interest which we derive each year from the Sun's seasonal action. And here I may be permitted to dwell on consider- ations which, though bearing rather on the economy of our Earth than on the general subject of solar physics, yet illustrate in a significant manner the work which the Sun has been appointed to do. I may premise, indeed, that we have no means of determining what the Sun's influence on the other planets may be, however clear it may appear to us that we are not the only, nor even the chief, recipients of those stores of force he lavishes so abundantly. It is on this account that while I give to this treatise a title indicating the Sun's position in the solar system, I deal only in this chapter — the sole one bearing on the Sun's office — with his position as our fire, light, and life. If in the considerations I am about to urge the Earth only seems concerned, it is none the less probable that results affecting the economy of the whole planetary scheme are in truth illustrated. We are accustomed to look upon the Earth, as an inexhaustible storehouse whence all our wants may be supplied. Year after year we till the soil, and still there is no lack in the growth of all the vegetable pro- ductions needed by man ; nor do our v flocks antl herds diminish, notwithstanding the enormous supplies of flesh-meat we are continually consuming. Taking the whole Earth, it is probable that the yearly produce of agricultural and pastoral labours increases at even a higher rate than that at which the human race is in- THE SUN OUR FIRE, LIGHT, AND LIFE. 449 creasing, so that were man content, as in old times, to draw upon the Earth's stores for the 'supply of his ordinary wants, there would be little fear of that store being ever exhausted. But of late a change has passed over the aspect of the world. On every side a multitude of new inven- . tions, and with them a multitude of new wants, are making their appearance. The stores which had been garnered up during long past ages of the Earth's history are being consumed with a rapidity which has already begun to alarm our men of science. It is true, indeed, that there is as yet little room for fearing that the terrestrial storehouse will soon be cleared of its contents. Even if the coal-mines of the world should be exhausted, there are still other force supplies; and doubtless the present rate of consumption might be continued, or even an increased rate maintained, for a period which seems indefinitely long when compared with the short span of life allotted to man. But, after all, what are a thousand years, or even several thousand years, when viewed with reference to the history of the globe on which we live ? If it could be shown that withiri two or three thousand years man will have exhausted all the stores of force which exist within the Earth, it surely might be urged with fairness that the present rate of consumption is unduly — selfishly great ; that the wants of future races should be considered, and that a check should be put upon those processes of over-rapid advance on which we are in the habit of priding ourselves. Precisely as we should g a 450 THE SUN. hold it to be blameworthy that a rich man should use, merely for purposes of luxury or convenience, that which could be shown to be absolutely essential to the existence of a large number of his fellow-men, so it might fairly be held to be wrong for the present in- habitants of the Earth to exhaust, in contrivances intended to add to the luxuries or conveniences of life, those stores which are absolutely necessary to the well- being of future races. In dealing, for example, with the question of terres- trial coal supplies, it will not suffice to point out that for a thousand or several thousand years they may be drawn upon as at present, or even more largely, with- out exhaustion. The thousand or thousands of years will pass as surely as those which have already passed, and the wants entailed by our wastefulness will be felt none the less, that for so many years there had been no failure in the supplies contained within the great terrestrial storehouse. What must be done, then, is to show that by the progress of that very course of events which results in the rapid use of those stores, the means will spring into existence for obtaining fresh and inexhaustible supplies. This is no idly speculative view, but the plain and obvious duty of the scientific world. Precisely as the superiority of civilised races over bar- barous tribes is shown in nothing more clearly than in the fact that the former are not content, as the latter are, merely to supply the wants of the moment, or of a few days, but seek to make provision, not only for future years, but for the wants of their immediate descendants, THE SUN OUR FIRE, LIGHT, AND LIFE. 4$ I so it behoves the leaders of the great movement which during the last few years has so greatly changed the aspect of the human race, to show the superiority of the new order of things by a careful provision for, and anticipation of, the wants of the races which will in- habit the Earth thousands of years hence. Without discussing the various forms of work which are being done upon the Earth, or considering the various agents employed in producing the motive power by which those forms of work are set in action, it may be simply stated that at present nearly all our motive force is obtained from stored Sun-force. It would be difficult to point to a single work accom- plished by the aid of modern scientific appliances which has not resulted in exhausting to a greater or less degree the force which the Earth has been garnering up in long past ages for our use. It is in this all-important respect that the more modern forms of machine- work differ from other forms of work. I refer, of course, to machines driven by inanimate motive powers, and not to those worked by the direct action of animal force. The machine draws upon the Earth's garnered stores,* while the living worker draws upon the Earth's periodical supplies of force. In the former case, that is being used up which cannot be replaced ; in the latter, what is consumed will be restored in the ordinary course of nature. In one * Those appliances in which advantage is taken of the action of the •wind, rainfall (rivers), tidal action, and a, few other natural processes, are to be excepted. Modern invention, however, is but seldom directed to the utilisation of these old-fashioned force-supplies. G G 2 452 THE SUN. case it is our ' force-principal,' in the other it is our ' force-income ' we are consuming. The distinction is all-important. This is not the place to enter into a discussion of the methods by which the great problem — a problem not requiring immediate solution, but which in the long run will surpass all others in. interest and impor- tance — is to be solved. But I may indicate what is, I take it, the direction in which a solution will be found. We are now utilising what Professor Tyndall calls the Sun of the Carboniferous Epoch: our de- scendants will have to employ the Sun of their own epoch. The heat of the solar rays — mayhap also their light and their actinic energy — must one day be ap- plied to work our machinery. Already men have felt the advantage of thus employing solar energy. They have not, indeed, as yet applied the direct action of the Sun systematically to their purposes.* But in an indirect manner they have utilised solar energy. The ships which sail upon our seas, the mills which are turned by water or by wind — these and many other devices of man have been contrived to utilise a portion of the Sun's heat. But the proportion thus utilised is almost indefinitely small by comparison with that which is actually available. It is only necessary to translate some of the ordinary phenomena of nature * Ericsson has constructed a machine in which the solar rays supply the primary motive force. It has not, however, yet been demonstrated (though I have not the least doubt it will be at some future epoch) that the solar heat can be employed in a profitable — that is, a ' mechanically advantageous,' manner. THE SUN OUR FIRE, LIGHT, AND LIFE. 453 into the language of the familiar forces in order to see that this is so. For instance, the amount of energy- involved in the production of rain is startlingly great when compared with our ordinary estimates of force. I have calculated that the force expended in the pro- duction of a day's steady rain over an area equal to that of the county of Middlesex would be equivalent to a mechanical power competent to raise 1,000,000,000 tons to a height of three miles ! Professor Tyndall has put in a striking form the relation which exists between the simpler processes of nature and those effects which seem to us the most apt exponents of power. ' I have seen,' he says, ' the wild stone-avalanches of the Alps, which smoke and thunder down the declivities with a vehemence almost sufficient to stun the observer. I have also seen snow- flakes descending so softly as not to hurt the fragile spangles of which they were composed ; yet to produce from aqueous vapour a quantity which a child could carry of that tender material, demands an exertion of energy competent to gather up the shattered blocks of the largest stone-avalanches I have ever seen, and pitch them to twice the height from which they fell.' And when we have thus seen what a tremendous amount of energy is involved in such processes as the formation of snow or rain in comparatively small quantities, we begin to recognise, though we are far from being able to conceive, how enormous is the potential energy which supplies the rainfall of the 454 THE SVN. whole Earth. We must remember, too, that a large amount of rain falls where it is not wanted, and that the energy of the Sun expended in the production of wind is in large part wasted. Clouds are raised by evaporation from the sea surface to fall on another part of the self-same waters. Storms are roused which blow with vehemence for a while, and then sink into rest without having accomplished any purpose neces- sary to the wants of terrestrial races. Here at once we see a large amount of energy not fully utilised. I do not indeed say that this apparently useless expendi- ture of force has no purpose in the economy of nature. Doubtless every natural event has its end and object. What I would dwell upon is, that if the energy which thus seems wasted could be made available to subserve human wants, it might be used without any fear that the economy of nature would suffer from such an ap- plication of her energies. And if this is true of the application of the indirect effects of solar energy, it is a fortiori true of the utilisation of the Sun's direct action — that is, of those solar rays to which the winds and the rains are due. Now, if we assume that with the progress of science the power of thus employing to the full — or much more fully than at present — the forces which the Sun really expends upon the Earth will be acquired by man, we recognise the probability that science viewed generally is one of the