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diff --git a/old/63615-0.txt b/old/63615-0.txt deleted file mode 100644 index d08fe41..0000000 --- a/old/63615-0.txt +++ /dev/null @@ -1,5857 +0,0 @@ -Project Gutenberg's A Century's Progress in Astronomy, by Hector MacPherson - -This eBook is for the use of anyone anywhere in the United States and most -other parts of the world at no cost and with almost no restrictions -whatsoever. You may copy it, give it away or re-use it under the terms of -the Project Gutenberg License included with this eBook or online at -www.gutenberg.org. If you are not located in the United States, you'll have -to check the laws of the country where you are located before using this ebook. - -Title: A Century's Progress in Astronomy - -Author: Hector MacPherson - -Release Date: November 3, 2020 [EBook #63615] - -Language: English - -Character set encoding: UTF-8 - -*** START OF THIS PROJECT GUTENBERG EBOOK A CENTURY'S PROGRESS IN ASTRONOMY *** - - - - -Produced by Charlene Taylor, Stephen Hutcheson, and the -Online Distributed Proofreading Team at https://www.pgdp.net -(This file was produced from images generously made -available by The Internet Archive/American Libraries.) - - - - - - - - - - A Century’s Progress - IN - Astronomy - - - BY - HECTOR MACPHERSON, Jun. - MEMBER OF THE SOCIÉTÉ ASTRONOMIQUE DE FRANCE; - MEMBER OF THE SOCIÉTÉ BELGE D’ASTRONOMIE; - AUTHOR OF ‘ASTRONOMERS OF TO-DAY’ - - - WILLIAM BLACKWOOD AND SONS - EDINBURGH AND LONDON - MCMVI - _All Rights reserved_ - - - - - PREFACE. - - -The present volume originated in a desire to present, in small compass, -a record of the marvellous progress in astronomy during the past hundred -years. Indebtedness should be acknowledged to the valuable works of -Professor Newcomb, Professor Schiaparelli, Professor Lowell, Professor -Young, Sir Robert Ball, Mr Gore, M. Flammarion, and Miss Clerke, who, as -the historian of modern astronomy, occupies a place at once -authoritative and unique. - -Portions of Chapters II. and XII. have already appeared in the form of -an article on the Construction of the Heavens, contributed by the writer -to the American periodical, ‘Popular Astronomy.’ - - Balerno, Mid-Lothian, - _October 1906_. - - - - - CONTENTS. - - - PAGE - CHAPTER I. 1 - HERSCHEL THE PIONEER. - Influence of Herschel’s work—His characteristics—Birth and - early years—Emigration to England—Caroline - Herschel—Discovery of Uranus—King’s Astronomer—Latter - years and death—Death of Caroline Herschel - CHAPTER II. 15 - HERSCHEL THE DISCOVERER. - Solar researches—Study of Venus—Of Mars—The - Asteroids—Jupiter—Saturn—Discovery of satellites—Uranian - satellites—Cometary researches—Motion of the Solar - System—Discovery of binary stars—Clusters and - nebulæ—Nebulous stars—The Nebular - Hypothesis—Star-gauging—The disc-theory—Subordinate - clusters—Abandonment of the disc-theory—Second method of - star-gauging—Estimate of Herschel’s work - CHAPTER III. 43 - THE SUN. - Schwabe and the sun-spot period—Researches of Wolf, Lamont, - Sabine, Gautier—Observations of Carrington and - Spörer—Career and work of Fraunhofer—Spectrum - analysis—Work of Kirchhoff—Solar eclipse work—The Solar - prominences—Janssen and Lockyer—Huggins and Zöllner—Work - of Young—The Italian spectroscopists, Secchi, Respighi, - Tacchini—Career of Tacchini—The reversing layer—The - Corona—Doppler’s principle—Rotation of the Sun—Work of - Dunér—Janssen’s solar atlas—Maunder and magnetism—Solar - theories—Distance of the Sun—Summary - CHAPTER IV. 65 - THE MOON. - Life and work of Schröter—Of Mädler—Of Schmidt—Changes on the - Moon—Selenography in England—Lunar atmosphere—Lunar - photography—Work of W. H. Pickering—The new - Selenography—The Moon’s heat—Motion of the - Moon—Acceleration of the Moon’s mean motion—Work of - Laplace, Adams, Delaunay - CHAPTER V. 80 - THE INNER PLANETS. - The problem of Vulcan—Mercury—Work of Schröter—Schiaparelli, - his life and work—Work of Lowell—Spectrum of - Mercury—Venus—Rotation period: work of Schröter, Di Vico, - Schiaparelli, Tacchini, Lowell—Atmosphere and surface of - Venus—The Earth: variation of latitude—Mars—Rotation of - Mars—Surface—Discovery of canals—Work of Schiaparelli and - Lowell—Interpretation of the canals—The theory of - intelligent life—Spectrum of Mars—Satellites—The - Asteroids—Bode’s law—Work of Piazzi and Olbers—Application - of photography by Wolf—Discovery of Eros - CHAPTER VI. 103 - THE OUTER PLANETS. - Physical condition of Jupiter—Work of Zöllner and Proctor—The - red spot—Satellites—Discovery of fifth satellite—Sixth and - seventh satellites—Rings of Saturn: Bond, Maxwell, - Keeler—Struve’s theory—Globe of Saturn—New - satellites—Uranus and its satellites—Discovery of - Neptune—Adams and Le Verrier—Satellite—Trans-Neptunian - planets - CHAPTER VII. 123 - COMETS. - Life and work of Olbers—His repulsion theory—Life and work of - Encke—His comet—Biela’s comet—Faye’s comet—Return of - Halley’s comet—Donati’s comet—Comet of 1861—Spectroscopic - study of comets—Theory of Brédikhine—Spectra of - comets—Comets of 1880 and 1882—The capture theory—Cometary - photography - CHAPTER VIII. 138 - METEORS. - Meteoric shower of 1833—Work of Olmsted—Work of Erman and - Kirkwood—Of H. A. Newton—Adams and the meteoric - orbit—Shower of 1866—Connection of comets and meteors—Work - of Schiaparelli—Shower of meteors in 1872—‘Le Stelle - Cadenti’—Meteoric observation—A. S. Herschel—Work of - Denning—Stationary radiants—Bolides and aerolites—Origin - of aerolites - CHAPTER IX. 150 - THE STARS. - Distance of the stars—Life and work of Bessel—Studies of - Struve—Life and work of Henderson—Work of Peters, Otto - Struve, Brünnow, and Ball—Measures of Gill—Parallax of - first-magnitude stars—Relative and absolute parallax—Work - of Kapteyn—Application of - photography—Star-catalogues—Argelander’s - ‘Durchmusterung’—Work of Schönfeld—Work of Gould—The ‘Cape - Photographic Durchmusterung’—Work of Gill and - Kapteyn—International chart of the heavens—Work of - Peck—Proper motions of the stars—Star-drift—Discoveries of - Proctor and Flammarion—Radial motion—Work of Huggins, - Vogel, and Campbell—Solar motion - CHAPTER X. 169 - THE LIGHT OF THE STARS. - Work of Fraunhofer and Donati—Life and work of Secchi—His - types of spectra—Life and work of Huggins—Photography of - spectra—Life and work of Vogel—His classification of - spectra—Work of Dunér—Of Pickering—Spectroscopic - catalogues—Analysis of spectra—Stellar photometry—Life and - work of E. C. Pickering—Variable stars—Work of - Goodricke—Of Argelander, Schmidt, Heis, Schönfeld—Studies - of Dunér—Of Gore—Photographic - discoveries—Classification—Algol variables: their - explanation—Explanation of other variables—η - Argus—Temporary stars—Of 1848—Of 1866—Of 1876—Of 1885—Of - 1892—Photographic discoveries—Nova Persei, 1901—New star - of 1903—Theories of temporary stars - CHAPTER XI. 197 - STELLAR SYSTEMS AND NEBULÆ. - Life and work of John Herschel—Binary stars—Computation of - orbits—Work of Wilhelm Struve—Of Otto Struve—Of - Burnham—Satellites of Sirius and Procyon—Astronomy of the - invisible—Work of Pickering and Vogel—Spectroscopic - binaries—Work of Bélopolsky and - Campbell—Star-clusters—Nature of nebulæ—Spectroscopic work - of Huggins—Of Copeland—Nebular photography—Work of - Roberts, Barnard, Wolf—Of Keeler - CHAPTER XII. 214 - STELLAR DISTRIBUTION AND THE STRUCTURE OF THE UNIVERSE. - Work of John Herschel—Researches of Wilhelm Struve—Extinction - of light—Mädler’s “central sun”—Distribution of - nebulæ—Work of Proctor—Aggregation of stars on the - Galaxy—Work of Gore and Schiaparelli—Studies of - Gould—Researches of Kapteyn—Of Newcomb—Is the Universe - limited? Newcomb’s argument—Observations of - Celoria—Researches of Seeliger—External Universes—Gore’s - speculations - CHAPTER XIII. 227 - CELESTIAL EVOLUTION. - Laplace’s nebular hypothesis—Helmholtz and solar - contraction—Theories of solar heat—Objections to Laplace’s - theory—Faye’s hypothesis—Ball’s exposition—The meteoritic - theory of Proctor—Its extension by Lockyer—Evolution of - the stars—Vogel’s order of evolution—Tidal friction: work - of Darwin—See’s explanation of double stars—Future of the - Universe - - - - - A CENTURY’S PROGRESS IN ASTRONOMY. - - - - - CHAPTER I. - HERSCHEL THE PIONEER. - - -In astronomy, as in other sciences, the past hundred years has been a -period of unparalleled progress. New methods have been devised, fresh -discoveries have been made, new theories have been propounded; the field -of work has widened enormously. In fact, the science of the heavens has -become not only boundless in its possibilities, but more awe-inspiring -and marvellous. - -To whom in the main is this great advance due? To the great pioneer of -what may be called modern astronomy—William Herschel. Not only did -Herschel reconstruct the science and widen its bounds, but his powerful -genius directed the course of nineteenth century research. As an -astronomical observer he has never been surpassed. In the breadth of his -views he was equalled only by Newton; and indeed he excelled Newton in -his unwearied observations and his sweeping conceptions of the Universe. -To quote his own remark to the poet Campbell, he “looked farther into -space than ever human being did before him.” - -Herschel studied astronomy in all its aspects. In all the branches of -modern astronomy he was a pioneer. He observed the Sun, Moon, and -planets, devoting special attention to Mars and Saturn. He doubled the -diameter of the Solar System by the discovery of Uranus. He discovered -several satellites and studied comets. He was pre-eminently the founder -of sidereal astronomy. He discovered binary stars, thus tracing the law -of gravitation in the distant star-depths; while to him is due the -credit of the discovery of the motion of the Solar System. He founded -the study of star-clusters and nebulæ, propounded the nebular -hypothesis, and devised two methods of star-gauging. Above all, he was -the first to attempt the solution of one of the noblest problems ever -attacked by man—the structure of the Universe. In fact, the latter -problem was the end and aim of his observations. As Miss Clerke remarks, -“The magnificence of the idea, which was rooted in his mind from the -start, places him apart from and above all preceding observers.” Most of -the departments of modern astronomy find a meeting-place in Herschel, as -the branches run to the root of the tree. He discussed astronomy from -every point of view. Before, however, proceeding to examine the work of -this great man, it is well to note a few of his characteristics. These -characteristics, once understood, give us the key to his researches. -Before we can master Herschel the astronomer we must understand Herschel -the man. - -Notwithstanding the fact that Herschel spent most of his life in -England, and that he is included in the ‘Dictionary of National -Biography,’ he was pre-eminently a German. Like most Germans his style -of writing was somewhat obscure, and this was emphasised when he wrote -in English, owing to his imperfect command of the language. Had he -written in German as well as in English, he would probably have been -better understood in his native country, where erroneous views of his -theories were long entertained. Even so distinguished an astronomer as -Wilhelm Struve, when translating Herschel’s papers into German, made a -mistake when translating a certain passage, which leaves the erroneous -impression that Herschel believed the Universe to be infinite—a mistake -which would not have arisen had he written in German. - -The student of Herschel should also be careful in quoting the views of -the great astronomer. Had Herschel at the close of his life written a -volume containing his final views on the construction of the heavens, -this would not have been necessary; but Herschel did not write such a -volume. His researches were embodied in a series of papers communicated -to the Royal Society from 1780 to 1818. As he observed the heavens his -opinions progressed, so that a statement of his views at any given time -was by no means a statement of his final opinions. The late R. A. -Proctor, who was the first great exponent of Herschel in England, has -well said: “It seems to have been supposed that his papers could be -treated as we might treat such a work as Sir J. Herschel’s ‘Outlines of -Astronomy’; that extracts might be made from any part of any paper -without reference to the position which the paper chanced to occupy in -the entire series.” - -Herschel, like the true student of nature, held theories very lightly. -They were to him but roads to the truth. Unlike many scientists, he did -not interpret observations by hypothesis: he framed his theories to fit -his observations. If he found that a certain theory did not agree with -what he actually saw in the heavens, he abandoned it: he did not -hesitate to change his views as his investigations proceeded. “No fear -of ‘committing himself,’” says Miss Clerke in her admirable work on ‘The -Herschels,’ “deterred him from imparting the thoughts that accompanied -his multitudinous observations. He felt committed to nothing but truth.” - -In the mind of Herschel imagination and observation were marvellously -blended. He was a philosophical astronomer. Although his imagination was -a very vivid one he did not allow his fancies to run away with him, as -Kepler sometimes did: on the other hand, he did not, like Flamsteed, -refrain from speculating altogether. “We ought,” he wrote in 1785, “to -avoid two opposite extremes. If we indulge a fanciful imagination, and -build worlds of our own, we must not wonder at our going wide from the -path of truth and nature. On the other hand, if we add observation to -observation, without attempting to draw not only certain conclusions but -also conjectural views from them, we offend against the very end for -which only observations ought to be made.” - -These characteristics—the lightness with which he held his theories, his -vivid imagination, and his philosophical reasoning—are the secrets of -Herschel’s success as an astronomer. Nearly all his ideas and -speculations have been confirmed. As Arago has said, “We cannot but feel -a deep reverence for that powerful genius that has scarcely ever erred.” -Herschel, like all other great students of Nature, was deeply religious. -He could not observe the heavens without feeling awed at the marvels -which his telescopes revealed. In his own words, “It is surely a very -laudable thing to receive instruction from the Great Workmaster of -Nature.” - -Friedrich Wilhelm Herschel, born in Hanover on November 15, 1738, was -the fourth child of Isaac Herschel, an oboist in the band of the -Hanoverian Guard. Isaac Herschel, a native of Dresden, was an -accomplished musician, and all his children, ten in number, inherited -his talent. Of these ten, six survived, and only two became famous. -These were William, the great astronomer, and his sister Caroline (born -on March 16, 1750), who became a student of the heavens only second to -her brother. - -At the garrison school in Hanover, where the Herschels were educated, -William Herschel showed intense love and aptitude for learning, and was -more diligent and persevering than his brother Jacob, his senior by four -years. In 1753 he became oboist in the band of the Hanoverian Guard in -which his father was now bandmaster. In her valuable memoirs, his sister -relates that her father was very interested in astronomy, and that he -taught his children the names of the constellations. William became -devoted to the science, and constructed a small celestial globe on which -equator and ecliptic were engraved. But his studies were much hampered. -His mother had a great dislike to learning: she had no sympathy with -aspirations, and tried to prevent her children becoming well educated. -Above all, the Hanoverian Guard was ordered to England in 1755, when a -French invasion was feared, and to that country Herschel proceeded, -along with his father and brother. - -Returning to Germany in 1756, the Hanoverian Guard was employed the -following year in the Seven Years’ War. Hanover was invaded by the -French, and, conscription being the rule, the musicians were not -exempted from service. Under the command of the Duke of Cumberland the -Guard suffered a terrible defeat at Hastenbeck. William Herschel spent -the night after the battle in a ditch, and decided that soldiering would -not be his profession. He deserted, and, with the consent of his -parents, he sailed for England. After his arrival at Dover, he wandered -through the country in search of musical employment. At length, in 1760, -he was appointed to train the band of the Durham Militia, and four years -later paid a secret visit to Hanover, where he was welcomed by his -father, whose health was now failing, and by his sister Caroline. In the -following year he was promoted to the post of organist at Halifax, and -in 1766 he removed to Bath as oboist in Linley’s Orchestra. Finally, in -1767, he became organist in the new Octagon Chapel at Bath. Herschel was -now twenty-nine years old, and known as a famous musician. As Miss -Clerke remarks: “The Octagon Chapel soon became a centre of fashionable -attraction, and he soon found himself lifted on the wave of public -favour. Pupils of high rank thronged to him, and his lessons often -mounted to thirty-five a-week.” - -In the year of his appointment his father died, aged sixty, after a life -of trouble and hardship. His death was a great blow to his daughter -Caroline, whom he had educated when her mother was from home. Caroline -Herschel was naturally possessed of musical ability, but her mother and -elder brother had determined that she should be a housemaid,—a -determination which William, who was devotedly attached to his sister, -opposed. Finally, in 1772, he visited Hanover, and took his sister to -England with him to act as his housekeeper. But for her unwearied -devotion it is doubtful whether William Herschel would have become the -great astronomer. - -About the time of his appointment in Bath Herschel commenced the study -of languages and mathematics, reading Maclaurin’s ‘Fluxions’ and -Ferguson’s ‘Astronomy.’ The perusal of the latter volume revived his -love for astronomy. After fourteen or sixteen hours’ teaching he would -retire to his bedroom and read of the wonders of the heavens. His -interest increased as he proceeded, until, in his own words, “I resolved -to take nothing upon trust, but to see with my own eyes all that other -men had seen before me.” Accordingly he hired a small reflector. -Inquiring the price of a larger instrument, he found it to be quite -beyond his means. Then in 1772, when his sister came to keep his house -for him, he resolved to make his own telescope. First he tried the -fitting of lenses into pasteboard tubes, but this being a total failure, -he bought the apparatus of a Quaker optician who had constructed, or -attempted to construct, reflecting telescopes. In June 1773, assisted by -his sister and by his brother Alexander, then in Bath, he commenced -work. His first speculum mirror was five inches in diameter; and, while -it was in process of construction, he was obliged to hold his hands on -it for sixteen hours at a stretch, while his sister supplied his food -and read ‘The Arabian Nights,’ ‘Don Quixote,’ and other tales aloud to -him to pass the time. At last, after two hundred failures, he finished a -5-inch reflector, and on March 4, 1774, he observed the Orion nebula. No -sooner had Herschel commenced his celestial explorations than he -resolved to survey the entire heavens, leaving no spot unvisited. - -In 1775 he commenced his review of the heavens, but finding his -telescope inadequate he began the work of telescope-making afresh. -Meanwhile he had much to distract him from astronomy. In 1776 he became -director of the Public Concerts at Bath. Yet his enthusiasm was -unbounded: he would run to his house between the acts at the theatre to -observe the heavens. In 1779, when observing the Moon from the street in -front of his house, a gentleman asked permission to see the celestial -wonders, a request which Herschel granted. The gentleman, Dr Watson of -Bath, introduced Herschel to the Literary Society, and we find him in -1780 contributing two papers to the Royal Society on Mira Ceti and the -Moon. In the same year he commenced his second review of the heavens, -and during its progress he made his first great discovery. On March 13, -1781, while surveying the constellation Gemini, he discovered a faint -object distinguished by a disc, which he concluded to be a tailless -comet, but which was soon shown to be a new planet beyond the orbit of -Saturn. This was the first planetary discovery made within the memory of -man. King George III. summoned Herschel to London, and gave him a -pension of £200 a-year, with the title of King’s Astronomer, pardoning -him also for his desertion from the army more than twenty years -previously. Herschel then named the new planet the “Georgium Sidus,” a -title now abandoned and replaced by Uranus. - -William and Caroline Herschel now moved to Datchet, near Windsor, in -1785 to Clay Hall, and finally, in 1786, to Slough,—“the spot of all the -world,” said Arago, “where the greatest number of discoveries have been -made.” Here Herschel and his sister worked for nearly forty years. He -communicated to the Royal Society paper after paper on astronomy in all -its aspects. He also continued the work of telescope-making, and -constructed, in 1789, his 40-foot reflector, the wonder of the age. In -1787 his sister was appointed his assistant, and together the Herschels -worked from dusk to dawn. Caroline Herschel herself detected eight -comets and numerous nebulæ. She relates in her memoirs that on one -occasion, while she was acting as assistant, the ink froze in her pen. -But such inconveniences mattered not to the Herschels. As Miss Clerke -has well remarked, “Every serene dark night was to him a precious -opportunity, availed of to the last minute. The thermometer might -descend below zero, ink might freeze, mirrors might crack; but, provided -the stars shone, he and his sister worked on from dusk to dawn.... On -one occasion he is said to have worked without intermission at the -telescope and the desk for seventy-two hours.” - -Honours were showered on Herschel. He was knighted in 1816, and became -President of the Royal Astronomical Society in 1820, besides receiving -several honorary degrees. But honours in no way elated him. Advancing -years in no way affected his wonderful mind. But his duties as King’s -Astronomer necessitated his acting as “showman of the heavens” on the -visits of royalties to Windsor, often after a whole day’s work, when -rest was absolutely necessary. This tremendous strain, which reflects -little credit on the Court, proved too much for the old man. His health -began to give way, although his mind was as vigorous as ever. - -Herschel contributed his last paper to the Royal Society in 1818, and -three years later sent a list of double stars to the new Astronomical -Society. He made his last observation on June 1, 1821. His strength had -now left him, and to this he could not reconcile himself. As Miss Clerke -puts it, “All his old instincts were still alive, only the bodily power -to carry out their behests was gone. An unparalleled career of -achievement left him unsatisfied with what he had done.... His strong -nerves were at last shattered.” After a prolonged period of failing -health he died at Slough, at the age of eighty-three, on August 25, -1822. On September 7 he was buried in the church-yard of St Laurence at -Upton. On his tombstone are engraved the words—“Cœlorum perrupit -claustra”—he broke through the barriers of the skies. - -The death of her brother was a terrible blow to Caroline Herschel. -Expecting to live only a twelvemonth, she returned to Hanover to the -home of her brother, Dietrich Herschel. But she lived twenty-five years -among people who cared nothing for astronomy. She was delighted at Sir -John Herschel’s continuation of his father’s work. She compiled a -catalogue of all the clusters and nebulæ observed by her brother, for -which she received the gold medal of the Astronomical Society, and she -was created an honorary member. In 1846 she received from the King of -Prussia the gold medal of science. But no honours made her in any way -elated. She always held that whoever said much of her said too little of -her brother. After a prolonged decline of health, she died on January 9, -1848, aged ninety-seven years, and was buried beside her father in the -churchyard of the Gartengemeinde at Hanover, leaving behind her a noble -example of self-sacrifice and devotion. - - - - - CHAPTER II. - HERSCHEL THE DISCOVERER. - - -One result of Herschel’s discoveries among the stars and nebulæ is that -his studies of the Sun and planets, with the exception of the discovery -of Uranus, have been completely thrown into the shade. Nevertheless, his -work in solar and planetary astronomy alone would have gained for him a -higher position in astronomy than his contemporaries. The planets, -satellites, and comets were all attentively studied by the great -astronomer; indeed, the scientific investigation of the surfaces of Mars -and Saturn began with Herschel. - -“His attention to the Sun,” Miss Clerke truly remarks, “might have been -exclusive, so diligent was his scrutiny of its shining surface.” -Sunspots were specially investigated by Herschel, who closely studied -their peculiarities, regarding them as depressions in the solar -atmosphere. He also paid much attention to the faculæ, but could not -observe them to the north and south of the Sun, thus proving their -connection with the spots which are confined to the regions north and -south of the equator. “There is all over the Sun a great unevenness,” -said Herschel, “which has the appearance of a mixture of small points of -an unequal light; but they are evidently a roughness of high and low -parts.” - -Herschel’s solar observations were very valuable, and did much for our -knowledge of the orb of day. His theory of the Sun’s constitution—a -development of the hypothesis put forward by _Alexander Wilson_ -(1714-1786), Professor of Astronomy in Glasgow—was, however, very far -from the truth. This was almost the only instance in which Herschel was -mistaken. He regarded the Sun as a cool, dark globe, “a very eminent, -large, and lucid planet, evidently the first, or, in strictness of -speaking, the only primary one of our system.” In his opinion an -extensive atmosphere surrounded the Sun, the upper stratum forming what -Schröter named the “photosphere.” This atmosphere, estimated as two or -three thousand miles in depth, was regarded as giving out light and -heat. Below this shining atmosphere there existed, Herschel believed, a -region of clouds protecting the globe of the Sun from the glowing -atmosphere, and reflecting much of the light intercepted by them. The -spots were believed to be openings in these atmospheres, caused by the -action of winds, the umbra or dark portion of the spot thus representing -the globe of the Sun, which Herschel believed to be “richly stored with -inhabitants.” This theory held its ground for many years. Newton, it is -true, believed the Sun to be gaseous, but he propounded no hypothesis of -its constitution. Herschel’s theory, on the other hand, was fully -developed, plausible, and attractive. It was held by eminent men of -science until 1860, when the revelations of the spectroscope showed it -to be quite untenable. The theory was supported for many years by Sir -John Herschel, who, however, abandoned it in 1864. Herschel made several -attempts to ascertain whether any connection existed between the state -of the Sun and the condition of the Earth. In 1801 he was inclined to -believe that “some temporary defect of vegetation” resulted from the -absence of sun-spots, which, he thought, “may lead us to expect a -copious emission of heat, and, therefore, mild seasons.” Herschel -believed, in fact, that food became dear at the times of spot-minima. It -may be remarked that Herschel never noted the spot-period of eleven -years, the discovery of which was afterwards made by Schwabe. - -Herschel closely scrutinised the surfaces of the planets. Mercury alone -was neglected by him. From 1777 to 1793 he observed Venus, with the -object of determining the rotation period, but he was unable to observe -any markings on the surface of the planet. He did not place reliance on -Schröter’s value of the rotation period (about twenty-three hours). -Meanwhile, Schröter announced the existence on Venus of mountains which -rose to five or six times the height of Chimborazo. As to these, said -Herschel, “I may venture to say that no eye which is not considerably -better than mine, or assisted by much better instruments, will ever get -a sight of them.” Herschel demonstrated the existence of an extensive -atmosphere round Venus. - -“The analogy between Mars and the Earth,” Herschel wrote in 1783, “is -perhaps by far the greatest in the whole Solar System.” In 1777 he -began, in his house at Bath, a series of observations on the red planet, -which yielded results of the utmost importance. Fixing his attention on -the white spots at the north and south poles,—discovered by Maraldi, -nephew of Cassini,—he soon ascertained the fact that they waxed and -waned in size, the north polar cap shrinking during the summer of the -northern hemisphere, increasing in winter, and _vice versa_ in the -southern hemisphere. He regarded the caps as masses of snow and ice -deposited from “a considerable, though moderate, atmosphere,” a theory -now generally accepted. Herschel gave an immense impetus to the study of -Mars. He carefully examined the planet’s surface, and the dark markings -were regarded by him as oceans. - -During Herschel’s lifetime the four small planets, Ceres, Pallas, Juno, -and Vesta, were discovered by Piazzi, Olbers, and Harding. The great -astronomer was much interested in these small worlds. He commenced a -search through the Zodiacal constellations for new planets, but failed. -He was of opinion that many minor planets would be discovered. Accepting -Olbers’ theory of the disruption of a primitive planet, Herschel -calculated that Mercury might be broken up into 35,000 globes equal to -Pallas. Meanwhile Herschel named the four new planets “Asteroids,” owing -to their minute size. He estimated the diameter of Ceres at 162 miles -and Pallas at 147 miles, but Professor Barnard’s measures have shown -them to be larger. - -In connection with the discovery of the Asteroids, Herschel showed a -very fine spirit. In ‘The Edinburgh Review’ Brougham declared that -Herschel had devised the word “asteroid,” so that the discoveries of -Piazzi and Olbers might be kept on a lower level than his own discovery -of Uranus. Many scientists would have been much offended at this -contemptible insult, but Herschel merely remarked that he had incurred -“the illiberal criticism of ‘The Edinburgh Review,’” and that the -discovery of the Asteroids “added more to the ornament of our system -than the discovery of another planet could have done.” - -In Herschel’s time astronomers were acquainted with three of the outer -planets,—Jupiter, Saturn, and Uranus,—all of which were closely studied -by the great astronomer. The belts of Jupiter were supposed by him to be -analogous to the “trade-winds” in the atmosphere of the Earth; while the -drifting-spots on Jupiter’s disc and their irregular movements were -carefully noted. His observations on the four satellites of Jupiter led -him to believe that, like our Moon, they rotated on their axes in a -period equal to that of their revolution round their primary—an opinion -shared by Laplace, and by many modern astronomers. - -Herschel’s researches regarding Saturn were, however, much more -important than those on Jupiter. The globe of the planet, the rings and -the satellites, were favourite objects of study at Bath and Slough. In -1794 he perceived a spot on the surface of Saturn, and made the first -determination of the rotation of the planet, which he fixed as 10 hours -16 minutes,—a result confirmed by modern astronomers. The rings were -subjected to the closest scrutiny. Herschel believed them to be solid, -and he also considered them to revolve round Saturn in about 10 hours. -It appears that he observed the famous “dusky ring,” but supposed it to -be a belt on the surface of the planet. He also studied Cassini’s -division in the ring, ascertaining its reality. - -On completing his famous 40-foot reflector, Herschel, on August 28, -1789, turned it on Saturn and its five known satellites. Near the -planet, and in the plane of the ring, was seen another object, which -Herschel believed to be a sixth satellite. To settle the question, he -watched the planet for several hours to see if the object would partake -in the planet’s motion. Finding that it did, he announced it as a new -satellite, which he found to revolve round Saturn in 1 day 8 hours. -About three weeks later, on September 17, Herschel discovered another -satellite yet closer to Saturn, revolving round the planet in about 22 -hours. These two satellites were not seen by any astronomers except -Herschel; and after his death they could not be observed. His son, -however, rediscovered them. - -The eighth satellite, Japetus, was shown by Herschel to rotate on its -axis in a period equal to that of its revolution, and his observations -were confirmed by modern observers. “I cannot,” Herschel said, “help -reflecting with some pleasure on the discovery of an analogy which shows -that a certain uniform plan is carried on among the secondaries of our -Solar System; and we may conjecture that probably most of the satellites -are governed by the same law.” In April 1805 Herschel observed the globe -of Saturn to present not a spherical but a “square-shouldered” aspect. -It was for long believed that this was an optical illusion; but Proctor -and others have shown that it is quite possible for storms in Saturn’s -atmosphere to cause the planet’s apparent distortion in shape. - -Herschel paid much attention to the planet Uranus, which he discovered -on March 13, 1781. The discovery of Uranus, which was mentioned in a -previous chapter, was in a sense the most striking of Herschel’s -achievements. Uranus was the first planet discovered within the memory -of man: besides, the discovery enlarged the diameter of the Solar System -from 886 to 1772 millions of miles. Throughout his lifetime Herschel -referred to the planet as the “Georgium Sidus,” out of gratitude to -George III. for appointing him King’s Astronomer; but the astronomers of -France and Germany, who, as Sir Robert Ball remarks, “saw no reason why -the King of England should be associated with Jupiter and Saturn,” -opposed this term. Lalande called the planet “Herschel,” but Herschel’s -countrymen, the Germans, named it Uranus, in keeping with the custom of -designating the planets from the Greek mythology. The name of Uranus -ultimately prevailed. - -In January 1787 Herschel discovered two satellites to Uranus, with the -aid of his 20-foot telescope. These satellites he believed to revolve -round Uranus in 8 days and 13 days respectively, and accordingly he made -a drawing of what their positions should be on February 10. On that day -he found them in their predicted places. In 1797 he announced that the -satellites revolved round Uranus in orbits at right angles to the -ecliptic, and in a retrograde direction. In subsequent years Herschel -believed that he had discovered other four satellites to Uranus, but he -was unable to confirm his belief. As Mr Gore says, some of the -satellites “must, therefore, have been either optical ‘ghosts’ or else -small fixed stars which happened to be near the planet’s path at the -time of observation. Herschel also suspected that he could see traces of -rings round Uranus like those round Saturn, but his observation was -never confirmed, either by himself or other observers.” - -Although Herschel made several important observations on the Moon, and -measured the heights of the lunar mountains, he was not a devoted -student of our satellite. Caroline Herschel remarks in her memoirs that -if it had not been for clouds or moonlight, neither her brother nor -herself would have got any sleep; adding that Herschel on the moonlight -nights prepared his papers or made visits to London. However, he did -make some investigations, and in 1783 and 1787 believed himself to have -witnessed the eruption of three lunar volcanoes. He afterwards -concluded, however, that what he believed to be eruptions was really the -reflexion of earth-shine from the white peaks of the lunar mountains. -Herschel never discovered a comet, leaving that branch of astronomy to -his sister, who discovered eight of these objects. He was, however, much -interested in comets, and attentively studied them, introducing the -terms of “head,” “nucleus,” and “coma.” Herschel anticipated the view -that comets are not lasting, but are partly disintegrated at their -perihelion passages. He was of opinion that they travelled from star to -star. The extent of their tails and appendages he thought to be a test -of their age. - -We have now completed our sketch of Herschel’s important labours -regarding our Solar System. As Miss Clerke says, “A whole cycle of -discoveries and successful investigations began and ended with him.” But -through observing the stars he made a further discovery in connection -with the Solar System; indeed, one of the greatest discoveries in the -history of astronomy—the movement through space of the Sun, carrying -with it planets and comets. - -“If the proper motion of the stars be admitted,” said Herschel, “who can -deny that of our Sun?” Of course it was plain that the motion of the Sun -could only be detected through the resulting apparent motion of the -stars. Thus, if the Sun is moving in a certain direction, the stars in -front will appear to open out, while those behind will close up. But the -problem is by no means so easy as this. The stars are also in motion, -and, before the solar motion can be discovered, the proper motions of -the stars—themselves very minute—have to be decomposed into two parts, -the real motion of the star, and the apparent motion, resulting from the -movement of the Solar System. To any astronomer but Herschel the problem -would have been insoluble. Only sixty years had elapsed since Halley had -announced the proper motions of the brighter stars which had been -previously supposed to be immovable—hence the name of “fixed stars.” -Herschel did not deal with the motions of many stars. Only a few proper -motions were known with accuracy when he attacked the problem in 1783. -Making use of the proper motions of seven stars, and separating the real -from the apparent motion, he found that the Solar System was moving -towards a point in the constellation Hercules, the “apex” being marked -by the star λ Herculis. The rate of the solar motion, Herschel thought, -was “certainly not less than that which the Earth has in her annual -orbit.” This extraordinary discovery was one of Herschel’s greatest -works. “Its directness and apparent artlessness,” Miss Clerke remarks, -“strike us dumb with wonder.” In 1805 Herschel again attacked the -subject, utilising the proper motions of thirty-six stars. His second -inquiry, on the whole, confirmed his previous result, the “apex” being -again situated in Hercules; but the determination of 1783 was probably -the more accurate of the two. - -Herschel was far in advance of his time regarding the solar motion. The -two greatest astronomers of the next generation, Bessel and Sir John -Herschel, rejected the results reached by Sir William Herschel. But in -1837 Argelander, after a profound mathematical discussion, confirmed -Herschel’s views, and proved the solar motion to be a reality. Since -that date the problem has been attacked by various methods by Otto -Struve, Gauss, Mädler, Airy, Dunkin, Ludwig Struve, Newcomb, Kapteyn, -Campbell, and others, with the result that the reality of the solar -motion and of the direction fixed by Herschel has been proved beyond a -doubt. As Sir Robert Ball well remarks, mathematicians have exhausted -every refinement, “but only to confirm the truth of that splendid theory -which seems to have been one of the flashes of Herschel’s genius.” - -In his volume ‘Herschel and his Work,’ Mr James Sime writes: “To -Herschel belongs the credit not merely of having suspected the -revolution of sun around sun in the far-distant realms of space, but -also of actually detecting that this was going on among the stars.” -Throughout his career double stars were favourite objects of -observation. The study of double stars was commenced by Herschel while a -musician in Bath. Before his day, of course, double stars had been -discovered and studied, but it was believed that the proximity of two -stars was merely an optical accident, the brighter star being much -nearer to us than the other. Herschel, at first sharing the general -view, observed double stars in the hope of measuring their relative -parallaxes; assuming one star to be much farther away from the Solar -System than another, he attempted to measure the parallactic -displacement of the brighter star relatively to the position of the -fainter. “This,” he afterwards wrote, “introduced a new series of -observations. I resolved to examine every star in the heavens with the -utmost attention, that I might fix my observations upon those that would -best answer my end. I took some pains to find out what double stars had -been recorded by astronomers; but my situation permitted me not to -consult extensive libraries, nor, indeed, was it very material; for as I -intended to view the heavens myself, Nature, that great volume, appeared -to me to contain the best catalogue.” - -Herschel, on January 10, 1782, submitted to the Royal Society a -catalogue of 269 double stars: of these he himself discovered 227. In -December 1784 he forwarded another catalogue, containing 434 stars. He -soon found that he was unable to measure stellar parallax, and the idea -dawned on him that the double stars were physically connected by the law -of gravitation, though he made no announcement to that effect for many -years. On July 1, 1802, Herschel informed the Royal Society that “casual -situations will not account for the multiplied phenomena of double -stars.... I shall soon communicate a series of observations, proving -that many of them have already changed their situation in a progressive -course, denoting a periodical revolution round each other.” In 1803 he -showed that many stars were revolving round their centres of gravity, -proving them, in his own words, to be “intimately held together by the -bond of mutual attraction.” In other words, Herschel discovered that the -law of gravitation prevailed in the Stellar Universe, as well as in our -Solar System—that the law which Newton ascertained to prevail in the -Solar System extended throughout the depth of space. - -Herschel did not merely prove the revolution of the binary stars; he -assigned periods to those which he had particularly studied. He believed -the period of Castor to be 342 years; γ Leonis 1200 years; δ Serpentis -375 years; and ε Böotis 1681 years. Herschel did not compute the orbits -mathematically. This was not done for nearly thirty years, when the -calculation of binary star-orbits was commenced by Savary, Sir John -Herschel, and Encke. - -In 1782 the French astronomer, _Charles Messier_ (1730-1817), published -a list of 103 nebulæ. In the following year Herschel commenced his -famous sweeps of the heavens with his large reflectors, and during these -he made many remarkable discoveries. In 1786 he published in the -‘Philosophical Transactions’ of the Royal Society a catalogue of a -thousand new nebulæ and star-clusters, in which he gave the position of -each object with a short description of its appearance, written by -Caroline Herschel while her brother actually had the object before his -eyes. In 1786 Herschel published a catalogue of another thousand -clusters and nebulæ, followed in 1802 by a list of 500; making a total -of 2500 clusters and nebulæ discovered by the great astronomer. This -alone would have gained a great name for William Herschel in this branch -of astronomy. In the space of only twenty years 2500 nebulæ and clusters -had been discovered. The various nebulæ and clusters were divided into -eight classes, as follows: the first class being “bright nebulæ,” the -second “faint nebulæ,” the third “very faint nebulæ,” the fourth -“planetary nebulæ,” so named by Herschel from their resemblance to -planetary discs, the fifth class contained “very large nebulæ,” the -sixth “very compressed and rich clusters of stars,” the seventh “pretty -much compressed clusters of large or small stars,” and the eighth -“coarsely scattered clusters of stars.” - -At first Herschel believed all nebulæ to be clusters of stars, the -irresolvable nebulæ being supposed to be farther from our system than -the resolvable nebulæ. As many of the nebulæ which Messier could not -resolve had yielded to Herschel’s instruments, Herschel believed that -increase of telescopic power would resolve the hazy spots of light which -remained nebulous. In the paper of 1785, in which Herschel dealt with -the construction of the heavens, he stated his belief that many of the -nebulæ were external galaxies—universes beyond the Milky Way; and in -1786 he remarked that he had discovered fifteen hundred universes! - -Arago, Mitchel, Nichol, Chambers, and other writers quite misinterpreted -Herschel’s views on the nebulæ when they said that he believed them to -be all external galaxies. In 1785 Herschel believed many to be connected -with the sidereal system; considering that in some parts of the Galaxy -“the stars are now drawing towards various secondary centres, and will -in time separate into different clusters.” He was coming to the view -that the star-clusters were secondary aggregations within the Galaxy, -probably the true theory. He pointed out that in Scorpio, the cluster -Messier 80 is bounded by a black chasm, four degrees wide, from which he -believed the stars had been drawn in the course of time to form the -cluster. His sister records that one night, after a “long, awful -silence,” he exclaimed on coming on this chasm—“Hier ist wahrhaftig ein -Loch im Himmel!” (Here, truly, is a hole in the heavens.) - -Herschel was now gradually giving up his theory of external galaxies and -his “disc-theory” of the Universe; but he still believed even the -nebulous objects to be irresolvable only through immensity of distance. -In 1791, however, he drew attention to a remarkable star in Taurus, -surrounded by a nebulous atmosphere, regarding which he wrote, “View, -for instance, the nineteenth cluster of my sixth class, and afterwards -cast your eye on this cloudy star. Our judgment, I will venture to say, -will be that _the nebulosity about the star is not of a starry_ -_nature_. We therefore either have a central body which is not a star, -or have a star which is involved in a shining fluid, of a nature totally -unknown to us.” And with caution he added that “the envelope of a cloudy -star is more fit to produce a star by its condensation than to depend -upon the star for its existence.” - -This was written in 1791, five years before Laplace propounded his -nebular theory. Meanwhile Herschel, believing that “these nebulous stars -may serve as a clue to unravel other mysterious phenomena,” found that -the theory of a “shining fluid” would suit the appearance of the -irresolvable planetary nebulæ and the great nebula in Orion much better -than the extravagant idea of “external universes.” Herschel now -considered the Orion nebula to be much nearer to the Solar System than -he formerly did, and ceased to regard it as external to the Galaxy. For -twenty years Herschel patiently observed the nebulæ, and it was not -until 1811 that he propounded his nebular hypothesis of the evolution of -the Sun and stars. He found the gaseous matter in all stages of -condensation, from the diffused cloudy nebulæ like that in Orion, -through the planetary nebula and the regular nebula, to the perfect -stars, like Sirius and the Sun. Herschel’s nebular theory was a grand -conception, and a magnificent attack on the secrets of nature. - -Sir Robert Ball says: “Not from abstract speculation like Kant, nor from -mathematical suggestion like Laplace, but from accurate and laborious -study of the heavens, was the great William Herschel led to the -conception of the nebular theory of evolution.” Herschel’s nebular -theory was wider and less rigorous than that of Laplace. Laplace reached -his theory by reasoning backwards; Herschel by observing the nebulæ in -process of condensation. Consequently, while Laplace’s theory has -required modification, Herschel’s, from its width, is universally -accepted, because there is nothing mathematically rigorous in it. The -great German did not go into details like his French contemporary. He -sketched the evolution of the stars in a wider sense. - -The astronomer’s “1500 universes,” Miss Clerke remarks, “had now -logically ceased to exist.” Herschel had gathered much evidence about -nebular distribution which shattered his belief in external universes, -although he still thought in 1818 that some galaxies were included among -the non-gaseous nebulæ. In 1784 Herschel pointed out that the clusters -and nebulæ “are arranged to run in strata”; and some time later he found -that the nebulæ were aggregated near the galactic poles; in other words, -where nebulæ are numerous, stars are scarce, and _vice versa_. So -rigorously did this rule hold, that when dictating his observations to -his sister Caroline, he would, on noting a paucity of stars, warn her to -“prepare for nebulæ.” - -“A knowledge of the construction of the heavens has always been the -ultimate object of my observations.” So Herschel wrote in 1811. All his -investigations were secondary to the problem which was constantly before -his mind—the extent and structure of the Universe. He aspired to be the -Copernicus of the Sidereal System. Although Bruno, Kepler, Wright, Kant, -and Lambert had speculated regarding the construction of the heavens, -they had not the slightest evidence on which to base their ideas. There -was no science of sidereal astronomy. The stars were observed only to -assist navigation, and the primary object of star-catalogues was to -further knowledge of the motions of the planets. In Herschel’s day, -also, the distances of the stars had not been measured, and he had to -base his views on the distribution of the stars. In 1784, therefore, he -commenced a survey of the heavens, in order to ascertain the number of -stars in various parts of the sky. This method, which he named -“star-gauging,” consisted in counting the number of stars in the -telescopic field. Totally he secured 3400 gauges. His studies showed -that in the region of the Galaxy the stars were much more numerous than -near the galactic poles. Sometimes he saw as many as 588 stars in a -telescopic field, at other times only 2. He remarked that he had “often -known more than 50,000 pass before his sight within an hour.” Assuming -that the stars were, on the average, of about the same size, and -scattered through space with some approach to uniformity, Herschel was -able to compute the extent to which his telescope penetrated into space; -and, assuming that the Universe was finite and that his -“gauging-telescope” was sufficiently powerful to completely resolve the -Milky Way, he was enabled to sketch the shape and extent of the -Universe. - -Thus Herschel concluded that the Universe extended in the direction of -the Galaxy to 850 times the mean distance of stars of the first -magnitude. In the direction of the galactic poles the thickness was only -155 times the distance of stars of the same magnitude. Herschel was thus -enabled to sketch the probable form of the Universe, which he regarded -as cloven at one of its extremities, the cleft being represented by the -famous gap in the Milky Way. The Universe was, in fact, supposed to be a -cloven disc, and the Milky Way was merely a vastly extended portion of -it and not a region of actual clustering. On this theory the clusters -and nebulæ were supposed to be galaxies external to the Universe. Even -in 1785, however, Herschel believed that there were regions in the Milky -Way where the stars were more closely clustered than others. “It would -not be difficult,” he wrote in 1785, “to point out two or three hundred -gathering clusters in our system.” - -Strange to say, Herschel’s original ideas regarding the Universe were -accepted for many years by astronomical writers. Arago accepted -Herschel’s original theory, unaware that he had in reality abandoned it, -and he was followed by a host of French and English writers who did not -take the trouble to read each of Herschel’s papers, merely quoting that -of 1785, and believing that it represented his final ideas on the -subject. Even Sir John Herschel seems to have been unaware that his -father gave up the disc theory of the Universe. The famous German -astronomer, Wilhelm Struve, after an exhaustive study of Herschel’s -papers, was enabled to prove in 1847 that the theory had been abandoned -by Herschel; and in England the late R. A. Proctor independently -demonstrated the same thing. Meanwhile, supposing Herschel had not given -up his theory, it would be quite untenable. After considering the fact -that the brighter stars, down to the ninth magnitude, aggregate on the -Milky Way, Mr Gore says: “As the stars are by hypothesis supposed to be -uniformly distributed throughout every part of the disc, and as the -limiting circles for stars to the eighth and ninth magnitudes fall well -within the thickness of the disc, there is no reason why stars of these -magnitudes should not be quite as numerous in the direction of the -galactic poles as in that of the Milky Way itself. We see, therefore, -that the disc theory fails to represent the observed facts, and that -Struve and Proctor were amply justified in their opinion that the theory -is wholly untenable, and should be abandoned.” - -The observations made by Herschel himself eventually proved fatal to the -disc theory—a hypothesis which he had all along held very lightly. His -ideas about subordinate clusters within the Milky Way were soon -confirmed, and though in 1799 he still adhered to the disc theory, he -wrote in 1802, “I am now convinced, by a long inspection and continued -examination of it, that the Milky Way itself consists of stars very -differently scattered from those which are immediately about us. This -immense starry aggregation is by no means uniform. The stars of which it -is composed are very unequally scattered”—a conclusion quite opposed to -the disc theory, where the Milky Way was supposed to be merely an -extended portion of the Universe. - -In 1811 Herschel wrote as follows: “I must freely confess that by -continuing my sweeps of the heavens, my opinion of the arrangement of -the stars, and their magnitudes, and some other particulars, has -undergone a gradual change; and, indeed, when the novelty of the subject -is considered we cannot be surprised that many things formerly taken for -granted should on examination prove to be different from what they were -generally but incautiously supposed to be. For instance, an equal -scattering of the stars may be admitted in certain calculations; but -when we examine the Milky Way, or the closely compressed clusters of -stars, of which my catalogues have recorded so many instances, this -supposed equality of scattering must be given up.” - -This was the virtual abandonment of the disc theory. Six years later -Herschel announced that in six cases he had failed to resolve the Milky -Way, stating that his telescopes could not fathom it. This was the -abandonment of his second assumption—namely, that his telescope was -sufficiently powerful to penetrate to the limits of the Universe. Yet he -still thought that some of the star-clusters might be external galaxies, -although he could not even dogmatically assert our Universe to be -limited. In an error of translation, Struve left the impression that -Herschel believed our Universe to be unfathomable or infinite, and was -obliged to devise a most artificial theory of the extinction of light to -account for the fact that the sky did not shine with the brilliance of -the Sun, which it would do were the stars infinite in number. Of course, -Herschel did not actually believe the Universe to be infinite, and, had -he lived, he would probably have shown that all the star-clusters which -we see are included within the bounds of our finite Galaxy. - -In 1814 Herschel was “still engaged in a series of observations for -ascertaining a scale whereby the extent of the Universe, as far as it is -possible for us to penetrate into space, may be fathomed.” In 1817 he -described another method of star-gauging, which Arago and other writers -have confused with that which he devised in 1785. The two methods, -however, were quite distinct from each other. In the first system, one -telescope was used on different regions of the heavens; whereas in the -second method, various telescopes were used on identical regions. The -principle was that the telescopic power necessary to resolve groups of -stars indicates the distance at which the stars of the groups lie. This, -however, also assumed an equal distribution of stars, and as the late Mr -Proctor says, “I conceive that no question can exist that the principle -is unsound, and that Herschel would himself have abandoned it had he -tested it earlier in his observing career.... In applying it, Sir W. -Herschel found regions of the heavens very limited in extent, where the -brighter stars (clustered like the fainter) were easily resolved with -low powers, but where his largest telescopes could not resolve the -faintest. These regions, if the principle were true, must be long, -spike-shaped star groups, whose length is directed exactly towards the -astronomer on Earth,—an utterly incredible arrangement.” - -Herschel, at the time of his death, left unsolved the problem of the -construction of the heavens. It is still unsolved, and will doubtless -remain so until astronomers know more about the distances and motions of -the stars. His last observation of the Galaxy showed that even with his -40-foot reflector he could not fathom it. Consequently, as we have -mentioned, Struve and his successors regarded the Universe as infinite—a -theory which has now received its death-blow. Herschel was undoubtedly -correct when he stated his belief in a limited Universe. - -Herschel’s star-gauges, and those of his son, still remain of immense -value to astronomers in any discussion of the construction of the -heavens. Thus, although they failed to reveal to Herschel the structure -of the Universe, they have been of much use to his successors. -Herschel’s discussion of the supreme problem—the ultimate object of his -observations—constitutes one of the most interesting chapters in the -history of science, and marks a new era in human thought. In the words -of Miss Clerke: “One cannot reflect without amazement that the special -life-task set himself by this struggling musician—originally a penniless -deserter from the Hanoverian Guard—was nothing less than to search out -the ‘construction of the heavens.’ He did not accomplish it, for that -was impossible; but he never relinquished, and, in grappling with it, -laid deep and sure the foundations of sidereal science.” - - - - - CHAPTER III. - THE SUN. - - -Four years after the death of Herschel, an apothecary in the little -German town of Dessau procured a small telescope, with which he began to -observe the Sun. The name of this apothecary was _Samuel Heinrich -Schwabe_ (1789-1875). In 1826 he commenced to observe the spots on the -Sun’s disc, counting them from day to day, more for self-amusement than -from any hope of discovery; for previous astronomers had agreed that no -law regulated the number of the sun-spots. Every clear day Schwabe -pointed his telescope at the Sun and took his record of the spots; this -he continued for forty-three years, until within a few years of his -death on April 11, 1875. As early as 1843 Schwabe hinted that a possible -period of ten years regulated the distribution of the spots on the Sun, -but no attention was given to his idea. In 1851, however, the result of -his twenty-six years of observation was published in Humboldt’s -‘Cosmos,’ and Schwabe was able to show that the spots increased and -decreased in a period of about ten years. Astronomers at once recognised -the importance of Schwabe’s work, and in 1857 he was rewarded by the -Gold Medal of the Royal Astronomical Society of London. - -_Rudolf Wolf_ (1813-1892) of the Zürich Observatory now undertook to -search through the records of sun-spot observation, from the days of -Galileo and Scheiner, to find traces of the solar cycle discovered by -Schwabe. He was successful, and was enabled to correct Schwabe’s -estimate of the length of the period, fixing it as on the average 11·11 -years. Additional interest, however, was given to Schwabe’s and Wolf’s -investigations by the remarkable discoveries which followed. In -September 1851 _John Lamont_ (1805-1879), a Scottish astronomer,—born at -Braemar in Aberdeenshire, but employed as director of the Munich -Observatory,—after searching through the magnetic records collected at -Göttingen and Munich, discovered that the magnetic variations indicated -a period of 10⅓ years. Soon after this Sir _Edward Sabine_ (1788-1883), -the English physicist, from a discussion of an entirely different set of -observations, independently demonstrated the same thing, proving -conclusively that once in about ten years magnetic disturbances reached -their height of violence; and Sabine was not slow to notice the -correspondence between the magnetic period and the sun-spot period. In -the same year (1852) Wolf and _Alfred Gautier_ (1793-1881) independently -made the same discovery, which had thus been made by four separate -investigators. - -In the same year an English amateur astronomer, _Richard Christopher -Carrington_ (1826-1875), commenced a series of solar observations which -led to some remarkable discoveries. From observations on the spots, -Carrington discovered that while the Sun’s rotation was performed in 25 -days at the equator, it was protracted to 27½ days midway between the -equator and the poles. In 1858 Carrington demonstrated the fact that -spots are scarce in the vicinity of the solar equator, but are confined -to two zones on either side, becoming scarce again at thirty-five -degrees north or south of the equator. Contemporary with Carrington was -_Friedrich Wilhelm Gustav Spörer_ (1822-1895), who was born in Berlin in -1822 and died at Giessen, July 7, 1895. He commenced his solar -observations about the same time as Carrington, and independently -discovered the Sun’s equatorial acceleration. From observations at his -little private observatory at Anclam in Pomerania, continued at the -Astrophysical Observatory in Potsdam, Spörer demonstrated a remarkable -law regarding sun-spots. This law is thus described by a well-known -astronomer: “The disturbance which produces the spots of a given -sun-spot period first manifests itself in two belts about thirty degrees -north and south of the Sun’s equator. These belts then draw in toward -the equator, and the sun-spot maximum occurs when their latitude is -about sixteen degrees; while the disturbance gradually and finally dies -out at a latitude of eight or ten degrees. Two or three years before -this disappearance, however, two new zones of disturbance show -themselves. Thus, at the sun-spot minimum there are four well-marked -spot-belts,—two near the equator, due to the expiring disturbance, and -two in high latitudes, due to the newly beginning outbreak.” These -remarkable discoveries, which resulted from the investigations of -Schwabe, Carrington, and Sporer, are a brilliant example of what may be -done by amateurs in astronomy. - -At the time when Carrington and Spörer were pursuing these researches, -the spectroscope came into use as an astronomical instrument, and since -1859 solar astronomy has been almost entirely spectroscopic. Before we -can rightly understand the principles of spectroscopic astronomy, we -must go back to the life and work of its founder—Joseph von Fraunhofer. - -The son of a poor glazier, _Joseph von Fraunhofer_ was born on March 6, -1787, at Straubing, in Bavaria. His father and mother having died when -their son was quite young, the boy, on account of his poverty, was -apprenticed to a looking-glass manufacturer in Munich named -Weichselberger, who acted tyrannically, keeping him all day at hard -work. Still the lad borrowed some old books, and spent his nights in -study. Young Fraunhofer lodged in an old tenement in Munich, which on -July 21, 1801, collapsed, burying in its ruins its occupants. All were -killed but Fraunhofer, who, though seriously injured, was dug out from -the ruins four hours later. The distressing accident was witnessed by -Prince Maximilian Joseph, Elector of Bavaria. He became interested in -Fraunhofer, and presented him with a sum of money. Of this he made good -use. He was already interested in optics, and he bought some books on -that subject, as well as a glass-polishing machine. The remainder of the -money served to procure his release from his tyrannical master, -Weichselberger. - -Fraunhofer became acquainted with prominent scientists at Munich, who -provided him with books on optics and mathematics. Meanwhile the young -optician occupied his time in shaping and finishing lenses. In 1806 he -entered the optical department of the Optical and Physical Institute of -Munich, and the following year, when only twenty years of age, was -appointed to the chief post in that department. In 1814 he commenced his -investigations with the prism, which have made his name famous. - -Newton had found that, in passing through a prism, white light is -dispersed into its primary colours, making up the band of coloured light -known as the solar spectrum. But he failed to recognise the existence of -dark lines in the spectrum. Casually seen in 1802 by _William Hyde -Wollaston_ (1786-1828), an English physicist, these lines were first -thoroughly examined by Fraunhofer. Allowing light from the Sun to pass -through a prism attached to the telescope, he was amazed to find several -dark lines in the spectrum. By the year 1814 he had detected no less -than 300 or 400 of these lines. Fraunhofer named the more prominent -lines by the letters of the alphabet, from A in the red to H in the -violet. They are now known as the Fraunhofer lines. At first he was much -perplexed regarding the nature of the dark lines. He suspected that they -might be an optical effect, depending on the quality of the glass used, -and he tried different prisms, but the lines were still to be seen. Then -he turned his prism to bright clouds to see if they were visible in -reflected sunlight, and he found that they were. He examined the Moon -and again perceived them, as moonlight is merely reflected sunlight; and -they were also conspicuous in the spectra of the planets. It was thus -proved that these lines were characteristic of sunlight, whether direct -or reflected. It was, however, still possible that they might be caused -by the passage of the rays of light from the celestial bodies through -the Earth’s atmosphere. In order to test this theory, Fraunhofer -examined the spectra of the brighter stars. He found that the lines -visible in the solar spectrum were not to be seen in the spectra of the -stars, thus proving that the lines were not merely an atmospheric -effect. Each star, Fraunhofer observed, had a different spectrum from -both the Sun and from other stars. These spectra were also characterised -by numerous dark lines, much fainter than those in the solar spectrum. - -Although he ascertained the existence of the dark lines in the Sun’s -spectrum, Fraunhofer never really found out what they represented. As -Miss Giberne expresses it, “Although he now saw the lines he could not -understand them: he could not read what they said. They spoke to him -indeed about the Sun, but they spoke to him in a foreign language, the -key to which he did not possess.” However, he expressed the belief that -the pair of lines in the solar spectrum, which he marked D, coincided -with the pair of bright lines emitted by incandescent sodium. Although -he doubtless suspected that the lines conveyed intelligence regarding -the elements in the Sun, he never was able properly to decipher their -meaning. Had he lived, he would probably have made the great discovery; -but these investigations were cut short by his sudden and untimely death -on June 7, 1826. - -After the death of Fraunhofer, very little was done to forward the study -of spectrum analysis. Investigations in this branch of research were -made, however, by Sir _John Herschel_ (1792-1871), _William Allen -Miller_ (1817-1870), Sir _David Brewster_ (1781-1868), and others. Two -famous men of science had partly discovered the secret. These were Sir -_George Stokes_ (1819-1903), of Cambridge, and _Anders John Angström_ -(1812-1872) of Upsala. Of Angström’s work, published in 1853, it has -been said that it would “have obtained a high celebrity if it had -appeared in French, English, or German, instead of Swedish.” - -It was not until 1859 that the principles of spectrum analysis were -fully enunciated by _Gustav Robert Kirchhoff_ (1824-1887), and his -colleague in the University of Heidelberg, _Robert Wilhelm Bunsen_ -(1811-1899). Kirchhoff demonstrated that a luminous solid or liquid -gives a continuous spectrum, and a gaseous substance a spectrum of -bright lines. In the words of Miss Clerke, “Substances of every kind are -opaque to the precise rays which they emit at the same temperature. That -is to say, they stop the kinds of light or heat which they are then -actually in a condition to radiate.... This principle is fundamental to -solar chemistry. It gives the key to the hieroglyphics of the Fraunhofer -lines. The identical characters which are written bright in terrestrial -spectra are written dark in the unrolled sheaf of sun-rays.” Kirchhoff -made several determinations of the substances in the Sun, proving the -existence of sodium, iron, calcium, magnesium, nickel, barium, copper, -and zinc. His great map of the solar spectrum was published by the -Berlin Academy in 1860, and represented an enormous amount of labour. It -was succeeded by another map by Angström, published in 1868. But both of -these maps have been recently superseded by the investigations of Sir -_Joseph Norman Lockyer_ (born 1836), and of the American physicist, -_Henry Augustus Rowland_ (1848-1901). Rowland largely increased our -knowledge of the elements in the solar atmosphere. - -The spectroscope had become, by 1868, a recognised instrument of -astronomical research, and in that year it was applied during the famous -total eclipse, visible in India. There were many eclipse problems, -arising from the observations made by the eclipse expeditions of 1842, -1851, and 1860. The eclipse of 1851 had finally proved that the red -flames seen surrounding the Sun during total eclipses belonged to the -Sun, and not to the Moon, as many astronomers had believed. At the -eclipse of 1860, visible in Spain, the Italian astronomer, _Angelo -Secchi_ (1818-1878), and the Englishman, _Warren De la Rue_ (1815-1889), -secured photographs of the solar prominences. The problem of 1868 was -the constitution of these prominences. - -_Pierre Jules César Janssen_, born in Paris in 1824, was stationed at -Guntoor, in India, to observe the eclipse. He succeeded in observing the -spectrum of the prominences during the progress of totality, and found -it to be one of bright lines, proving the gaseous nature of the -sun-flames. During the progress of the eclipse, Janssen was specially -struck by the brilliancy of the bright lines, and it occurred to him -that the prominence-spectrum could be observed in full daylight, if -sufficient dispersive power was used to enfeeble the ordinary continuous -spectrum. At ten o’clock on the following morning, August 19, 1868, -Janssen applied his spectroscope to the sun, and observed the -prominence-spectrum. After a month’s observation in India, he sent to -the French Academy an account of his success. A short time, however, -before his report arrived, the Academy had received a similar one from -Lockyer, who had independently made the same discovery. Two years -previously, in 1866, the new method had occurred to him, but his -spectroscope was not powerful enough; and although he ordered a more -powerful one at once, it was not until October 16, 1868, that he had the -instrument in his hands. Four days later he observed the -prominence-spectrum in full daylight. - -The next advance in the study of the prominences was announced in 1869. -Janssen and Lockyer had shown astronomers how to observe the spectrum of -the prominences; but the researches of other two famous astronomers -enabled observers to see the forms of the prominences. These two men -were _William Huggins_ (born 1824) and _Johann Carl Friedrich Zöllner_. -The latter astronomer, born in Leipzig in 1834, was one of the most -successful students of the solar prominences. He was Professor of -Astrophysics in the University of Leipzig, a position which he filled -with success until his untimely death on April 25, 1882. Independently -of Huggins, he found that by opening the slit of the spectroscope wider, -the forms of the prominences themselves could be seen. The study of the -prominences was at once taken up by the most famous solar observers: -these were Huggins and Lockyer in England, Spörer and Zöllner in -Germany, Janssen in France, Secchi, Respighi, and Tacchini in Italy, -Young in America. To _Charles Augustus Young_ (born 1834) we owe the -careful study of individual prominences. On September 7, 1871, he -observed the most gigantic outburst on the sun ever witnessed, fragments -of an exploded prominence reaching a height of 100,000 miles: Young, -also, made the first attempt to photograph the prominences. - -To the Italian school of astronomers, however, we owe the persistent and -systematic study of the prominences. Among them the three greatest names -are _Angelo Secchi_ (1818-1878), _Lorenzo Respighi_ (1824-1889), and -_Pietro Tacchini_ (1838-1905). After the death of Secchi, the recognised -head of spectroscopy in Italy was Pietro Tacchini. Born at Modena in -1838, he was appointed director at Modena in 1859, assistant at Palermo -in 1863, and director at Rome in 1879. In 1870 he commenced to take -daily observations of the prominences, noting their sizes, forms, and -distribution, and these observations were continued for thirty-one -years, until within four years of Tacchini’s death, which took place on -March 24, 1905. Tacchini did for the study of the prominences what -Schwabe did for the spots. The Italian spectroscopists found that the -prominences increased and decreased every eleven years in harmony with -the spots. Tacchini demonstrated that the streamers of the solar corona -originate in regions where the prominences are most numerous, and that -the shape of the corona, on the whole, varies in sympathy with the -prominences. - -The researches of Lockyer indicated that the prominences originated in a -shallow gaseous atmosphere which he termed the chromosphere. Formerly -astronomers had to observe only isolated prominences, but in 1892 an -American astronomer, _George Ellery Hale_ (born 1868), formerly director -of the Yerkes Observatory, and now director of the Solar Observatory in -California, succeeded in photographing, by an ingenious process, the -whole of the chromosphere, prominences, and faculæ visible on the solar -surface. - -Another solar envelope was discovered in 1870 by Dr Charles Augustus -Young, who from 1866 to 1877 directed the Observatory at Dartmouth, New -Hampshire, and from 1877 to 1905, that at Princeton, New Jersey. During -the eclipse of December 22, 1870, Young was stationed at Tenez de -Frontena, Spain. As the solar crescent grew apparently thinner before -the disc of the Moon, “the dark lines of the spectrum,” he says, “and -the spectrum itself gradually faded away, until all at once, as suddenly -as a bursting rocket shoots out its stars, the whole field of view was -filled with bright lines, more numerous than one could count. The -phenomenon was so sudden, so unexpected, and so wonderfully beautiful, -as to force an involuntary exclamation.” The phenomenon was observed for -two seconds, and the impression was left on the astronomer that a bright -line had taken the place of every dark one in the solar spectrum, the -spectrum being completely reversed. Hence the name which was given to -the hypothetical envelope—“the reversing layer.” For long the existence -of the reversing layer was disputed by numerous astronomers. In 1896 -photographs taken during the solar eclipse of that year finally -demonstrated the existence of the “flash spectrum” as seen by Young. - -The last of the solar appendages, the corona, can only be seen during -total eclipses. The researches of Young and Janssen indicate that it is -partly gaseous and partly meteoric in its constitution; and various -photographs, taken at the eclipses since 1870, have demonstrated its -variation in shape, which is in harmony with the eleven-year period. -Several attempts have been made to observe the corona without an -eclipse. In 1882 Huggins made the attempt, but failed, and Hale, with -his photographic process, had no better success. More recently, in 1904, -a Russian astronomer, _Alexis Hansky_, observing from the top of Mont -Blanc, secured some photographs on which he believes the corona is -represented, but so far his observations have not been confirmed by -other astronomers. - -The application of the spectroscope to the motions on the solar surface -is perhaps one of the most wonderful triumphs in astronomical science. -In 1842 _Christian Doppler_ (1803-1853), Professor of Mathematics at -Prague, had expressed the view that the colour of a luminous body must -be changed by its motion of approach or recession. It was obvious to -Doppler that if the body was approaching, a larger number of light waves -must be entering the eye of the observer than if it were retreating. -Miss Clerke thus illustrates Doppler’s principle: “Suppose shots to be -fired at a target at fixed intervals of time. If the marksman advances, -say, twenty paces between each discharge of his rifle, it is evident -that