means by which the efficiency of the solar energies is enlarged and extended. What is true of our Earth may be regarded as in all probability true THE SUN OUH FISH, LIGHT, AND LIFE. 455 of other worlds than ours. As on our Earth so pro- bably in other worlds there are or have been eras during which the beneficent power given to qur great luminary is used without any consciousness of its value. There are or have been more advanced eras when the return of day and night, the progress of the seasons, the nourishment of the lately sown seed by spring rains, and the whitening of the fields into har- vest under the summer Sun, are watched with anxiou a interest. Then later follow the eras when the annual supply of truly vital energies seems to become insuffi- cient, and when garnered stores of force are utilised by the thoughtful or ransacked by the too eager. And lastly, it may well be that in other worlds, as one day doubtless on our Earth, there will be eras when a more advanced degree of science will enable intelligent beings to derive from direct solar action the means of obtaining even larger supplies of force than they had been able to gather from the Sun- work of past epochs garnered by nature for their benefit. The ideas of the Sun's true position in the solar system thus suggested lend an enhanced interest to the question whence the Sun himself derives and recruits his energies. As Tyndall has finely written, ' How is the perennial loss made good ? We are apt to overlook the wonderful in the common. Possibly to many of us — and even to some of the most enlightened among us — the Sun appears as a fire, differing from our terrestrial fires only in the magnitude and intensity of its combus- tion. But what is the burning matter which can thus 45 6 THE SUN. maintain itself? All that we know of cosmical phe- nomena declares our brotherhood with the Sun — affirms that the same constituents enter into the composition of his mass as those already known to chemistry. But no earthly substance with which we are acquainted — no substance which the fall of meteors has landed on the Earth — would be at all competent to maintain the Sun's combustion. The chemical energy of such sub- stances would be too weak, and their dissipation too speedy. Were the Sun a block of burning coal, and were it supplied with oxygen sufficient for the observed emission, it would be utterly consumed in 5,000 years. On the other hand, to imagine it a body originally endowed with a store of heat — a hot globe now cooling — necessitates the ascription to it of qualities wholly different from those possessed by terrestrial matter. If we knew the specific heat of the Sun * we could calculate its rate of cooling. Assuming the specific heat to be the same as that of water — the terrestrial substance which possesses the highest specific heat — then, at its present rate of emission, the entire mass of the Sun would cool down 15,000 degrees in 5,000 years. In short, if the Sun be formed of matter like * The absolute quantity of heat necessary to raise the average tem- perature of the Sun by any giran amount — say one degree, and there- fore the absolute quantity of heat which corresponds to the loss of say one degree of temperature from the average temperature of the Sun — •will depend on the physical constitution of the Sun. To say merely that the Sun's substance subsists at such and such a temperature, is by no means sufficient to indicate the amount of heat which the Sun is capable of imparting. A mass of iron, for example, may be at a temperature pre- cisely equalling that of an equal mass of boiling water; yet the boiling water will give out far more heat white passing to any given lower tem- perature than the heated iron will. THE SUN OUR FIRE, LIGHT, AND LIFE. 457 our own, some means must exist of restoring to it its wasted power.' We have not as yet the means of satisfactorily- answering the question thus suggested. Answers have been suggested, but no answer has yet seemed so satisfactory that men could regard the problem as dis- posed of. Indeed ' the facts are so extraordinary,' as Tyndall has said, ' that the soberest hypothesis must appear wild.' Whether we conceive, with Mayer and Thomson, that the Sun's heat is maintained by the incessant downfall of cosmical bodies gathered out of space by the Sun's mighty attractive energies ; or whether we follow Helmholtz in supposing that the gradual contraction of the solar orb is the mainspring of the solar energies ; or whether we believe, with Secchi, that the dissociation of compound bodies in the Sun's substance is a fund of force to be gradually exhausted only as the dissociated elements unite in chemical combinations ; or, lastly, whether we prefer the idea thrown out by Sir John Herschel that may- hap the vital energies of monstrous creatures — the willow-leaves of Nasmyth — are the true source of the great luminary's might, we have not overpassed by a step the amazing field of conjecture appertaining to our subject. Startling as these theories- appear, they are not a whit more startling than the known facts which they are intended to interpret. I have no wish to enter here into a detailed con- sideration of any of the theories above referred to ; but I wish to make a few remarks respecting those of Helmholtz and Thomson. 458 THE SUN. When we consider the evidence forced upon us by the present condition of the solar system, and by the nature of the observed motions taking place within it, we find it difficult not to believe that two great pro- cesses have for uncounted ages been at work within its limits. On the one hand, the evidence is very strong in favour of the view that a process of contraction from a nebulous condition has taken place not only in the case of the Sun, but in that of the planets and other members of the solar system. On the other hand, the evidence is absolutely demonstrative that at the present time uncounted millions of minute cosmical bodies are streaming in upon the Sun. We have, then, clear evidence that at least some portion of the Sun's energy is derived from each of the two sources now dealt with. We can hardly conceive that the process of solar con- traction has come to an end ; and certainly we have no proof in the apparent constancy of the Sun's volume that the process has ceased, since Helmholtz has shown ' that the shrinking of the Sun's diameter by one- 1 0,000th part of its present length would generate an amount of heat competent to cover the solar emission for 2,000 years.' Nor can we question that whatever energy may correspond to the velocity, mass, and distance of a meteoric body at any epoch must have been transferred to the Sun if at some later epoch the mass of the meteor has come (after whatever processes) to form part of the solar globe. Now, without com- mitting myself to the opinion that the whole solar emission can be accounted for by combining these two THE SUN as seenbythe author on. September 25,1870. .'./ ■_'. V'. : -'. .'-,;'-..' ■ ■ * 'l vr'r'Tk A TELESCOPE 2t'INCH£S IN APERTURE, ANB A POWER Or Om.Y 26 DIAMETCR&. THE SUN OUJR FIRE, LIGHT, AND LIFE. 459 causes, I must yet express the conviction that to forget the reality of these causes, their competence to account for some aliquot portion (let its amount be what it may) of the solar energies, would be a mistake. Nor can I see any valid reasons for asserting positively that the two causes combined may not account for a very large proportion of the Sun's activity. The irregular and perhaps intermittent supply of meteoric matter affords doubtless but an insufficient explanation of the Sun's copious and steady emission of heat ; but the pro- cess of contraction would act the combined part of a ' governor ' and an independent source of heat. Checked during the arrival of large meteoric supplies, and proceeding more rapidly when those supplies were temporarily diminished, it would account for that ob- served steadiness of emission which forms so important a characteristic of solar action. I feel that I cannot bring •this chapter more aptly to a conclusion than by quoting that noble passage in which Tyndall closes his discussion of the same subject : — ' Presented rightly to the mind,' he says, ' the discoveries and generalisations of modern science constitute a poem more sublime than has ever yet been addressed to the imagination. The natural philosopher of to-day may dwell amid conceptions which beggar those of Milton. So great and grand are they, that in the contemplation of them a certain force of character is requisite to preserve us from bewilderment. Look at the integrated energies of our world — the stored power of our coal-fields, our winds, 460 THE SUN. and rivers; our fleets, armies, and guns. What are they ? They are all generated by a portion of the Sun's energy which does not amount to the two- millionth of the whole. This is the entire fraction of the Sun's force intercepted by the Earth, and we con- vert but a small fraction of this fraction into mechanical energy. Multiplying all our powers by millions of millions, we do not reach the Sun's expenditure. And still, notwithstanding this enormous drain in the lapse of human history, we are unable to detect a diminution of his store. Measured by our largest terrestrial standards, such a reservoir of power is infinite ; but it is our privilege to rise above these standards, and to regard the Sun himself as a speck in infinite extension — a mere drop in the universal sea. "We analyse the space in which he is immersed and which is the vehicle of his power. We pass to other systems and other suns, each pouring forth energy like our own, but still without infringement of the law which reveals immutability in the midst of change, which recognises incessant transference or conversion, but neither final gain nor loss. The law generalises the aphorism of Solomon, that " there is nothing new under the Sun," by teaching us to detect everywhere,, under its infinite variety of appearances, the same primeval force. To Nature nothing can be added; from Nature nothing can be taken away ; the sum of her energies is con- stant, and the utmost man can do in the pursuit of physical truth, or in the applications of physical knowledge, is to shift the constituents of the never- THE SUN OUR FIRE, LIGHT, AND LIFE. 461 varying total. The law of conservation rigidly ex- cludes both creation and annihilation. Waves may change to ripples and ripples to waves — magnitude may be substituted for number and number for mag- nitude — asteroids may aggregate to suns, suns may resolve themselves into florae and faunae, and floras and faunas melt in air — the flux of power is eternally the same. It rolls in music through the ages, and all ter- restrial energy — the manifestations of life, as well as the display of phenomena — are but modulations of its rhythm.' 462 CHAPTER IX. THE SUN AMONG HIS PEERS. We have hitherto regarded the Sun with reference to his position in the solar system — as ruler, fire, light, and life of that wonderful scheme whose real magni- ficence and complexity have but recently begun to be recognised by astronomers. We have now — but very briefly, for already the space allotted to our subject has been exceeded — to consider him as a member of the sidereal system. What he is to the scheme of de- pendent worlds we have seen ; it remains that we should endeavour to form some conception of his position among his peers. We have to contemplate him as a Sun among many suns, exerting an influence indeed over his fellow orbs, but, swayed in like sort by their attractions, still surrounded — as when we considered him with reference to the solar system — by orbs travelling with enormous velocity, but no longer at rest, or almost at rest, amidst a scheme of moving worlds. We are to see him taking part in a scheme of movement too wondrously com- plicated to be as yet interpreted by astronomers. We must not pause here to consider the processes — interesting though their history may be — by which THE SUN AMONG HIS PEERS. 463 astronomers have been enabled to determine the distances of certain stars, and so to form a general estimate of the scale on which the sidereal system is constructed. Let it suffice to mention that the funda- mental fact on which our estimate of the distances of the fixed stars from us and from each other has been based, is the circumstance that while our Earth sweeps round the Sun on an orbit more than 180,000,000 miles in diameter, the stars remain all but unchanged in their apparent position, all the powers of our modern instruments only revealing in a very few instances the minutest conceivable displacement. Setting this fact clearly before us, the grandeur of the sidereal system becomes more real and present to our minds. From the nearest fixed star, the vast orbit of our Earth is reduced to little more than a point — to a circle so minute that 2,000 such circles could be placed side by side along the apparent diameter of the Sun or Moon. But the great majority of the stars lie at distances far vaster ; so vast indeed, that the Earth's orbit is reduced to a mere point as viewed from beyond the vast abysms which separate us from those orbs. Nor is it likely that in general, the distance of star from star, of any star in the heavens, for instance, from the nearest of its neighbours, falls short of the distance by which our Sun is separated from the nearest of his fellow orbs. 'We are thus brought face to face with a problem full of interest but enormously difficult— a problem which belongs perhaps rather to the astronomy of the 464 THE SUN. future than to that of our day. How are men to de- termine the figure and dimensions of the sidereal sys- tem, to understand its structure and complexities, to trace out the motions taking place within its limits, when as yet they seem to have scarce any means of even attempting to solve these problems ? Yet here, unless I mistake, is a work from which future astrono- mers will not shrink, a problem whose solution (for it will be solved) cannot but reveal results altogether surpassing in interest any which astronomers have yet obtained. It is true that if we consider the means we have for attacking this noble problem, they seem ineffective indeed ; if we look at the results of past research we find little to encourage present confidence. Yet it is only necessary to consider the amazing in- terest of the problem to set doubt and irresolution on one side, and at least patiently to test the means we have at our disposal. I have elsewhere* pointed out reasons for regarding the views hitherto accepted respecting the sidereal system as unsatisfactory. The results obtained by Sir "William Herschel, and apparently confirmed by the labours of Sir John Herschel, the elder Struve, and others, seem, according to the evidence I have adduced, to be self-contradictory and not accordant with other equally reliable researches. I confess I can no longer entertain any doubt that there is an error in the * In Other Worlds than Ours, and more especially the second edition, where additional, and, I think, conclusive arguments are brought forward. THE SUN AMONG HIS PEERS. 465 hypothesis which underlies Sir William Herschel's reasoning. It is absolutely essential, if we would form any adequate conceptions at all respecting the nature of the scheme to which the Sun belongs, that that hypo- thesis and its results should be re-considered. I must avoid here, however, all reference to arguments already enforced, and indeed I propose but to sum up here the results I have exhibited elsewhere, and then to pass on to consider one special circumstance connected with the Sun's relation to his fellow suns — the proper motion by which he speeds through interstellar space. Sir "William Herschel, fired with the noble thought of gauging the celestial depths, took as the fundamental hypothesis on which his gaugings were to rest, the conception that the stars are spread with a certain general uniformity within a definite region of space.* If this one hypothesis be admitted, it becomes possible, by means of a telescope powerful enough to reach the most distant and the smallest stars of the system, to gauge the extent of the system. All that is necessary is to count the number of stars seen in the telescopic field of view when the instrument is directed towards different parts of the heavens. Where many stars are seen, there the system must necessarily have its greatest * It is sometimes added that Sir William Herschel supposed a cer- tain general uniformity of size and brilliancy to exist among the stars. This, however, is a mistake. The only general hypothesis made by Sir William Herschel was the one stated above ; he added, as necessary to the admission of his own star-gaugings, the theory that his tele- scope reached — at least in most directions —the limits of the sidereal system. H H 466 THE SUN. extension ; where there are few stars, there the limits of the system must be nearest to us. It is well known that by this process of star-gauging Sir William Herschel was led to the conclusion that the sidereal system has the form of a cloven disc. The extension of this disc is towards the region of the heavens occupied by the Milky Way, the parts of the heavens where no milky light is seen corresponding to the flattened sides of the disc. According to the essential principle of this method of star-gauging, applied to the observed numerical relations, it follows inevitably that the stars visible to the naked eye lie far within the limits of the cloven disc. The same conclusion follows, also, from Sir John Herschel's gauges of the southern heavens ; though his view of the sidereal system differed in this respect from his father's, that he considered the stars visible to the naked eye, and others down to about the tenth magni- tude, to be less richly spread through space than those whose united lustre produces the milky light of the galaxy. But whether the richer parts of the sidereal system form, as Sir William Herschel thought, a cloven disc in space, or, as Sir J. Herschel supposes, a cloven ring, surrounding the lucid stars — in either case, accepting only Sir William Herschel's fundamental hypothesis, we are bound to admit that the lucid stars lie far within the limits of the sidereal system.* In * For convenience, astronomers speak of the stars visible to the naked eye as the lucid stars. The title has no reference, it will be understood, to the intrinsic brilliancy of the light of the visible stars. THE SUA AMONG HIS PEERS. 467 whatever direction we turn our eyes to look upon the lucid stars, we may be quite certain, if only this hypo- thesis be true, that the bounds of the star-system lie far beyond the constellations we are regarding. Now it is at this point that my study of the stars has led to the recognition of evidence which opposes itself in the most striking manner to the views usually accepted. I find among the lucid stars the most con- vincing signs of aggregation along certain definite regions, and of segregation from others. I have applied to these signs the strictest principles of mathe- matical calculation, in order to determine whether they can by any possibility be due to chance distribution, and I find that it is wholly impossible so to interpret them. But in this result, regarded by itself, there is in truth nothing opposed to the accepted theories. It is indeed an interesting circumstance that such traces of aggregation and segregation should be recognisable, and perhaps it may seem to many a perplexing circum- stance that these signs should so long have escaped recognition.* But, apart from the interest thus attach- ing as I think to the discovery, there is nothing which may not be conceived to accord very well with the views of the Herschels. For such peculiarities of structure, if one may so speak, within the sphere of * This may be ascribed wholly to the strange nature of the star- atlases hitherto constructed, in which the authors seem to have studied how they might best (by distorting the celestial spaces and by covering them over with monstrous figures of men and animals) conceal alto- gether from view any laws of association which may really exist among the stars. H H 2 468 THE SUN. the lucid stars might extend throughout the whole of the ' cloven-disc star-system ' conceived by Sir William Herschel, and yet the averages on which he based his conclusions might not be disturbed. It is when another and most unexpected relation is mentioned that the accepted theories are found to fail. The aggregation of stars distinctly recognised in some regions and very marked in others is most marked of all along the Milky Way. Not only are lucid stars so richly strewn on the Milky Way that for the whole heavens to be as richly spread 6,000 new lucid stars would be wanted, but the gaps and lacunse in the Milky Way are so bare and vacant that were the whole heavens no richer 4,600 stars now visible would have to be blotted from our view. Such, briefly stated, is the statistical evidence on this point. There can be no question that it is of the most convincing character. The probabilities against such a result if chance distribution were alone in question — that is, if no real relation existed between the lucid stars seen amid the milky light of the galaxy and the clustering groups of telescopic stars which produce that light — may be readily shown to be so overwhelming that no illustration can be devised to convey an adequate idea of their immensity. The chance that the Sun will rise to-morrcw is ridiculously small (at least as Quetelet calculates it) by comparison. So that as long as the laws of probability are to be our guide in such matters (and in every scientific conclu- sion ever yet adopted we have had no other evidence) it must be regarded as certain that the lucid stars seen THE SUN AMONG HIS PEERS. 