the shots will fall faster on the target than if he stood still; -if, on the contrary, he retires by the same amount, they will strike at -correspondingly longer intervals.” It occurred to various astronomers -that it would be possible to measure cyclones and hurricanes in the Sun, -not by change of colour in the spectrum, but by the shifting of the -lines; and in 1870 this was successfully done by Lockyer. In the next -few years efforts to measure the solar rotation were made by Young, -Zöllner, and others, who succeeded in measuring the displacement of the -lines, but not the time of rotation. This was reserved for the famous -Swedish astronomer, Dunér. - -_Nils Christopher Dunér_, born in 1839 in Scania, was employed as an -assistant at Lund Observatory from 1858 to 1888, when he was appointed -director of the Observatory at Upsala. In that year he commenced a study -of the solar rotation, measuring it by means of Doppler’s principle. He -confirmed the telescopic work of Carrington and Spörer on the equatorial -acceleration, and measured the displacement up to within fifteen degrees -of the poles. He brought out the surprising fact that the rotation -period of the Sun is there protracted to 38½ days. These remarkable -researches were published in 1891. - -In 1873 the Astronomer-Royal of England commenced at Greenwich -Observatory to photograph the Sun daily. This work has been carried on -there by _Edward Walter Maunder_ (born 1851), and Greenwich Observatory -possesses a photographic record of sun-spots. At the Meudon -Astrophysical Observatory, near Paris, Janssen has, since 1876, secured -photographs of the solar surface, which were comprised in a great atlas, -published by him in January 1904. These photographs have revealed a -remarkable phenomenon—the “réseau photospherique,” the distribution over -the solar surface of blurred patches of light, which Janssen considers -are inherent in the Sun. The Greenwich records of sun-spots and of -magnetic disturbances have been made use of by Maunder in his remarkable -studies, promulgated in 1904, of the connection between sun-spots and -terrestrial magnetism. Maunder finds that on the average magnetic storms -are dependent on the presence of sun-spots, and on the size of the spot. -The magnetic action, he finds, does not radiate equally in all -directions from the sun-spots, but along definite and restricted lines. - -Herschel’s hypothesis of a dark and cool globe beneath the solar -photosphere was seen to be untenable after the introduction of the -spectroscope. The first important theory as to the solar constitution -was that advanced in 1865 by the French astronomer, _Hervé Faye_ -(1814-1902). Numerous other theories were afterwards advanced by Secchi, -Zöllner, Young, and others, but a complete description of the various -developments in solar theorising cannot be given here. There is no -complete “theory” of the exact constitution of every part of the Sun, -but the unpretentious “Views of Professor Young on the Constitution of -the Sun,” which appeared in April 1904 in ‘Popular Astronomy,’ represent -the latest ideas of the foremost solar investigator. Professor Young -regards the reversing layer and the chromosphere as “simply the -uncondensed vapours and gases which form the atmosphere in which the -clouds of the photosphere are suspended.” He says that the contraction -theory of Helmholtz,—explained in another chapter,—advanced to explain -the maintenance of the Sun’s heat, is true so far as it goes; but that -it is all the truth is now made doubtful by the discovery of radium, -which “suggests that other powerful sources of energy may co-operate -with the mechanical in maintaining the Sun’s heat.” - -The important question of the distance of the Sun was thoroughly -investigated in 1824 by _Johann Franz Encke_ (1791-1865), then of -Seeberg, near Gotha, who, from a discussion of the transits of Venus in -1761 and 1769, found a parallax of 8″·571, corresponding to a mean -distance of 95,000,000 miles. This value was accepted for thirty years, -until _Peter Andreas Hansen_ (1795-1874), in 1854, and _Urban Jean -Joseph Le Verrier_ (1811-1877), in 1858, found from mathematical -investigations that the distance indicated was too great. Preparations -were accordingly made for the observation of the transits of Venus, -which took place respectively on December 8, 1874, and December 6, 1882. -On the first occasion many expeditions were sent to view the transit, -consisting of French, German, American, English, Scottish, Italian, -Russian, and Dutch astronomers, and it was hoped that the solar parallax -would be accurately measured once for all. However, the transit, -although favoured with good weather, was not successful, owing to the -difficulty of making exact measurements, by reason of the illumination -and refraction in the atmosphere of Venus. Accordingly the values -deduced for the parallax were far from unanimous. The transit of 1882 -was not observed so extensively, as astronomers had found the transit of -Venus to be by no means the best method. In 1877 Sir _David Gill_ (born -1843), the great Scottish astronomer, determined the solar parallax -successfully from measures of the parallax of Mars in opposition. His -value was 8″·78, corresponding to 93,080,000 miles. Some years previous -to this _Johann Gottfried Galle_ (born 1812), the German astronomer, -had, from measurements of the parallax of the asteroid Flora, deduced a -solar parallax of 8″·87. Gill’s work at the Cape in 1888, on the -Asteroids, was successful in giving a more accurate value than the -transit of Venus: in 1900 and 1901 measures of the parallax of the -asteroid Eros, the nearest minor planet, were made by many different -observatories, and agree with the other results. The values which have -been derived from the velocity of light, and from the constant of -aberration, are fairly in agreement with those derived from direct -measurement. On the whole, the most probable value of the parallax is -about 8″·8, indicating a mean distance of about 92,700,000 miles, with a -“probable error” of about 150,000 miles. - -What a different picture the sun presents to us at the beginning of the -twentieth century from that which it presented to Herschel and his -contemporaries at the beginning of the nineteenth! To Herschel, the Sun -was a cool dark globe, surrounded by a luminous atmosphere. As the -outcome of the researches and discoveries outlined in this chapter, the -Sun is now seen to be a vast central world, which is over a million -times larger than the Earth. In the words of an able writer, “It is most -probably a world of gases, where most of the metals and metallic gases -that we know exist only as vapours, even at the Sun’s surface, hotter -than any furnace on earth, and getting a still fiercer heat for every -mile of descent lower. Of that heat in the Sun’s interior we can form no -conception. The pressure within the Sun is equally inconceivable. A -cannon-ball weighing 100 lb. on earth would weigh 2700 on the Sun. Thus -a mighty conflict goes on unceasingly between imprisoned and expanding -gases and vapours struggling to burst out, and massive pressures holding -them down. For reasons we cannot fully understand, no equilibrium is -reached. For millions of years up-rushes and down-rushes of the -white-hot materials have been proceeding on that bright photosphere -which gives us light, and looks a picture of calm and quiescence. Above -that is a comparatively thin rose-coloured layer, the chromosphere, -agitated with fiery ‘prominences,’ and outside all these the coronal -glory—all alike pointing to immeasurable activities.” - -The following remark of Professor Newcomb shows our inability to realise -the solar activity. “Suppose,” he says, “every foot of space in a whole -country covered with 13-inch cannon, all pointed upward, and all -discharged at once. The result would compare with what is going on -inside the photosphere about as much as a boy’s popgun compares with the -cannon.” - - - - - CHAPTER IV. - THE MOON. - - -It is somewhat remarkable that the one celestial body which Herschel -neglected was our satellite, the Moon; and it is also remarkable that -the Moon was for many years the chief object of study of his -contemporary astronomer, _Johann Hieronymus Schröter_ (1745-1816). Born -at Erfurt, near Hanover, on August 30, 1745, Johann Hieronymus Schröter -was originally intended for the study of law, for which he was sent to -the University of Göttingen. At the same time he studied mathematics, -and particularly astronomy, under the mathematician, Kaestner of -Göttingen. Deeply interested in music, he became acquainted with the -Herschel family, and, inspired by William Herschel’s example, determined -to study the heavens. In 1779 he became the possessor of a small -achromatic refractor, and commenced to observe the Sun and Moon. In 1778 -he entered the legal profession at Hanover, and four years later he was -appointed “oberamtmann” or Chief Magistrate of Lilienthal—“the Vale of -Lilies”—in the Duchy of Bremen. At Lilienthal Schröter erected a small -observatory, and acquired in 1785 one of Herschel’s 7-foot reflectors. -In 1792 the astronomer superintended the construction of a 13-foot -reflector, made by Schrader of Kiel, who transferred his workshop to -Lilienthal. With these instruments the great work of Schröter was -accomplished. - -Schröter directed his powers of observation to the study of the Moon. He -originated the study of the surface of the Moon, and founded the branch -of astronomy known as selenography, or the study of the Moon’s surface. -The foundations of this branch were laid in 1791 with the publication of -Schröter’s ‘Seleno-topographische Fragmente’. The astronomer determined -to make a comparative study of the surface of our satellite, and before -1801 discovered eleven “rills” or clefts on the Moon’s surface, and -recognised a large number of craters. He likewise believed that he had -seen a lunar atmosphere, an observation of which was made by him in -February 1792. Schröter seems never to have doubted what Herschel and -his contemporaries believed—that the Moon was a living world with -volcanoes in active eruption, surrounded by an atmosphere, and inhabited -by beings like ourselves. Unfortunately, Schröter was not good at making -drawings of what he saw; nevertheless, he accomplished a vast amount of -work. In the little observatory at Lilienthal the foundations were laid -of the comparative study of the surface of the Moon. - -But these observations were destined to be rudely interrupted. In 1810 -Hanover was occupied by the invading troops of Napoleon, and Schröter -lost his appointment as Chief Magistrate of Lilienthal, and also his -income. But there was worse to follow. On April 20, 1813, three years -after, the French, under Vandamme, with that cruelty which seems to -belong to warfare, occupied Lilienthal, and set fire to the little -village. A few days later the French soldiers entered the observatory -and burned it to the ground. All Schröter’s precious observations, -accumulated after thirty-four years’ labour, were destroyed with a few -exceptions, the observations on Mars narrowly escaping the -conflagration. Unable to forget the destruction of his observatory, and -without the means to repair the loss, he lived only three years after -the disaster. He died on August 29, 1816, “leaving behind him,” says Mr -Arthur Mee, “an imperishable record, and a noble example to observers of -all time.” - -_Wilhelm Gotthelf Lohrmann_, a land-surveyor of Dresden, continued the -observations of Schröter, and in 1824 published four of the twenty-five -proposed sections of a large lunar chart. In 1827, however, his sight -began to fail, and he was obliged to abandon his intention. But a -successor had already appeared on the scene. _Johann Heinrich von -Mädler_ (1794-1874) was born in Berlin in 1794, and, after a severe -struggle to earn a living, entered the University of Berlin in 1817. In -1824 he became acquainted with _Wilhelm Beer_ (1797-1850), a wealthy -banker, who had come to him for instruction in astronomy, and who -erected in 1829 an observatory near his villa in Berlin, where pupil and -tutor pursued their studies. - -In 1830 Mädler, with Beer’s assistance, commenced a great -trigonometrical survey of the surface of the Moon. The observations of -Beer and Mädler were made with no larger instrument than a 3¾-inch -refractor. They ascertained the positions of 919 lunar spots, and -measured the height of 1095 mountains. Their great chart of the -Moon—which was afterwards followed by a smaller one—was issued in four -parts during 1834-36. “The amount of detail,” wrote Proctor, “is -remarkable, and the labour actually bestowed upon the work will appear -incredible.” The chart has neither been revised nor superseded, and it -remains to this day one of the standard works on the subject. - -The chart was succeeded in 1837 by a descriptive volume entitled ‘Der -Mond.’ In this work Beer and Mädler did much for the progress of lunar -astronomy. Their observations led to a change of opinion regarding our -satellite’s physical condition. Herschel, Schröter, Olbers, and other -astronomers seem to have considered the Moon a living world. Mädler -declared that it was a dead world. He believed it to be destitute of -life of any kind, and the changes observed by Schröter and other -observers were put down as illusions. ‘Der Mond’ was the end of Mädler’s -work in lunar astronomy, for, receiving an appointment at Dorpat, he -went there in 1846, and retained his post until within a few years of -his death, which took place at Hanover on March 14, 1874. - -Mädler’s successor in the field of lunar astronomy was _Johann Friedrich -Julius Schmidt_ (1825-1884), who was born at Eutin in Lübeck in 1825. At -a very early age he gave indications of a taste for astronomy. -Fortunately his father possessed a small hand telescope, with which -young Schmidt commenced his lunar studies. Appointed assistant at Bonn -and Olmütz and director at Athens successively, he kept up his -persistent study of the surface of the Moon for over forty years. In -1839, when fourteen years of age, he began the valuable series of -observations which were destined to form the basis of his great chart of -the surface of the Moon. Between 1853 and 1858, when employed at Olmütz, -Schmidt made and calculated no fewer than 4000 micrometrical measures of -the altitudes of lunar mountains. Before 1866 Schmidt had found no fewer -than 278 “rills,” and his discoveries were the means of augmenting the -number of these curious objects to nearly a thousand. - -In a word, it may be said that Schmidt drew out a lunar geography, and -the result of his labours, together with those of Schröter and Mädler, -is that in a sense we now know the features of the Moon better than -those of the Earth. For instance, astronomers see the whole surface of -the Moon spread before their eyes, while geographers can never have a -similar view of the terrestrial features: we have never seen the poles -of the Earth, while the lunar poles are well known to astronomers. For -twenty years after his appointment at Athens, Schmidt worked at fixing -the positions of lunar objects, measuring the heights of mountains and -the depths of craters. An idea of his enthusiasm in constructing his -great chart may be gained from the fact that he made almost a thousand -original sketches. - -Mädler’s dogmatic assertion that the Moon was entirely a dead world was -generally believed until Schmidt made observations to the contrary. From -1837 to 1866 the popular opinion was that our satellite was an -absolutely dead world. Consequently there was little progress in lunar -astronomy during those thirty years. Although Mädler’s view was much -nearer the truth than the opinions of his predecessors, it was also too -positive. His confident assertion, which was received without -hesitation, was never questioned until Schmidt came upon the scene. To -Schmidt the Moon was not entirely dead, and it was he who brought -forward indisputable evidence as to the existence of changes on its -surface. In October 1866 he announced that the crater Linné had lost all -appearance of such, and that it had become entirely effaced. Lohrmann -and Mädler had observed it under a totally different aspect, as also had -Schmidt himself from 1840 to 1843. There was great excitement in the -astronomical world on Schmidt’s announcement, and many astronomers -denied the change, although Schmidt’s observation was confirmed by -Secchi and Webb. The evidence in favour of it preponderated, and very -few observers now consider the Moon’s surface to be absolutely -changeless. - -In 1865 Schmidt had begun to arrange his observations on the Moon into -the form of a chart. At first he decided to have a chart of six feet -diameter, divided, like that of Mädler, into four sections. But in April -1868, on making an estimate of the value of such a chart, he was -dissatisfied, and determined to construct a map of the same size divided -into twenty-five sections instead of four. He began the work in 1868, -and after six years the great map was completed. After some delay the -German Government undertook to issue the chart at their expense, and it -was published in 1879, after fourteen years of preparation. It contained -no fewer than 30,000 objects, and its completed diameter was six feet -three inches—more than double the size of any previous map of the Moon. -Indeed, it was probably the greatest contribution ever made to lunar -astronomy. Schmidt lived only a few years after the publication of his -great chart. He died at Athens, in his fifty-ninth year, February 8, -1884. - -Schmidt’s announcement of the change in the appearance of Linné was -followed in 1878 by a statement by _Hermann Joseph Klein_ (born 1842) of -Cologne, to the effect that a new crater had been formed to the north of -the well-known lunar crater, Hyginus. The change in this case, however, -is by no means so certain as in that of Linné. It will be observed that -the majority of the students of the Moon were Germans. In England the -study was not taken up until 1864, when a Lunar Committee of the British -Association was appointed. Some good lunar work was done by the -well-known astronomer, _Thomas William Webb_ (1807-1885), while the -study was popularised by _James Nasmyth_ (1808-1890), the famous -engineer, who published, in 1874, in conjunction with _James Carpenter_ -of Greenwich Observatory, a beautifully-illustrated volume entitled ‘The -Moon.’ This was succeeded, in 1876, by the larger work of _Edmund -Neison_ (now Nevill), Government Astronomer of Natal. About this time -several English astronomers, devoted to the study of the Moon, formed -themselves into the Selenographical Society. After a few years, however, -the society came to an end, and the enthusiasts formed themselves into -the lunar section of the British Astronomical Association, on the -foundation of that society in 1890. Chief among those English -selenographers was _Thomas Gwyn Elger_ (1837-1897), whose observations -of the Moon and drawings of the various craters were of the utmost -value. Two years before his death, in 1895, Elger published his -important work, ‘The Moon,’ along with an exhaustive chart of the -visible face of our satellite. - -Herschel and Schröter firmly believed in the existence of a lunar -atmosphere, the latter believing that he had actually observed the -Moon’s atmospheric envelope. Early in the nineteenth century it was soon -observed, however, that on the Moon passing over and occulting stars, -these stars disappeared suddenly behind the Moon’s limb, instead of -gradually, as they should have done, had an atmosphere of any density -existed. Accordingly astronomers gave up believing in a lunar -atmosphere. On January 4, 1865, Huggins observed with his spectroscope -the occultation of a small star in Pisces. There was not the slightest -sign of absorption in a lunar atmosphere; the entire spectrum vanished -at once. - -Lunar photography was introduced as long ago as 1858 by _Lewis Morris -Rutherfurd_ (1816-1892), the well-known American astronomer; but for -years very little was done in this matter, although Rutherfurd secured -fairly good photographs. Rutherfurd, De la Rue, and the older -astronomical photographers took photographs of the entire Moon, but this -plan was abandoned in favour of what Miss Clerke calls “bit by bit -photography.” About 1890 this method was introduced, and has been -followed with success by _Maurice Loewy_ (born 1833), and his assistant, -Pusiex, at the Paris Observatory; by _Ladislas Weinek_ at Prague; by the -astronomers of the Lick Observatory; and by _William Henry Pickering_ -(born 1858), the distinguished astronomer of Harvard, whose discoveries -and investigations have created quite a new interest in lunar astronomy. -These investigations were commenced in 1891 at Arequipa, on the slope of -the Andes, in Peru. An occultation of Jupiter, witnessed by W. H. -Pickering on October 12, 1892, gave support to the view that a very -tenuous lunar atmosphere does exist. In 1900 he established, near -Mandeville, Jamaica, a temporary astronomical station, where he obtained -many excellent photographs. Totally he secured eighty plates. These -appeared, as the first complete photographic lunar atlas ever published, -in his work ‘The Moon’ (1903), in which he sums up all his observations -since 1891, and concludes that “the evidence in favour of the idea that -volcanic activity upon the Moon has not yet ceased is pretty strong, if -not fairly conclusive.” - -Pickering points out that the density of the lunar atmosphere is not -greater than one ten-thousandth of that at the Earth’s surface, and, -under these circumstances, water cannot exist above freezing-point, -which of course brings us to the subject of snow. He considers that snow -is observed on the mountain peaks and near the poles of the Moon, and he -believes his conclusion to be verified by observations on the well-known -crater, Linné. He brings forward evidence of the probable existence on -the Moon of organic life, pointing out that the difference between the -conditions of the Earth and the Moon is not so great as that above and -below the ocean on our own planet. He has collected evidence of the -existence of something resembling vegetation on the Moon “coming up, -flourishing, and dying, just as vegetation springs and withers on the -Earth.” - -The first successful attempt to measure the heating power of moonlight -was made in 1846 on Mount Vesuvius by _Melloni_, an Italian physicist, -whose results were confirmed four years later by _Zantedeschi_, another -Italian. The most important work in this direction was accomplished by -the present _Earl of Rosse_ (born in 1840), who in the years 1869-72 -believed himself to have measured the lunar heat; but these conclusions -were not altogether confirmed by the observations of Dr _Otto -Boeddicker_ (Lord Rosse’s astronomer), during the total lunar eclipse of -October 4, 1884. Further investigations on this subject were afterwards -made by _Samuel Pierpont Langley_ (1834-1906), of Alleghany, and by his -assistant, _Frank Very_. - -The motion of the Moon and its perturbations were made the subject of -deep study by the famous _Pierre Simon Laplace_ (1749-1827), the -contemporary of Herschel, and the worthy successor of Newton. He devoted -much attention to the secular acceleration of the Moon’s mean motion, a -problem which had baffled the greatest mathematicians. After a profound -discussion he found, in 1787, that the average distance of the Earth and -Moon from the Sun had been slowly increasing for several centuries, the -result being an increase in the Moon’s velocity. In the third volume of -the ‘Mécanique Céleste’ Laplace worked out the lunar theory in great -detail, although he calculated no lunar tables. After his death the -subject was taken up by _Charles Theodore Damoiseau_ (1768-1846), and -the most important advance was made by _Giovanni Antonio Amadeo Plana_ -(1781-1864), the director of the Turin Observatory, who published in -1832 a very complete lunar theory. The work of Plana was followed by -that of _Peter Andreas Hansen_ (1795-1874), whose lunar tables were used -for the Nautical Almanac, and whom Professor Simon Newcomb considers to -be the greatest master of celestial mechanics since Laplace. The theory -of the Moon’s motion was worked out in detail by the famous astronomer -_Charles Eugene Delaunay_ (1816-1872), who from 1870 till 1872 occupied -the post of director of the Paris Observatory. Delaunay was about to -work out the lunar tables when, in 1872, he was accidentally drowned by -the capsizing of a pleasure-boat at Cherbourg. The work accomplished in -this direction by _Simon Newcomb_ (born 1835) is of great importance, -particularly in his correction of Hansen’s tables. _John Couch Adams_ -(1819-1892), one of the discoverers of Neptune, while at work on the -lunar theory, had occasion to correct Laplace’s supposed solution of the -acceleration of the lunar motion. On going over the calculation Adams -found that several quantities, omitted by Laplace as unimportant, showed -that the Moon has a minute increase of speed for which the theory of -gravitation will not account,—a conclusion opposed by Plana, Hansen, and -Pontécoulant, but fully confirmed by Delaunay. Delaunay suggested in -1865 that the minute apparent increase was due to the retardation of the -Earth’s rotation by tidal friction. This brings us to the subject of -celestial evolution, which is discussed in another chapter. - - - - - CHAPTER V. - THE INNER PLANETS. - - -Much progress has been made during the last hundred years in our -knowledge of the planets. In fact, the study of Mercury only dates from -the commencement of the nineteenth century. Our knowledge of the -vicinity of the Sun is very limited, and Mercury is difficult of -observation. So limited, in fact, is our knowledge of the Sun’s -surroundings, that it is not yet known for certain whether there is a -planet, or planets, between Mercury and the Sun. Perturbations in the -motion of the perihelion of Mercury’s orbit led Le Verrier in 1859 to -the belief that a planet of about the size of Mercury, or else a zone of -asteroids, existed between Mercury and the Sun. It was, however, obvious -that such a planet could only be seen when in transit across the Sun’s -disc, or during a total eclipse. Meanwhile a French doctor, Lescarbault, -informed Le Verrier that he had seen a round object in transit over the -Sun’s disc. Le Verrier, certain that this was the missing planet, named -it “Vulcan,” and calculated its orbit, assigning it a revolution period -of twenty days. But it was never seen again. Transits of “Vulcan” were -fixed for 1877 and 1882, but nothing was seen on these dates. During the -total eclipse of July 29, 1878, two observers—_James Watson_ -(1838-1880), the well-known astronomer, and _Lewis Swift_ (born -1820)—believed themselves to have discovered two separate planets, and -ultimately claimed two planets each, which were never heard of again. -During the total eclipse of 1883 an active watch for “suspicious -objects” was kept, but with no result. At the eclipses of 1900 and 1901 -respectively, photographs were exposed by the American astronomers, W. -H. Pickering and _Charles Dillon Perrine_ (born 1867), but on none of -these plates could any trace of “Vulcan” be found. At the total eclipse -of August 30, 1905, plates were again exposed, but no announcement has -been made of an intra-Mercurial planet; and the prevalent opinion among -astronomers is that no planet comparable with Mercury in size exists -between that planet and the Sun. - -The study of the physical appearance of Mercury was inaugurated by -Schröter, who in 1800 noticed that the southern horn of the crescent -presented a blunted appearance, which he attributed to the existence of -a mountain eleven miles in height. From observations of this mountain he -came to the conclusion that the planet rotated in 24 hours 4 minutes. -This was afterwards reduced by _Friedrich Wilhelm Bessel_ (1784-1846) to -24 hours 53 seconds. - -After the time of Schröter there was no astronomer who paid much -attention to either Mercury or Venus until the arrival on the scene of -the most persistent planetary observer and one of the foremost -astronomers of the nineteenth century. _Giovanni Virginio Schiaparelli_ -was born at Savigliano, in Piedmont, in 1835, and graduated at Turin in -1854. Called to Milan as assistant in the Brera Observatory in 1860, he -became director in 1862, and there for thirty-eight years he studied -astronomy in all its aspects, making a great name for himself in various -branches of the science. In 1900 he retired from the post of director, -and pursues his astronomical researches in his retirement. - -In 1882 Schiaparelli took up the study of Mercury in the clear air of -Milan. Instead of observing the planet through the evening haze, like -Schröter and others, he examined it by day, and was enabled to follow it -hourly instead of looking at it for a short period when near the -horizon. At length, after seven years’ observation, he announced, on -December 8, 1889, that Mercury performs only one rotation during its -revolution round the Sun—in fact, that its day and year coincide. As a -consequence, the planet keeps the same face towards the Sun, one side -having everlasting day and the other perpetual night; but owing to the -libratory movement of Mercury—the result of uniform motion on its axis -and irregular motion in its orbit—the Sun rises and sets on a small zone -of the planet’s surface. Schiaparelli’s observations indicated that -Mercury is a much spotted globe, with a moderately dense atmosphere, and -he was enabled to form a chart of its surface-markings. - -Schiaparelli’s conclusions remained until 1896 unconfirmed and yet not -denied, although most astronomers were sceptical on the subject. In 1896 -the subject was taken up by the American astronomer, _Percival Lowell_ -(born 1855), who, in the clear air of Arizona, confirmed Schiaparelli’s -conclusions, fixing 88 days as the period of rotation. He remarked, -however, that no signs of an atmosphere or clouds were visible to him. -The surface of Mercury, he says, is colourless,—“a geography in black -and white.” The determination of the rotation period by Schiaparelli and -Lowell is now generally accepted, and is confirmed by the theory of -tidal friction. It is only right to add that _William Frederick Denning_ -(born 1848) in 1881 suspected a rotation period of 25 hours, but this -remains unconfirmed. In April 1871 the spectrum of Mercury was examined -by _Hermann Carl Vogel_ (born 1842) at Bothkamp. He suspected traces of -an atmosphere similar to ours, but was not certain. Of more interest are -the photometric observations of Zöllner in 1874. These observations -indicated that the surface of Mercury is rugged and mountainous, and -comparable with the Moon,—a conclusion supported by Lowell’s -observations in 1896. - -Venus, the nearest planet to the Earth, has been attentively studied for -three centuries, and still comparatively little is known regarding it. -This is due to its remarkable brilliancy, combined with its proximity to -the Sun. The great problem at the beginning of the nineteenth century -was the rotation of the planet. In 1779 the subject was taken up by -Schröter at Lilienthal. Nine years later, from a faint streak visible on -the disc, he concluded that rotation was performed in 23 hours 28 -minutes, and in 1811 this was reduced by seven minutes; but as Herschel -was unable to observe the markings seen by Schröter, many astronomers -were inclined to be sceptical regarding the accuracy of the Lilienthal -observers results. Schröter also observed the southern horn of Venus -when in the crescent form to be blunted, and he ascribed this to the -existence of a great mountain, five or six times the elevation of -Chimborazo; while he observed irregularities along the terminator, which -he considered to be more strongly marked than those on the Moon. -Schröter’s opinion on this point, although rejected by Herschel, was -confirmed by Mädler, Zenger, Ertborn, Denning, and by the Italian -astronomer _Francesco Di Vico_ (1805-1848), director of the Observatory -of the Collegio Romano. In 1839 Di Vico attacked the problem of the -rotation, and his results were confirmatory of those of Schröter. He -estimated that the axis of Venus was inclined at an angle of 53° to the -plane of its orbit. Meanwhile a series of important observations had -been made on Venus by the Scottish astronomer and theologian, _Thomas -Dick_ (1772-1857), who suggested daylight observations on Venus to solve -the problem of the rotation. - -In 1877 the question was attacked by Schiaparelli, who commenced a -series of observations on Venus at Milan in that year. The results of -his studies were summed up in 1890 in five papers contributed to the -Milan Academy. He came to the conclusion that the markings observed by -Schröter, Di Vico, and others were not really permanent, and -concentrated his attention on round white spots, which remained fixed in -position. Instead of observing Venus in the evening, Schiaparelli -followed it by day, watching it continuously on one occasion for eight -hours. But the markings remained fixed. Schiaparelli accordingly -concluded that the planet’s rotation was performed in probably 225 days, -equal to the time of revolution. One face is turned towards the Sun -continually, while the other is perpetually in darkness. - -The announcement was so startling that, as Miss Clerke says, “a clamour -of contradiction was immediately raised, and a large amount of evidence -on both sides of the question has since been collected.” Perrotin at -Nice, Tacchini at Rome, Cerulli at Teramo, Mascari at Catania and Mount -Etna, and Lowell in Arizona, all in favourable climates, confirmed -Schiaparelli’s results, as also did a second series of observations by -the Milan astronomer himself in 1895. On the other hand, Neisten, -Trouvelot, _Camille_ _Flammarion_ (born 1842), and others, under less -favourable climatic conditions, arrived at a period of 24 hours. -_Aristarch Bélopolsky_ (born 1854), from spectroscopic observations at -Pulkowa, by means of Doppler’s principle, found a period of 12 hours. -Lowell, by the same principle, found, in 1901-03, a period of 225 days, -in agreement with Schiaparelli’s results. This is the last word on the -subject. Schiaparelli’s rotation period, confirmed by the theory of -tidal friction, is generally accepted. - -That Venus has an atmosphere was one of the conclusions reached by -Schröter in 1792; and in this at least he was correct, as the atmosphere -of Venus, illuminated by the solar rays, has been seen extending round -the entire disc of the planet. Spectroscopic observations by Tacchini, -Ricco, and Young, during the transits of 1874 and 1882, indicated the -existence of water-vapour in the planet’s atmosphere. Very little has -been discovered regarding the “geography” of Venus. White patches at the -supposed “poles” of the planet were observed in 1813 by _Franz von -Gruithuisen_, and in 1878 by the French astronomer _Trouvelot_ -(1827-1895). The secondary light of Venus, similar to the “old Moon in -the new Moon’s arms,” was repeatedly observed since the time of Schröter -by Vogel, Lohse, Zenger, and others. Vogel attributed it to twilight, -and Lamp, a German observer, to electrical processes analogous to our -auroræ. In 1887 a Belgian astronomer, _Paul Stroobant_, submitted to a -searching examination all the supposed observations of a satellite of -Venus, and was enabled to explain nearly all the supposed satellites as -small stars which happened to lie near the planet’s path in the sky at -the time of observation. - -The study of our own planet can hardly be said to belong to the realm of -astronomy. Nevertheless, it is through astronomical observation that the -motion of the North Pole has been discovered. For many years it has been -a problem whether there is a variation of latitude resulting from the -motion of the pole. Euler had declared, from theoretical investigation, -that, were there such a motion, the period must be 10 months. The -question was revived in 1885 by the observations of _Seth Carlo -Chandler_ (born 1846) at Cambridge, Mass., with his newly-invented -instrument, the “almucantar,” which indicated an appreciable variation -of latitude. This was confirmed by _Friedrich Küstner_ (born 1856), now -director of the Observatory at Bonn. The idea now occurred to Chandler -to search through the older records to discover if there was any trace -of the variation of latitude, with the result that he brought out a -period of 14 months instead of 10. This aroused much interest, and many -prominent astronomers denied Chandler’s results, which were announced in -1891. As a well-known astronomer has expressed it, “Euler’s work had -shown what period the motion must have, and any appearance of another -period must be due to some error in the observations. Chandler replied -to the effect that he did not care for Euler’s mathematics: the -observations plainly showed 14 months, and if Euler said 10, _he_ must -have made the mistake. I do not exaggerate the situation in the least; -it was a deadlock: Chandler and observation against the whole weight of -observation and theory.” It was now shown by Newcomb that Euler had -assumed the Earth to be an absolutely rigid body, while modern -investigations show that it is not so. Chandler’s discovery is now -accepted, and proves that the North Pole is not fixed in position, but -has a small periodic motion, though never twelve yards from its mean -position. That the small resulting variation in the position of the -stars has been noticed at all is a striking illustration of the accuracy -of astronomical observation. - -Of all the planets Mars has been most studied during the nineteenth -century. Many illustrious astronomers have devoted years to the study of -the red planet, with the result that more is known of the surface of -Mars than of any other celestial body, with the exception of the Moon. -After the time of Herschel, the leading students of Mars were Beer and -Mädler, who carefully studied the planet from 1828 to 1839. They -identified at each opposition the same dark spots, frequently obscured -by mists, and they also made the most accurate determination of the -rotation period, which they fixed at 24 hours 37 minutes 23 seconds. -This estimate was confirmed in 1862 by _Friedrich Kaiser_ (1808-1872) of -Leyden, in 1869 by _Richard Anthony Proctor_ (1837-1888), and in 1892 by -_Henricius Gerardus van de Sande Bakhuyzen_ (born 1838), director of the -Leyden Observatory. In 1862 Lockyer identified the various markings seen -by Beer and Madler in 1830. The other great names in Martian study prior -to 1877 are Angelo Secchi and _William Rutter Dawes_ (1799-1868), who -studied Mars from 1852 to 1865 and secured a very valuable series of -drawings. These drawings were used by Proctor for the construction of -the first reliable map of Mars, which was published in 1870 in his work, -‘Other Worlds than Ours.’ Proctor gave names to the various Martian -features, the reddish-ochre portions of the disc being named continents -and the bluish-green portions seas; and Proctor’s views on Mars found -favour for many years. In 1877, however, Schiaparelli opened a new era -in the study of Mars. In September of that year, during the very -favourable opposition of the planet, Schiaparelli, while executing a -trigonometrical survey of the disc, discovered that the continents were -cut up by numerous long dark streaks, which he called _canali_. In 1879, -to his surprise, he found that some of the canals had become double; and -he confirmed this in 1881 and at subsequent oppositions. Meanwhile, as -Schiaparelli was the only observer who had hitherto seen the canals, -there was much scepticism as to their reality. In 1886, however, they -were seen at the Nice Observatory by _Henri Perrotin_ (1845-1904), who -also observed their duplication. Since 1886 they have been observed by -many astronomers, including Camille Flammarion in France, _William -Frederick Denning_ (born 1848) in England, _Vincenzo Cerulli_ (born -1859) in Italy, Percival Lowell and W. H. Pickering in the United -States. In 1892 W. H. Pickering successfully observed the canals, and -discovered at the junctions of two or more canals round black spots, to -which he gave the name of “lakes,” in keeping with the view that the -dark regions of the planet were seas. - -In 1894 Percival Lowell erected at Flagstaff, Arizona, an observatory -for the specific purpose of observing Mars and its canals in good and -steady air. He was assisted by W. H. Pickering and by _Andrew Ellicott -Douglass_ (born 1867). During a year’s study Douglass measured the -Martian atmosphere and discovered canals crossing the dark regions of -the planet, finally disproving the idea of their aqueous character. -Lowell recognised all Schiaparelli’s canals, and discovered many more. -He also attentively studied the south polar cap of Mars, which -disappeared entirely on October 12, 1894. Lowell noticed, also, that as -the cap melted the canals became darker, as if water was being conveyed -down; and accordingly he adopted the view put forward by Schiaparelli, -that the canals are waterways lined on either side by banks of -vegetation. His observations were published in the end of 1895 in his -work ‘Mars.’ He is of opinion that the reddish-ochre regions or -“continents” are deserts, and the greenish areas marshy tracts of -vegetation. The lakes are named by him “oases,” and, as Miss Clerke -observes, he “does not shrink from the full implication of the term.” He -regards the canals as strips of vegetation fertilised by a small canal, -much too small to be seen, an idea which originated with W. H. -Pickering. The canals are believed by Lowell to be waterways down which -the water from the melting polar cap is conveyed to the various oases. -He considers, in fact, that the canals are constructed by intelligent -beings with the express purpose of fertilising the oases, regarded by -him as centres of population. He remarks that water is scarce on the -planet, owing to its small size, and as a consequence the inhabitants -are forced to utilise every drop. The canal system is the result. - -Lowell’s theory has not been cordially received—although it is now -gradually gaining popularity,—and several other hypotheses have been -propounded to explain the canals. Proctor, who died some years before -Lowell’s theory was given to the world, regarded them as rivers, but -this view may now be looked upon as abandoned. It was suggested that the -canals might be cracks in the surface of Mars or meteors ploughing -tracks above it: and Professor _John Martin Schaeberle_ (born 1853) of -the Lick Observatory put forward the view that the canals were chains of -mountains running over the light and dark regions. None of these -theories, however, gained popularity, and had to give way to a more -popular theory, the “illusion” hypothesis, put forward by the Italian -astronomer Cerulli, and supported by Newcomb and Maunder. On the basis -of the illusion theory, Newcomb explains that the “canaliform” -appearance “is not to be regarded as a pure illusion on the one hand or -an exact representation of objects on the other. It grows out of the -spontaneous action of the eye in shaping slight and irregular -combinations of light and shade, too minute to be separately made out -into regular forms.” Experiments were made by Maunder in 1902, and the -results pointed to the truth of the theory that the canals were really -illusions. But the studies of Lowell at the oppositions of 1903 and 1905 -have seriously weakened the hypothesis of Cerulli and Maunder, and -strongly confirm the theory of the artificial origin of the canals. In -1903 Lowell was enabled, from a study of the development of the canals, -to show the probability of their artificial nature, and his study of the -double canals showed a distinct plan in their distribution. Finally, on -May 11, 1905, several photographs of Mars were secured at the Lowell -Observatory, on which the canals appeared, not as dots of light and -shade, as on the illusion theory, but as straight dark lines. This goes -far to prove the reality of the canals,—in spite of the ridicule cast on -them and their observers,—and consequently the truth of the theory of -intelligent life in Mars. - -Meanwhile the old-fashioned Martian observations have been continued in -less favourable climates than Arizona and Italy by various astronomers, -among them the famous Camille Flammarion, the American astronomers -_James Edward Keeler_ (1857-1900), _Edward Emerson Barnard_ (born 1857), -the English astronomer W. F. Denning, and others. These conscientious -and painstaking observers have done much for Martian study in increasing -the number of accurate delineations of the Martian surface. - -The spectrum of Mars was first examined by Huggins in 1867. He found -distinct traces of water-vapour, and this was confirmed by Vogel in -1872, and by Maunder some years later. In 1894, however, _William -Wallace Campbell_ (born 1862), the American astronomer, observing from -the Lick Observatory, California, was unable to detect the slightest -difference between the spectra of Mars and the Moon, indicating that -Mars had no appreciable atmosphere; and from this he deduced that the -Martian polar caps could not be composed of snow and ice, but of frozen -carbonic acid gas. In 1895, however, Vogel confirmed his previous -observations, and reaffirmed the presence of water-vapour in the Martian -atmosphere. - -During the opposition of 1830, Mädler undertook an extensive search for -a Martian satellite, but was unsuccessful. In 1862 the search was -resumed by _Heinrich Louis D’Arrest_ (1822-1875), the famous German -observer, who was also unsuccessful. Accordingly the red planet was -referred to by Tennyson as the “moonless Mars.” In 1877 the search was -taken up by _Asaph Hall_, the self-made American astronomer, born at -Goshen, Connecticut, in 1829, and employed from 1862 to 1891 at the -Naval Observatory, Washington. During the famous opposition of August -1877, favoured by the great 26-inch refractor, he succeeded in -discovering two very small satellites of Mars, to which he gave the -names of Phobos and Deimos. He determined the time of revolution of -Phobos at 7 hours 39 minutes, and that of Deimos at 30 hours 17 -minutes,—Phobos revolving round Mars more than three times for one -rotation of the planet on its axis. These two satellites are very small, -not more than thirty miles in diameter. After Hall’s successful search, -photographs were exposed at the Paris Observatory for other Martian -satellites, but none was discovered. No further moons have been found -belonging to the red planet, nor is it likely that any further -satellites of Mars are in existence. - -The discovery of a zone of small planets in the space between Mars and -Jupiter belongs completely to the nineteenth century, although the -existence of a planet in the vacant space was suspected three centuries -ago. In 1772 the subject was taken up by _Johann Elert Bode_ -(1747-1826), afterwards director of the Berlin Observatory, who -investigated a curious numerical relationship, since known as Bode’s -Law, connecting the distances of the planets. If four is added to each -of the numbers—0, 3, 6, 12, 24, 48, 96, and 192, the resulting series -represents pretty accurately the distances of the planets from the Sun, -thus—4 (Mercury), 7 (Venus), 10 (The Earth), 16 (Mars), 28, 52, -(Jupiter), and 100 (Saturn). After the discovery of Uranus, in 1781, it -was found that it filled up the number 196. Bode, however, saw that the -number 28, between Mars and Jupiter, was vacant, and predicted the -discovery of the planet. Aided by _Franz Xavier von Zach_ (1754-1832), -he called a congress of astronomers, which assembled in 1800 at -Schröter’s observatory at Lilienthal, when, for the purpose of searching -for the missing planet, the zodiac was divided into twenty-four zones, -each of which was given to a separate astronomer. One of them was -reserved for _Giuseppe Piazzi_ (1746-1826), director of the Observatory -of Palermo. - -Born in 1746 at Ponte, in Lombardy, Giuseppe Piazzi, after entering the -Theatine Order of monks, became in 1780 Professor of Mathematics at -Palermo, where an observatory was erected in 1791; and at that -observatory Piazzi worked till his death in 1826. In 1792 he commenced a -great star-catalogue, and while making his nightly observations he -discovered, on January 1, 1801—the first night of the nineteenth -century,—what he took to be a tailless comet, but which proved to be a -small planet revolving round the sun in the vacant space. The discovery -was hailed by Bode and Von Zach with much enthusiasm, and Piazzi named -the planet Ceres. The little planet was, however, soon lost in the rays -of the sun before sufficient observations had been made; but the great -mathematician, _Friedrich Gauss_ (1777-1855), came to the rescue, and -pointed out the spot where the planet was to be rediscovered. In that -spot it was found on December 31, 1801, by Von Zach at Gotha, and on the -following evening by _Heinrich Olbers_ (1758-1840) at Bremen. - -On March 28, 1802, while observing Ceres from his house at Bremen, -Olbers was struck by the presence of a strange object near the path of -the planet. At first he supposed it to be a variable star at maximum -brilliance, but a few hours showed him that it was in motion, and was -therefore another planet. He named it Pallas, and propounded the theory -that the two “Asteroids”—so named by Herschel—were fragments of a -trans-Martian planet, which, through some accident, had been shattered -to pieces in the remote past. Olbers urged the necessity of searching -for more small planets. His advice was taken. In 1804 _Karl Ludwig -Harding_ (1765-1834), Schröter’s assistant, discovered Juno, and Olbers -himself detected Vesta, March 29, 1807. - -After 1816 the search was relinquished, as no more planets were -discovered. In 1830, however, a German amateur, _Karl Ludwig Hencke_ -(1793-1866), ex-postmaster of Driessen, commenced a search for new -planets, which was rewarded, after fifteen years, by the discovery of -Astræa, December 8, 1845. On July 1, 1847, he made another discovery, -that of Hebe. A few weeks later, _John Russell Hind_ (1823-1895), the -English astronomer, discovered Iris. Since 1847 not a year has passed -without one or more planets being found, sometimes as many as twenty -being discovered in a single year. Some astronomers have made the search -for asteroids their chief business. The principal asteroid discoverers -have been _Christian H. F. Peters_ (1813-1890), Henri Perrotin, _Paul -Henry_ (1848-1905), _Prosper Henry_ (1849-1903), James Watson, _Robert -Luther_ (1822-1900), _Johann Palisa_ (born 1848), and _Max Wolf_ (born -1863). - -In 1891 a new impulse was given to asteroid study by the application of -photography by Max Wolf to the discovery of the minor planets. It -occurred to Wolf that the asteroid would be represented on the plate by -a trail, caused by its motion during the time of exposure; and assisted -by _Arnold Schwassmann_ (born 1870), _Luigi Carnera_ (born 1875), and -others, Wolf has discovered over a hundred asteroids, and he has the -whole field of asteroid hunting to himself. Few minor planets are now -discovered by the older method. In 1901 Wolf invented his new instrument -of research, the stereo-comparator, which, on the principle of the -old-fashioned stereoscope, represents the planetary bodies as suspended -in space far in front of the stars. In this way this ingenious -astronomer has been enabled to discover asteroids at the first glance: -year by year fresh discoveries are announced from the Heidelberg -Observatory, until more than five hundred asteroids are now known. - -Waning interest in the ever-increasing family of asteroids was revived -in 1898 by the discovery by _Karl Gustav Witt_ (born 1866) of a small -planet, to which he gave the name of Eros, which comes nearer to the -Earth than Mars, and which is of great assistance to astronomers in the -determination of the solar parallax. For some time prior to 1898 -astronomers had considered it a waste of time to search for new -asteroids; but this idea is not now so popular, in view of the benefit -conferred on astronomy by the discovery of Eros. - -Of the physical nature of the asteroids astronomers know nothing. Only -the four largest have been measured. For many years it was supposed that -Vesta, the brightest of the asteroids, was also the largest. The -measures of Barnard with the great Lick refractor in 1895, however, -showed that Ceres is the largest, with a diameter of 477 miles. Pallas -comes next, with a diameter of 304 miles; while the diameters of Vesta -and Juno are respectively 239 and 120 miles. Barnard saw no traces of -atmosphere round any of the asteroids. It should be stated that in 1872 -Vogel thought he could detect an “air-line” in the spectrum of Vesta: he -admitted that the observation required confirmation, but it has not been -corroborated either by himself or any other observer. - - - - - CHAPTER VI. - THE OUTER PLANETS. - - -Jupiter, the greatest planet of the Solar System, has perhaps been more -persistently studied by astronomers than any other. In the early -nineteenth century the prevalent idea was that Jupiter was a world -similar to the Earth, only much larger,—a view held by Herschel and -other famous astronomers, and put forward by Brewster in ‘More Worlds -than One.’ This view prevailed for many years, although Buffon in 1778, -and Kant in 1785, had stated their belief in the idea that Jupiter was -still in a state of great heat—in fact, that the great planet was a -semi-sun. This idea, however, was long in being adopted by astronomers, -and very little attention was paid to Nasmyth’s expression of the same -opinion in 1853. The older view still held the field—namely, that the -belts of Jupiter represented trade-winds, and that a world similar to -the terrestrial lay below the Jovian clouds. In 1860 _George Philip -Bond_ (1826-1865), director of the Harvard Observatory, found from -experiments that Jupiter seemed to give out more light than it received, -but he did not dare to suggest that Jupiter was self-luminous, -considering that the inherent light might result from Jovian auroras. - -In 1865 Zöllner showed that the rapid motions of the cloud-belts on both -Jupiter and Saturn indicated a high internal temperature. At the -distance of Jupiter sun-heat is only one twenty-seventh as great as on -the Earth, and would be quite incapable of forming clouds many times -denser than those on the Earth. In 1871 Zöllner drew attention to the -equatorial acceleration of Jupiter, analogous to the same phenomenon on -the Sun. In 1870 these opinions of Zöllner’s were adopted and supported -by Proctor in his ‘Other Worlds than Ours.’ In his subsequent volumes -Proctor did much to popularise the idea, which is now accepted all over -the astronomical world. - -During the century many valuable observations on Jupiter were made by -numerous observers, among them Airy, Mädler, Webb, Schmidt, and others. -Much time was devoted to the accurate determination of the rotation -period, which was fixed at 9 hours 55 minutes 36·56 seconds by Denning -in observations from 1880 to 1903. No really important discovery was -made till 1878, when Niesten at Brussels discovered the “great red -spot,” a ruddy object 25,000 miles long by 7000 broad, attached to a -white zone beneath the southern equatorial belt. This remarkable object -has been observed ever since. In 1879 its colour was brick-red and very -conspicuous, but it soon began to fade, and Riccó’s observation at -Palermo in 1883 was thought to be the last. After some months, however, -it brightened up, and, notwithstanding changes of form and colour, it is -still visible, a permanent feature of the Jovian disc. In 1879 a group -of “faculæ,” similar to those on the Sun, was observed at Moscow by -_Theodor Alexandrovitch Brédikhine_ (1831-1904), and at Potsdam by -_Wilhelm Oswald Lohse_ (born 1845). It was soon observed that the -rotation period, as determined from the great red spot, was not -constant, but continually increasing. A white spot in the vicinity -completed its rotation in 5½ minutes less, indicating the differences of -rotation on Jupiter. - -The great red spot has been observed since its discovery by Denning at -Bristol and _George Hough_ (born 1836) at Chicago. Twenty-eight years of -observation have not solved the mystery of its nature. The researches -made on it, in the words of Miss Clerke, “afforded grounds only for -negative conclusions as to its nature. It certainly did not represent -the outpourings of a Jovian volcano; it was in no sense attached to the -Jovian soil—if the phrase have any application to the planet; it was not -a mere disclosure of a glowing mass elsewhere seethed over by rolling -vapours.” - -In 1870 _Arthur Cowper Ranyard_ (1845-1894), the well-known English -astronomer, began to collect records of unusual phenomena on the Jovian -disc to see if any period regulated their appearance. He came to the -conclusion that, on the whole, there was harmony between the markings on -Jupiter and the eleven-year period on the Sun. The theory of inherent -light in Jupiter, however, has not been confirmed. The great planet was -examined spectroscopically by Huggins from 1862 to 1864, and by Vogel -from 1871 to 1873. The spectrum showed, in addition to the lines of -reflected sunlight, some lines indicating aqueous vapour, and others -which have not been identified with any terrestrial substance. A -photographic study of the spectrum of Jupiter was made at the Lowell -Observatory by Slipher in 1904, probably the most exhaustive -investigation on the subject. The spectroscope has, however, given -little support to the theory of inherent light, and “we are driven to -conclude that native emissions from Jupiter’s visible surface are local -and fitful, not permanent and general.” - -Herschel’s idea, that the rotations of the four satellites of Jupiter -were coincident with their revolutions, has on the whole been confirmed -by recent researches, although in the case of the two near satellites -(Io and Europa) W. H. Pickering’s observations in 1893 indicated shorter -rotation periods. There is much to learn regarding the geography of the -satellites, although in 1891 Schaeberle and Campbell at the Lick -Observatory observed belts on the surface of Ganymede, the third -satellite analogous to those on Jupiter. Surface-markings on the -satellites have also been seen by Barnard at the Lick Observatory, and -by Douglass at Flagstaff. - -Since the time of Galileo no addition had been made to the system of -satellites revolving round Jupiter. Profound surprise was created, -therefore, by the announcement of the discovery of a fifth satellite by -Barnard at the Lick Observatory, on September 9, 1892. The satellite, -one of the faintest of telescopic objects, was discovered with the great -36-inch telescope, and its existence was soon confirmed by _Andrew -Anslie Common_ (1841-1903), with his great 5-foot reflector at Ealing, -near London. The new satellite was found by Barnard to revolve round -Jupiter in 11 hours 57 minutes at a mean distance of 112,000 miles. - -Although the existence of other satellites of Jupiter was predicted by -Sir _Robert Stawell Ball_ (born 1840) soon after the discovery of the -fifth, much surprise was created by the announcement, in January 1905, -that a sixth satellite had been discovered by Perrine, who, in the -following month, announced the discovery of a seventh. These discoveries -were made by photography, the objects being very faint. The periods of -revolution were found to be 242 days and 200 days for the sixth and -seventh satellites respectively, the mean distances being 6,968,000 and -6,136,000 miles. It is possible that they may belong to a zone of -asteroidal satellites. In fact, the fifth moon may belong to a similar -zone, so that Jupiter may have two asteroidal zones; but this is -anticipating future discovery. - -A particular charm has always attached itself to the study of Saturn, -the ringed planet. The magnificent system of rings has for two and a -half centuries been the object of wonder and admiration in the Solar -System, and accordingly they have been exhaustively studied by many -eminent observers. While observing the two bright rings of Saturn on -June 10, 1838, Galle noticed what Miss Clerke calls “a veil-like -extension of the lucid ring across half the dark space separating it -from the planet.” No attention, however, was paid to Galle’s -observation. On November 15, 1850, _William Cranch Bond_ (1789-1859), of -the Harvard Observatory in Massachusetts, discovered the same phenomenon -under its true form—that of a dusky ring interior to the more brilliant -one. A fortnight later, before the news of Bond’s observation, Dawes -made the same discovery independently at Wateringbury in England. This -ring is known as the dusky or “crape” ring. - -The discovery of the dusky ring brought to the front the problem of the -composition of the ring-system. Laplace and Herschel considered the -rings to be solid, but this was denied in 1848 by _Edouard Roche_ -(1820-1880), who believed them to consist of small particles, and in -1851 by G. P. Bond, who asserted that the variations in the appearance -of the system were sufficient to negative the idea of their solidity; -but he suggested that the rings were fluid. In 1857 the question was -taken up by the Scottish physicist, _James Clerk-Maxwell_ (1831-1879), -who proved by mathematical calculation that the rings could be neither -solid nor fluid, but were due to an aggregation of small particles, so -closely crowded together as to present the appearance of a continuous -whole. Clerk-Maxwell’s explanation—which had been suggested by the -younger Cassini in 1715, and by Thomas Wright in 1750—was at once -adopted, and has since been proved by observation. In 1888 _Hugo -Seeliger_ (born 1849), director of the Munich Observatory, showed from -photometric observations the correctness of the satellite-theory; while -Barnard in 1889 witnessed an eclipse of the satellite Japetus by the -dusky ring. The satellite did not disappear, but was seen with perfect -distinctness. The final demonstration of the meteoric nature of the -rings was made by Keeler at the Alleghany Observatory in 1895, with the -aid of the spectroscope. By means of Doppler’s principle, he found that -the inner edge of the ring revolved in a much shorter time than the -outer, proving conclusively that they could not be solid. This was -confirmed by the observations of Campbell at Mount Hamilton, _Henri -Deslandres_ at Meudon, and Bélopolsky at Pulkowa. - -In 1851 a startling theory regarding Saturn’s rings was put forward by -the famous _Otto Wilhelm von Struve_ (1819-1905). Comparing his -measurements on the rings made at Pulkowa in 1850 and 1851 with those of -other astronomers for the past two hundred years, he reached the -conclusion that the inner diameter of the ring was decreasing at the -rate of sixty miles a-year, and that the bodies composing the rings were -being drawn closer to the planet. Accordingly, Struve calculated that -only three centuries would be required to bring about the precipitation -of the ring-system on to the globe of Saturn. In 1881 and 1882 Struve, -expecting a further decrease, made another series of measures, but these -did not confirm his theory, which was accordingly abandoned. - -The study of the globe of Saturn has made less progress than that of the -rings. The surface of the planet had been known since before the time of -Herschel to be covered with belts, but as spots seldom appear on Saturn, -only one determination of the rotation period had been made, that by -Herschel. Much interest was aroused, therefore, by the discovery, by -Hall, at Washington, on December 7, 1876, of a bright equatorial spot. -Hall studied this spot during sixty rotations of the planet, determining -the period as 10 hours 14 minutes 24 seconds. This was confirmed by -Denning in 1891, and by _Stanley Williams_, an English observer, in the -same year. On June 16, 1903, Barnard, at the Yerkes Observatory, -discovered a bright spot, from which he deduced a rotation period of 10 -hours 39 minutes,—a period considerably longer than that found by Hall. -In the same year various spots on Saturn were observed by Denning, who -found a period of 10 hours 37 minutes 56·4 seconds, and at Barcelona by -_José Comas Sola_, now director of the Observatory there, who may be -considered Spain’s leading astronomer. The result of these observations -has been to show that the spots on Saturn have probably a proper motion -of their own, apart from the rotation of the planet. As to the spectrum -of Saturn, little has been learned. It closely resembles that of -Jupiter. In 1867 Janssen, observing from the summit of Mount Etna, found -traces of aqueous vapour in the planet’s atmosphere. - -In the chapters on Herschel we have seen that he discovered the sixth -and seventh satellites of Saturn. The next discovery was made on -September 19, 1848, by W. C. Bond, at Harvard, Massachusetts, and -independently by _William Lassell_ (1799-1880), at Starfield, near -Liverpool. The new satellite received the name of Hyperion, and was -found to be situated at a distance of about 946,000 miles from Saturn. -Its small size led Sir John Herschel to the idea that it might be an -asteroidal satellite. Fifty years elapsed before another satellite of -Saturn was discovered. In 1888 W. H. Pickering commenced a photographic -search for new satellites of the planet. At last, on developing some -photographs of Saturn, taken on August 16, 17, and 18, 1898, he found -traces of a new satellite which he named “Phœbe.” But, as the satellite -was not seen or photographed again for some years, many astronomers were -sceptical as to its existence. However, photographs taken in 1900, 1901, -and 1902 revealed the satellite, which was again photographed in 1904, -and seen visually by Barnard in the same year with the 40-inch Yerkes -telescope. At that time the discoverer brought out the amazing fact that -the motion of the satellite is retrograde—a fact which he attempts to -explain by a new theory of the former rotation of Saturn. He likewise -demonstrated that its distance from Saturn varied from 6,120,000 to -9,740,000 miles. Early in 1905 Pickering announced the discovery of a -tenth satellite of Saturn, which received the name of Themis, with a -period and mean distance nearly similar to Hyperion, so that Sir John -Herschel’s idea of Hyperion being an asteroidal satellite is being -confirmed after a lapse of half a century. - -If little is known of the globe of Saturn, still less is known regarding -Uranus. Dusky bands resembling those of Jupiter were observed by Young -at Princeton in 1883. In the following year Paul and Prosper Henry -discerned at Paris two grey parallel lines on the disc of the planet. -This was confirmed by the observations of Perrotin at Nice, which also -indicated rotation in a period of ten hours. In 1890 Perrotin again took -up the study and re-observed the dark bands. On the other hand, no -definite results regarding the planet were obtained by the Lick -observers in 1889 and 1890. Measurements of the planet by Young, -Schiaparelli, Perrotin, and others indicate a considerable polar -compression. The spectrum of the planet has been studied by Secchi, -Huggins, Vogel, Keeler, Slipher, and others. The spectrum shows six -bands of original absorption, a line of hydrogen, which, says Miss -Clerke, “implies accordingly the presence of free hydrogen in the -Uranian atmosphere, where a temperature must thus prevail sufficiently -high to reduce water to its constituent elements.” From a photographic -study of the spectrum at the Lowell Observatory in 1904, Slipher -observed a line corresponding to that of helium, indicating the presence -of that element in the planet’s atmosphere. - -Herschel left our knowledge of the Uranian satellites in a very -uncertain state. The two outer satellites, Titania and Oberon, were -rediscovered in 1828 by his son, but the other four, which he was -believed to have discovered, were never seen again. In 1847 two inner -satellites, Ariel and Umbriel, were discovered by Lassell and Otto -Struve respectively, their existence being finally confirmed by -Lassell’s observations in 1851. - -After the discovery of Uranus by Herschel, mathematical astronomers -determined its orbit and calculated its position in the future. _Alexis -Bouvard_, the calculating partner of Laplace, published tables of the -planet’s motions, founded on observations made by various astronomers -who had considered it a star before its discovery by Herschel; but as -the planet was not in the exact position which Bouvard predicted, he -rejected the earlier observations altogether. For a few years the planet -conformed to the Frenchman’s predictions, but shortly afterwards it was -again observed to move in an irregular manner, and the discrepancy -between observation and the calculations of mathematicians became -intolerable. Did the law of gravitation not hold good for the frontiers -of the Solar System? Gradually astronomers arrived at the conclusion -that Uranus was being attracted off its course by the influence of an -unseen body, an exterior planet. Bouvard himself was one of the first to -make the suggestion, but died before the planet was discovered. An -English amateur, the Rev. _T. J. Hussey_, resolved to make, in 1834, a -determination of the place of the unseen body, but found his powers -inadequate; and in 1840 Bessel laid his plans for an investigation of -the problem, but failing health prevented him carrying out his design. - -In 1841 a student at the University of Cambridge resolved to grapple -with the problem. John Couch Adams, born at Lidcot in Cornwall in 1819, -entered in 1839 the University of Cambridge, where he graduated in 1843. -From 1858 Professor of Astronomy at Cambridge, and from 1861 director of -the Observatory, he died on January 21, 1892, after a life spent in -devotion to mathematical astronomy. In 1843, on taking his degree, he -commenced the investigation of the orbit of Uranus. For two years he -worked at the difficult question, and by September 1845 came to the -conclusion that a planet revolving at a certain distance beyond Uranus -would produce the observed irregularities. He handed to _James Challis_ -(1803-1882), the director of the Cambridge Observatory, a paper -containing the elements of what was named by Adams “the new planet.” On -October 21 of the same year he visited Greenwich Observatory, and left a -paper containing the elements of the planet, and approximately fixing -its position in the heavens. But the Astronomer-Royal of England, Sir -_George Biddell Airy_ (1801-1892), had little faith in the calculations -of the young mathematician. He always considered the correctness of a -distant mathematical result to be a subject rather of moral than of -mathematical evidence: in fact, regarding Uranus, the Astronomer-Royal -almost called in question the correctness of the law of gravitation. -Besides, the novelty of the investigations aroused scepticism, and the -fact that Adams was a young man, and inexperienced, went against Airy’s -acceptance of the theory. However, he wrote to Adams questioning him on -the soundness of his idea. Adams thought the matter trivial, and did not -reply. Airy, therefore, took no interest in the investigations, and no -steps were taken to search for the unseen planet. Meanwhile the Rev. W. -R. Dawes happened to see Adams’ papers lying at Greenwich, and wrote to -his friend, the well-known astronomer Lassell, who was in possession of -a very fine reflector, erected at his residence near Liverpool, asking -him to search for the planet. But Lassell was suffering from a sprained -ankle, and Dawes’ letter was accidentally destroyed by a housemaid. So -Adams’ theory remained in obscurity. - -The question now came under the notice of _François Jean Dominique -Arago_ (1786-1853), the director of the Paris Observatory. He recognised -in a young friend of his a rising genius, who was competent to solve the -problem. Urban Jean Joseph Le Verrier, born at Saint Lo, in Normandy, in -1811, became in 1837 astronomical teacher in the École Polytechnique, -and in 1853 director of the Paris Observatory. In consequence of -differences with his staff he was obliged, in 1870, to resign from this -position, but two years later was restored to the post, which he held -till his death on September 23, 1877. - -In 1845, ignorant of the fact that Adams had already solved the problem, -Le Verrier began his investigations of the irregular motions of Uranus. -In a memoir communicated to the Academy of Sciences in November of that -year, he demonstrated that no known causes could produce these -disturbances. In a second memoir, dated June 1, 1846, he announced that -an exterior planet alone could produce these effects. But Le Verrier had -now before him the difficult task of assigning an approximate position -to the unseen body, so that it might be telescopically discovered. After -much calculation Le Verrier, in his third memoir (August 31, 1846), -assigned to the planet a position in the constellation Aquarius. - -Meanwhile one of Le Verrier’s papers happened to reach Airy. Seeing its -resemblance to Adams’ papers, which had been lying on his desk for -months, his scepticism vanished, and he suggested to Challis that the -planet should be searched for with the Cambridge equatorial. In July -1846 the search was commenced. The planet was actually observed on -August 4 and 12, but, owing to the absence of star maps, it was not -recognised. “After four days of observing,” he wrote to Airy, “the -planet was in my grasp if I had only examined or mapped the -observations.” - -Le Verrier wrote to Encke, the illustrious director of the Berlin -Observatory, desiring him to make a telescopic search for a planetary -object situated in the constellation Aquarius, as bright as a star of -the eighth magnitude and possessed of a visible disc. “Look where I tell -you,” wrote the French astronomer, “and you will see an object such as I -describe.” Encke ordered his two assistants, Galle and D’Arrest, to make -a search on the night of September 23, 1846. In a few hours Galle -observed an object not marked in the star-maps of the Berlin -Observatory, which had been recently published. The following night -sufficed to show that the object was in motion, and was therefore a new -planet. On September 29 Challis found the planet at Cambridge, but he -was too late, as the priority of the discovery was now lost to Adams. -The planet received the name of “Neptune.” - -For some time, indeed, it appeared as if the French astronomer alone was -to receive the honour of the discovery. But on October 3, 1846, a letter -from Sir John Herschel appeared in the ‘Athenæum’ in which he referred -to the discovery made by Adams. The French scientists were extremely -jealous. Indeed, Arago actually declared that, when Neptune was under -discussion, the entire honour should go to Le Verrier, and the name of -Adams should not even be mentioned,—Arago’s line of reasoning being that -it was not the man who first made a discovery who should receive the -credit, but he who first made it public. However, the credit of the -discovery is now given equally to Adams and Le Verrier, both of whom are -regarded as among the greatest of astronomers. - -Only a fortnight after the discovery of Neptune, the astronomer Lassell -observed a satellite to the distant planet on October 10, 1846. This -discovery was confirmed in July 1847 by the discoverer himself, and -shortly afterwards by Bond and Otto Struve. Regarding the globe of -Neptune, we know practically nothing. No markings of any kind have been -observed on its surface. However, in 1883 and 1884, _Maxwell Hall_, an -astronomer in Jamaica, noticed certain variations of brilliance which -suggested a rotation-period of eight hours, but this was not confirmed -by any other astronomer. The spectrum of Neptune has been investigated -by various observers, who have found it to be similar to that of Uranus. - -The existence of a trans-Neptunian planet has been suspected by many -astronomers. In November 1879 the first idea of its existence was thrown -out by Flammarion in his ‘Popular Astronomy.’ Flammarion noticed that -all the periodical comets in the Solar System have their aphelion near -the orbit of a planet. Thus Jupiter owns about eighteen comets; Saturn -owns one, and probably two; Uranus two or three; and Neptune six. The -third comet of 1862, however, along with the August meteors, goes -farther out than the orbit of Neptune. Accordingly, Flammarion suggested -the existence of a great planet, assigning it a period of 330 years and -a distance of 4000 millions of miles. - -Two independent investigators, _David Peck Todd_ (born 1855) in America -and _George Forbes_ in Scotland, have since undertaken to find the -planet. Todd, utilising the “residual perturbations” of Uranus, assigned -a period of 375 years for his planet. Forbes, on the other hand, working -from the comet theory, stated his belief in the existence of two planets -with periods of 1000 and 5000 years respectively. In October 1901 he -computed the position of the new planet on the celestial sphere, fixing -its position in the constellation Libra, and computing its size to be -greater than Jupiter. A search was made by means of photography, in -1902, but without success. Nevertheless, astronomers are pretty -confident of the existence of one or more trans-Neptunian planets. -Lowell is very definite on this subject when he says in regard to meteor -groups, “The Perseids and the Lyrids go out to meet the unknown planet, -which circles at a distance of about forty-five astronomical units from -the Sun. It may seem strange to speak thus confidently of what no mortal -eye has seen, but the finger of the sign-board of phenomena points so -clearly as to justify the definite article. The eye of analysis has -already suspected the invisible.” - - - - - CHAPTER VII. - COMETS. - - -At the time of Herschel the ancient superstitions in regard to comets -had to a great extent vanished, thanks mainly to the return of Halley’s -comet in 1758. Yet, although comets had ceased to be objects of terror, -no explanation or rational theory of their nature was put forward until -the appearance of the great comet of 1811. This comet was visible from -March 26, 1811, to August 17, 1812, a period of 510 days. It was one of -the most magnificent comets ever seen, its tail being 100 millions of -miles in length and its head 127,000 miles in diameter. This wonderful -phenomenon was the subject of much investigation, particularly by -Olbers, the great German astronomer. - -Heinrich Wilhelm Matthias Olbers was born at Arbergen, a village near -Bremen, October 11, 1758. His father was a clergyman who, in addition to -considerable mathematical powers, was an enthusiastic lover of -astronomy. At the age of thirteen young Olbers became deeply interested -in that science. While taking an evening walk in the month of August, he -observed the Pleiades, and determined to find out to which constellation -they belonged. He therefore bought some books on astronomy, along with a -few charts of the sky, and he began to study the science with much -enthusiasm. He read every book he could lay his hands on, and a few -months sufficed to make him acquainted with all the constellations. - -In 1777, when in his nineteenth year, Olbers entered the University of -Göttingen to study medicine, and at the same time he learned much -regarding mathematics and astronomy from the mathematician Kaestner. -When twenty-one years of age he observed the stars at Göttingen, and -devised a method of calculating the orbits of comets, the idea coming to -him while he was attending at the bedside of a fellow-student who had -taken ill. “Although not made public until 1797,” writes Miss Clerke, -“‘Olbers’ method’ was then universally adopted, and is still regarded as -the most expeditious and convenient in cases where absolute rigour is -not required. By its introduction, not only many a toilsome and -thankless hour was spared, but workers were multiplied and encouraged in -the pursuit of labours more useful than attractive.” - -Towards the end of 1781 he returned to Bremen, settled as a medical -doctor, and continued in practice for about forty-one years. But -although he had adopted perhaps the most toilsome profession, his love -of science prevailed, and night after night he explored the heavens with -untiring zeal. He never slept more than four hours, and the upper part -of his house in the Sandgasse, in Bremen, was fitted up with -astronomical instruments. The largest telescope which he possessed was a -refractor 3¾ inches in aperture. He remained in active practice till -1823, when he retired, and was enabled to devote more attention to his -beloved science. He died on March 2, 1840, at the advanced age of -eighty-one. - -Miss Clerke says of Olbers, “Night after night, during half a century -and upwards, he discovered, calculated, or observed the cometary -visitants of northern skies.” He was the discoverer of the comet of -1815, known as Olbers’ comet. It moves round the Sun in a period of over -seventy years, and returned to perihelion in 1887, forty-seven years -after the death of its discoverer. The great comet of 1811 was the -subject of a memoir which Olbers published the following year, and in -which he originated the “electrical repulsion” theory of comets’ tails. -Even after the fulfilment of Halley’s great prediction, comets were -still looked upon with profound awe, and the popular fear regarding them -was still prevalent. Olbers, however, showed that the tails of comets -resulted from purely natural causes. He regarded the Sun as possessed of -a repulsive as well as an attractive force, and considered the tails to -be vapours repelled from the nucleus of the comet by the Sun. He -calculated that in the comet of 1811 the particles of matter expelled -from the head reached the tail in eleven minutes, with a velocity -comparable to that of light. The theory of electrical repulsion, since -elaborated by other observers, is now generally accepted among -astronomers. No other hypothesis represents in such a complete manner -the formation and growth of the luminous appendages of the celestial -bodies so picturesquely called “pale-winged messengers” as that put -forward by the physician of Bremen. - -Some years after Olbers’ famous theory was given to the world, a great -advance was made in cometary astronomy by another great German -astronomer, his friend and pupil Encke. The son of a Hamburg clergyman, -Johann Franz Encke was born in that city in 1791, and died in 1865 at -Spandau. After taking part in the war against Napoleon, he was in 1822 -appointed director of the Gotha Observatory, being called to Berlin in -1825. In early life he was the pupil of Olbers and Gauss, and his -investigations and discoveries formed an epoch in astronomy. His most -famous discovery related to the little comet which bears his name. The -comet was discovered by _J. L. Pons_ (1761-1831) at Marseilles, although -it had previously been seen by Méchain and Caroline Herschel. In 1819 -Encke computed the orbit of the comet, and boldly announced that it -would reappear in 1822, its period being about 3¼ years, or 1208 days. -In 1822 the comet, true to Encke’s prediction, returned to perihelion, -and was observed at Paramatta in Australia, the perihelion passage -taking place within three hours of the time predicted by Encke. As Miss -Clerke remarks, “The importance of this event will be better understood -when it is remembered that it was only the second instance of the -recognised return of a comet; and that it, moreover, established the -existence of a new class of celestial bodies, distinguished as comets of -short period.” - -In 1825 the comet was again observed by Valz, passing perihelion on -September 16, and in 1828 it was seen by Struve. Encke now made a very -remarkable discovery. Determining its period with great accuracy, in -1832 he found that his comet returned to perihelion two and a half hours -before the predicted time. As this repeatedly happened, Encke put -forward the theory that the acceleration was due to the existence of a -resisting medium in the neighbourhood of the Sun, too rarefied to retard -the planetary motions, but quite dense enough to make the comet’s path -smaller, and to eventually precipitate it on the Sun. The theory was -widely accepted, but after 1868 the acceleration began to decrease, -diminishing by one-half; besides, no other comet is thus accelerated, -and the hypothesis has accordingly been abandoned. - -The second comet recognised as periodic was that discovered on February -27, 1826, by an Austrian officer, _Wilhelm von Biela_ (1782-1856), and -ten days later by the French observer, _Gambart_ (1800-1836), both of -whom, in computing its orbit, noticed a remarkable similarity to the -orbits of comets which appeared in 1772 and 1805. Accordingly, they -concluded it to be periodic, with a period of between six and seven -years. The comet returned in 1832. In 1828 Olbers had published certain -calculations showing that portions of the comet would sweep over the -part of the Earth’s orbit a month later than the Earth itself. This gave -rise to a panic that the comet would destroy the Earth, which did not -subside till it was announced by Arago that the Earth and the comet -would at no time approach to within fifty million miles of each other. -The comet returned again in the end of 1845. It was kept well in view by -astronomers in Europe and America. On December 19, 1846, Hind noticed -that the comet was pear-shaped, and ten days later it had divided in -two. The two comets returned again in 1852 and were well observed; but -they were never seen again, at least as comets. Their subsequent history -belongs to meteoric astronomy. - -A comet discovered by Faye at Paris in 1843 was found to have a period -of seven and a half years. It has returned regularly since its -discovery, true to astronomical prediction. Its motion was particularly -investigated for traces of a resisting medium, by _Didrik Magnus Axel -Möller_ (1830-1896), director of the Lund Observatory, who reached a -negative conclusion. - -In 1835 Halley’s comet returned to perihelion, and was attentively -studied by the most famous astronomers of the age. It was particularly -studied by Sir John Herschel and by Bessel, who assisted in developing -Olbers’ theory of electrical repulsion. But the most brilliant comet of -the century was that which suddenly appeared on February 28, 1843, in -the vicinity of the Sun. This great comet, whose centre approached the -Sun within 78,000 miles, rushed past its perihelion at the speed of 366 -miles a second. The comet’s tail reached the length of 200 millions of -miles. The comet of 1843 was however outshone, not in brilliance but as -a celestial spectacle, by the great comet discovered on June 2, 1858, by -_Giovanni Battista Donati_ (1826-1873) at Florence, and since known by -his name. It became visible to the naked eye on August 19, and was -telescopically observed until March 4, 1859. There was abundance of -time, therefore, to study the comet, which was exhaustively observed by -G. P. Bond at Harvard. His observations convinced him that the light -from Donati’s comet was merely reflected sunshine, and this was -generally accepted. Another great comet appeared in 1861. Like that of -1843, its appearance was sudden, being observed after sunset on June 30, -1861, when, says Miss Clerke, “a golden yellow planetary disc, wrapt in -dense nebulosity, shone out while the June twilight in these latitudes -was still in its first strength.” On the same evening the Earth and the -Moon passed through the tail of the great comet. The vast majority of -people never knew that such a phenomenon had taken place, and even the -astronomers only noticed a singular phosphorescence in the sky—a proof -of the extreme tenuity of comets. - -The first application of the spectroscope to the light of comets was -made by Donati in 1864. The spectrum was found to consist of three -bright bands, but Donati was unable to identify them. However, his -observation gave the death-blow to the theory that comets shone by -reflected light alone, for it implied the existence of glowing gas in -them. On the appearance in 1868 of the periodic comet discovered by -_Friedrich August Theodor Winnecke_ (1835-1897), the spectrum was -examined by Huggins, who identified the bright bands with the spectrum -of hydrocarbon. This was confirmed in regard to Coggia’s comet of 1874 -by Huggins himself, and also Brédikhine and Vogel. The hydrocarbon -spectrum is characteristic of comets, and has been recognised in all -those spectroscopically studied. - -The time had now come for a more complete theory of comets than that of -Olbers. The theory of electrical repulsion was developed in 1871 by -Zöllner, whose principle of investigation is thus described by Miss -Clerke: “The efficacy of solar electrical repulsion relatively to solar -attraction grows as the size of the particle diminishes.” If the -particle is small enough, it will obey the repulsive, and not the -attractive, power of the Sun. Zöllner considered that the smallest -particles of comets obeyed the repulsive power, and thus formed the -tails of comets. The development of a complete cometary theory is due, -however, to the genius of a Russian astronomer. Theodor Alexandrovitch -Brédikhine, born in 1831 at Nicolaieff, was employed at Moscow -Observatory from 1857 to 1890, when he was promoted to the position of -director at Pulkowa. He resigned in 1895, and spent his last years in St -Petersburg, where he died on May 14, 1904. From the beginning of his -astronomical career he was devoted to the study of comets and their -tails, but it was the appearance of Coggia’s comet in 1874 which marked -the commencement of his most important observations. In that year, on -making certain calculations regarding the hypothetical repulsive force -exerted by the Sun on various comets, he reached the conclusion that the -values representing the intensity of the repulsion fell into three -classes. This was the first hint of a classification of cometary tails. -Meanwhile he carefully studied the tails of comets both from direct -observation and from drawings. - -In 1877 he wrote: “I suspect that comets are divisible into groups, for -each of which the repulsive force is perhaps the same.” Subsequent -investigations led Brédikhine to divide the tails of comets into three -types. The first type consists of long, straight tails, pointed directly -away from the Sun, represented by the tails of the great comets of 1811, -1843, and 1861. In the second type, represented by Donati’s and Coggia’s -comets, the tails, although pointed away from the Sun, appear -considerably curved. In the third type the tails are, to quote Miss -Clerke, “short, strongly-bent, brushlike emanations, and in bright -comets seem to be only found in combination with tails of the higher -classes.” - -In 1879 Brédikhine fully developed his cometary theory. Assuming the -reality of the repulsive force, he concluded that to produce tails of -the first type, the repulsion requires to be twelve times greater than -the solar attraction; the production of tails of the second type -necessitates a repulsive force about equal to gravity; while the force -producing third-type tails has only one-fourth the power of gravitation. -It was concluded that the tails are formed by particles of matter -repelled from the comet by the repulsive force of the Sun, and in tails -of the first type the velocity with which these particles leave the body -of the comet is four or five miles a second. Brédikhine reached the -conclusion that the Sun’s repulsive force is invariable, and that the -different types of tails are formed by the same force acting on -different elements. The numbers 12, 1, and ¼, are inversely proportional -to the atomic weights of hydrogen, hydrocarbon gas, and iron vapour. -Here, then, was the key to the mystery. Brédikhine pointed out that in -all probability the first-type tails are formed of hydrogen, the second -of hydrocarbon, and the third of iron, with a mixture of sodium and -other elements. - -Within a few years of the publication of Brédikhine’s theory, five -bright comets made their appearance, and there was abundant chance of -testing the theory spectroscopically. In 1882 Well’s comet was -particularly studied at Greenwich by Maunder, who discerned a -sodium-line in its spectrum. The magnificent comet which appeared in -1882 was spectroscopically studied at Dunecht in Aberdeenshire by _Ralph -Copeland_ (1837-1905), Astronomer-Royal of Scotland, who identified in -its spectrum the prominent iron-lines as well as the sodium-line. These -observations were certainly confirmatory of Brédikhine’s theory. It -should also be stated, however, that several comets have shown, in -addition to the hydrocarbon spectrum, that of reflected sunlight, which -proves that the light we receive from comets is of a compound nature. - -The comet which appeared in 1880 was announced by _Benjamin Apthorp -Gould_ (1824-1896) to be a return of the great comet of 1843. -Calculations by Gould, Copeland, and Hind revealed a close similarity -between the elements of the two orbits. Eventually it had to be admitted -that the comets were separate bodies travelling in the same orbit. Then, -two years later, the great September comet of 1882 was found to revolve -in the same orbit as those of 1668, 1843, and 1880. Four years later, -another comet, discovered in 1887, was found to move in the same path. - -Closely allied to this subject is the existence of “comet families,” -demonstrated by Hoek of Utrecht in 1865, and mentioned in our chapter on -the Outer Planets. These comets are found to be dependent on the -planets, Jupiter, Saturn, Uranus, and Neptune, each possessing a -comet-group. Various theories have been advanced to account for the -existence of these groups. One of these theories is that the comets have -been captured by the various planets, who have forced them into their -present orbits. A mathematical study by _Jean Pierre Octave Callandrean_ -(1852-1904) shows that the large number of comets possessed by the -various planets may be explained by the disintegration of large comets -into small ones. The capture theory, it must be remembered, is purely -hypothetical, and must not be regarded as anything but a theory. All -that we really know is the existence of comet-families, and of comets -moving in the same orbits. - -The first photograph of a comet was that of Donati’s, taken in 1858 by -Bond. In 1881 Tebbutt’s comet was photographed in England by Huggins, -and in America by _Henry Draper_ (1837-1882), while in 1882 Gill secured -excellent photographs of the great September comet. The first -photographic discovery of a comet was made by Barnard in 1892. Since -then photography has been much used in cometary astronomy. No bright -comets have appeared since 1882,—if we except the comet of 1901, only -seen in the southern hemisphere,—although several have been just visible -to the naked eye, among them Swift’s comet of 1892 and Perrine’s in the -autumn of 1902. Telescopic comets, however, are very numerous, and a -year never passes without one or more being discovered. The ordinary -periodic comets, such as Encke’s, Faye’s, and others, are very faint, -and are becoming fainter at each return—a clear proof that comets die, -as Kepler said three centuries ago. This brings us to the subject of the -next chapter, Meteoric Astronomy. - - - - - CHAPTER VIII. - METEORS. - - -There is no more interesting chapter in the history of astronomy than -that relating to meteors. A hundred years ago shooting-stars were not -considered to be astronomical phenomena. They were supposed to be merely -inflammable vapours which caught fire in the upper regions of our -atmosphere, although both Halley and the scientist _Ernst Chladni_ -(1756-1827) had notions of their celestial origin. For thirty-three -years after the beginning of the century, however, nothing was heard of -meteoric astronomy, nor was the subject considered as part of the -astronomer’s labours. - -A great meteoric shower took place on the night of November 12 and -morning of November 13, 1833. The shower was probably the grandest ever -witnessed, the shooting-stars being literally innumerable. The display -was best observed in America, and was attentively watched by _Denison -Olmsted_ (1791-1859), Professor of Mathematics at Yale, and by the -American physicist, _A. C. Twining_ (1801-1884). These investigators -discovered that all the meteors which fell during the great shower -seemed to come from the same part of the celestial vault. In other -words, their paths, when traced back, were found to converge to a point -near the star γ Leonis. This observation gave the death-blow to the -theory of their terrestrial origin. The point known as the “radiant” was -clearly a point independent of the Earth. Olmsted also recognised the -fact that the shower had taken place in the previous year, and he -regarded it as produced by a swarm of particles moving round the Sun in -a period of 182 days. Soon after this it was noticed that the phenomenon -took place in 1834 and subsequent years with gradually decreasing -intensity. It was then remembered that Humboldt had observed in November -1799 a very brilliant shower, and accordingly Olbers suggested that -another shower might be seen in 1867. - -The falling stars of August were next proved by _Adolphe Quetelet_ -(1791-1874) to form another meteoric system; and accordingly the theory -of Olmsted that the November meteors moved round the Sun in 182 days had -to be abandoned, for, says Miss Clerke, “If it would be a violation of -probability to attribute to _one_ such agglomeration a period of an -exact year or sub-multiple of a year, it would be plainly absurd to -suppose the movements of _two_ or more regulated by such highly -artificial conditions.” Accordingly Erman suggested in 1839 the theory -that meteors revolved in closed rings, intersecting the terrestrial -orbit; and that when the Earth crossed through the point of -intersection, it met some members of the swarm. The subject now remained -in abeyance for thirty-four years, if we except some wonderful ideas put -forward in 1861 by _Daniel Kirkwood_ (1813-1896), an American -astronomer, who stated his belief in the disintegration of comets into -meteors; but little attention was paid to his opinions. In 1864 the -subject was taken up by _Hubert Anson Newton_ (1830-1896), Professor at -Yale, who undertook a search through ancient records for the -thirty-three-year period of the Leonids or November meteors. His search -was highly successful, and having demonstrated the existence of the -period, Newton set himself to determine the orbit. He indicated five -possible orbits for the swarm, ranging from 33 years to 354½ days. -Newton was unable to solve the question mathematically; but here Adams, -the discoverer of Neptune, came to the rescue, and demonstrated that the -period of 33¼ years was alone possible, and that the others were -untenable. These investigations, completed in March 1867, proved the -existence of a great meteoric orbit extending to the orbit of Uranus. - -Meanwhile Newton had predicted a meteoric shower on the evening of -November 13 and morning of November 14, 1866. His prediction was -fulfilled. The shower was inferior to that of 1833, but was still a -magnificent spectacle. Sir Robert Ball, then employed at Lord Rosse’s -Observatory, observed the shower, and records the impossibility of -counting the meteors. This great shower attracted the attention of -astronomers all over the world to the study of meteors. Meanwhile -Schiaparelli had been working at the subject for some time, and in four -letters addressed to Secchi, towards the end of 1866, he showed that -meteors were members of the Solar System, possessed of a greater -velocity than that of the Earth, and travelling in orbits resembling -those of comets, in the fact that they moved in no particular plane, and -that their motion was both direct and retrograde. Schiaparelli computed -the orbit of the Perseids or August meteors, and was astonished to find -it identical with the comet of August 1862. This was a proof of the -connection between these two apparently widely different types of -celestial bodies. Early in 1867 Schiaparelli found that Le Verrier’s -elements for the orbit of the Leonids were identical with those of the -comet of 1866, discovered by _Ernst Tempel_ (1821-1889). Peters of -Altona had meanwhile reached the same conclusion; while _Edmund Weiss_ -(born 1837) of Vienna pointed out the similarity of the orbit of a -star-shower on April 20 and that of the comet of 1861. He also drew -attention, independently of Galle and D’Arrest, to the close connection -between the orbits of the lost Biela’s comet and the Andromedid meteors -of November. - -All doubt as to the connection of comets and meteors was removed by the -great shower on November 27, 1872. Biela’s lost comet was due at -perihelion in 1872, and although searched for was not observed; but when -the Earth crossed its orbit, a great meteoric shower took place. “It -became evident,” says Miss Clerke, “that Biela’s comet was shedding over -us the pulverised products of its disintegration.” The shower was little -inferior to that of 1866. Meanwhile _Ernst Klinkerfues_ (1827-1884), -Professor at Göttingen, believing that Biela’s comet itself had -encountered the Earth, telegraphed to _Norman Robert Pogson_ -(1829-1891), Government astronomer at Madras, to search for the comet in -the opposite region of the sky. Pogson did observe a comet, but -certainly not Biela’s, although probably another fragment of the missing -body. - -The theory of the actual disintegration of comets was enunciated by -Schiaparelli in 1873, and developed in his work ‘Le Stelle Cadenti.’ He -was led to regard comets as cosmical clouds formed in space by “the -local concentration of celestial matter.” He then remarks that a -cosmical cloud seldom penetrates to the interior of the Solar System, -“unless it has been transformed into a parabolic current,” which may -occupy years, or centuries, in passing its perihelion, “forming in space -a river, whose transverse dimensions are very small with respect to its -length: of such currents, those which are encountered by the earth in -its annual motion are rendered visible to us under the form of showers -of meteors diverging from a certain radiant.” - -Schiaparelli next pointed out that when the current of meteors -encounters a planet, the resulting perturbations cause some of the -meteoric bodies to move in separate orbits, forming the bolides and -aerolites which fall from the sky at intervals. “The term _falling -stars_” he says, “expresses simply and precisely the truth respecting -them. These bodies have the same relation to comets that the small -planets between Mars and Jupiter have to the larger planets.” In the -third chapter of his ‘Le Stelle Cadenti’ he explicitly states that “the -meteoric currents are the products of the dissolution of comets, and -consist of minute particles which certain comets have abandoned along -their orbits, by reason of the disintegrating force which the Sun and -planets exert on the rare materials of which they are composed.” - -In 1878 _Alexander Stewart Herschel_ (born 1836), son of Sir John -Herschel, and a famous meteoric observer, published a list of known or -suspected coincidences of meteoric and cometary orbits, amounting to -seventy-six. Meanwhile much progress has since been made in the -observation of meteoric showers and the determination of their radiant -points. In this branch of astronomy, by far the greatest name is that of -William Frederick Denning, the self-made English astronomer. Born at -Redpost, in Somerset, in 1848, his career of meteoric observation -commenced in 1866. For the past forty years he has attentively devoted -himself to the observation of meteors. From 1872 to 1903 he determined -the radiant points of no fewer than 1179 meteoric showers. In addition -to this, he published, in 1899, a catalogue of meteoric radiants, -containing 4367; and he has carefully studied the remarkable objects -known as fireballs or “sporadic meteors.” He has occasionally been able -to trace a connection between fireballs and weak meteoric showers, but -he concludes that they “must either be merely single sporadic bodies, or -else the survivors of some meteor group, nearly exhausted by the waste -of its material during many past ages.” All of Denning’s meteoric work -has been done in his spare time, for it must be borne in mind that he -pursues the profession of accountant in Bristol, and that only his -leisure hours have been devoted to the science of astronomy. His -researches have been entirely conducted with the unaided eye. His only -instrument is a perfectly straight wand, which he uses as a help and -corrective to the eye in ascribing the paths of the meteors. Thanks to -the laborious work of this able English astronomer, the observation of -meteors is now a _scientific_ branch of astronomy. In the words of -Maunder, “for six thousand years men stared at meteors and learned -nothing, for sixty years they have studied them and learned much, and -half of what we know has been taught us in half that time by the efforts -of a single observer.” - -Further meteoric showers from Biela’s comet were seen in 1885 and 1892. -The Leonid shower was confidently predicted for 1899, in accordance with -the thirty-three-year period, but the great display did not come off, -either in 1899 or 1900. In 1901 there was a certain weak shower observed -in America; and similar displays took place in 1903 and 1904. Many -explanations have been given as to the failure of the shower, the most -probable idea being that the attraction of Jupiter diverted the meteors -from their course. - -Denning’s observations on meteors resulted, as early as 1877, in the -discovery of so-called “stationary radiants.” The radiant-point of a -long enduring shower usually exhibits an apparent motion, resulting from -the combined orbital motions of the Earth and the meteors; but Denning -found that in some cases the shower, though lasting for months, -persistently exhibited the same radiant-point, implying that the motion -of the Earth must be insignificant compared with that of the meteors, -computed by Ranyard at 880 miles per second. The difficulty of admitting -so great a velocity led the French astronomer, _François Felix -Tisserand_ (1845-1896), to doubt the existence of these stationary -radiants; but the fact of their existence cannot be doubted, although no -really satisfactory explanation has been offered. - -Another type of meteors comprises the bodies termed respectively as -bolides, uranoliths, and aerolites,—stones which fall to the Earth from -the sky. In 1800 the French Academy declared the accounts of stones -having fallen from the heavens to be absolutely untrue. Three years -later an aerolite fell at Laigle, in the Department of Orne, on April -26, 1803, attended by a terrific explosion. In the words of Flammarion, -“Numerous witnesses affirmed that some minutes after the appearance of a -great bolide, moving from south-east to north-east, and which had been -perceived at Alençon, Caen, and Falaise, a fearful explosion, followed -by detonations like the report of cannon and the fire of musketry, -proceeded from an isolated black cloud in a very clear sky. A great -number of meteoric stones were then precipitated on the surface of the -ground, where they were collected, still smoking, over an extent of -country which measured no less than seven miles in length.” - -Some aerolites, instead of being shattered into fragments, have been -observed to fall to the Earth intact, and bury themselves in the ground. -Numerous instances have been observed during the last century, and -masses of meteoric stones have been found in positions which clearly -indicate that they must have fallen from the sky. Chemists have made -analyses of the elements in these remarkable bodies, and have found them -to contain iron, magnesium, silicon, oxygen, nickel, cobalt, tin, -copper, &c. The spectrum of these aerolites, raised to incandescence, -has been studied by Vogel and by the Swedish observer, _Bernhardt -Hasselberg_ (born 1848), who detected the presence of hydrocarbons, -which are also present in cometary spectra. - -When the existence of aerolites as celestial bodies was first -recognised, Laplace suggested that they had been ejected from volcanoes -on the Moon. This theory, although supported by Olbers and other -astronomers, was soon rejected. Next, it was suggested that they were -ejected from the Sun, and Proctor believed them to come from the giant -planets. A very detailed discussion of the subject is to be found in -Ball’s ‘Story of the Heavens’ (1886), in which he expresses views in -harmony with those of the Austrian physicist Tschermak. Ball -demonstrated that the meteors which fall to the Earth cannot have come -from any other planet, nor from the Sun. Accordingly, he concluded that -they were originally ejected by the volcanoes of the Earth many ages -ago, when they were active enough to throw up pieces of matter with a -velocity great enough to carry them away from the Earth altogether. Such -meteors would, however, intersect the terrestrial orbit at each -revolution. - -The alternative theory to this, supported by Schiaparelli and Lockyer, -is that the aerolites are merely larger members of the meteor-swarms, -which have been deflected from their paths. The chief objection to this -theory is the absence of connection between the meteoric showers and the -falls of aerolites and bolides. Only on one occasion was a meteoric -stone observed to fall during a shower. On November 27, 1885, during the -shower of Andromedid meteors from Biela’s comet, a large bolide, -weighing more than eight pounds, fell at Mazapil, in Mexico. This, -however, was the only case hitherto observed; and it may have been -merely a coincidence. - - - - - CHAPTER IX. - THE STARS. - - -The most remarkable progress in astronomy during the past century has -been in the department of sidereal science, or the study of the Suns of -space, observed for their own sakes, and not merely for the purpose of -determining the positions of the Sun and Moon, and to assist navigation. -Thanks to Herschel, the nineteenth century witnessed the steady -development of stellar astronomy, combined with many important -discoveries and investigations. - -The one pre-Herschelian problem in sidereal astronomy was the distance -of the stars. Owing to its bearing on the Copernican theory, the problem -was attacked by the astronomers of the seventeenth and eighteenth -centuries. Herschel made numerous attempts to detect the parallax of the -brighter stars, but failed. Meanwhile there had been many illusions. -Piazzi believed that his instruments—which in reality were worn out and -unfit for use—had revealed parallaxes in Sirius, Aldebaran, Procyon, and -Vega; Calandrelli, another Italian, and _John Brinkley_ (1763-1835), -Astronomer-Royal of Ireland, were similarly deluded; and in 1821 it was -shown by _Friedrich Georg Wilhelm Struve_ (1793-1864), the great German -astronomer, that no instruments then in use could possibly be successful -in measuring the stellar parallax. A few years later, however, -Fraunhofer brought the refractor to a degree of perfection surpassing -all previous efforts. In 1829 he mounted for the observatory at -Königsberg a heliometer, the object-glass of which was divided in two, -and capable of very accurate measurements. This heliometer eventually -revealed the parallax of the stars in the able hands of Friedrich -Wilhelm Bessel. - -Friedrich Wilhelm Bessel was born at Minden, on the Weser, south-west of -Hanover, on July 22, 1784. His father was an obscure Government -official, unable to provide a university education for his son. Bessel’s -love of figures, together with an aversion to Latin, led him to pursue a -commercial career. At the age of fourteen, therefore, he entered as an -apprenticed clerk the business of Kuhlenkamp & Sons, in Bremen. He was -not content, however, to remain in that humble position. His great -ambition was to become supercargo on one of the trading expeditions sent -to China; and so he learned English, Spanish, and geography. But he -never became a supercargo. In order to be fully equipped for such a -position, he determined to learn how to take observations at sea, and -his acquaintance with observation aroused a desire to study astronomy. -He constructed for himself a sextant, and by means of this, along with a -common clock, he determined the longitude of Bremen. - -Such enthusiasm could not be long without its reward. For several years -Bessel remained a clerk, and the hours devoted to study were those -spared from sleep. He studied the works of Bode, Von Zach, Lalande, and -Laplace, and in two years was able to compute the orbits of comets by -means of mathematics. From some observations of Halley’s comet at its -appearance in 1607, Bessel calculated its orbit, and forwarded the -calculation to Olbers, then the greatest authority on cometary -astronomy. Olbers was delighted at this work, and he sent the results to -Von Zach, who published them. The self-taught young astronomer had -accomplished a piece of work which fifteen years before had taxed the -skill and patience of the French Academy of Sciences. - -In 1805, Harding, Schröter’s assistant at Lilienthal, resigned his -position for a more promising one at Göttingen. Olbers procured for -Bessel the offer of the vacant post, which the latter accepted. At -Lilienthal Bessel received his training as a practical astronomer. He -remained in Schröter’s observatory until 1809. Although only twenty-five -years of age, he had become so well known in Germany that in that year -he was appointed Professor of Astronomy in the University of Königsberg, -and was chosen to superintend the erection of the new observatory there. -Within a few years a clerk in a commercial office had worked his way -from obscurity to fame. - -In 1813 the Königsberg Observatory was completed, and here Bessel worked -for thirty-three years, until his death, on March 17, 1846. It was only -about ten years before his death that he commenced his search for the -stellar parallax, with the aid of Fraunhofer’s magnificent heliometer. -He determined to make a series of measures on a small double star of the -fifth magnitude in the constellation Cygnus, named 61 Cygni, the large -proper motion of which led him to suspect its proximity to the Solar -System. From August 1837 to September 1838 he made observations on 61 -Cygni, and he found that there was an annual displacement which could -only be attributed to parallax. In order to have no mistake, he made -another year’s observations, which confirmed the results he arrived at -previously, and all doubt was removed by a third series. The resulting -parallax was 0·3483″, corresponding to a distance of 600,000 times the -Earth’s distance from the Sun. This was confirmed some years later by C. -A. F. Peters at Pulkowa, and still later by Otto Struve, who estimated -the distance at forty billions of miles. Meanwhile, F. G. W. Struve, -working at Pulkowa, found a parallax of 0·2613″ for Vega, but this was -afterwards found to be considerably in error. Accordingly, Struve does -not rank with Bessel as a successful measurer of star-distance. But -independently of Bessel, another accurate measure had been made by -_Thomas Henderson_, the great Scottish astronomer. - -Born in Dundee in 1798, Thomas Henderson was the youngest of five -children of a hard-working tradesman. After education in his native town -he went to Edinburgh, where he worked for years as an advocate’s clerk, -pursuing studies in astronomy as a recreation from his boyhood. In 1831 -he had become so well known, that he received the appointment of -Astronomer-Royal at the new observatory at the Cape of Good Hope. But -the climate of South Africa did not suit his health, and after a year he -returned to Scotland. In 1834 he became Professor of Astronomy in the -University of Edinburgh, and Astronomer-Royal of Scotland, which -position he held till his death on November 23, 1844, at the early age -of forty-six. - -During a year’s work at the Cape, Henderson undertook a series of -observations on the bright southern star, α Centauri, with a view to -determining its parallax. These observations were made in 1832 and 1833, -but were not reduced until Henderson’s return to Scotland. At length, on -January 3, 1839, he announced to the Royal Astronomical Society that he -had succeeded in measuring the parallax of α Centauri, which he -determined as about one second of arc, corresponding to a distance of -about twenty billions of miles. This result was confirmed by the -observations of _Thomas Maclear_ (1794-1879), his successor at the Cape, -and by those of later observers, notably Sir David Gill, who has reduced -the parallax to 0·75″. - -Other determinations of stellar parallax, some genuine and others -illusory, were made soon after these successful observations. C. A. F. -Peters and Otto Struve at Pulkowa were among the most famous -parallax-hunters in the middle of the century. One of the most -successful searchers after parallax was the German astronomer _Friedrich -Brünnow_ (1821-1891), who was employed from 1865 to 1874 as -Astronomer-Royal of Ireland. He determined the parallax of Vega as -0·13″, and this was confirmed in 1886 by Hall at Washington: while he -measured the parallax of the star Groombridge 1830, which turned out to -be 0·09″. He resigned his post in 1874, and his successor at Dublin -Observatory proved to be his successor also in this branch of astronomy. -_Robert Stawell Ball_, born in Dublin in 1840, was astronomer to Lord -Rosse in 1865 and 1866, and became in 1874 Astronomer-Royal of Ireland -in succession to Brünnow, a position which he filled until his -appointment in 1892 as Professor of Astronomy at Cambridge, and director -of the observatory there. During his term of office in Dublin he -undertook, in 1881, a “sweeping search” for large parallaxes, thereby -disproving certain ideas as to the proximity to the Earth of red and -temporary stars; while he also determined the parallax of the star 1618 -Groombridge. - -But the greatest extension of our knowledge of stellar distances, in -recent years, is due to a Scottish astronomer, who has maintained the -reputation of Scotland, and also of the Cape Observatory, in this line -of research. Born in Aberdeen in 1843, _David Gill_ directed Lord -Lindsay’s private observatory at Dunecht, in Aberdeenshire, from 1876 to -1879. In the latter year he succeeded _Edward James Stone_ (1831-1897) -as Astronomer-Royal at the Cape, a position which he has since filled -with conspicuous ability. From 1881 he has been engaged in the hunt for -parallax. In conjunction with _William Lewis Elkin_ (born 1855), now -director of Yale College Observatory, he determined the parallaxes of -nine stars with the aid of Lord Lindsay’s heliometer. In 1887, with a -larger instrument, he resumed the search, while Elkin worked in -co-operation with him, but at Yale Observatory, where he undertook the -measurement of the parallaxes of northern stars. He fixed in 1888 an -average parallax for first-magnitude stars, which was determined at -0·089″, corresponding to a journey for light of thirty-six years. - -Most of the successful determinations of parallax have been made by the -“relative” method—that is, the determination of the displacement of a -star in reference to another star, assumed to be situated at an -immeasurable distance. The method of absolute parallax, on the other -hand,—the star’s displacement in right ascension and declination,—has -been seldom used, owing to the laborious reduction which has to be gone -through before the result can be reached. In 1885, however, a series of -observations were undertaken at Leyden by _Jacobus Cornelius Kapteyn_ -(born 1851), who determined by the absolute method the parallaxes of -fifteen northern stars. - -The first application of photography to the problem was due to the zeal -and energy of _Charles Pritchard_ (1808-1893), Professor of Astronomy at -Oxford, who determined by this method the parallax of 61 Cygni, which he -announced in 1886 to be 0·438″, in agreement with Ball’s determination. -He also determined the average parallax of second-magnitude stars, which -came out as 0·056″. Since the time of Pritchard’s observations various -other more or less satisfactory determinations of parallax have been -made. Few of the parallax determinations are probably very accurate, and -none exact; but an idea of the difficulty of the measurement may be -gathered from the remark of an American writer, Mr G. P. Serviss, that -the displacement “is about equal to the apparent distance between the -heads of two pins, placed an inch apart, and viewed from a distance of a -hundred and eighty miles.” - -Closely allied to the question of parallax is the determination of the -exact positions of the stars and the formation of star-catalogues. In -this branch, too, much is due to the genius of Bessel. The observations -of Bradley at Greenwich from 1750 to 1762 were reduced by Bessel into -the form of a catalogue, which was published in 1818, with the title of -‘Fundamenta Astronomiæ.’ During the years 1821 to 1823 Bessel took -75,011 observations, by which he brought up the number of accurately -known stars to 50,000. At the same time notable catalogues had been -constructed, particularly by the English astronomer, _Francis Baily_ -(1774-1844), and by _Giovanni Santini_ (1786-1877), director of the -observatory at Padua; but Bessel’s successor in this branch of research -was _Friedrich Wilhelm August Argelander_ (1799-1875). In 1821 he became -assistant to Bessel at Königsberg, in 1823 director of the Observatory -at Abo, in Finland, and in 1837 of that at Bonn. Here he commenced in -1852 the great ‘Bonn Durchmusterung,’ a catalogue and atlas of 324,198 -stars visible in the northern hemisphere. The great catalogue was -published in 1863. After Argelander’s death it was extended so as to -include 133,659 stars in the southern hemisphere, by his assistant -_Eduard Schönfeld_ (1828-1891), who succeeded him in 1875 as director of -Bonn Observatory, where he died in 1891. Meanwhile a greater undertaking -was commenced in 1865 by the Astronomische Gesellschaft. This was the -co-operation of thirteen observatories in Europe and America for the -exact determination of the places of 100,000 of Argelander’s stars. - -In the southern hemisphere, working at Cordova in Argentina, was the -great American astronomer, _Gould_, whose ‘Uranometria Argentina,’ -published in 1879, gives the magnitudes of 8198 stars, and whose -Argentine General Catalogue, containing reference of 32,448 stars, was -published in 1886. The late Radcliffe observer, Stone, published a -useful catalogue in 1880 from his observations at the Cape. - -The application of photography to the work of star-charting dates from -1882, when Gill photographed the comet of 1882, and was struck with the -distinctness of the stars on the background. For some time he had -contemplated the extension of the ‘Durchmusterung,’ from the point where -Schönfeld left it, to the southern pole, and the idea struck him to -utilise photography for the purpose. In 1885, accordingly, Gill -commenced work, and in four years all the photographs were taken. The -reduction of the observations into the form of a catalogue was -spontaneously undertaken by the great Dutch astronomer, Kapteyn, who was -occupied with the work for fourteen years, until in 1900 the great -catalogue, known as the ‘Cape Photographic Durchmusterung,’ was -completed. Half a million stars are represented on the plates taken at -the Cape. - -By the time the ‘Durchmusterung’ was completed, a greater undertaking -was in progress. Paul and Prosper Henry, astronomers at the Paris -Observatory, when engaged in continuing Chacornac’s ecliptic charts, -applied photography to their work, and found it very successful. -Accordingly Gill’s proposal, on June 4, 1886, of an International -Congress of Astronomers, to undertake a photographic survey of the -heavens, was enthusiastically received by the French astronomers. The -Congress met at Paris in 1887, under the presidentship of _Amédée -Mouchez_ (1821-1892), director of the Paris Observatory, fifty-six -astronomers of all nations being present. The Congress resolved to -construct a Photographic Chart, and a Catalogue, the former containing -twenty million stars, the latter a million and a quarter. Meetings were -held in Paris in 1891, 1893, 1896, and 1900 to superintend the progress -of the work, which is now (1906) well advanced towards completion. - -A unique star catalogue is in course of preparation by the Scottish -astronomer, _William Peck_ (born 1862), astronomer to the City of -Edinburgh since 1889. Mr Peck’s catalogue is accompanied by a series of -charts. His star-magnitudes are those of all famous catalogues reduced -to a standard scale. This catalogue, the result of more than fifteen -years’ work, will be an important addition to the many valuable works of -the kind already in existence, and will further increase the already -great reputation of Scotsmen in practical astronomy. - -The determination of the proper motions of the stars is another -important branch of practical astronomy in which much progress has been -made since the time of Herschel. Stars with much larger proper motions -than those of the first magnitude have been discovered. For many years -the small sixth-magnitude star in Ursa Major, 1830 Groombridge, was -supposed to be the swiftest of the stars, and was named by Newcomb the -“runaway star.” But in 1897, on examining the plates of the ‘Cape -Durchmusterung,’ Kapteyn discovered a still swifter star of the eighth -magnitude, situated in the southern constellation, Pictor. The rate of -its motion is over eight seconds of arc yearly; and an idea of the vast -distance of the stars may be obtained by the statement that it would -take 200 years for the star—known as Gould’s Cordova Zones, V Hour -243—to move over a space equal to the moon’s diameter. Important -observations have been made on the stellar motions, and on their bearing -on the structure of the Universe, by various astronomers, including J. -C. Kapteyn and _Ludwig Struve_ (born 1858), son of Otto Struve; but -these must be reserved for a later chapter. - -Richard Anthony Proctor, born at Chelsea, in London, in 1837, graduated -at Cambridge in 1860. For the next twenty-eight years he earned his -living by publishing many volumes on astronomy, popular and technical, -fifty-seven having appeared at the time of his death, which took place -at New York on September 12, 1888. Notwithstanding the vast amount of -work bestowed on his books, his original investigations were permanent -contributions to astronomical science. In 1870 he undertook to chart the -directions and amounts of 1600 proper motions. While engaged on this -work, it occurred to him that it would be “desirable and useful to -search for subordinate laws of motion.” He found, from the laborious -process of charting, that five of the seven stars of the Plough had a -motion in common—that is to say, were moving in the same direction at -the same rate. This phenomenon was termed by Proctor “star-drift.” He -also recognised other instances of star-drift in other portions of the -heavens. - -The subject was soon afterwards taken up by the French astronomer, -Camille Flammarion. Born in 1842 at Montigny-le-Roi, in Haute Marne, -Flammarion was appointed assistant to Le Verrier in 1858, but gave up -his post in 1862. Employed successively at the Bureau des Longitudes, -and as editor of scientific papers, he founded in 1882 his private -observatory at Juvisy-sur-Orge, where he has since continued his -investigations. - -Following up Proctor’s discovery of star-drift, Flammarion drew charts -of proper motions. He demonstrated the “common proper motion” of Regulus -and an eighth-magnitude star, Lalande 19,749, from a comparison of his -measures in 1877 with those of Christian Mayer a century previously; -while he discovered many other instances. His reflections on these -motions, as given in his ‘Popular Astronomy,’ are worthy of -reproduction: “Such are the stupendous motions which carry every sun, -every system, every world, all life, and all destiny in all directions -of the infinite immensity, through the boundless, bottomless abyss; in a -void for ever open, ever yawning, ever black, and ever unfathomable; -during an eternity, without days, without years, without centuries, or -measures. Such is the aspect, grand, splendid, and sublime, of the -universe which flies through space before the dazzled and stupefied gaze -of the terrestrial astronomer, born to-day to die to-morrow, on a -globule lost in the infinite night.” - -Measures of proper motion only enable us to determine the motion of -stars across the line of sight. They do not tell us whether the star is -advancing or receding. Here, however, the spectroscope comes to our aid -by means of Doppler’s principle, described in the chapter on the Sun. It -occurred to Huggins that, by observing the displacement of the lines in -the spectra of the stars, he could determine their motion in the line of -sight. His first results were announced in 1868. In the case of Sirius, -the displacement of the line marked F was believed to indicate a -velocity of recession of 29 miles a second. Some time later Huggins -announced that Betelgeux, Rigel, Castor, and Regulus were retreating, -while Arcturus, Pollux, Vega, and Deneb were approaching. Soon after -this successful work the subject was taken up by Maunder at Greenwich -and by Vogel at Bothkamp; but the delicacy of the measurements prevented -satisfactory results from being reached through visual observations, and -accordingly the measurements were very discordant. - -In 1887 H. C. Vogel, working at Potsdam Astrophysical Observatory, -applied photography to the measurement of radial motion. Assisted by -_Julius Scheiner_ (born 1858), he determined the radial motions of -fifty-one bright stars by photographing the stellar spectra and -measuring the photographs. Vogel found 10 miles a second to be the -average velocity of stars in the line of sight, the tendency of the eye -being to exaggerate the displacements. The swiftest of the stars -measured by Vogel proved to be Aldebaran, with a velocity of recession -of 30 miles a second. Since 1892 the subject has been pursued by Vogel -himself with the new 30-inch refractor at Potsdam, by Campbell at the -Lick Observatory, Bélopolsky at Pulkowa, and other observers. Towards -the end of 1896 Campbell undertook, with the 36-inch Lick refractor, a -series of measures on radial motion, and many important discoveries were -made. These, however, must be reserved for the chapter dealing with -double stars. - -Herschel’s great discovery, from the apparent motions of the stars, of -the movement of the Solar System was not accepted by the next generation -of astronomers. Bessel declared in 1818 that there was absolutely no -evidence to show that the Sun was moving towards Hercules. Even Sir John -Herschel rejected his father’s views, although some confirmatory results -had been reached by Gauss. At length, in 1837, Argelander, in a -memorable paper, based on his observations at Abo, in Finland, attacked -the problem, and demonstrated, from a discussion of the motions of 390 -stars, quite independently of Herschel’s work, that the Solar System was -moving towards Hercules. This was confirmed in 1841 by Otto Struve, in -1847 by _Thomas Galloway_, and in 1859 and 1863 by Airy and _Edwin -Dunkin_ (1821-1898), assistant at Greenwich Observatory. - -Meanwhile, in 1886, _Arthur Auwers_, permanent Secretary of the Berlin -Academy of Sciences, completed the re-reduction of Bradley’s -observations at Greenwich, and brought out 300 reliable proper motions, -which were utilised by Ludwig Struve, whose investigation removed the -solar apex from Hercules to the neighbouring constellation Lyra: this -slight change was confirmed by _Oscar Stumpe_, of Bonn, and _Lewis_ -_Boss_ (born 1847), director of the Observatory at Albany, New York. An -investigation by Newcomb fully confirmed the previous results. In 1900, -1901, and 1902 Kapteyn made three distinct investigations on the solar -motion, and still further confirmed the previous investigations. - -These investigations are fully confirmed by the application to the -question of Doppler’s principle of measuring radial motion. The -spectroscopic researches of Campbell at the Lick Observatory place the -solar apex very near the position assigned to it by Newcomb and Kapteyn. -Campbell finds the solar velocity to be about 12 miles a second, and -Kapteyn thinks a velocity of about 11 miles a second is “the most -probable value that can at present be adopted.” - - - - - CHAPTER X. - THE LIGHT OF THE STARS. - - -“That a science of stellar chemistry should not only have become -possible, but should already have made material advances, is assuredly -one of the most amazing features in the swift progress of knowledge our -age has witnessed.” So writes Miss Agnes Mary Clerke, the historian of -modern astronomy. As long ago as 1823 Fraunhofer observed the spectra of -the brighter stars, and gathered the first hint of the grouping of the -stars into three classes. Then, after Fraunhofer’s death, the subject -lay in abeyance for thirty-seven years. At length, in 1860, on -Kirchhoff’s explanation of the Fraunhofer lines, the study of stellar -spectra was inaugurated at Florence by Donati, who carefully fixed the -positions of the more important lines. His instrumental means, however, -were very limited, and his observations were not successful. In 1862 -Rutherfurd, in New York, commenced the study of stellar spectra, but -shortly afterwards turned his attention to astronomical photography. The -actual founders of stellar spectroscopy were the eminent Italian -observer, Angelo Secchi, and the illustrious Englishman, William -Huggins. - -Angelo Secchi was born in 1818 at Reggio, in the Emilia. Educated in the -Collegio Romano, he was ordained priest in 1847, but his love of -science, and particularly astronomy, dates from the beginning of his -career. In 1849 he succeeded Di Vico as director of the Observatory of -the Collegio Romano. This post he filled with conspicuous ability for a -period of twenty-nine years, until his death on February 26, 1878. To -Secchi is due the credit of the first spectroscopic survey of the -heavens. He reviewed the spectra of 4000 stars, and classified them into -four distinct groups, which are recognised to this day. The first type -embraces over half of those which Secchi examined. This type is -represented by Sirius, Vega, Altair, and other bluish-white stars, and -is characterised by the intensity of the hydrogen lines. The second type -embraces the yellow stars, such as Capella, Arcturus, Aldebaran, Pollux, -and the Sun itself, and is known as the Solar type. The spectra of these -stars closely resemble that of the Sun, and are distinguished by -innumerable lines. Secchi’s third type, or red stars, represented by -Betelgeux, Antares, and others, are characterised by strong absorption -bands, and the spectra have been described as “fluted.” The third-type -stars are comparatively scarce compared with the first and second, and -the fourth is even less numerous. The fourth-type stars are also red -with broad absorption lines. To Secchi’s four types a fifth was added in -1867 by Wolf and Rayet of Paris Observatory—namely, the gaseous stars. -Secchi aimed at a comprehensive survey of the stellar spectra, and he -accomplished much valuable work. He did not devote his time to analysing -individual stars. This branch of study—analysis of spectra and the -determination of the elements in the stars—was undertaken by his -contemporary, William Huggins, one of the greatest astronomers whom -England has ever produced. - -Born in London in 1824, William Huggins commenced his astronomical -researches at the age of twenty-eight. In 1856 he erected, at Tulse -Hill, London, an observatory which he equipped at great expense. He -commenced observations on the usual astronomical lines, taking times of -transits and making drawings of the surfaces of the planets. But he soon -tired of the routine of ordinary astronomical work, and on the -publication of Kirchhoff’s explanation of the Fraunhofer lines in the -solar spectrum, he commenced to investigate the spectra of the stars. -Having constructed a suitable spectroscope, he commenced observations in -1862 in conjunction with his friend, William Allen Miller, Professor of -Chemistry in London. He exhaustively investigated the two red stars, -Betelgeux and Aldebaran, ascertaining the existence in the former star -of sodium, iron, calcium, magnesium, and bismuth; and in the latter star -the same elements, with the addition of tellurium, antimony, and -mercury. - -In 1863 Huggins made an attempt to photograph the spectra of the stars, -and, indeed, obtained prints of Sirius and Capella, but no lines were -visible in them. In 1874 Draper of New York obtained a photograph of the -spectrum of Vega, showing four lines. Two years later Huggins again -attacked the problem, and secured a photograph of the spectrum of Vega, -showing seven strong lines. In 1879 he was enabled to communicate -satisfactory results of his work to the Royal Society, and since then he -has secured many admirable representations. In 1899 the monumental work, -‘An Atlas of Representative Stellar Spectra,’ the joint work of Sir -William and Lady Huggins, was published. - -In 1874 the German Government established at Potsdam the Astrophysical -Observatory, for the spectroscopic study of the Sun and stars. A -position on the staff was given to Hermann Carl Vogel, whose researches -in astronomical spectroscopy rank with those of Secchi and Huggins. Born -in Leipzig in 1842, he was from 1865 to 1869 employed in the Leipzig -Observatory. Called to Bothkamp as director in 1870, he resigned his -post in 1874 to accept a position on the staff at Potsdam Observatory. -In 1882 he became director of that Institution, which position he still -retains. - -In 1874 Vogel revised Secchi’s classification of stellar spectra, and in -1895 he further improved on it. His classification improves rather than -supersedes the previous work of Secchi; nevertheless, he approached the -question from a different standpoint. Vogel concluded in 1874 that a -rational scheme of stellar classification “can only be arrived at by -proceeding from the standpoint that the phrase of development of the -particular body is, in general, mirrored in its spectrum.” Vogel divides -Secchi’s first type into three classes. In the first type, designated -I_a_,—represented by Sirius and Vega,—the metallic lines are “very faint -and fine,” and the hydrogen lines conspicuous. In I_b_ no hydrogen lines -are visible, while in I_c_ the hydrogen lines are bright. This class -includes the gaseous stars. In 1895, after the recognition of helium in -the stars by his assistant, Scheiner, Vogel separated the stars of class -I_b_ from the first type altogether. These stars are sometimes -designated as “Type O,” and sometimes as helium stars and Orion stars, -as the majority of the stars in Orion are of that type. The solar type -is divided into two classes, II_a_ being represented by the Sun, -Capella, and other well-known stars, while II_b_ includes the Wolf-Rayet -stars. Secchi’s third and fourth types are both classified by Vogel as -of the third type. These red stars were specially studied from 1878 to -1884 by Dunér at Lund. His results were published in a descriptive -catalogue which appeared at Stockholm in 1884. His researches related to -the spectra of 352 stars, 297 of Secchi’s third type and 55 of his -fourth. Dunér is perhaps the greatest authority on stars with banded -spectra. - -Vogel’s classification of spectra is generally adopted by astronomers, -although others have been proposed by Lockyer and by _Edward Charles -Pickering_ (born 1846), director of the Harvard Observatory. Lockyer’s -classification was designed to fit in with his “meteoritic hypothesis,” -discussed in the chapter on Celestial Evolution. The stars were divided -by Lockyer into seven groups, according to his views of their -temperature, rising through gaseous stars, red stars of Secchi’s third -type, and a division of solar stars to the Sirian type, and falling -through a second division of the solar type to red stars of Secchi’s -fourth type. - -The first spectroscopic star-catalogue was published in 1883 by Vogel, -assisted by _Gustav Müller_ (born 1851), a son-in-law of Spörer. The -catalogue contained details of 4051 stars to the seventh magnitude, and -more than half of these proved to be of Secchi’s first type. Vogel’s -work was completed in different latitudes by Dunér at Upsala, and by -_Nicolaus Thege von Konkoly_ (born 1842) at O’Gyalla in Hungary. - -The famous ‘Draper Catalogue’ ranks as the greatest catalogue of stellar -spectra. It was undertaken at Harvard Observatory by E. C. Pickering, in -the form of a memorial to Henry Draper, the successful spectroscopist. -Commenced in 1886, and published in 1890, it contains photographs of the -spectra of no fewer than 10,351 stars, down to the eighth magnitude. -Pickering subdivided Secchi’s types into various classes, the first or -Sirian into four classes, the second into eight, while the third and -fourth types each constitute a separate class. Pickering designated his -classes by the capital letters of the alphabet. - -Much useful work has been done also in the analysis of the various -spectra. Julius Scheiner, now “chief observer” at Potsdam Astrophysical -Observatory, has, since 1890, done much valuable work in this direction. -Special attention was devoted to the spectrum of Capella, 490 lines in -the spectrum of which were measured by Scheiner. In his own words, “he -believes a complete proof of the absolute agreement between its spectrum -and that of the Sun to be thereby furnished.” Other stars of the Sirian -and solar classes were exhaustively studied by Scheiner. - -The study of the exact brilliance of the stars was a branch of research -long neglected, yet it is of much importance in astronomy, for it is -only through exact measurement of stellar brilliance that stellar -variation can be detected. Herschel commenced the study, which was -continued by his son at the Cape, but it is only within the last twenty -years that stellar photometry has become a recognised branch of -astronomy; and the credit of this is due to the energy and zeal of the -great American observer, Edward Charles Pickering. - -Born in Boston in 1846, Edward Charles Pickering was appointed in 1865 -Instructor of mathematics in the Lawrence Scientific School at Harvard, -after a distinguished university career. In 1876 he succeeded Winlock as -director of the Harvard Observatory, and in the following year he -commenced his photometric studies. He invented an instrument named the -meridian photometer, with the aid of which he succeeded in determining, -in the years 1879 to 1882, the exact brilliance of 4260 stars to the -sixth magnitude between the north celestial pole and thirty degrees of -south declination. At a later date he devised a larger photometer, with -which he made over one million observations. Pickering next extended his -survey to the southern hemisphere, erecting the photometer on the slope -of the Andes, where the Harvard auxiliary station at Arequipa is now -located, and where 8000 determinations of stellar brilliance were made. -Meanwhile Pritchard, at Oxford, published in 1885 his ‘Uranometria Nova -Oxoniensis,’ with photometric determinations of the brilliance of 2784 -stars from the pole to ten degrees of south declination. Both of these -catalogues were epoch-making works, and testify to the enthusiasm and -perseverance of the astronomers who designed them. - -The study of stellar photometry glides into that of stellar variation. -At the beginning of the nineteenth century the number of known variable -stars was very small, as a glance at the list given in Brewster’s -edition of Ferguson’s Astronomy (1811) will show. Some remarkable -investigations were due to the English astronomer, _John Goodricke_ -(1764-1786), who rediscovered the variability of the star Algol, and -accurately determined its period in 1782. Goodricke suggested that the -regular variations in the light of Algol were due to the partial eclipse -of its light by a dark satellite, a hypothesis now fully confirmed. Two -years later, in 1784, Goodricke discovered other two variables, δ Cephei -and β Lyræ. He died in 1786 at the age of twenty-one, and thus -variable-star astronomy was deprived of its founder. - -The foundation of variable-star astronomy as an exact branch of the -science is due to Argelander. In the years 1837-1845, while residing at -Bonn during the erection of the observatory, of which he had been made -director, he erected a temporary observatory, and there he carefully -determined the magnitudes of all stars visible in Central Europe. From -this he was led to the discussion of stellar variation, to which subject -he continued to give much attention. He was the first to describe a -method of observing variable stars scientifically and accurately,—a -method consisting in estimating in “steps” or “grades” the difference in -brilliance between the variable, or suspected variable, and other stars -which are selected for comparison, and which are of various degrees of -brilliance, so that they may be available for comparison with the -variable throughout its fluctuations. Argelander’s “steps” are tenths of -a magnitude, and Gore describes the method of observation as follows: -“If we call _a_ and _b_ the comparison stars, and _v_ the variable, _a_ -being brighter than _b_, and if _v_ is judged to be midway in brightness -between _a_ and _b_, we write _a_5_v_5_b_. If _v_ is slightly nearer to -_b_, we write _a_6_v_4_b_. We may also write _a_3_v_7_b_, or -_a_7_v_3_b_, the sum of the steps being always ten.” - -This method, described in 1844, led to many discoveries at Bonn in the -following twenty years by Argelander and his assistants Schmidt and -Schönfeld. At this time Eduard Heis (1806-1877), at Münster, who also -ranks as one of the founders of meteoric astronomy, while engaged on the -construction of his great atlas, attentively determined the change of -magnitude of stars visible to the naked eye; and by means of the naked -eye, the opera-glass, and a small telescope, he amassed a large number -of observations. At the same time two English observers, Hind and -Pogson, were making remarkable discoveries which greatly increased the -number of known variables. Among Hind’s discoveries were S Cancri of the -Algol type; while Schmidt discovered another of the same class, δ Libræ, -and also the famous ζ Geminorum. While director of the Observatory of -Mannheim, an institution equipped with very antiquated instruments, -Schönfeld devoted himself to the study of variable stars, and increased -the number of known variables considerably. In the southern hemisphere -Gould, in South America, did for the observation of variable stars what -Argelander did in the northern. - -In 1874 a very important, although not so obvious, service to -variable-star astronomy was rendered by the Danish observer, _Hans Carl -Fredrik Christian Schjellerup_ (1827-1887). He translated from Arabic -into French the works of the Persian astronomer of a thousand years ago, -Al-Sufi, and thus rendered his observations available to modern -astronomers. Al-Sufi was a most accurate observer, and, by comparing his -catalogue with those of modern observers, it can be found whether stars -have changed in brilliance during the past thousand years. - -The study of variable stars has been pursued in recent years by many -astronomers, both by means of photography and by the visual method. The -most important names in the visual discovery of variables are Gustav -Müller and _Paul Friedrich Ferdinand Kempf_ (born 1856) of Potsdam; -_Alexander William Roberts_ of Lovedale, South Africa; Seth Carlo -Chandler of Boston; Nils Christopher Dunér at Upsala; and _John Ellard -Gore_ (born 1845) in Dublin. - -The researches of J. E. Gore are a brilliant example of how much may be -done for astronomy by means of very moderate instruments. Born in 1845 -at Athlone, in Connaught, he went to India in 1868 as engineer on the -Sirhind Canal in the Punjab. While in India he erected his small -telescopes on brick pillars, and took observations, many of them of -stellar brilliance. In 1879 he returned to Ireland, and since then has -devoted himself to astronomy with zeal and enthusiasm. His discoveries -and investigations of variables have been made by means of the -binocular. On December 13, 1885, he discovered a remarkable star in -Orion, which was at first considered to be temporary, and called “Nova -Orionis,” but was afterwards found to be a long-period variable star. - -Recently photography has come much to the front in the discovery of -variable stars. Pickering at Harvard, and Wolf at Heidelberg, have -particularly distinguished themselves in this branch, and the number of -known variables is now very large, as every year brings fresh -discoveries, mostly by aid of photography. Many of these -newly-discovered variables are in star-clusters and nebulæ. - -Pickering proposed in 1880 the following classification of variable -stars, which has been adopted all over the scientific world: Class I., -temporary star; Class II., stars undergoing in several months large -variations, such as Mira Ceti and U Orionis; Class III., irregular -variables, such as Betelgeux and α Herculis; Class IV., short-period -variables, such as δ Cephei, ζ Geminorum, and β Lyræ; Class V., “Algol -variables,” which undergo variations lasting but a few hours. It is -doubtful whether new stars should be included in a classification of -variables, although in one case, at least, a new star was found to be a -long-period variable. To these a sixth class may now be added. This -class, the detection of which is mainly due to the profound -investigations of Gore, is composed of what have been termed “secular -variables,” which undergo slow fluctuations in periods of many years, -and sometimes of centuries. This Class includes δ Ursæ Majoris, Al-Fard, -λ Draconis, θ Serpentis, ε Pegasi, 83 Ursæ Majoris, ζ Piscis Australis, -β Leonis, α Ophiuchi, η Crateris, and others. The secular variations of -some of these stars have been detected by Gore himself during the past -thirty years, while in other cases he has detected them by comparison of -the most important star-catalogues, from Hipparchus and Al-Sufi down to -our own time. In some cases the star in question seems to be slowly -gaining in brilliance, in others slowly diminishing. - -Thanks to the application of the spectroscope, much is now known of the -cause of the light changes in variable stars. Goodricke’s theory of the -variations of Algol was theoretically confirmed by the researches of E. -C. Pickering in 1880. In 1889 Vogel proved beyond a doubt that the -variation in the light of Algol is due to the partial eclipse of its -light by a dark satellite. It was obvious to Vogel that, as both Algol -and its companion are in revolution round their common centre of -gravity, the motion of Algol in the line of sight might be detected by -the spectroscopic method of observation. Vogel found that before each -eclipse Algol was retreating from our system, while on recovering it -gave signs of rapid approach, proving conclusively that both the star -and its dark satellite were in revolution round their centre of -gravity,—Algol suffering partial eclipse only because the plane of the -orbit lies in our line of sight. Algol, therefore, is not inherently a -variable star, but merely a binary. Following up his researches, Vogel, -assuming that the bright and dark stars are of equal density, arrived at -the conclusion that Algol is a globe about one and a half million miles -in diameter, the satellite equalling the size of the Sun, and the -centres of the stars being separated by about 3,230,000 miles. Thus, -variable stars of the Algol type are not variable in the true sense of -the word. Even the most irregular of the Algol variables have been -explained. Perhaps the most irregular was Y Cygni, discovered by -Chandler in 1886. It was soon found, however, that the variations -recurred with great irregularity: in less than two years the phases -differed by as much as seven hours from the predicted times. At length -the subject was taken up by Dunér at Upsala. A series of observations -made with the 14-inch refractor at Upsala in 1891 and 1892 convinced him -in the latter year that two eclipses take place in the course of one -revolution: one star occults the other. Dunér showed that the intervals -between minima were thus—1 day 9 hours; 1 day 15 hours; 1 day 9 hours, -and so on. Thus, the first, third, fifth, and seventh sets of minima -obeyed a different law from the second, fourth, sixth, and eighth. Dunér -proved that two stars revolve round their centre of gravity in less than -three days, alternately occulting each other, while the ellipticity of -the orbit explains the irregularity of the light changes. In April 1900 -Dunér gave his final conclusions as follows: “The variable star Y Cygni -consists of two stars of equal size and equal brightness, which move -about their common centre of gravity in an elliptical orbit, whose major -axis is eight times the radius of the stars.” He also stated the exact -period of revolution and the eccentricity of the orbit. - -In the case of the short-period variables, such as β Lyræ, δ Cephei, ζ -Geminorum, and η Aquilæ, the variations do not seem to be due to -eclipse. It was discovered by Professor Pickering that β Lyræ is a -spectroscopic binary, but Vogel and Keeler showed that the supposed -orbit is incompatible with the eclipse theory. Vogel says: “I am -convinced that β Lyræ represents a binary or multiple system, the -fundamental revolutions of which in 12 days 22 hours in some way control -the light change.” The eclipse theory, however, is still maintained by -Bélopolsky, who has framed a hypothesis according to which the chief -minimum of the star’s light corresponds with the obscuration of the -lesser star, the lesser minimum with that of the primary, implying that -the primary is much less luminous in proportion to its light than its -satellite,—a state of affairs which Miss Clerke concludes to be -improbable. - -The variable stars, δ Cephei and η Aquilæ, were both found in 1894 by -Bélopolsky to be binaries; but as the times of minimum light do not -correspond with those of eclipses in the hypothetical orbits, he -concludes that the variations cannot be explained on the eclipsing -satellite theory. Miss Clerke is inclined to the theory that the -increase of luminosity in short-period variables is due to tidal action, -so that while the revolutions of the stars control their variability, -they are inherently unstable in light. A large number of these stars are -known, and it is a remarkable fact that the majority of these variables -lie on or near the Galaxy, so that their variations have probably -something to do with their vicinity. - -We now come to the long-period variables of which Mira Ceti, χ Cygni, -and U Orionis are examples. Although varying in regular periods, -generally of about a year, they are subject to remarkable -irregularities, so that an exact period cannot be assigned even to Mira -Ceti, of which the maxima are at times retarded and at others -accelerated with no apparent law. The spectroscopic investigations of -Campbell in 1898 have shown that Mira Ceti is a solitary star, while -bright lines of hydrogen appear in its spectrum at maximum, showing that -the variations are due to periodical conflagrations in its atmospheres. -In many other long-period variables bright lines have been observed. - -A remarkable fact regarding these stars is the amount of their light -change. Mira Ceti, for instance, varies from the first to the ninth -magnitude, and U Orionis from the sixth to the twelfth. As M. Flammarion -says, “the longer the period the greater the variation.” Another -remarkable fact is that their light curves show a curious resemblance to -the curves of the solar spots, only on a vastly greater scale, which -indicates that, relatively, these long-period variables are much older -than our Sun, the small variations in the light of which are -imperceptible. “Here, if anywhere,” says Miss Clerke, “will be found the -secret of stellar variability.” - -To the irregular variables no period can be assigned. Betelgeux, in -Orion, the variation of which was noted by Sir John Herschel in 1840, is -a typically irregular variable. But the most extraordinary of all -variables is η Argus, in the southern hemisphere, which