469 on the Milky Way are for the most part immersed among the crowds of minute stars forming; the diffused light of the galaxy. These galactic stars then are much nearer • than had been supposed, and they are really minute, not reduced merely to apparent minute- ness by the vastness of their distance. When we add to the considerations thus suggested that the nebulae have been shown by unmistakable signs of association to form a system intimately connected with the system of stars, we begin to see that the side- real system regarded as a whole is very different from that scheme of suns pictured in the accepted theories. Our Sun and his fellow suns are associated with groups of minor suns, with clusters of star-dust, with masses of star-mist. We trace amid the complex system thus disclosed the signs of as yet unthought-of laws. Here the large siins gather into well-marked clusters ; here they form streams amid the celestial depths. In one region we find them associated with that strange spiral of minute stars forming the galaxy* (see fig. 96) ; in another they are grouped with discrete nebulas ; and yet elsewhere they are immersed amid the whorls and convolutions of nebulous matter. Lastly, in two regions we see suns and minor stars, star-clusters and discrete nebulas, and masses of nebulous matter com- bined into vast spherical aggregations — the Magellanic Clouds of the seaman — and these aggregations them- * Such is at least a figure which (as shown in my Other Worlds) accounts in a satisfactory manner for all the observed peculiarities of the Milky Way. 470 THE SUN. selves forming the centre of a remarkable group of lucid stars. This group, numbering more than 2,500 orbs, covers one half of the southern heavens. It sweeps in a mighty spiral * around the greater Magellanic Cloud. It gathers its host of lucid orbs so densely along one part of its course that that region of the heavens alone suffices to light up the southern skies as with the light of a young moon. It presents, in fine, phenomena which leave little room for question that it Fib flfi. The Milky Way regarded as a Spiral. forms a great and distinct system, within whose bounds are included all the characteristic features of the sidereal system itself, if indeed we are not to regard it as form- ing the noblest half of that portion of the universe of which we have hitherto become cognisant. * The features here referred to are very strikingly exhibited in the isographic maps of the northern and southern heavens accompanying the second edition of my Other Worlds. These maps show all the stars visible to the naked eye (in white on a black ground) truly distributed area for area. THE SUN AMONG HIS PEERS. 471 It is not without a purpose that I have thus directed the reader's attention to the vast southern star-system which constitutes the most striking and instructive feature of the heavens. If there is no feature of the northern heavens which to ordinary vision seems to correspond to this southern star-system, yet statistical research reveals the fact that the southern region has its true analogue in our northern heavens. The widely- extended group of stars surrounding the projection of the Milky Way in Cepheus, and including within its limits the singularly rich portion of the Milky Way in Cygnus, has not only well-defined limits, hut presents a well-marked superiority to the rest of the northern heavens as regards richness of star-distribution. Though smaller in extent, it is not less rich on the average than the great southern rich region. It corresponds also with that region in some other and rather peculiar respects. It covers a region where the Milky Way throws out projections and shows vast vacuities. The Milky Way reaches it on one side as a single stream, on the other as a double stream ; and further, the brightest portions of the Milky Way, in the northern and southern heavens, lie near these two rich regions, and both also towards that edge of the rich region whence the double stream of milky light extends. Now, I would invite attention to the circumstance that the Sun's proper motion, according to the best estimates hitherto made, is carrying him from the borders of the southern rich region towards the borders 472 THE SUN. of the northern rich region. He is passing away from the neighbourhood of Canis Major, Columba, and Lepus (not to define too precisely the as yet scarcely determined path along which he travels), and he is urging his way with inconceivable velocity towards the region between Hercules and Lyra. Of the true habi- tudes of those regions of space through which he is bearing, and has lately borne, his family of planets we know little. But as we look back along the extended track he has pursued, and see the richness of those regions he has left, and as we look onwards and trace his course in imagination towards the borders of that rich region whose glories gather into their chiefest splendour in Cygnus, the conception is suggested that he is now winging his way through a relatively barren region, that he has left and will again visit more glo- rious star-depths than those through which he now pursues his course. And here we may pause for a moment to consider the nature of that path along which we ourselves are borne as the Earth sweeps on her course round the Sun. Let the foreshortened circle in fig. 97 represent the path of the Earth about the Sun, the globe N s represent- ing on a large scale the slope of the Earth's axis through- out her annual revolution. When the Earth is at 55 it is mid-winter ; when she is at *± it is spring ; at yf it is summer ; and lastly, when she is at T it is autumn. Then the path of the Sun has the position indicated in the figure, being inclined some 53 degrees to the plane of the Earth's orbit, and some 15 degrees in advance THE SUN AMONG HIS PEERS. 473 of yf .* Along this path the Sun pursues his course at a rate which has been estimated at about 150 millions of miles, or five-sixths of the diameter of the Earth's orbit in each year. Hence, since the Earth's orbit-plane is carried along at this rate, while the Earth circles around that orbit once in each year, it follows that the actual path pursued by the Earth in Fin. 37. Illustrating the motion of the Earth's orbit through space. space f is such as is indicated in fig. 98. It is in fact a skew spiral or helicoidal path. J * Its projection on the ecliptic, that is, lies in longitude 285°, or thereabouts. t Or rather within the sidereal system, which itself doubtless has some motion — perhaps an inconceivably rapid motion. \ Some persons have expressed great anxiety lest, if the Sun is really travelling so swiftly through space, he should leave the Earth behind. There is not the least fear of this, any more than there is that the Earth should leave the Moon behind. Whatever forces have caused the Sun to follow his present career have acted upon the Earth and all the planets with equal effect. And as the Sun's course through 474 THE SUN. The other members of the planetary system pursue paths of different figures. The coils of the helicoids traversed by Mercury and Venus lie relatively close together ; those of Mars are not so near as the coils of the Earth's path. The asteroidal helicoid paths — amazingly complicated they must be — are yet more drawn out (so to speak). But it is when we pass beyond the asteroidal orbits that we get the most ex- Fra. 98. The Earth's motion through space. tended helicoids. Jupiter is carried some 1,700,000,000 miles onward with the advancing Sun, while he cir- cuits once around his orbit of less than 1,000,000,000 miles in diameter ; Saturn sweeps on through some 4,400,000,000 miles, while circuiting his orbit, less than 1,850,000,000 miles in diameter; and the paths space becomes modified by the varying attractions to which he may be subjected, every planet in the solar system, every satellite, meteor, planetary comet, and asteroid, -spill experience the same influences, and accompany the Sun just as faithfully as at present. THE SUN AMONG HIS PEERS. 475 traversed by Uranus and Neptune amid the depths of sidereal space are even more remarkably drawn out, regarding them in their helicoidal character. As the Sun travels through space the planets sweep onward with him.* But has he, besides his planetary dependants, any companions on his voyage ? Do any of his brother suns travel along with him ? As yet we have no means of knowing, for a strange difficulty arises. If the Sun has companions, these must, of course, be relatively near to him. It does not follow that they will appear brighter than other stars, because if they are no larger than the Sun, we know that other orbs (as Sirius and Arcturus) must largely exceed them in real size, and so their relative proximity may not be rendered apparent. But this is not all. The stars which astronomers select as most likely to afford measurable indications of proximity are those whose apparent motions on the heavens are exceptionally large. Now the companions of our Sun on his voyage through the sidereal system doubtless travel on a nearly parallel course ; and therefore, setting aside their orbital motions around each other, or around the common centre of gravity of the family, they must appear, as viewed by us, to be almost at rest. They would of course indicate in a more marked manner than any other stars the effect of the Earth's annual * It is -worthy of notice that the Sun's northern hemisphere travels forwards. Is it not conceivable that in this peculiarity we may find some explanation of the greater heat which has been said to be emitted from the northern solar hemisphere ? See also p. 211, foot note. 47 6 THE SUN. motion (or, technically, they would have a large annual parallactic displacement) ; but then astronomers would not be led to look for such effects, since we know as a matter of fact that the stars hitherto examined for signs of annual parallax are those which, either through Fig. 99. \ ., •» •^ •» •^ \ s i *-• *-• —ft ,.*-» */ ^ "--.. "'•--- -• y ,,--- / / Observed proper motions in Ursa Major and neighbourhood exceptional brilliancy or through exceptionally large proper motion, seem likely to be near to us. We find signs in the heavens leading us to regard the existence of such ' companions of the Sun ' as at least not wholly improbable. Here, for example, is a picture (tig. 99) borrowed from my ' Other Worlds,' in which the chief stars of the constellation Ursa THE SUN AMONG HIS PEERS. 477 Major are depicted, and some few others belonging to Draco and Bootes. To each star is attached a small arrow indicating the direction of its motion, and the amount of such motion in 36,000 years. We see here decided signs of star-drift. We can scarcely doubt that the five principal stars of Ursa Major included within the dotted line are travelling together through space ; while the four stars above, belonging to Draco, Fig. 100. s s f r r Observed proper motion of Stars in head of Aries. seem similarly to be companion suns. The re- maining stars may also not improbably form a single family. The group of stars shown in fig. 100 seem to form a system within which probably there are orbital motions of considerable magnitude. It is worthy of notice that in the two instances here referred to there are evidences of association apart from the observed proper motions. For in the second the stars forming the system seem segregated in a somewhat marked manner from neighbouring star- 478 THE SUN. groups. The stars in Ursa Major, again, have been noted by Fr. Secchi as having very similar spectra — in other words, as resembling each other very closely in structure and condition. In this circumstance we have a peculiarity which may one day enable us to select with some confidence those stars which are our Sun's special companions, which voyage along with him through the sidereal system, and share with him perchance in a reign over some special domain of space. And here I have to draw this work to a conclusion. Too limited by far has been the space I have allotted to my subject, and yet, for many reasons, it would have been unwise to have exceeded this space. I have followed so closely the course I marked down in the beginning that I see no occasion to change one word in the introductory pages. Yet at many stages where I had promised myself a pause for survey and reflection I have been compelled to pass on without stay. Many inviting paths opening out on one hand and the other have been perforce left unexplored. As I have proceeded I have become more and more sensible of the vastness of the subject I have under- taken to discuss. Not a book, but a library of books, is needed to deal adequately even with only what is at present known about the Sun ; not a few students of science, but all the astronomers and physicists now living, might devote their powers to the study of solar phenomena, and yet find that the army of labourers THE SUN AMONG HIS PEERS. 479 needed to be largely recruited. Only in the coming time, when the students of science will be the majority instead of a minute minority of mankind, will this subject, and those countless other kindred subjects which await investigation, be adequately dealt with. Let us hope that time is not far off. APPENDIX A. THE APPROACHING TRANSITS OF VENUS AND THE BEST MEANS FOR OBSERVING THEM. On account of the interest attaching to the approaching tran- sits, and more especially to the transit of 1874, for which a sum of more than 10,000J. has been voted by the Govern- ment, it seems desirable to exhibit here in a popular form the results to which I have been led by a very careful mathema- tical investigation of the conditions under which the coming transits may be most satisfactorily observed.* Already in Chapter I. I have exhibited the general prin- ciples on which the determination of the Sun's distance by observation of transits of Venus depend. But as it is always advantageous in discussing astronomical relations to view them in as many aspects as possible, I now present a mode of * I may remark here that I have gone over, at the expense of a con- siderable amount of time (for the work was altogether new to me), the process of calculation by which the elements of the transit are deduced from the tables of Venus and the Sun ; but (as I anticipated when I began) the results I obtained accord so closely with Mr. Hind's, that the labour (save for the practice it gave me) was in a sense thrown away. It is worthy of notice, however, that M. Puiseux having obtained some- what different results, this confirmation of Mr. Hind's results has a value which ordinarily would be wanting to researches of the sort. Mr. Plummer, of Mr. Bishop's Observatory, Twickenham, had already, at Mr. Hind's request, tested the published elements ; and there can be now no doubt whatever that in this matter (as always) Mr. Hind's calculations are beyond question. I I 482 THE SUN. considering transits of Venus which is perhaps the best and simplest conceivable. Let s (fig. 101) be the Sun, v Venus and e (the Earth) travelling in the same direction and nearly in the same plane round the Sun as shown by the arrows, Venus the more swiftly; and let it be noted that the dimensions of the Sun, Venus, and the Earth are here necessarily exaggerated, since even the Sun, if presented on the same scale as the distances, would be scarcely perceptible in our figure. Now, Venus, like every other body in the solar system, throws a shadow, and the shadow is represented by the tri- angular space behind Venus in fig. 101. It comes to a point not very far (relatively) from Venus. Suppose this cone produced beyond its apex so as to form the shaded cone shown in the Fio. 101. figure. It is obvious that to an eye placed within this shaded region Venus will be seen fully within the Sun's disc ; and to an eye placed on the surface of this region Venus will be seen just touching the Sun's disc on the inside. Now conceive a double cone touching both Venus and the Sun, but having its apex between these bodies, as shown in the figure. Then obviously to any eye on the surface of the region between this apex and v »', Venus would be seen just touching the Sun's disc ore the outside.* Clearly, then, to any eye between this * For instance, to an eye at v Venus would seem just to touch the Sun's disc on the outside at s, while to an eye placed at t/, Venus would seem just to touch the Sun's disc on the outside at s'. This is obvious, because the lines v v, 1/ v, just touch the Sun and Venus, whereas any other lines from v and 1/ to points on Venus pass clear of the Sun. TEE APPROACHING TRANSITS. 483 surface and the region shaded in the figure, a part only of the disc of Venus will be seen on the disc of the Sun. So that if an observer were carried through the double cone shown in fig. 101 behind Venus, he would see the follow- ing successive phenomena. When he came to the outer surface Venus would be in exterior contact, or as at a (fig. 14, p. 42) ; as he passed on to the inner surface Venus would enter more and more on the Sun's disc, until when he reached that surface she would be in interior contact, or as at b (fig. 14). Then as he travelled on through the inner cone Venus would seem to cross the Sun's disc, and she would just touch it on the inside when our observer reached the surface of this inner region on his passage outwards. Next, as he passed onwards to the surface of the outer region, Venus would be seen cross- ing the edge of the Sun's disc. And, lastly, as he passed that surface he would again see Venus in exterior contact, the transit thereupon coming to an end. Now, during a transit of Venus the Earth does actually pass in such a way through these regions ; or rather these regions overtake and pass over the Earth. In ordinary conjunctions the cones of fig. 101 pass above or beneath the Earth ; but when the Earth is sufficiently near to the plane in which Venus is moving, the cones do not pass without encountering the Earth, and so a transit takes place. Further, the considera- tions in the preceding paragraph suffice to exhibit the general circumstances of a transit. Since the cones overtake the Earth in the direction shown by the arrows, we may consider that the Earth passes through the cones in the contrary direction. Suppose v v' (fig. 102) to represent the same section of the outer cone as v v' in fig. 101 ; v v' the section of the inner cone; and e (fig. 102) the Earth, as shown at e in fig. 10]. Then v v' is really moving towards the left ; but we are to suppose that E is moving towards the right through v v'. Furthermore, if Venus is near an ascending node, as she will be during both the approaching transits, we must suppose the Earth to pass descendingly along such a course as e e' through 11 2 484 THE SUN. the region v v'. The actual course, both aa respects position and direction, is determined from the calculated elements of the transit. With this calculation we need not here concern ourselves.* The figure shows the course actually traversed by the Earth in 1874 and 1882. Now, taking the Earth through v v' for the 1874 transit, let us consider the various critical points, so to speak, of her course.. When she first touches the outer oircle v v' external Fra. 102. Illustrating the transits of Venus. contact will have begun at that point of the Earth which first reaches this circle. She passes on, falling more and more with- in v v 1 , until she is just wholly within. All this time external contact is taking place wherever the outline v v' intersects the Earth's disc ; at parts within the line Venus is seen partly * As to the size of v 1/ compared with that of the Earth, it is easily seen from fig. 101 that v v? is less than a great circle of the Snn, very nearly in the proportion that the Earth's distance from Venus exceeds the Sun's. If the cone of which v 1/ is a section had its vertex at Venus's centre this proportion would be exact. THE APPROACHING TRANSITS. 485 within the Sun's disc, and at parts outside of it external contact has not yet taken place. When the Earth has passed wholly within the circle v v', external contact has taken place at all parts of the visible hemisphere. But as at this time no part of the Earth has reached the circle v v',* internal contact has nowhere commenced. In other words, Venus is not yet fully upon the Sun's disc as seen from any part of the Earth. Now, this part of the Earth's motion is not illustrated in fig. 102, because external contacts and the passage of Venus across the Sun's outline are not phases which the observers of transits pay great attention to. We now come to the impor- tant phases. When the Earth just reaches the inner circle v v', interior contact has just begun at the point on the Earth which first touches this circle. Here, then, earliest of all, internal contact begins, and we have at this point the phenomenon called by astronomers first internal contact most accelerated. The Earth is then in the position numbered 1 in fig. 102. She passes on, the outline v v' encroaching more and more over her face until she is wholly within this outline or in position 2. All this time internal contact is taking place wherever the outline v v' intersects the Earth's disc. At parts of the Earth within that line internal contact has passed, or Venus is already fully upon the Sun's disc. At parts of the Earth outside that line Venus still breaks the outline of the Sun's disc. When the Earth is at 2, internal contact has taken place for all places on the Earth's illuminated hemisphere. This contact takes place latest of all at that point on the Earth's surface which at this moment touches v v'. It is here, then, that there occurs the phase which astronomers call first internal contact most retarded. * The distance between the circles v 1/ and vV is obviously greater than the Earth's diameter, if we consider how the two circles v 1/ and v v' are obtained, For the diameter of Venus is very nearly equal to the Earth's ; so that the diverging lines from s or s' (fig. 101) are already separated at v by a distance nearly equal to the Earth's diameter, and therefore at v or v' are wider apart. 