is probably a -connecting link between variable and temporary stars. The traveller -Burchell, from 1811 to 1815, observed the star as of the second -magnitude, but in 1827 he noted it to be of the first magnitude. In the -following year it fell to the second magnitude. In 1834 Sir John -Herschel noted the star to be between the first and second magnitude, -and in 1838 it rose to the first, being equal to α Centauri. After a -decline, it became in 1843 equal to Canopus, and not much inferior to -Sirius. Then it began to fade, and in 1868 it was only of the sixth -magnitude. In 1899 Innes estimated it as 7·71. Rudolf Wolf suggested a -period of 46 years, and Loomis 67 years; but astronomers generally agree -with Schönfeld that the star has no regular period. - -The first temporary star of the nineteenth century was discovered by -Hind, in London, April 28, 1848. It was of the fifth magnitude at -maximum, and soon after began to fade, falling to the tenth magnitude. -In 1860 a new star appeared in the cluster Messier 80 in Scorpio, and -was discovered by Auwers at Königsberg. It reached only the seventh -magnitude. - -On the night of May 12, 1866, a new star of the second magnitude blazed -out in the constellation Corona Borealis. It was first observed at Tuam, -in Ireland, by the Irish astronomer, _John Birmingham_. Four hours -earlier Schmidt had been observing that part of the heavens, and it was -not then visible. Birmingham at once communicated the discovery to -Huggins, at Tulse Hill, who had commenced his spectroscopic -observations. On May 16 Huggins observed its spectrum. In the words of -Miss Clerke, “The star showed what was described as a double spectrum. -To the dusky flutings of Secchi’s third type, four brilliant rays were -added. The chief of these agreed in position with lines of hydrogen; so -that the immediate cause of the outburst was plainly perceived to have -been the eruption, or ignition, of vast masses of that subtle kind of -matter.” Nine days after the appearance of the new star it was invisible -to the naked eye, and afterwards fell to the tenth magnitude. In 1856 -Schönfeld had observed it at Bonn as a telescopic star, so that it was -not a “new star” in the true sense of the word. - -The next temporary star observed was discovered by Schmidt, at Athens, -November 24, 1876. It was of the third magnitude, situated in the -constellation Cygnus. On December 2 its spectrum was examined at Paris -by _Alfred Cornu_ (1841-1902), and some days later at Potsdam by Vogel -and Lohse. It was closely similar to that of the new star of 1866, -bright lines of hydrogen and other elements standing out in front of an -“absorption” spectrum. By the end of 1876 the star was of the seventh -magnitude. On September 2, 1877, Nova Cygni was observed at Dunecht, and -its spectrum was found to have been transformed into that of a planetary -nebula. Three years later, however, the ordinary stellar spectrum -reappeared. - -A new star appeared in the centre of the great nebula in Andromeda in -August 1885. The first announcement of the discovery was by _Karl Ernst -Albrecht Hartwig_ (born 1851), who observed the new star on August 31; -but it had been previously seen by several other observers. On September -1 it was of the seventh magnitude, and by March of the following year -had fallen to the sixteenth. Observed by Vogel, Young, and Hasselberg, -the new star gave a continuous spectrum, but Huggins and Copeland -succeeded in discerning bright lines. Hall, at Washington, undertook a -series of measures to detect the parallax of Nova Andromedæ, but his -efforts were unsuccessful. - -The discovery of the next temporary star was announced February 1, 1892, -by the Rev. _Thomas_ _D. Anderson_, a Scottish amateur astronomer, in a -post-card to the Astronomer-Royal of Scotland. The star was situated in -the constellation Auriga. An examination of photographs, taken at -Harvard Observatory, showed that the new star had appeared December 10, -1891, and had risen to a maximum of the fourth magnitude ten days later. -On a photograph taken by Max Wolf on December 8 the new star was not -visible. After Anderson’s visual discovery, the spectrum of the new star -was studied by Copeland, Huggins, Lockyer, Vogel, Campbell, and others. -Bright hydrogen lines were visible in the spectrum, which appeared to be -actually double, giving support to the theory that the outburst was the -result of a collision between two dark bodies; and this was confirmed by -the measurements of radial motion by the Potsdam astronomers. - -After March 9, 1892, the new star steadily faded, falling to the -sixteenth magnitude on April 26. But on August 17 _Edward Singelton -Holden_ (born 1846), director of the Lick Observatory, and his -assistants, Schaeberle and Campbell, found it of the tenth magnitude. On -August 19 Barnard found it transformed into a planetary nebula: while -various spectroscopic observations of the revived Nova revealed the -nebular lines. By the end of 1894 the new star had faded to the eleventh -magnitude, and early in 1901 was observed as a minute nebula. - -After 1892 several new stars appeared, and were detected on photographic -plates by _Mrs Fleming_ (born 1857), of Harvard Observatory. The first -of these, in the southern constellation Norma, was discovered in 1893 by -its peculiar spectrum on a Draper spectrographic plate taken at Harvard. -But the new star rose only to the seventh magnitude. Other new stars -were discovered in Carina (Argo) in 1895, in Centaurus in 1895, in -Sagittarius in 1898, and in Aquila in 1900. Nova Sagittarii was, at its -brightest, fully equal to Nova Aurigæ, and was plainly visible to the -naked eye, but was never observed visually. - -A temporary star, appropriately designated “the new star of the new -century,” blazed out in Perseus on the night of February 21, 1901. It -was discovered independently by several observers: on February 21, by -Borisiak, a student at Kiev, in Russia; on the following morning, by -Anderson in Edinburgh; and on the next evening, by Gore at Dublin, -Nordvig in Denmark, Grimmler at Erlangen, and other observers. When -first seen by Anderson, it was equal to Algol, of the second magnitude. -A photograph by Williams at Brighton showed that it must have been -fainter than the twelfth magnitude on February 20. On the evening of -February 23 the star was brighter than Capella, and was then the -brightest star in the northern hemisphere. On February 25 it fell to the -first magnitude; on March 1 to the second, and on March 6 to the third. -During the spring and summer the light fluctuated considerably, but in -September and October faded to the 6·7 magnitude. In March 1902 it was -of the eighth magnitude, and in July 1903 of the twelfth. - -The spectrum of Nova Persei was found by Pickering to be of the Orion -type on February 22 and 23. On February 24 the spectrum had become one -of the bright and dark lines, and the hydrogen lines indicated a -velocity of 700 to 1000 miles a second. Measures of the sodium and -calcium lines, by Campbell and others, indicated a velocity of only -three miles a second, so that the displacements of the hydrogen lines -may have been due to an outburst of hydrogen in the star. The spectrum -was carefully studied during the spring and summer by Pickering, -Lockyer, Huggins, Vogel, and others. On June 25 Pickering reported that -the spectrum was slowly changing into that of a gaseous nebula. In -August and September 1901 the nebular spectrum became more apparent. - -In August 1901 Wolf at Heidelberg discovered a faint trace of nebula -near the nova. On September 20 this nebula was photographed by _George -Ritchey_ at the Yerkes Observatory, and was seen to be of a spiral form. -This was confirmed by Perrine, who also found, from plates taken in -November, that the nebula was moving at the rate of eleven minutes of -arc a year. This extraordinary velocity was exceedingly puzzling to -astronomers, and at length Kapteyn suggested that the nebula shone only -by reflected light from the new star, and that the apparent motion was -an illusion caused by the flare of the explosion travelling out from the -nova. - -On March 16, 1903, _Herbert Hall Turner_ (born 1861), Professor of -Astronomy at Oxford, discovered a new star of the seventh magnitude in -the constellation Gemini, from an examination of photographic plates. -Photographs taken at Harvard showed that on March 1 it must have been -fainter than the twelfth magnitude, while five days later it was of the -fifth. In August 1903 Pickering found its spectrum nebular. In August -1905 another small nova was found by Mrs Fleming on the Harvard -photographs, situated in Aquila. - -Many theories have been advanced to account for temporary stars. -Flammarion has shown that a body surrounded by a hydrogen atmosphere, on -grazing a dark body enveloped in oxygen, would produce a tremendous -explosion. In 1892 Huggins suggested that the outburst of Nova Aurigæ -was due to the near approach of two bodies with large velocities, -disturbances of a tidal nature resulting and producing enormous -outbursts. Vogel suggested that the new star was due to the encounter of -a dark star with a worn-out system of planets; while Lockyer believes -all new stars to be due to the collision of swarms of meteors. Perhaps -the most probable theory is that of Seeliger, which attributes these -outbursts to the movement of a dark body through nebulous matter, which -is extensively diffused throughout space. This theory explains the -changes in the spectra as well as the revivals of brightness which -characterised Nova Aurigæ and the fluctuations of Nova Persei. In a -paper read to the Royal Society of Edinburgh in November 1904, the -German astronomer, _Jacobus Halm_, of the Royal Observatory, Edinburgh, -extended and developed Seeliger’s theory, showing also that the -necessary consequence of such an encounter as the theory assumes is the -formation of an atmosphere of incandescent gases, followed by that of a -revolving ring of nebulous matter. In the hands of Halm, therefore, -Seeliger’s theory leads to the nebular hypothesis as advanced by Laplace -and Herschel. - - - - - CHAPTER XI. - STELLAR SYSTEMS AND NEBULÆ. - - -The study of double stars, commenced by Herschel, was taken up after his -death by several of the foremost astronomers, and has since been pursued -by quite a number of observers and computers. Herschel’s immediate -successor in the study of double stars was his son, who ranks only -second to his father as a student of stellar systems. Born at Slough on -March 7, 1792, John Frederick William Herschel passed his childhood -“within the shadow of the great telescope.” Although his early life was -spent with his father and aunt, astronomy does not appear to have taken -up his attention as a boy. Chemistry, however, always interested him, -and, as his aunt recorded, even while a child he was fond of making -experiments. He was educated at Hitcham, and afterwards at Eton. He was -delicate, however, so his mother removed him from school, and he was -trained at Slough by Mr Rogers, a Scottish mathematician. At the age of -seventeen Herschel entered the University of Cambridge, and Caroline -Herschel, who was exceedingly proud of him, recorded in her memoirs that -he gained all the first prizes without exception. He left the University -in 1813. - -John Herschel did not turn his attention to astronomy until he had -attained the age of twenty-four. In a letter to a friend, September 10, -1816, he said, “I am going, under my father’s directions, to take up -star-gazing.” It was only reverence for his father that made him turn to -astronomy, and he gave up the science he loved most—chemistry. But his -unselfishness received its reward. In 1820 John Herschel constructed his -first reflector under his father’s guidance. Four years previously he -had begun to observe double stars, which had been for long studied by -his father, who discovered their revolutions. These observations were -continued from 1821 to 1823 at the Observatory of Sir _James South_ -(1786-1867). John Herschel and South measured 380 of the elder -Herschel’s double stars. These investigations gained for Herschel and -South the Lalande Prize of the French Academy and the Gold Medal of the -Royal Astronomical Society. - -When his mother died Sir John Herschel decided to sail to the Cape of -Good Hope to make an investigation of the stars of the southern -hemisphere, which until then had been much neglected. He was offered a -free passage in a ship of war, but declined. In November 1833 he left -England, taking with him his great telescopes. In two months he arrived -at Cape Town, and erected his astronomical instruments at Feldhausen, a -short distance off. In October 1835 he informed his aunt that he had -almost completed his survey of the southern hemisphere. During his -“sweeps” of the heavens he discovered 1202 double stars, and 1708 nebulæ -and star-clusters. In 1838 he returned to England, and devoted the -remainder of his life to the publication of his results, as well as to -other branches of science. He died at Collingwood, in Kent, on May 11, -1871, at the age of seventy-nine. - -John Herschel’s favourite objects of study were double stars, of which -he discovered 3347 in the northern hemisphere, and 1202 in the southern. -He also computed several stellar orbits; but the first calculation of a -stellar orbit was made by the French astronomer _Felix Savary_ -(1797-1841), who computed the orbit of ξ Ursæ Majoris, and found the -period to be about sixty years. Contemporary with John Herschel was his -great rival in double-star astronomy, Friedrich Georg Wilhelm Struve. -Born at Altona in 1793, Struve took his degree in 1811 at the Russian -University of Dorpat. In 1813 he became director of the Dorpat -Observatory, and was in 1839 promoted to Pulkowa, as director of the -great Observatory there, remaining at its head until within three years -of his death, on November 23, 1864. Struve’s first recorded observation -was on the double star Castor. In 1819 he commenced to measure the -position-angles of double stars, of which he published a catalogue of -795. In 1825 he commenced a review of the heavens down to fifteen -degrees south, and thus discovered 2200 previously unknown objects. The -results were published in Struve’s ‘Mensuræ Merometricæ,’ which appeared -in 1836, giving the places, distances, colours, position-angles, and -relative brilliance of 3112 double and multiple stars. - -Struve’s successor in this branch of astronomy was his son, Otto Wilhelm -von Struve, born in 1819 at Dorpat, who became in 1837 assistant to his -father, and in 1861 succeeded him as director of the Pulkowa -Observatory. In 1890 he retired from this post, settling in Germany, at -Carlsruhe, where, on April 14, 1905, he died in his eighty-sixth year. -Otto Struve detected 500 double stars, among them γ Andromedæ, -discovered in 1842, and δ Equulei, discovered in 1852, within a period -of between five and eleven years. - -Various other astronomers have devoted themselves to the observation of -double stars, among them _Ercole Dembowski_ (1815-1881), of Milan; _Karl -Hermann Struve_ (born 1854), son of Otto Struve; _William Doberck_ (born -1845); _William J. Hussey_ (born 1864), now director of the Detroit -Observatory; Camille Flammarion; N. C. Dunér; G. V. Schiaparelli; -_Thomas Jefferson Jackson See_ (born 1866). But the greatest living -_discoverer_ is _Sherburne Wesley Burnham_ (born 1838), now employed at -the Yerkes Observatory, in Wisconsin. Born in 1838 at Thetford, Vermont, -he commenced his career as a shorthand reporter, studying astronomy in -his leisure hours. With a small 6-inch refractor, mounted in a home-made -observatory, Burnham commenced in 1871 his discoveries of double stars, -which soon attracted the attention of noted astronomers, who permitted -him to use larger telescopes, with which he continued his researches. -His first official appointment was in 1888, when he became chief -assistant at the Lick Observatory, which position he resigned in 1892. -Some years later he became astronomer in the Yerkes Observatory. -Altogether he has discovered 1308 double stars, with telescopes ranging -from a 6-inch refractor to the gigantic 40-inch of the Yerkes -Observatory. - -The computation of double-star orbits has been undertaken by various -astronomers, among them Mädler, Klinkerfues, Dunér, Flammarion, -Seeliger, See, Gore, Burnham, _Robert Grant Aitken_ (born 1864) of the -Lick Observatory, and _Giovanni Celoria_ (born 1842), who was, from 1866 -to 1900, assistant in the Brera Observatory of Milan, and since 1900 -director of that institution. On June 9, 1890, Gore presented to the -Royal Irish Academy a catalogue of computed binaries containing -reference to fifty-nine stars. - -In 1844 Bessel discovered a remarkable irregularity in the proper motion -of Sirius. He ascribed this to the gravitational influence of some -obscure body, probably a large satellite. In 1857 Peters calculated an -orbit for the supposed satellite with a period of 50 years. In 1861 an -orbit was computed by _Truman Henry Safford_ (1836-1901), which -indicated the position of the satellite. Close to this position it was -accidentally discovered by _Alvan Clark_ (1832-1897), the famous -American optician. The period of the star seems to be about 50 years. In -1844 Bessel noticed irregularities in the proper motion of Procyon, and -put forward the idea of a disturbing satellite, as in the case of -Sirius. This was confirmed by Mädler, and in 1874 an orbit was computed -by Auwers, who found a period of 40 years. In 1896 the satellite was -found by Schaeberle with the 36-inch refractor of the Lick Observatory. -A period of 40 years was found by See, in agreement with the -hypothetical orbit. - -In putting forward these theories as to invisible stellar satellites, -Bessel remarked that “light is no real property of mass,” and that the -existence of countless visible stars is nothing against the existence of -countless invisible and dark ones. In this he laid the foundation of the -branch of science termed by Mädler the “Astronomy of the invisible.” In -recent years the astronomy of the invisible has become a recognised -branch of astronomical research, through the application and -interpretation of Doppler’s principle in spectroscopic observations. In -the course of photographing the stellar spectra for the Draper -Catalogue, E. C. Pickering photographed the spectrum of Mizar (ζ Ursæ -Majoris) in 1887 and again in 1889. On some of these photographs the -line K was seen double, while on others it was seen under its normal -aspect. This doubling of the lines indicated that the star which we see -as single is in reality composed of two bodies in revolution round their -centre of gravity, so close together that even the largest telescopes -cannot divide them. Pickering assigned a period of 104 days, but in 1901 -Vogel diminished this to 20 days. In the same year the star β Aurigæ was -similarly found to be double; and in 1890 Vogel, from photographs taken -at Potsdam, independently inaugurated the discovery of spectroscopic -binaries. In the spectrum of Spica he discovered the spectral lines to -be, not doubled, but periodically displaced, indicating the existence of -a dark or nearly dark companion, both stars revolving round their centre -of gravity. Spica was seen to belong to the same class as Algol, only -that in the case of Algol the plane of the satellite’s orbit passes -through the Earth and eclipses the star, while in the case of Spica the -orbit is inclined, and the star is constant in light. - -The line of research commenced by Vogel and Pickering was soon followed -up by these investigators, as well as by Bélopolsky at Pulkowa, Campbell -at the Lick Observatory, Slipher at the Lowell Observatory, and by -_Edwin Brant Frost_ (born 1866), now director of the Yerkes Observatory, -and his assistant, _Walter Adams_. In 1894 Bélopolsky discovered the -duplicity of several variable stars, and in 1896 that of Castor, in -Gemini. Late in 1896 Campbell undertook a systematic investigation of -radial motions, and has since discovered about sixty spectroscopic -binaries,—among them, in 1899, the Pole Star, and in 1900 Capella. The -latter discovery was made independently by _Hugh Frank Newall_ (born -1857) at Cambridge, in England. It was found by Campbell that the -revolution of the stars round their centre of gravity is performed in -104 days; and it soon became apparent that, owing to the large size of -the orbit, the duplicity of Capella might be observed telescopically. At -Greenwich the star was seen to be elongated, but at the Lick Observatory -it was seen persistently single. - -Campbell finds that of 285 stars observed by him, more than one in nine -is a spectroscopic binary. He concludes that at least one star in five -or six will be found to be spectroscopically double, and considers that -“the proven existence of so large a number of stellar systems, differing -so widely in structure from the Solar System, gives rise to a suspicion -at least that our system is not of the prevailing type of stellar -systems.” - -The study of triple and multiple stars is of deep interest, but the -orbits of these objects cannot be said to be fully investigated by any -means. The first application of the problem of three bodies to stellar -astronomy was made by Seeliger in 1889. His researches, relating to the -famous star, ζ Cancri, disclosed the existence of three stars revolving -round a dark body, apparently the most massive in the system. The system -of ζ Cancri, at least, seems to be modelled on the Ptolemaic design. - -In the study of star-clusters and nebulæ, as in the investigation of -double stars, Herschel’s successor was his son. His observations, both -in England and at the Cape of Good Hope, resulted in a large number of -new discoveries, and the results of his studies in this direction were -published in 1864 in his catalogue of all known clusters and nebulæ, -amounting to 5079. This catalogue was enlarged and revised in 1888 by -_John Louis Emil Dreyer_ (born 1852), a Danish astronomer, but director -of the Observatory at Armagh, in Ireland; and the same observer -published from 1888 to 1894 a supplementary list, bringing the number of -known clusters and nebulæ to about 10,000. - -In the early part of his career, John Herschel held firmly to the views -of his father of the difference between star-clusters and nebulæ, -considering the latter to be composed of “shining fluid.” But he fell -off from this view with the resolution into stars of many irresolvable -nebulæ. In 1845 _William Parsons_, third _Earl of Rosse_ (1800-1867), -erected at Birr Castle, in Ireland, his great 6-foot reflector, which -still surpasses all other telescopes in point of size. With this -instrument Lord Rosse believed himself to have resolved the Crab nebula -in Taurus and the Nebula in Orion, which was also said to have been -resolved by Bond with the 15-inch refractor at Harvard; and in 1854 -Olmsted declared the “resolution” of these nebulæ to be the signal for -the renunciation of Herschel’s nebular theory. Most astronomers fell in -with the view that all the nebulæ were distant clusters, which would -eventually be resolved into stars, although it is only right to state -that the Scottish astronomer, _John Pringle Nichol_ (1804-1859), and -some other investigators, held to the theory of Herschel. - -The solution of the great problem was in 1864, when on August 29 of that -year Huggins turned his spectroscope on a bright planetary nebula in -Draco. To his amazement the spectrum was one of bright lines, proving -conclusively that the nebula was not a star-cluster, but a mass of -glowing gas,—hydrogen, and some other unknown substance, now named -“nebulium.” By 1868 Huggins had observed the spectra of seventy nebulæ. -Of these one-third proved to be gaseous, among them the great Orion -nebula which Lord Rosse was believed to have resolved into stars. In the -spectrum of the latter, the “chief nebular line” was at first ascribed -by Huggins to nitrogen, but this was a mistake. Later, it was believed -by Lockyer to coincide with the fluting of magnesium, but this was -disproved by Huggins in 1889-90, and by Keeler in 1890-91. The great -nebula in Andromeda and the great spiral in Canes Venatici were found by -Huggins to display a continuous spectrum, and a similar discovery was -made in regard to the cluster M 13 in Hercules, and other star-clusters. -In the case of the nebulæ, it is not believed that the continuous -spectrum is due to the existence of sun-like bodies, as a gas under -pressure would give a continuous spectrum. - -The Orion nebula has been more thoroughly studied than any other object -of its class. The application of photography to spectroscopy has done -much to further the study of the lines in the nebular spectrum. In 1886 -Copeland detected in the spectrum of the Orion nebula the yellow ray of -helium. On February 13, 1890, Scheiner announced an important discovery, -namely, the possession by both the nebula and the stars in Orion—with -the exception of Betelgeux—of a line, which appeared bright in the -nebular spectra and dark in the stellar. This line was identified by -Vogel, Lockyer, and others with that of helium. - -Nebular photography was inaugurated in 1880 by Draper, who obtained a -remarkably good representation of the Orion nebula in that year. His -work in this direction, cut short by his death in 1882, was taken up by -Janssen at Meudon, and by Common in England, who obtained, in 1883, -several excellent photographs. Later photographs have shown the Orion -nebula to be much more extended than visual observations would lead one -to expect. A photograph secured in 1890 by W. H. Pickering revealed the -nebulous matter in Orion in its true form, that of a gigantic spiral, -starting from near Bellatrix, sweeping past κ Orionis and Rigel to η, -and joining with the great nebula surrounding θ; the entire -constellation being thus shown to be enwrapped in nebulous haze. - -In 1885 nebular photography was commenced by _Isaac Roberts_ -(1829-1904), the English amateur astronomer, who secured admirable -representations of clusters and nebulæ. He published, in 1893 and 1900, -two volumes of collected photographs of clusters and nebulæ. This -monumental work was thus referred to by Dr _William James Lockyer_: “Dr -Roberts has not only nobly enriched astronomical science, but has raised -a monument to himself which will last as long as astronomy has any -interest for mankind.” - -Perhaps the most remarkable revelation made by photography in this -branch of research has been the discovery of the nebulæ in the Pleiades. -In 1859 Tempel observed at Florence an elliptical nebula south of the -star Merope. On November 16, 1885, the brothers Henry obtained at Paris -a photograph of the Pleiades, revealing the existence of a small spiral -nebula. This was confirmed by visual observations, and particularly by -the photographs of Roberts, which also showed the entire cluster to be -nebulous, and that “the nebulosity extends in streamers and fleecy -masses, till it seems almost to fill the spaces between the stars, and -to extend far beyond them.” In 1888 a further advance was made by the -brothers Henry, who found seven stars to be strung on a nebulous streak. - -Since 1890 nebular photography has been pursued by Max Wolf in Germany, -and by E. E. Barnard and J. E. Keeler in America. Wolf’s photographs of -the constellation Cygnus brought out the close connection between the -stars and the extensively diffused nebulosities discovered by him. In -1901 Wolf discovered a “nebelhaufen” or cluster of nebulæ, and in 1902 -published a catalogue of 1528 nebulæ round the pole of the Galaxy, -showing them to be systematically distributed. Keeler made his memorable -observations with the great 36-inch reflecting telescope, which was -constructed in England many years ago by Common. It afterwards passed -into the hands of Mr Crossley of Halifax, who presented it to the Lick -Observatory. With this great instrument Keeler commenced to take -photographs of the heavens. On one occasion he photographed a well-known -nebula, and on developing the plate was surprised to find seven new -nebulæ besides that which he had photographed. On another occasion he -exposed a plate to a nebula in Pegasus. He was amazed to find altogether -twenty-one nebulæ included in the photograph. To give another instance, -a plate directed to the constellation Andromeda contained no fewer than -thirty-two nebulous objects. This has given an enormous extension to our -knowledge of the nebulæ. But even this is not all. Keeler found on his -plates numerous points of light which seem to be also nebulæ, either too -small or too remote to appear as such. Apparently, however, they are not -stars. Keeler’s work convinced him that, on a modest estimate, there -must be at least _one hundred and twenty thousand_ new nebulæ within -reach of the Crossley reflector. Half of these, he announced, were -probably spiral. An idea of the vast importance of Keeler’s work may be -gained if we reflect that the observations of all the earlier -astronomers resulted in the discovery of six thousand nebulæ. The -investigations of Keeler, in all probability, were the means of adding -120,000 more. - -Many observations have been made on nebulæ, for the purpose of -ascertaining their proper motions—but without success. Measurements were -made by D’Arrest in 1857 and by Burnham in 1891, but none of these -revealed any motion of the nebulæ across the line of sight. Even the new -spectroscopic method of determining motions in the line of sight, in the -hands of Huggins, failed in the case of the nebulæ. With the great Lick -refractor at his disposal, Keeler attacked the subject in 1890, and -measured the radial velocities of ten nebulæ. He found that the -well-known planetary nebula in Draco was moving towards the Solar System -at the rate of 40 miles a second; for the Orion nebula he found a motion -of recession of 11 miles a second; but probably this belongs chiefly to -the movement of the Solar System in the opposite direction. - -Unfortunately Keeler did not live to carry on his investigations in -nebular astronomy. His early death brought to an abrupt end these -fruitful investigations. Appointed director of the Lick Observatory in -1898, he died suddenly at San Francisco on August 12, 1900, at the early -age of forty-two. - - - - - CHAPTER XII. - STELLAR DISTRIBUTION AND THE STRUCTURE OF THE UNIVERSE. - - -After the death of Herschel there was little done in the direction of -furthering our knowledge of stellar distribution, or the construction of -the heavens. Here, as elsewhere, Herschel’s immediate successor was his -son, whose star-gauges, both in England and in South Africa, were a -worthy sequel to those of his father; but John Herschel, in his books on -astronomy, reproduced his father’s disc-theory, unaware that the elder -Herschel had himself abandoned it. The work of the younger Herschel was -entirely supplementary to that of his father. - -To Wilhelm Struve belongs the credit of showing the disc-theory to be -untenable, and of demonstrating that Herschel had abandoned it. This he -was able to do after a perusal of Herschel’s papers, presented to him by -John Herschel. Having demonstrated this, he undertook a series of -investigations which resulted in his famous theory of the Universe. This -was published in his work ‘Études d’Astronomie Stellaire,’ which was -published in 1847. His researches were based on the star-catalogues of -Bessel, Piazzi, and others; and dealing with 52,199 stars, he discussed -the number of stars in each zone of Right Ascension. He found, in the -words of Mr Gore, “that the numbers increase from hour i to hour vi, -where they attain a maximum. They then diminish to a minimum at hour -xiii, and rise to another but smaller maximum at hour xviii, again -decreasing to a second minimum at hour xxii. As the hours vi and xviii -are those crossed by the Milky Way, the result is very significant.” He -concluded the Galaxy to be produced by a collection of -irregularly-condensed clusters, the stars condensed in parallel planes. -Next, he considered the Universe as perhaps infinitely extended in the -direction of the Galaxy, and accordingly he put forward the idea that -the light from the fainter and more distant stars was extinguished in -its passage through the ether of space, which he regarded as imperfectly -transparent. The theory, as Struve propounded it, was disposed of by Sir -John Herschel, who remarked that we were not permitted to believe that -at one part of the sky our view was limited by extinction, while at -another a clear view right through the Galaxy could be had; and by -_Robert Grant_ (1814-1892), director of the Glasgow Observatory, who -showed that, were the theory true, the Galaxy should present a uniform -appearance throughout its course. On the whole, Struve’s theory was no -improvement on Herschel’s; for, as Encke pointed out, Struve’s theory -was built on five assumptions, all of which were questionable. - -At the time of Struve’s investigation Mädler, at Dorpat, was engaged in -an attempt to solve the question of the construction of the heavens by -quite another method, that of stellar proper motion. He determined to -investigate the subject of proper motion in order to discover the -central body of the Milky Way. If such a centre existed, however, the -motions near it would be somewhat different from those in the Solar -System. In our Solar System the planets nearest the Sun move swiftest, -owing to the strength of the force of gravitation. In the Sidereal -System, on the other hand, the movements at the centre, as Mädler -pointed out, would be slowest. As there would be no very large -preponderating body, the mutual attractions of the different stars would -cause the bodies at the boundaries of the Universe to move faster than -those at the centre, the central sun—the object of Mädler’s search—being -in a state of rest relative to the Sidereal System. Mädler accordingly -began to search the heavens for a region of sluggish proper motions. - -In the constellation Taurus, Mädler noticed that the proper motions of -the stars were very slow. The idea occurred to him that the bright red -star Aldebaran might be the central sun, but its very large proper -motion was obviously against this inference. Star after star was now -subjected by Mädler to the most careful scrutiny. At length, after a -laborious investigation, he announced that the star which fulfilled the -conditions of a central body was Alcyone, the brightest of the Pleiades, -a group possessed of no proper motion except that due to the sun’s drift -in the opposite direction. In 1846 Mädler published his hypothesis in -his elaborate work, ‘The Central Sun.’ He announced that his -observations had led him to the conclusion that Alcyone occupied the -centre of gravity of the Sidereal System, and was the point round which -the stars of the Galaxy were all revolving. His profound imagination, -however, did not stop here. This speculation led him to the sublime -thought that the centre of the Universe was the Abode of the Creator. In -1847 Struve rejected Mädler’s theory as “much too hazardous,” and this -has been the general opinion of astronomers. Mädler’s theory is now -regarded as quite untenable. - -Herschel’s earlier idea that the nebulæ were external galaxies was long -held by the majority of astronomers, in preference to his later and more -advanced ideas. The supposed resolution of the nebulæ by Lord Rosse’s -telescope gave support to this external galaxy theory. It was clearly -shown, however, by _William Whewell_ (1794-1866) in 1853, and by -_Herbert Spencer_ (1820-1903) in 1858, that the systematic distribution -of the nebulæ in regard to the stars precluded the possibility of their -being external galaxies. This was confirmed by the spectroscopic -discovery of the gaseous nature of some of the nebulæ, and by the later -researches of R. A. Proctor. Not only did Proctor make fresh -discoveries, but it fell to him to clear away the erroneous ideas -regarding the construction of the heavens, and to put the study on a new -basis. In 1870 Proctor plotted on a single chart all the stars, to the -number of 324,198, contained in Argelander’s ‘Durchmusterung’ charts. -This work gave the death-blow to the “disc-theory.” In his own words, -“In the very regions where the Herschelian gauges showed the minutest -telescopic stars to be most crowded, my chart of 324,198 stars shows the -stars of the higher orders (down to the eleventh magnitude) to be so -crowded, that by their mere aggregation within the mass they show the -Milky Way with all its streams and clusterings. It is utterly impossible -that excessively remote stars could seem to be clustered exactly where -relatively near stars were richly spread.” - -Proctor showed also that in all probability the stars composing the -nebulous light of the Galaxy are much smaller than the brighter stars, -and not at such a great distance as their faintness would lead us to -suppose,—a conclusion confirmed by the work of Celoria. Proctor was not -so fortunate in theorising as in direct investigation. He thought that -the Magellanic clouds were probably external galaxies; and further, he -put forward the idea that the Milky Way is a spiral, the gaps and -coal-sacks being due to loops in the stream, but neither of these ideas -has found favour with astronomers. But the chief work accomplished by -Proctor was a revision of our knowledge of the Universe, which he thus -describes: “Within one and the same region coexist stars of many orders -of real magnitude, the greatest being thousands of times larger than the -least. All the nebulæ hitherto discovered, whether gaseous and stellar, -irregular, planetary, ring-formed, or elliptic, exist within the limits -of the Sidereal System.” - -Proctor’s discovery of the excess of bright stars on the Galaxy was -confirmed by _Jean Charles Houzeau_ (1820-1888), director of the -Brussels Observatory. Some time later J. E. Gore carefully examined the -positions of all the brighter stars in the northern and southern -hemisphere. Following this, he made an enumeration of the stars in the -atlas of Heis and in the charts constructed by Harding; the outcome of -the investigation being to show that stars of each individual magnitude -taken separately tend to aggregate on the Galaxy, the aggregation being -noticed even in first-magnitude stars. Gore further pointed out many -cases of close connection between the lucid stars and the galactic -light. A similar investigation was undertaken by Schiaparelli in 1889. -Schiaparelli, basing his work on the catalogue of Gould and the -photometric measures of Pickering, constructed a series of planispheres -which demonstrated the crowding of the lucid stars towards the plane of -the Galaxy. These investigations were still further continued by Simon -Newcomb, who demonstrated that “the darker regions of the Galaxy are -only slightly richer in stars visible to the naked eye than other parts -of the heavens, while the bright areas are between 60 and 100 per cent -richer than the dark areas.” The Dutch astronomer, _Charles Easton_, -finds a connection between the distribution of ninth-magnitude stars and -the luminous and obscure spots in the Galaxy. - -It was noticed by Gould, from observations made at Cordova, that “a belt -or stream of bright stars appears to girdle the heavens very nearly in a -great circle which intersects the Milky Way.” According to Gould, the -belt includes Orion, Canis Major, Argo, Crux, Centaurus, Lupus, and -Scorpio in the southern hemisphere, and Taurus, Perseus, Cassiopeia, -Cepheus, Cygnus, and Lyra in the northern. This was interpreted by -Celoria as indicating the existence of two galactic rings, but Gould -considered the zone of bright stars to form with the Sun a subordinate -cluster of about five hundred stars within the Galaxy. - -Perhaps the most elaborate investigations on the structure of the -Universe have been those of Kapteyn, commenced in 1891. In that year he -demonstrated that stars are bluer and more easily photographed in the -Galaxy than elsewhere, a discovery independently made by Gill at the -Cape, and Pickering at Harvard. In 1893 Kapteyn announced his -conclusions, derived from a novel method of studying the distance of the -stars from their proper motions. In order to reach a definite idea of -the distances of the stars, he made use of the component of the proper -motion, measured at right angles to a great circle of the sphere which -passes through a given star and the apex of the solar motion. He found -that stars of the first spectral type have smaller proper motions than -those of the second, indicating that stars of the second type are on the -average nearer to the Solar System than those of the first, the near -vicinity containing almost exclusively second-type stars. Kapteyn -concluded that the group of second-type stars formed one system, named -the solar cluster, which he considered to be roughly spherical in shape. -In 1902 he abandoned this idea, retaining, however, his opinions as to -the relative distances of the different types. That the second-type -stars are nearer to the Sun than the first is, he remarked in a letter -to the writer, incontrovertible. - -In the investigation of the motions in, and extent of, the Universe, the -name of Simon Newcomb stands out pre-eminently. Born in 1835 at Wallace, -in Nova Scotia, he went to the States in 1853. In 1862 he received an -appointment at Washington Observatory, and he retained an official -position until 1897. Throughout his scientific career he has been -specially attracted by the question of the construction of the heavens, -which he fully discussed in his book on ‘The Stars’ in 1901. Newcomb’s -investigations have shown that some of the stars are not permanent -members of the Sidereal System, among them the swiftly-moving 1830 -Groombridge. He has shown that the Stellar Universe does not possess -that form of stability which is seen in the Solar System. Newcomb -considers the Universe to be limited in extent, as opposed to the -opinions of Struve and others, who believed it to be infinite. He has -brought clearly before his readers a calculation, based on the known law -that there are three times as many stars of any given magnitude as of -that immediately brighter, the increase of number compensating for the -decrease of brilliance. Were the Universe infinitely extended, the whole -heavens would shine with the brilliance of the Sun. Newcomb, therefore, -concludes that “that collection of stars which we call the Universe is -limited in extent.” - -Positive evidence that this is the case was obtained by Giovanni -Celoria, now director of the Milan Observatory, in the course of a -series of star-gauges at the north galactic pole. Using a small -refractor, showing stars barely to the eleventh magnitude, he found he -could see exactly the same number of stars as Herschel’s large -reflector, indicating that increase of optical power will not increase -the number of stars visible in that direction. Celoria’s observation can -only be explained on the assumption that the Universe is limited in -extent, as otherwise Herschel’s telescope should have shown more stars -than Celoria’s, even granting an extinction of light,—a theory which -Newcomb, Schiaparelli, and others have shown to be quite untenable. That -the Universe is limited in extent is about all that is known for -certain, although even this has been called in question, notably by E. -W. Maunder and H. H. Turner. The problem of the construction of the -heavens is by no means solved, although several more or less probable -theories have been advanced. - -A series of investigations on stellar distribution, from 1884 to 1898, -led Hugo Seeliger, director of the Munich Observatory, to some -remarkable deductions. He believes the Universe to be flattened at the -galactic poles. The Galaxy is the zone of stellar condensation, and he -concludes the distance of the Solar System from the inner border of the -zone to be 500 times the distance of Sirius, while the external border -is 1100 times that distance. The Universe is finite in extent, its -limits being about 9000 light years from the Solar System. In Seeliger’s -opinion the extinction of light may come into play beyond our Universe, -and prevent us seeing other collections of stars. - -The question of external universes is purely a hypothetical one, -although there is undoubtedly much to be said in its favour. These -universes have never been seen, and we can only speculate as to their -existence. The last word on the subject is by Gore, in 1893, in his -elaborate work, ‘The Visible Universe.’ He regards the Solar System as a -system of the first order, and the Galaxy and its fellow-universes of -the second. He makes a calculation of the possible distance of an -external universe of his second order. He assumes the distance of the -nearest universe from our Galaxy as proportional to that separating the -Sun from α Centauri, and reaches the amazing conclusion that the -distance of the nearest Galaxy is no less than -520,149,600,000,000,000,000 miles,—a distance which light, with its -inconceivable velocity of 186,000 miles a second, would take almost -ninety millions of years to traverse. - -These calculations absolutely overwhelm the mind, which is unable to -comprehend such vast distances. Our universe is indeed, as Flammarion -expresses it, a point in the infinite. The calculations of J. E. Gore -represent our highest scientific conception of the universe. He sums up -his investigations with the following words: “Although we must consider -the number of _visible_ stars as strictly finite, the numbers of stars -and systems really existing, but invisible to us, may be practically -infinite. Could we speed our flight through space on angel wings beyond -the confines of our limited universe to a distance so great that the -interval which separates us from the remotest fixed star might be -considered as merely a step on our celestial journey, what further -creations might not then be revealed to our wondering vision? Systems of -a higher order might there be unfolded to our view, compared with which -the whole of our visible heavens might appear like a grain of sand on -the ocean shore,—systems perhaps stretching out to infinity before us, -and reaching at last the glorious ‘mansions’ of the Almighty, the Throne -of the Eternal.” - - - - - CHAPTER XIII. - CELESTIAL EVOLUTION. - - -In the second chapter we outlined the nebular hypothesis as propounded -by Herschel. Some time earlier the French mathematician, Laplace, had -put forward his theory of the evolution of the Solar System. _Pierre -Simon Laplace_ was born at Beaumont-en-Auge, near Honfleur, in 1749, and -was educated in the Military School of his native town. In 1767 he -became Assistant Professor of Mathematics at Beaumont, and some years -later at the Military School in Paris, which position he retained for -many years. Member of the Institute and Minister of the Interior under -Napoleon, he was created a Marquis by Louis XVIII., and died at Arcuile -on March 5, 1827. - -In the last chapter of his popular work, the ‘Système du Monde,’ Laplace -put forward his nebular theory “with that distrust which everything -ought to inspire that is not the result of observation or calculation.” -Laplace noticed that in the Solar System all the planets revolved round -the Sun in the same direction, from west to east, and that the -satellites of the planets obeyed the same law. He also observed that the -Sun, Moon, and planets rotated on their axes in the same direction as -they revolved round the Sun; also that the planets moved round the Sun, -and the satellites round their primaries, in almost the same plane as -the Earth’s orbit, the plane of the ecliptic. It was evident that these -remarkable congruities were not the result of chance, and accordingly -Laplace expressed his belief that the Solar System originated from a -great nebula, which in condensing detached various rings in the process -of rotation. These rings condensed into the various planets and their -satellites. - -Laplace’s theory was powerfully supported by Herschel’s observations of -the various nebulæ in the heavens. But, with the supposed resolution of -the various nebulæ after the erection of the Rosse reflector in 1845, -the evidence in favour of the nebular theory seemed to be greatly -reduced. In 1864, however, the discovery of the gaseous nebulæ, by means -of the spectroscope, gave further support to the theory. Powerful aid -was lent to the nebular hypothesis by the famous German physicist, -_Hermann Ludwig Ferdinand von Helmholtz_ (1821-1894), in 1854, in his -theory of the maintenance of the Sun’s heat. Many theories had been -already advanced to account for this. After the discovery of the -conservation of energy, _Julius Robert Mayer_, one of the discoverers, -put forward the theory that the Solar heat was sustained by the inflow -of meteorites from space, and this idea was developed in 1854 by Sir -_William Thomson_, now _Lord Kelvin_ (born 1824), but it was soon -apparent that the supply of meteors required to sustain the Solar heat -was such as would have increased the mass of the Sun very considerably. -Accordingly the hypothesis was partially abandoned, and was succeeded by -that of Helmholtz, who pointed out that the radiation of the Sun’s heat -was the result of its contraction through cooling. The rate was then -estimated at 380 feet yearly, or a second of arc in 6000 years. This -theory was at once generally accepted. It assumes the Sun to be still -contracting, and therefore, on going backwards in imagination, we reach -a period when the Sun must have been much larger than now, and, in fact, -extended beyond the orbit of Neptune. - -Several objections to Laplace’s nebular theory were urged by various -investigators. Among these was the retrograde motions of the satellites -of Uranus and Neptune, and the extremely rapid revolution of the inner -satellite of Mars. Other objections were urged by Babinet, Kirkwood, and -others, and at length a sweeping reform of the nebular theory was -proposed by Faye in 1884, in his work, ‘Sur l’Origine du Monde.’ Faye -put forward the idea that all the planets interior to the orbit of -Uranus were formed inside the solar nebula, while Uranus and Neptune -came into existence after the development of the Sun was far advanced. -But the objections to Faye’s theory are formidable, and the hypothesis -has not been accepted. - -A popular exposition of the nebular theory was given in 1901 in Ball’s -work on ‘The Earth’s Beginning.’ He exhaustively discusses the whole -question, and explains the retrograde motion of the satellites of Uranus -and Neptune as due to the fact that the planes of the orbits of the -satellites will eventually be brought to coincide with the ecliptic. -These motions, says Ball, do not disprove the nebular theory. “They -rather illustrate the fact that the great evolution which has wrought -the Solar System into its present form has not finished its work: it is -still in progress.” - -The theory that the Sun’s heat was maintained by meteors, was extended -by Proctor in 1870 to explain the growth of the planets through meteoric -aggregation as well as nebular condensation. Certainly the theory, as -developed by Proctor, accounted fairly well for the various features of -the Solar System; but the highest development of the meteoritic theory -is due to Lockyer, who published his views in 1890, in his work, ‘The -Meteoritic Hypothesis.’ Lockyer claims that his views are merely -extensions of Schiaparelli’s ideas regarding the concentration of -celestial matter. He considered the chief nebular line to be identical -with the remnant of the magnesium fluting, which is conspicuous in -cometic and meteoric spectra; but Huggins and Keeler, with more powerful -instruments, disproved the supposed coincidence. Lockyer considers that -“all self-luminous bodies in the celestial space are composed either of -swarms of meteorites or of masses of meteoric vapour produced by heat. -The heat is brought about by the condensation of meteor swarms, due to -gravity, the vapour being finally condensed into a solid globe.” - -Lockyer divided the stars into seven groups, according to temperature, -the order of evolution being from red stars through a division of -second-type stars to Sirian stars, regarded as the hottest stars; -through a second division of solar stars to fourth-type stars. In fact, -the theory aspires to give a complete explanation of all celestial -phenomena, from meteors to nebulæ. Newcomb, however, considers that the -objections to the theory are insuperable, and his opinion is shared by -the majority of astronomers, many of whom, however, consider that there -are elements of truth in the theory; but Lockyer undoubtedly carried his -ideas to an extravagant extent. - -Lockyer’s evolutionary order of the stars is not supported by Vogel. -Zöllner suggested in 1865 that yellow and red stars are simply white -stars in a further stage of cooling; but Angström showed that -atmospheric composition is a safer criterion of age than colour. Vogel’s -classification, first published in 1874, and further developed in 1895, -is from the standpoint of evolution. He considers Orion stars and Sirian -stars to be the youngest orbs. Solar stars are considered by Vogel to -have wasted much of their store of radiation, and red stars are viewed -as “effete suns, hastening rapidly down the road to final extinction.” -He considers stars of Secchi’s fourth type to be also dying suns, both -types representing alternative roads for stars of the Solar type in -their decline into dark stars. This view is supported by Dunér, and is -distinctly confirmed by Hale’s observations with the Yerkes telescope. -Vogel’s views, in fact, are generally accepted among astronomers. The -nebular theory, modified by subsequent research, seems destined to hold -its own against all attacks. - -Distinctly supplementary to the nebular theory are the remarkable -researches, commenced in 1879, by Sir _George Howard Darwin_ (born -1845), son of Charles Darwin the great biologist. George Howard Darwin -was born in 1845, at Downe in Kent, was educated at Cambridge, and -studied for the law; but in 1873 he returned to Cambridge, where he -became Plumian Professor of Astronomy in 1883. In 1879 he communicated -to the Royal Society the first of his papers on tidal friction, which -were summed up in his book on ‘The Tides,’ published in 1898. He finds -that the tides act upon the Earth as a brake does upon a machine,—they -tend to retard its rotation. Consequently, the day is growing longer, -the Moon’s orbit is becoming enlarged, and its period of revolution is -being lengthened. - -At present the day is about twenty-four hours long, and the month about -twenty-seven days. The day, however, will be lengthened at a more rapid -rate than the month, and in the remote future the day and month will -both last fifty-five of our present days. The Moon will revolve round -the Earth in the same period that the Earth rotates on its axis, and the -two bodies will perform their circuit round the Sun as if united by a -bar. - -Not only can we foresee the future of the Earth-Moon System, but we can -also read the past. According to Darwin’s theory, the Earth, in the -remote past, was probably rotating on its axis in a very short period, -between three and five hours. The Moon must then have been much nearer -us than it is now, and was probably revolving round its primary in the -same period that the Earth took to rotate on its axis. The two globes, -then gaseous, must have been revolving almost in actual contact. Had the -month been even a second shorter than the day, the Moon must inevitably -have fallen back on the Earth. As it was, the condition of affairs could -not endure. The condition of the Moon resembled that of an egg balanced -on its point. The Moon must either recede from the Earth or fall back -upon it. The solar tide here interfered, and caused the Moon to recede -from its primary until it reached its present distance of 239,000 miles. - -The fact that the Earth and Moon were almost in contact suggests that -they were probably in contact. In other words, the Moon originally -formed part of the Earth, which, in consequence of its short-rotation -period, and probably also owing to the interference of the solar tide, -split into two portions, and the smaller of these now forms the Moon. It -is likely that the matter now forming the Moon was detached from the -Earth in separate particles. Just as the tides raised by the Moon tend -to retard the motion of the Earth, so the Earth tides raised in the Moon -have already done their work. The Moon now rotates on its axis in the -same time as it revolves round the Earth. Part of the evolution of the -Earth-Moon system is completed. Schiaparelli’s discovery that the -rotation periods of both Venus and Mercury coincide with their times of -revolution is distinctly confirmatory of Darwin’s theory. - -In his chapter on the “Evolution of Celestial Systems” in his book on -‘The Tides,’ Darwin discusses the distribution of the satellites of the -Solar System. He says of the evolution of a planet: “We have seen that -rings should be shed from the central nucleus when the contraction of -the nebula has induced a certain degree of augmentation of rotation. -Now, if the rotation were retarded by some external cause, the genesis -of a ring might be retarded or entirely prevented.” He then remarks that -probably the formation of the Moon was retarded, and in the case of -Mercury and Venus, solar tidal friction prevented satellite formation. -This explains why Mercury and Venus have no satellites, the Earth only -one, Mars two, while the exterior planets have each several satellites. - -The theory of tidal friction was extended in 1892 to the explanation of -the double stars by the American astronomer, See. See showed by -mathematical calculation the effects of tidal friction in shaping the -eccentric orbits of the binary stars, the course of evolution being -traced from double stars, revolving almost in contact, which the -spectroscope reveals, to the telescopic doubles. See’s researches have -done much to supplement those of Darwin, who considers that there are -two types of cosmical evolution,—the Laplacian, and the “second” or -lunar type. - -Lowell, in his work on ‘The Solar System’ (1903), adds six congruities -to those remarked by Laplace and his successors. These are, “All the -satellites turn the same face to their primaries (so far as we can -judge); Mercury, and probably Venus, do the same to the Sun; one law -governs position and size in the Solar System and in all the satellite -systems; orbital inclinations in the satellite systems increase with -distance from the primary; the outer planets show a greater tilt of axis -to orbit-plane with increased distance from the Sun (so far as -detectable); the inner planets show a similar relation.” - -The fate of the average solar star is sketched out by Vogel’s -classification, and by any evolutionary hypothesis which we may adopt. -In the words of Lowell: “Though we cannot as yet review with the mind’s -eye our past, we can, to an extent, foresee our future. We can with -scientific confidence look forward to a time when each of the bodies -composing our Solar System shall turn an unchanging face in perpetuity -to the Sun. Each will then have reached the end of its evolution set in -the unchanging stare of death. Then the Sun itself will go out, becoming -a cold and lifeless mass; and the Solar System will circle unseen, -ghostlike, in space, awaiting only the resurrection of another cosmic -catastrophe.” - -As to what this cosmic catastrophe will be, science gives no definite -idea; nor can astronomers say with certainty whether the Universe will -come to an end by the extinction of its luminaries, or whether the suns -and planets will be brought back to luminosity again; but the human mind -shrinks from the idea of a dead Universe. At this point science has said -its last word, and must give place to religion. In our day we may repeat -with deeper meaning the words of the Scottish astronomer, Thomas Dick: -“Here imagination must drop its wing, since it can penetrate no further -into the dominions of Him who sits on the Throne of Immensity. -Overwhelmed with a view of the magnificence of the Universe, and of the -perfections of its Almighty Author, we can only fall prostrate in deep -humility and exclaim, ‘Great and marvellous are Thy works, Lord God -Almighty.’” - - - - - INDEX. - - - A - Absolute parallax, 158. - Adams, J. C., 78, 116, 117, 118, 119, 120, 140. - Adams, W., 205. - Aerolites, 147, 148, 149. - Airy, Sir G. B., 27, 104, 117, 120. - Aitken, R. G., 202. - Alcyone (η Tauri), 217. - Aldebaran (α Tauri), 151, 166, 170, 172. - Algol (β Persei), 178, 182, 183, 184, 193, 204. - Al-Sufi, 180, 183. - Altair (α Aquilæ), 170. - Anderson, T. D., 191, 192. - Andromeda nebula, 180, 208. - Andromedæ (γ), 201. - Andromedæ (Nova), 180. - Andromedid meteors, 142, 149. - Angström, A. J., 50, 51. - Antares (α Scorpii), 171. - Aquila, 195. - Aquilæ (η), 185, 186. - Arago, F. J. D., 6, 11, 31, 37, 40, 118, 120, 129. - Arcturus (α Bootis), 165, 170. - Arequipa Observatory, 75. - Argelander, F. W. A., 27, 159, 167, 178, 179, 180, 218. - Argo Navis, 221. - Argus (η), 187, 188. - Armagh Observatory, 206. - Asteroids, 19, 62, 97-102. - Astronomer-Royal of Scotland, 134, 155, 191; - of England, 59, 17; - of Ireland, 151, 156. - Astronomy of the invisible, 203. - Aurigæ (Nova), 191, 192, 195. - Auwers, A., 167, 188, 203. - - - B - Babinet, 230. - Baily, F., 159. - Bakhuyzen, H. G., 91. - Ball, Sir R. S., 23, 34, 108, 141, 149, 156, 158, 230. - Barnard, E. E., 19, 95, 107, 108, 110, 111, 113, 136, 191, 211. - Beer, W., 68, 69, 90. - Bellatrix (γ Orionis), 209. - Bélopolsky, A., 87, 110, 166, 185, 186, 204, 205. - Berlin Observatory, 119, 120. - Bessel, F. W., 24, 82, 116, 151, 152, 153, 154, 159, 167, 202, - 203. - Betelgeux (α Orionis), 165, 171, 172, 182, 187. - Biela, W., 128. - Biela’s comet, 128, 129, 142, 143, 146, 149. - Birmingham, J., 189. - Bode, J. E., 97, 98, 152. - Bode’s Law, 97. - Boeddicker, O., 77. - Bond, G. P., 103, 109, 130, 136, 207. - Bond, W. C., 109, 112, 120. - ‘Bonn Durchmusterung,’ 159, 160, 218. - Bonn Observatory, 88, 97, 160. - Böotis (ε), 30. - Borisiak, 192. - Boss, L., 168. - Bouvard, A., 115, 116. - Bradley, J., 159, 167. - Brédikhine, T. A., 105, 131, 132, 133, 134, 135. - Brewster, Sir D., 50, 101, 178. - Brinkley, J., 151. - Brünnow, F., 156. - Bruno, G., 35. - Buffon, 103. - Bunsen, R. W., 51. - Burchell, 188. - Burnham, S. W., 201, 202, 212. - - - C - Callandreau, O., 136. - Callandrelli, 151. - Cambridge Observatory, 116. - Campbell, T. (Poet), 2. - Campbell, W. W., 24, 107, 110, 166, 168, 187, 191, 193, 204, 205. - Canals of Mars, 91, 92, 93, 94, 95. - Cancri (ζ), 206. - Cancri (S), 180. - Canis Major, 188. - ‘Cape Durchmusterung,’ 161, 162. - Cape Observatory, 155, 157. - Capella (α Aurigæ), 170, 176, 193, 205. - Carnera, L., 100. - Carpenter, J., 73. - Carrington, R. C., 45, 46, 59. - Cassini, D., 21. - Cassiopeia, 221. - Castor (α Geminorum), 30, 200, 205. - Celoria, G., 202, 218, 221, 223, 224. - Centauri (α), 155, 188, 225. - Centaurus, 221. - Cephei (δ), 178, 182, 185, 186. - Cepheus, 221. - Ceres, 19, 98, 101. - Cerulli, V., 86, 91, 94. - Chacornac, 161. - Challis, J., 116, 119, 120. - Chambers, G. F., 31. - Chandler, S. C., 88, 89, 181, 184. - Chladni, E., 138. - Chromosphere, solar, 55, 56. - Clark, A., 202. - Clerke, Miss A. M., 3, 5, 8, 12, 13, 15, 25, 26, 34, 42, 58, 75, - 86, 92, 105, 109, 124, 125, 131, 132, 133, 140, 142, 169, - 186, 187, 189. - Clerk-Maxwell, J., 109, 110. - Coggia’s comet, 131, 132, 133. - Comet families, 135. - Comets, 24, 123-137, 141, 142, 143, 144, 146, 149, 152. - Common, A. A., 107, 209. - Copeland, R., 134, 135, 190, 208. - Cornu, A., 189. - Corona Borealis, 188. - Corona, solar, 55, 57, 64. - Coronæ (Nova), 188, 189. - Crossley, E., 211. - Crux, 221. - Cygni (61), 152, 158. - Cygni (Y), 184, 185. - Cygni (Nova), 189, 190. - Cygnus, 152, 189, 221. - - - D - Damoiseau, 78. - D’Arrest, H. L., 96, 119, 142, 212. - Dartmouth Observatory, 56. - Darwin, Sir G. H., 233, 234, 235, 236. - Dawes, W. R., 90, 117. - De la Rue, W., 52, 75. - Delaunay, C. E., 78, 79. - Dembowski, E., 201. - Deneb (α Cygni), 165. - Denning, W. F., 84, 85, 91, 95, 105, 111, 112, 144, 145, 146. - Deslandres, H., 110. - Dick, T., 85, 238. - Disc-theory, 32, 36, 38, 39, 214, 218. - Di Vico, F., 85, 86, 170. - Doberck, W., 201. - Donati, G. B., 130, 131, 169. - Donati’s comet, 130, 133, 136. - Doppler, C., 57, 58. - Doppler’s Principle, 58, 59, 87, 110, 165, 168, 203. - Douglass, A. E., 92, 107. - Draconis (λ), 182. - Draper, H., 136, 172, 175. - Dreyer, J. L. E., 206. - Dunecht Observatory, 157. - Dunér, N. C., 58, 59, 174, 175, 181, 184, 185, 201, 202, 233. - Dunkin, E., 27, 167. - Dunsink Observatory, 156. - - - E - Earth, 76, 97, 103, 104, 147, 148, 149, 153, 154, 156, 236. - Earth-Moon system, 234, 235. - Easton, C., 221. - Eclipses, lunar, 77. - Eclipses, solar, 56, 57, 80, 81. - Edinburgh (Royal) Observatory, 195. - Electrical repulsion theory, 126. - Elger, T. G., 74. - Elkin, W. L., 157. - Encke, J. F., 30, 61, 119, 127, 128, 216. - Encke’s comet, 127, 128, 137. - Erman, 140. - Eros, 62, 101. - Ertborn, 85. - Euler, L., 88, 89. - Evolution, planetary, 228, 229, 230, 231. - Evolution, stellar, 33, 34, 231, 232. - - - F - Faye, H., 60, 129, 230. - Faye’s comet, 129, 137. - Ferguson, J., 9, 178. - Flammarion, C., 87, 91, 95, 121, 147, 164, 187, 195, 201, 202, - 226. - Flamsteed, J., 5. - Fleming, Mrs, 192, 195. - Forbes, G., 122. - Fraunhofer, J. 47, 48, 49, 50, 3, 151, 153, 169. - Fraunhofer lines, 48, 49, 50, 51, 169, 172. - Frost, E. B., 205. - - - G - Galactic poles, 35, 224. - Galaxies, external, 32, 218, 225, 226. - Galaxy, or Milky Way, 32, 36-42, 186, 211, 215, 216, 217, 219, - 220, 221, 224, 225. - Galileo, 44, 107. - Galle, J. G., 62, 108, 109, 119, 142. - Galloway, T., 167. - Gambart, 128. - Gauss, C. F., 27, 98, 167. - Gautier, A., 45. - Gemini, 11, 194. - Geminorum (Nova), 194. - Geminorum (ζ), 180, 182, 185. - George III., 11, 23. - Gill, Sir D., 62, 136, 155, 157, 160, 161, 221. - Glasgow Observatory, 216. - Goodricke, J., 178, 183. - Gore, J. E., 24, 38, 179, 181, 182, 183, 192, 202, 215, 220, 225, - 226. - Gould, B. A., 135, 160, 163, 180, 220, 221. - Grant, R., 216. - Gravitation, law of, 29. - Greenwich Observatory, 59, 117. - Grimmler, 192. - Groombridge (1830), 156, 162, 223. - Groombridge (1618), 156. - Gruithuisen, 87. - - - H - Hale, G. E., 55, 57, 233. - Hall, A., 96, 111, 112, 156, 190. - Hall, Maxwell, 121. - Halley, E., 138. - Halley’s comet, 123, 130, 152. - Halm, J., 195, 196. - Hansen, P. A., 61, 78, 79. - Hansky, A., 57. - Harding, K. L., 99, 153, 220. - Hartwig, E., 190. - Harvard Observatory, 174, 175, 191. - Hasselberg, B., 148, 190. - Heis, E., 179, 220. - Heliometer, 153, 157. - Helium stars, 174. - Helmholtz, H., 61, 229. - Hencke, K. L., 99. - Henderson, T., 154, 155. - Henry, Paul and Prosper, 100, 114, 210. - Hercules, 167. - Herculis (α), 182. - Herculis (λ), 26. - Herschel, William, 1-42, 43, 60, 63, 65, 69, 74, 77, 85, 90, 99, - 103, 109, 111, 112, 115, 123, 150, 162, 167, 176, 196, - 197, 207, 214, 216, 218, 224, 227. - Herschel, A., 144. - Herschel, Caroline, 6, 8, 9, 12, 13, 14, 30, 35, 127, 198. - Herschel, Sir J., 4, 17, 27, 30, 37, 50, 112, 113, 120, 130, 144, - 167, 187, 188, 197, 198, 199, 200, 214, 215. - Hind, J. R., 99, 129, 135, 180, 188. - Hoek, 135. - Holden, E. S., 191. - Hough, G., 105. - Houzeau, J. C., 220. - Huggins, Lady, 172. - Huggins, Sir W., 54, 57, 74, 95, 106, 114, 131, 136, 165, 170, - 171, 172, 173, 189, 190, 191, 193, 195, 207, 208, 212, - 231. - Humboldt, A., 44, 139. - Hussey, W. J., 116, 201. - - - I - Innes, R., 188. - Intra-Mercurial planet, 80, 81. - Italian spectroscopists, 54, 55. - - - J - Janssen, P. J. C., 52, 53, 54, 57, 59, 112, 209. - Juno, 19, 99, 101. - Jupiter, 20, 75, 97, 101-108, 112, 114, 121, 122, 135, 144, 146. - Juvisy Observatory, 164. - - - K - Kaestner, 65, 124. - Kaiser, F., 90. - Kant, I., 34, 35, 101, 103. - Kapteyn, J. C., 27, 158, 161, 162, 168, 221, 222. - Keeler, J. E., 95, 114, 185, 208, 211, 212, 213, 231. - Kelvin, Lord, 229. - Kempf, P., 181. - Kepler, J., 5, 35, 137. - Kirchoff, G. R., 61, 169, 172. - Kirkwood, D., 140, 230. - Klein, H. J., 73. - Klinkerfues, E., 142, 202. - Konkoly, N., 175. - Küstner, F., 88. - - - L - Lalande, 23, 152. - Lambert, J. H., 34. - Lamont, J., 44. - Langley, S. P., 77. - Laplace, P. S., 20, 33, 34, 77, 109, 148, 152, 195, 227, 228, 229. - Lassell, W., 112, 115, 117, 120. - Latitude, variation of, 88, 89. - Leipzig Observatory, 173. - Leonid meteors, 139, 140, 142. - Leonis (β), 183. - Lescarbault, 80. - Le Verrier, U. J. J., 61, 80, 81, 118, 119, 120, 142, 164. - Leyden Observatory, 91. - Libræ (δ), 180. - Lick Observatory, 93, 107, 166, 168, 191, 213. - Light, extinction of, 40, 215, 216, 224, 225. - Lindsay, Lord, 157. - Linné, 71, 72. - Lockyer, Sir J. N., 52, 53, 54, 55, 58, 149, 174, 191, 193, 195, - 208, 209, 231, 232. - Lockyer, W. J. S., 210. - Loewy, M., 75. - Lohrmann, W. G., 68, 71. - Lohse, W. O., 88, 105. - Loomis, 188. - Lowell, P., 83, 84, 86, 87, 91, 92, 93, 94, 122, 236, 237. - Lowell Observatory, 92, 94, 106, 114. - Lund Observatory, 59. - Lupus, 221. - Luther, R., 100. - Lyra, 221. - Lyra (β), 178, 182, 185. - Lyrid meteors, 122. - - - M - Maclaurin, C., 9. - Maclear, Sir T., 155. - Mädler, J. H., 27, 68, 69, 71, 96, 104, 202, 203, 216, 217, 218. - Magellanic clouds, 219. - Magnetism, 44, 60. - Mars, 18, 19, 90-97, 101, 144, 236. - Maunder, E. W., 59, 60, 94, 95, 134, 145, 166, 224. - Mascari, A., 86. - Mayer, C., 164. - Mayer, J. R., 229. - Mazapil meteorite, 149. - Méchain, 127. - Mee, A., 68. - Melloni, 76. - Mercury, 18, 80, 81-84, 97, 236. - Messier, C., 30. - Meteorites, 147, 148, 149, 229, 231. - ‘Meteoritic Hypothesis,’ 231, 232. - Meteors, 138-149. - Meudon Observatory, 59. - Milan Observatory, 82, 202, 224. - Milky Way. See Galaxy. - Miller, W. A., 50, 172. - Mira Ceti, 11, 182, 186, 187. - Mitchel, O. M., 31. - Mizar (ζ Ursæ Majoris), 204. - Möller, A., 129. - Moon, the, 10, 24, 65-79, 90, 95, 148, 228. - Moscow Observatory, 132. - Mouchez, A., 161. - Müller, G., 175, 181. - Munich Observatory, 44, 224. - - - N - Napoleon, 67, 127. - Nasmyth, J., 73, 103. - Nebulæ, 30, 31, 207-213, 228. - Nebular Hypothesis, 33, 195, 227, 228, 229, 230, 233. - Neison (Nevill), E., 73. - Neisten, 86, 105. - Neptune, 120, 121, 135, 229, 230. - Newall, H. F., 205. - Newcomb, S., 27, 64, 78, 89, 94, 162, 168, 220, 222, 223, 224, - 232. - Newton, H. A., 140, 141. - Newton, Sir I., 2, 17, 29, 77. - Nichol, J. P., 31, 207. - Nordvig, L., 192. - - - O - Olbers, H. W. M., 19, 20, 69, 98, 99, 123, 124, 125, 126, 127, - 129, 130, 139, 148, 152, 153. - Olbers’ comet, 125. - Olmsted, D., 138, 207. - Ophiuchi (α), 183. - Orion, 221. - Orion nebula, 10, 33, 207, 208, 209, 213. - Orion stars, 174, 193, 209, 232. - Orionis (κ), 209. - Orionis (η), 209. - Orionis (θ), 209. - Orionis (U), 181, 182, 186, 187. - - - P - Palisa, J., 100. - Pallas, 19, 99, 101. - Parallax, solar, 61, 62, 63, 101. - Parallax, stellar, 150-158, 190. - Paris Congresses, 161. - Paris Observatory, 78, 118, 171. - Perrine, C. D., 81, 108, 194. - Perrine’s comet, 136. - Perrotin, H., 86, 91, 100, 114. - Peck, W., 162. - Persei (Nova), 192, 193, 194, 195. - Perseid meteors, 122, 141. - Perseus, 192, 221. - Peters, C. H. F., 100. - Peters, C. A. F., 142, 153, 155, 202. - Photography, astronomical, 54, 56, 57, 59, 75, 81, 94, 108, 113, - 136, 158, 160, 161, 172, 175, 192, 193, 194, 203, 208, - 209, 210, 211, 212. - Photometry, 176, 177. - Piazzi, G., 19, 20, 98, 150. - Pickering, E. C., 174, 175, 176, 177, 181, 182, 183, 185, 193, - 194, 203, 204, 220, 221. - Pickering, W. H., 75, 76, 81, 91, 92, 93, 107, 113, 209. - Plana, G., 78, 79. - Pleiades, 124, 210, 217. - Pogson, N. R., 142, 180. - Pole Star, 205. - Pollux, 165, 170. - Pons, J. L., 127. - Pontécoulant, 79. - Potsdam Observatory, 46, 173, 176. - Pritchard, C., 158, 177. - Proctor, R. A., 4, 20, 38, 41, 90, 91, 104, 148, 163, 164, 218, - 219, 220, 231. - Procyon (α Canis Minoris), 151, 203. - Prominences, solar, 52, 53, 55, 64. - Puiseux, P., 75. - Pulkowa Observatory, 200. - - - Q - Quetelet, A., 139. - - - R - Radiant points, meteoric, 139, 144, 145, 146. - Ranyard, A. C., 106, 146. - Red spot on Jupiter, 105, 106. - Regulus (α Leonis), 164, 165. - Relative parallax, 157. - Réseau, Photospherique, 59. - Resisting medium, 128. - Respighi, L., 55. - Reversing layer, 56, 57. - Ricco, A., 87, 105. - Rigel (β Orionis), 165, 209. - Ritchey, G., 194. - Roberts, A. W., 181. - Roberts, I., 209, 210. - Roche, E., 109. - Roman College Observatory, 85. - Rosse, third Earl of, 141, 156, 207, 208, 218. - Rosse, fourth Earl of, 77. - Rotation of the Sun, 58, 59; - of the planets, 82, 83, 84, 85, 86, 87, 104, 111, 112. - Rowland, H. A., 52. - Rutherfurd, L. M., 75, 169. - - - S - Sabine, Sir E., 44. - Safford, T. H., 202. - Santini, G., 159. - Savary, F., 30, 199. - Satellites, 96, 107, 108, 112, 113, 115, 120, 121, 236. - Saturn, 20, 21, 22, 97, 103, 108-113, 121, 135. - Schaeberle, J. M., 93, 107, 191, 203. - Scheiner, C., 44. - Scheiner, J., 166, 174, 176. - Schiaparelli, G. V., 82, 83, 84, 85, 86, 87, 91, 92, 114, 141, - 143, 149, 201, 220, 224, 231, 235. - Schjellerup, H., 180. - Schmidt, J. F. J., 69, 70, 71, 72, 73, 104, 179, 180, 189. - Schönfeld, E., 160, 179, 180, 188, 189. - Schröter, J. H., 16, 65, 66, 67, 68, 69, 70, 74, 81, 82, 84, 85, - 86, 87, 97, 99, 153. - Schwabe, S. H., 18, 43, 44, 46, 55. - Schwassman, A., 100. - Secchi, A., 52, 54, 55, 60, 72, 90, 114, 141, 170, 171, 173. - Secchi’s types of stellar spectra, 170, 171, 173, 174, 175, 189, - 232. - See, T. J. J., 201, 202, 236. - Seeliger, H., 110, 195, 196, 202, 206, 224, 225. - Serviss, G. P., 158. - Sirius (α Canis Majoris), 151, 170, 173, 188, 202, 225. - Slipher, V. M., 106, 114, 204. - Sime, J., 27. - Sola, J. C., 112. - Solar cluster, 221, 222. - Solar system, motion of, 26, 27, 167, 168. - South, Sir J., 198. - Spectroscopic binaries, 203, 204, 205. - Spencer, H., 218. - Spica (α Virginis), 204. - Spörer, F. W. G., 45, 46, 54, 59. - Star-catalogues, 159, 160, 161, 162. - Star-clusters, 30, 31, 32, 206, 210. - Star-drift, 164. - Star-gauging, 36, 40, 41, 224. - Stars, distance of, 150-158. - Stars, distribution of, 35, 39, 40, 198-214. - Stars, double, 28, 29, 30, 197-206. - Stars, gaseous, 171, 174. - Stars, proper motion of, 162, 163, 164, 165. - Stars, radial motion of, 165, 166. - Stars, temporary, 156, 182, 188-196. - Stars, triple and multiple, 206. - Stars, variable, 177-188. - Stellar spectra, 169-176, 187, 189, 190, 191, 193, 194. - Stellar universe, 35-42, 214, 215-226. - Stereo-comparator, 100, 101. - Stokes, Sir G., 50. - Stone, E. J., 157, 160. - Stroobant, P., 88. - Struve, F. G. W., 3, 37, 38, 40, 42, 128, 151, 153, 200, 214, 215, - 216, 218. - Struve, H., 201. - Struve, L., 27, 163, 167. - Struve, O. W., 27, 110, 115, 120, 153, 156, 163, 200, 201. - Stumpe, O., 167. - Sun, 15, 16, 17, 40, 43-64, 65, 80, 81, 105, 125, 128, 170, 222, - 228, 229, 230, 237. - Swift, L., 81. - Swift’s comet, 136. - - - T - Tacchini, P., 55, 86, 87. - Taurus, 217, 221. - Tempel, E., 210. - Tennyson, 96. - Tidal friction, 79, 87, 233, 234, 235, 236. - Tisserand, F. F., 146. - Todd, D. P., 122. - Trans-Neptunian planet, 121, 122. - Trouvelot, E., 86, 87. - Tschermak, 148. - Tulse Hill Observatory, 171. - Turner, H. H., 194, 224. - Twining, A. C., 139. - - - U - Upsala Observatory, 59. - Uranometria Argentina, 160. - Uranus, 11, 20, 22, 23, 97, 113, 114, 115, 118, 121, 135, 141, - 230. - Ursa Major, 162, 164. - Ursæ Majoris (δ), 182. - Ursæ Majoris (ξ), 199. - - - V - Venus, 18, 84-88, 97, 235, 236. - Venus, transits of, 61, 62, 87. - Vega (α Lyræ), 151, 165, 170, 172, 173. - Very, F. W., 77. - Vesta, 19, 99, 101, 102. - Vogel, H. C., 84, 88, 95, 102, 106, 114, 131, 148, 166, 173, 174, - 175, 183, 184, 185, 190, 191, 193, 195, 204, 209, 232, - 233, 237. - Vulcan, 81. - - - W - Washington Observatory, 96, 223. - Watson, J. C., 81, 100. - Webb, T. W., 72, 73, 104. - Weinek, L., 75. - Weiss, E., 142. - Well’s comet, 134. - Whewell, W., 218. - Williams, A. S., 110, 193. - Wilson, A., 16. - Winlock, J., 177. - Winnecke, F. A. T., 131. - Witt, K. G., 101. - Wolf, Max, 100, 181, 191, 194, 211. - Wolf, R., 44, 45, 188. - Wolf and Rayet, 171. - Wolf-Rayet stars, 171, 174. - Wollaston, W. H., 48. - Wright, T., 34, 110. - - - Y - Yale Observatory, 157. - Yerkes Observatory, 55, 111, 202. - Young, C. A., 54, 56, 57, 58, 60, 87, 114, 190. - - - Z - Zach, F. X., 97, 98, 152. - Zantedeschi, 77. - Zenger, 85, 88. - Zöllner, J. C. F., 54, 58, 60, 84, 103, 132, 232. - - -CORRIGENDA. - - P. 30, l. 5, _for_ “objects” _read_ “orbits.” - P. 36, l. 13, _for_ “unable” _read_ “able.” - P. 61, l. 17, _for_ “8″.371” _read_ “8″.571.” - P. 63, l. 21, _for_ “bases” _read_ “gases.” - P. 100, l. 16, _for_ “Schwussmann” _read_ “Schwassmann.” - P. 167, l. 28, _for_ “Strumpe” _read_ “Stumpe.” - P. 184, l. 11, _for_ “star-variables” _read_ “variable stars.” - P. 199, l. 23, _for_ “2102” _read_ “1202.” - - THE END. - - PRINTED BY WILLIAM BLACKWOOD AND SONS. - - - - - Transcriber’s Notes - - -—Silently corrected a few typos, and incorporated the corrigenda into - the text. - -—Retained publication information from the printed edition: this eBook - is public-domain in the country of publication. - -—In the text versions only, text in italics is delimited by - _underscores_. - - - - - - - -End of the Project Gutenberg EBook of A Century's Progress in Astronomy, by -Hector MacPherson - -*** END OF THIS PROJECT GUTENBERG EBOOK A CENTURY'S PROGRESS IN ASTRONOMY *** - -***** This file should be named 63615-0.txt or 63615-0.zip ***** -This and all associated files of various formats will be found in: - http://www.gutenberg.org/6/3/6/1/63615/ - -Produced by Charlene Taylor, Stephen Hutcheson, and the -Online Distributed Proofreading Team at https://www.pgdp.net -(This file was produced from images generously made -available by The Internet Archive/American Libraries.) - -Updated editions will replace the previous one--the old editions will -be renamed. - -Creating the works from print editions not protected by U.S. copyright -law means that no one owns a United States copyright in these works, -so the Foundation (and you!) can copy and distribute it in the United -States without permission and without paying copyright -royalties. 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