486 THE SUN. Then the Earth passes onwards through the positions shown severally along her track in fig. 93. As the Earth passes out of the spaces v v', v i/, similar phases occur in reverse order. "We need note only the posi- tions numbered severally 14 and 15. The first shows where the Earth first reaches v v', and the point on her surface which first touches v v' is the place where occurs the phase called second internal contact most accelerated; while 15 shows where the Earth just passes clear of vv', and the point on her surface which is the last to touch v v' is the place where the phase occurs called second internal contact most retarded. The circumstances of the progress of the Earth from one position to the other precisely correspond to those already considered in dealing with the Earth's motion from 1 to 2, only they take place in reverse order. Such a passage as I have described lasts altogether some four or five hours (a passage through the centre o about eight hours), and in this interval the Earth's axial rotation is not inconsiderable. This rotation has to be taken into account in dealing with the real circumstances of any transit. The reader wiil see, then, that to present in an exact manner all the relations thus involved would require not merely a large amount of space, but the discussion of mathematical considerations of some complexity. And yet there is absolutely no simpler way of exhibiting these relations in a descriptive manner. This is, then, one of those cases where, if we are to give views at once exact and generally intelligible, we must employ that too much neglected aid of the astronomical teacher, truth- ful pictorial illustration. We must have representations of the Earth as she is actually placed at the most important parts of her course through v v', truly poised, with true axial slope, and in the true position as respects axial rotation. Such pictures will do what verbal description in this case can never do, save for the mathematical reader. The illustrative plates VIII., IX., and X. have been specially constructed to meet this difficulty. PASSAGE OF THE EARTH THROUGH 17h.21m.30s. 17 h, 43 m DEC: 8, 1874. Plate Vffl lGh.36m.30s. 16h.51m.30s. <- \ W xx v ' \ \ \ \ \ V \ \ \ \ "> \ N N V \\\\\\\\\\\Vv 1\\\\\\\\\\.V >\ \ \ \ \ \ \ ^ .'A \ \ \ » \ > \ \ y s \ ) ^ : >' s 18h. 9m.4s. Malby&Sons.lith. THE APPROACHING TRANSITS. 4.87 The long folding plate pictures the Earth on her course through the positions marked 1, 2, . . . 14, 15, in fig. 102. The path of the Earth in this figure is, for convenience of engrav- ing, broken up into three parts, as shown in fig. 103, and the Earth is represented at each part of her progress, precisely poised and rotated, as she would appear'if she could be viewed from the Sun during the course of the transit of 1874. Leaving the cross lines out of consideration for the present, let the student study this plate, interpreting it by reference to figs. 101 and 103, and he will be able to form more exact con- ceptions of the real relations presented during the transit than he could from a very long and recondite explanation. He sees in the first picture of the Earth those regions whence the S 6 7 & M 11 30DOOOH: .12 13 14 15/ ZggjjBfZZ Fig. 103, explaining Plate VIII. beginning of the transit will be visible. He sees in the last those regions where the end will be visible. Those parts of the Earth which appear in both these views are those from which the whole of the transit will be visible. And, finally, those parts which do not appear in either of these views (nor, therefore, in any of the fifteen) are those whence no part of the transit will be seen. But now, having done this, and understanding also that he will presently be invited to return to the consideration of this plate, let him examine the larger and more elaborate views Plate IX. and X., representing the Earth as supposed to be seen from the Sun at the beginning and end of the transit; Of these views the first represents the Earth as she would appear from the Sun when her centre is just crossingMhe circle v v of fig. 93 at ingress, and the second represents her as she 488 THE SUN. would appear when her centre is just crossing the same circle at egress. So that the first corresponds to an epoch between those represented in theirs* two Earth pictures of the folding plate, while the second corresponds to an epoch between those represented in the last two pictures of that plate. The seemingly parallel cross-lines in Plate IX. represent the encroaching outline of the circle v v' (fig. 98) at intervals of a single minute of time between the epochs represented by the first two figures in the folding plate. The corre- sponding cross-lines in Plate X. represent the same outline gradually passing off the Earth's face between the epochs corresponding to the last two figures in the folding plate. The encroachment and the passing off not being strictly uniform,* these lines are not equi-distant, nor are they strictly parallel. (They should not be absolutely straight, since they really form short arcs of circles ; but this consideration is relatively unimportant.) Now, these two plates give us all we require for determining what are the best stations, whether for Delisle's or Halley's method.t For Delisle's, applied as at ingress, consider Plate IX. "We * The reason of this will be seen by a reference to fig. 102. Obviously the rate at which the Earth's centre is approaching the centre of Venus (which rate really measures the rate of encroachment) diminishes during ingress, while for like reason the rate of passing off increases during egress. f Properly speaking Plates IX. and X. only represent the Earth accu- rately for the moment when the outline of v Y (fig. 102) crosses the Earth's centre. Since, as we see by the eross'lines, no less than 25m. 6s. are occupied by the passage of the outline of v V over the Earth's face, both at ingress and egress, the Earth's rotation has to be considered. This, however, can very easily be done, since the latitude circles are shown, and the longitude circles are separated by ten degrees, corre- sponding to the Earth's rotation in forty minutes. Thus from Plate IX. we see that the cross-line marked 7m. on the right of the centre passes near Jeddo. But as the cross-line occupies this position seven minutes before it crosses the Earth's centre, we must put Jeddo back through an amount corresponding to seven minutes' rotation, or about one-sixth of the distance separating two longitude circles in this neighbourhood. THE APPROACHING TRANSITS. 489 want stations where the acceleration is greatest and where the retardation is greatest. The first lie at the point where ' ac-: celerated ingress ' is written. We must not choose a station at this point, because there the Sun would be on the horizon, and therefore distorted; but taking the line representing the places where ingress is beginning a minute or so later we see that it passes near Woahoo and Hawaii. These, then, are good stations for observing this phase. Three or four minutes later the line passes Jeddo, Bonin, Marquesas, Otaheite, and so on ; and these, therefore, though not such good stations as Hawaii or Woahoo, are still excellent. We may note, too, that at these stations the Sun will have a greater elevation, the actual elevation at different stations being indicated by the concentric circles marked with degrees. As respects retarded ingress, we see that the best station is Crozet Island, close by the point marked retarded ingress. But Kerguelen's Island, Macdonald Island, Amsterdam Island, as also Eodriguez, Mauritius, and Bourbon, are all good stations for observing this phase. Now let us turn to Plate X. to determine what stations are best for observing accelerated and retarded egress. It will be seen that the place marked ' accelerated egress ' falls inconveniently near to the south pole. Only when we reach the cross-lines marked 11m. and 10m. do we come on places where stations could be conveniently taken. The lines marked 9m. and 8m. bring us past several excellent stations in New Zealand ; and then we come to stations in South Australia ; and on the other side of the arrow-line we find Kemp Island, and (inferior, but still serviceable) the Macdonald Islands, Kerguelen, and Crozet. Lastly, as respects retarded egress, we find an abundance of excellent stations, the best being in Siberia and Eastern European Kussia ; but there are several excellent stations in India ; * while Alexandria will supply a very suitable place of observation. * The very best station in English territory, namely Peshawur— far superior to Alexandria both as respects the amount of retardation and 49° THE SUN. Tables at the close of this Appendix exhibit the actual amount of acceleration and retardation at the several stations indicated in these maps, and at some others not named in either plate. Now, as respects Halley's method, it will be remembered (see page 33) that we have to consider that the whole transit (or at least the beginning and end) * should be visible. We want stations (these will obviously be northern ones) where the transit will last as long a time as possible, and other stations where the transit will last as short a time as possible. We should therefore naturally look for northern stations where the transit begins as early as possible and ends as late as possible, and vice versa as respects southern stations. But when we consider Hawaii and Woahoo, where the transit begins at the earliest, we find, on turning to Plate X., that these stations will not suit our purpose ; for in Plate X. they are not visible ; in other words, before the end of transit Hawaii and Woahoo pass to the un-illuminated side of the Earth, the Sun sets, in fact, and the end of the phenomenon cannot be seen. In like manner, if we take those stations shown in Plate X. which are most suitable for observing the retarded egress, we find that at the epoch represented in Plate X. they are not visible ; in other words, they are on the un-illuminated ^ide of the Earth, or the Sun has not yet risen at these stations when transit begins. It needs but a brief study of the two Plates to see that the stations which give the longest duration are those in Man- solar elevation — had wholly escaped notice until my construction of Plate X. (reduced from the sixth plate of the original series drawn by me for the Koyal Astronomical Society, and published in vol. xxix. of their Proceedings) exhibited the advantages of this station. * This parenthetical remark may seem strange at first sight ; but it must be remembered that there are southern stations (though I do not say any in this case are available) where the beginning of the transit can be seen before sunset and the end after sunrise. It is only neces- sary to study Plates IX. and X. thoughtfully in order to see that this is the case. THE APPROACHING TRANSITS. 491 chouria, Japan, and North China, whose names are shown in both maps. Now, as regards the southern stations where the shortest duration is to be observed, we have in some respects a wider selection, for the obvious reason that day lasts longer at these southern stations in December (a relation corresponding, of course, to the longer portions of southern latitude-parallels shown in both maps). We require to find a southern station where the transit will begin as late and end as early as possible. All the stations by the place of most retarded ingress in Plate IX. are shown also in Plate X., but in the latter plate they are seen to be very far from the place of most accelerated egress. On the other hand, the stations near the latter place in Plate X., though all visible in Plate IX., are seen in the latter plate to be very far from the place of most retarded ingress. The best stations as respects proximity to both the place of retarded ingress and that of accelerated egress are Kemp Island, Enderby Land, Sabrina Land, and those in the neighbourhood of these spots. All are in very unsatisfactory and almost inaecessible regions of the Earth's surface. It happens, however, that Crozet Island, Kerguelen Land, Eoyai Co. Island, and Macquarie Island give sufficiently short- ened transit-periods to afford very satisfactory means of com- parison, by Halley's method, with the lengthened transit-periods at Nertchinsk and other neighbouring northern stations. As regards the elevation of the Sun the difficulty is of course greater at the northern than at the southern stations. But at all the northern stations marked in the maps the Sun will have an elevation exceeding ten degrees at the epochs of internal contact. On the whole, as will be inferred from the tables at the end of this Appendix, Halley's method will be applicable under very favourable conditions during the transit of 1874.* * It will be seen that I express in the above paragraphs the opinion that Halley's method, which had been pronounced ' wholly inapplicable ' 49 * THE SVN. Before proceeding to consider certain other matters belong- ing to the general subject, I will briefly discuss the transit of 1882. to the transit of 1874, can be applied under very favourable conditions. In vol. xxix. of the Monthly Notices of the Royal Astronomical Society, I have exhibited the calculations requisite to indicate the probable rela- tive values of Delisle's and Halley's method in 1874; and the conclusion to which these calculations point is that Halley's method is on the whole superior to Delisle's. Somewhat before my papers appeared, the French astronomer Puiseux had published a paper expressing his belief that Halley's method could be applied under conditions sufficiently favourable to render it advisable that that method, as well as Delisle's, should be employed. The actual results obtained by Puiseux gave, however, a, slightly inferior position to Halley's method. The difference is due to the fact that M. Puiseux employed approximate instead of exact modes, considering the passage of Venus' s centre, for example, instead of internal contacts, and taking no account of the equation of time. My results, not only as relates to the several methods, but as respects those cases in which I deduce different relative values for certain stations suitable for applying either Delisle's and Halley's method, have been now abundantly confirmed by the calculations of Peters, Hansen, and others. They were never indeed seriously questioned, because I was able to point to the exact places where my processes diverged from former and less exact computations, and to show how differences of considerable importance came thus to be discernible in the results. But I must disavow all desire to dwell upon or to magnify errors either of computation or of plan in the work of the eminent astronomer who pre- ceded me in dealing with this problem. The work was not undertaken by me, as I fear the Astronomer Boyal judged at the time, in any spirit of captious criticism. Deeply imbued with a sense of the extreme interest and importance of the problem of determining the Sun's distance, and attracted to it also by the exceedingly beautiful nature of the geometrical considerations it involves, I worked at it without any refer- ence in the first place to the labours of others. Only when I found that my results differed in many respects (which seemed to me, and still seem, important) from the Astronomer Eoyal's, was I led to compare his pro- cesses with my own, and to trace out the causes which led to the differ- ence in the results. I was prepared to find I had fallen into some error. As the reverse appeared, and as his results had been made widely public, and were, as I believed, to be made the basis of the choice of stations and methods for English observers in 1874, 1 should have been wanting in my allegiance to the cause of science had I failed to THE APPROACHING TRANSITS. 493 In the first place it is well to have a picture indicating the relations which Plates IX. and X. indicate for the transit of 1874. The time has not yet come, perhaps, when so carefully- constructed a drawing is needed ; but a drawing of a somewhat similar nature is essential to the adequate illustration of the sub- ject. I might avail myself here of the Astronomer Eoyal's two drawings, which will be found in Guillemin's ' Heavens ' (and are repeated, with Mr. Airy's statements, in Mr. Lockyer's ' Elementary Lessons of Astronomy ') ; for the corrections in- volved by the considerations I have attended to in the case ot the transit of 1874, have (for reasons which need not be entered into) a far less important effect in the case of the later transit. But I do not find myself on the whole content to adopt this course, partly because the differences (as anyone will see by comparing figs. 104 and 105 with the Astronomer Royal's drawings) are quite appreciable ; secondly, because the cross-lines which indicate the passage of the boundary of Venus's shadow-cone over the face of the Earth have not been separated by minute intervals, as in my maps (but by tenths of the total interval of passage), and are not quite correctly placed ; and, thirdly, because I think it on the whole more worthy of the student of science to give his own work in such instances. Figs. 104 and 105 show the exact presentation of the Earth publish the results of my researches. One thing alone would have forced me to publish my results — namely, the fact that it had been widely announced that in 1882 Halley's method could be applied if certain Antarctic stations were reached, whereas my calculations serve to prove that there is absolutely no station where Halley's method can be applied in 1882 under conditions sufficiently favourable to warrant the danger- ous expeditions and the protracted stay in Antarctic stations by which alone the requisite observations could be made. I repeat here, and urge a's the main reason for the earnestness with which I have pressed my views, that to send expeditions to survey the neighbourhood of the proposed stations near Victoria Land and Kepulse Bay, and to select either of these neighbourhoods for wintering, in anticipation of the summer (Antarctic) transit of 1882, would be to risk the lives of British seamen and men of science without any prospect of adequate return. 494 THE SUN. at the beginning and end of the transit of 1882. The general relations indicated correspond precisely to those dealt with already. As regards the application of Delisle's method at ingress, we have for the accelerated phase few convenient stations, those bestsituated being very near the Antarctic regions. Kerguelen's TRANSIT OF 1882. (INGRESS.) Fig. 104. Illumiuuted side of the Earth at ingress, Dec. 6, 2k. loin. 56s. (Greenwich mean time.) Land, parts of Madagascar, and the Cape of Good Hope seem the best. The retarded phase can be viewed under singularly favourable circumstances, however, since the whole seaboard of the United States and many inland towns there and in Canada fall close by the place of most retarded ingress. Many West Indian stations also seem not unsuitable. As regards egress, we also find a number of well-suited THE APPROACHING TRANSITS. 495 stations, many of which are identical with those just referred to as well suited for observing the retarded ingress. It is well that these doubly fortunate stations are American, since our American brethren in science are far more warmly sup- ported by their Government than ours are (or perhaps I TRANSIT OF 1882. (EGKRESS.) Fio. 105. Illuminated side of the Earth at egress, Dec. 6, 8h. 0m. 32s. (Greenwich mean time.) should say their cause is far more warmly advocated before their Government). Eetarded egress can be observed favourably in New Zea- land, and parts of Australia. Now, as respects the application of Halley's method, I find myself approaching a delicate subject. It has been so long asserted (fourteen years at least) that Halley's method is only 496 THE SVN. applicable in 1882, that many will be disposed to ridicule the notion that there can have been any mistake in the matter. Yet, confident in the simple mathematical relations dealt •with (and confirmed also by the view of the matter now steadily growing among mathematicians), I point to figs. 104 and 105, and ask where the stations are to be placed ? So far as the shortened transit observed in northern stations is con- cerned, there is no difficulty ; since, as we have seen, the commencement of the transit will be most retarded at the very stations where the end will be most accelerated — that is, in the United States, Bermuda, the West Indies, and so on. But as regards a southern station where the transit shall be most lengthened, the case is not so simple. The Astronomer Royal has pointed to two southern stations — one at Repulse Bay, near Sabrina Land, and one near Victoria Land. These are shown on the map. There is a double objection to Re- pulse Bay. First, the Sun will have an elevation of but about 4 degrees both at ingress and egress ; and, secondly, there is no known station there. The ' suggested station ' marked in Mr. Airy's maps (December number of ' Monthly Notices of Royal Astronomical Society ' for. 1868-69) at Victoria Land, occupies a spot on a shore line (explored by Captain Sir J. C. Ross) so precipitous that Ross could not approach it, though he would ' readily have given his right arm ' for an anchorage there. At Possession Island near by, a place where his men landed but were compelled to leave by the fearful odour of the accumulated penguin guano, the Sun would have the totally insufficient elevation of but about 5 degrees at ingress. Now, remembering that it has been shown by skilful Arctic and Antarctic seamen that to make observations at these so- called stations on December 6 r 1882, astronomers would have to winter there (making their way to them in February or March 1882, at latest), it surely will not be thought that the poor observations which could alone be made on a Sun but 4 or 5 degrees above the horizon are worth the expense and great risk which must needs be involved. But even if they were, then a fortiori would the application of Halley's method in THE APPROACHING TRANSITS. 497 1374 be' called for; since "I have'been able to demonstrate that without seeking such dismal regions, a more favourable opportunity will be afforded in 1874 than in 1882, even on (he supposition that the selected stations were visited during tho latter transit. And this result has been confirmed by mathematicians of eminence. ' As regards other southern stations in 1882, it needs but a brief study of figs. 104 arid 105, to see that none are available. AH the stations on the right of the arrow-line in fig. 104, and close by the arrow-feather, are unseen in fig. 105 — that is, the Sun has set at those stations. The barren Antarctic land south of Cape Horn (Graham Land) is not very favourably situated in fig. 104, but in fig. 105 has passed over to the wrong side of the central cross-line; in other words, there is acceleration of egress instead of the needed retardation. Similar remarks apply to the southern parts of South America.* So far, then, is it from being true that Halley's method is wholly inapplicable in 1874 but is applicable under favour- able circumstances in 1882, that the exact reverse is the case. Halley's method is even better than Delisle's in 1874, and is wholly inapplicable in 1882/f 1 * The reasoning in the note beginning on p. 37 will he found to be well illustrated by a comparison of Plates IX. and X. with figs. 101 and 10S. It will be seen that in the transit, of 1874 the rotation of the Earth- carries tho northern stations the wrong way as regards that lengthening which is required ; while in the transit of 1882 the northern stations are carried the right way for the shortening which is required. As to the southern stations, it will be seen how. according as they He above or below the pole (as shown in the maps), the lengthening or shortening can be increased or diminished. It is the fact that no stations below the pole are suitable in 1882, which spoils the otherwise sound reasoning heretofore employed. f It will be seen from fig. 102 that the path of the Earth crosses the sur- face of the shadow-cone much more obliquely in 1874 than in 1882; and,as this path is as it were the reflexion of the apparent path of Venus over the Sun's disc, Venus crosses the limb much more obliquely in 1874 than in 1882. Thus the actual progress of ingress and egre6S must take place much more slowly. Mr. Stone believes that all time errors will be correspondingly increased. The comparison above referred to is founded, K K 498 THE SUN. And now I propose to indicate, in conclusion, considerations -which lead me to believe that the direct method of deter- mining the actual parallactic displacement of Venus on the Sun's disc can be applied under very favourable circumstances during the approaching transits. Let us first consider if we cannot in part diminish the difficulties which seem inherent in this method. Suppose an observer at one station sees Venus at a given epoch as at a (fig. 106, I.), while at the same epoch another observer sees her at b. Then it is perfectly obvious that in such a case as is illustrated by I., when the two observations Fig. 106. Illustrating the displacement of Venus as seen from different terrestrial stations during transit. are brought into comparison, any error affecting the bearing of b or a from-c will tend to produce its full effect in increas- ing or diminishing the parallactic displacement separating a and b. But now suppose that a and b in II. represent the on this opinion, very unfavourable to my views. I must, however, ex- press my own conviction that though the time-errors will be larger in 1874 they will not be proportionately increased. The slowness of ingress and egress will give the observer an opportunity of observing the features of ingress and egress more satisfactorily. Especially if my suggestion is attended to as regards the breadth of the ligament at the moment of breaking or forming (see p. 63) will this be the case. Apart from this, . as all the time differences are increased precisely in proportion to the slowness of ingress or egress, a very slanting passage is cteteris paribus as favourable as a more direct one. THE APPROACHING TRANSITS. 499 true apparent positions of Venus as seen from the two stations — a, b, and c (the centre of the Sun's disc), lying on, or nearly on, a line. Then it is clear that a small error of bearing will produce no appreciable effect in increasing or diminishing the parallactic displacement separating a and b. Hence, since distances are much more easily determined than bearings in observations such as are here considered, it is obvious that if we select pairs of stations where the paral- lactic displacement of Venus will be along a radius of the Sun's disc, the resulting estimate of the distance of Venus is much more likely to be correct than in any other possible case. Now, if we inquire what are the conditions to be fulfilled for this purpose, we shall find it exceedingly easy to obtain a geometrical means of determining what the answer should be. Lines drawn to Venus from two stations on the Earth meet the Sun's disc on a radius, if they are to fulfil the required conditions. In other words, these lines are in a plane which passes through the Sun's centre, and of course they are in a plane through Venus's centre, since each passes through that centre. Hence they are in a plane through the axis of the cones shown in fig. 101 ; for this axis passes through the cen- tres of Venus' and the Sun. But a plane through the axis of these cones must intersect such a section as v v' in figs. 101 and 102 in a radial line. Hence the two terrestrial stations must at the moment of observation be on a line through o in fig. 102. Now, the slanting lines over Plate VIII. are such radial lines ; and therefore any two stations which in any figure of this plate lie along one of the slanting lines fulfil the required conditions. They must, however, be as far apart as possible. For example, taking the fifth picture of the Earth in Plate VIII. , we see that Japanese stations and a station near Crozet Island could be very effectively combined at the hour named under this particular presentation of the Earth. Of course the best of all such opportunities would arise when the two' stations lay near opposite extremities of one of the cross-lines traversing the middle of the Earth's disc. 500 THE SUN. The various stations which fulfil the required conditions, can be very readily determined from a study of Plate VIII. - The application of photography, as proposed by Mr. De La Rue and Lieut.-Col. Tennant, would be greatly facilitated by the choice of stations on the principle here enunciated. (See, further, my paper in the ' Monthly Notices of the Kdyal Astro- nomical Society,' No. 3, vol. xxx.) The following tables, which I have calculated with great care, supplement this essay, and serve to exhibit the qualities of the principal stations suited for observing the transit of 1874, either as respects de Lisle's or Halley's method : — TRANSIT OF 1874. Table I. — Places where Ingress is accelerated. Table II.- -Places where Ingress is. retarded. Sun's Accele- ration Sun's Retarda- Station eleva- Station eleva- tion in tion minutes tion minutes deg. m. deg. m. i Woahoo 19-8 11-2 Crozet Island 150 12-6 Hawaii . 19-7 111 Enderby Land 200 11-8 Aitou Id., Aleutian 10-8 10-3 Kerguelen Land . 27-5 11-6 Marquesas Island . 177 7-9 Macdonald Island . 310 11-2 Mouth of Amoor K. 140 7-6 Kemp Island 30.0 11-1 Jeddo . 321 68 Bourbon Island 12-4 11-1 Otaheite . . 297 6-4 Mauritius . 14-1 10-7 Nertchinsk . 10-1 5-8 Amsterdam Island. 34.1 10-3 Tsitsikar 17-0 5-8 Rodriguez 190 9-9 Kirin-Onla . 195 5-7 Sabrina Land 45-0 8-2 Nagasaki 327 5-3 Adelie Land . 45-0 68, Tientsin < 22-2 50 South Victoria Ld. 38-5 6-0 Pekin . 20-8 4-3 Perth (Australia) . 65-0 53 Shanghai 285 3-9 Royal Co. Island . 620 4:5 Nankin , 27-1 36 Madras . 21-0 40 Canton . 355 1-6 Bombay 12-5 38 Hongkong 362 1-6 Macquarie Land . 520 3-5 Hobart Town . 67-0 2-8 Adelaide 75-0 2-5 Melbourne 75-0 2-2 THE APPROACHING TRANSITS. 501 TRANSIT OF 1874. Table III. — Races where Egress is accelerated: Table IV.; — Places where Egress is retarded. Sun's Accele- ration SunV Eetarda- Station eleva- Station eleva- tion in tion. minutes tion. minutes deg. m. deg. m. South Victoria Land (Possession Ilnd.) . 250 11-4 Orsk . 12.5 11-8 Adelie Land . 34-0 10-6 Omsk . 11-5 11-7 Campbell Island 26-0 103 Astracan 120 11-6 Emerald . 300 10-3 Aleppo 14 6 10-5 Macquarie Island . 320 98 Peshawur 31-5 10-3 Chatham Island 160 9-8 Alexandria 14-0 100 Canterbury (N.Z.) . 22-5 9-3 Suez 16-1 98 Wellington 20-0 9-2 Nertchinsk . 10-1 9-8 Sabrina Land . 430 92 Delhi . 38-0 9-4 Enderby Land 39-0 85 Tsitsikar 15-0 87 Royal Co. Island 42-0 8o Bombay 45-0 8-5 Auckland 192 85 Pekin . 21-0 86 Kemp Island . 51-0 7-6 Kirin-Oula 14-0 8-4 ' Hobart Town . 40-0 7-6 Tientsin 17-1 8-4 Melbourne 43-0 66 Calcutta 45-3 8-2 Sydney . 37-2 66 Aden . 300 7-8 Adelaide 47-8 58 Nankin 27-0 7-6 Kerguelen Land 57-1 5 Madras 52-0 7-4 Crozet Island . 47'5 42 Shanghai 26-0 7-2 Perth (Australia) . 66-2 3-6 Canton 37-0 66 Hongkong . 37-0 6-5 502 THE SUN. TRANSIT OP 1874. Table V.- the actual difference of duration when comparison made between different stations. Stations where the Duration is Shortened. cS tj •o Stations where IH if A v. S3 Id > & 1 o w < a Ik 3 £ 2 a ■4 Differ. Differ. Differ, Differ. Differ. Differ. Differ. Differ. Differ. m. m. m. in. m. m. m. m. m. Nertehinsk 33-0 33-0 33-0 28-9 28-6 26-0 24-5 24-4 23-9 Tsitsikar . 31-7 31-7 31-7 27-6 27-3 24-7 23-2 23-1 22-6 Kirin-Oula 31-5 31-5 31-5 271 26-8 24-5 23-0 22-9 22-4 Tientsin . 30'8 30-8 30-8 26-7 264 23-8 22-3 22-2 21-7 Pekin . . 30-3 30-3 30-3 26-2 25-9 23-3 21-8 21-7 21-2 Nankin 28-6 28-6 28-6 24-5 24-2 21-6 20-1 20-0 19-5 Shanghai . 28-5 28-5 - 28-5 24-4 24.1 21-5 20-0 19-9 19-4 Canton 25-6 25-6 25-6 21-0 21-2 18-6 17-1 17-u 16-5 Hongkong . 25-5 25-5 25-5 21-4 21-3 18-5 17-0 16-9 16-4 THE APPROACHING TRANSITS. 503 Table VI. Principal Elements of the Sun. Equatorial horizontal parallax at mean distance from theEarth 8"-9 Diameter in miles 850 000 Diameter (Earth's as 1) 'l08 Volume (Earth's as 1) 1,260,000 Mass (Earth's as 1) 318,000 Density (Earth's as 1) 0-250 Density (Water's as 1) 1-42 Surface (Earth's as 1) , 11,650 Gravity at surface (Earth's as 1) 27-1 Fall of bodies in feet in one second 436'3 „ „ miles „ 0-085 Greatest apparent diameter viewed from the Earth . . 32' 36"-41 Mean „ „ . 32' 3"-64 Least „ „ . 31' 31"79 Linear value of 1" at mean distance from Earth . . . 440 miles. The elements of the Sun's rotation are given in Chapter IV., pp. 211, 212. Table VII. For determining the effect of changes in the value of the Sun's equatorial horizontal parallax (at his mean distance) on the estimated mean distance. Parallax (in seconds) Mean Distance (in miles) Difference for 0"-01 (in miles) 8-0 8-1 8-2 8-3 84 8-5 86 8-7 8-8 89 90 9-1 92 9-3 9-4 95 9-6 9-7 9-8 9-9 10-0 102,173,020 100,911,600 99,680,990 98,480,020 97,307,620 96,162,840 95,044,640 93,952,200 92,884,540 91,840,920 90,820,470 89,822.420 88,846,100 87,890,780 86,955,740 86,040,440 85,144,200 84,266,400 83,406,540 82,564,060 81,738,420 126,142 123,061 120,097 117,240 114,478 111,820 109,244 106,766 104,362 102,045 99,805 97,632 95,532 93,504 91,530 89,624 87,780 85,986 84,248 82,564 Spottistcoode & Co., Printers, New-street Square and Parliament Street.