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diff --git a/.gitattributes b/.gitattributes new file mode 100644 index 0000000..d7b82bc --- /dev/null +++ b/.gitattributes @@ -0,0 +1,4 @@ +*.txt text eol=lf +*.htm text eol=lf +*.html text eol=lf +*.md text eol=lf diff --git a/LICENSE.txt b/LICENSE.txt new file mode 100644 index 0000000..6312041 --- /dev/null +++ b/LICENSE.txt @@ -0,0 +1,11 @@ +This eBook, including all associated images, markup, improvements, +metadata, and any other content or labor, has been confirmed to be +in the PUBLIC DOMAIN IN THE UNITED STATES. + +Procedures for determining public domain status are described in +the "Copyright How-To" at https://www.gutenberg.org. + +No investigation has been made concerning possible copyrights in +jurisdictions other than the United States. Anyone seeking to utilize +this eBook outside of the United States should confirm copyright +status under the laws that apply to them. diff --git a/README.md b/README.md new file mode 100644 index 0000000..3ad94a6 --- /dev/null +++ b/README.md @@ -0,0 +1,2 @@ +Project Gutenberg (https://www.gutenberg.org) public repository for +eBook #63615 (https://www.gutenberg.org/ebooks/63615) 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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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.) - - - - - - -</pre> - -<div id="cover" class="img"> -<img id="coverpage" src="images/cover.jpg" alt="A Century’s Progress in Astronomy" width="800" height="1375" /> -</div> -<div class="box"> -<h1><span class="small">A Century’s Progress -<br /><span class="smallest">IN</span></span> -<br />Astronomy</h1> -<p class="tbcenter"><span class="smallest">BY</span> -<br />HECTOR MACPHERSON, <span class="sc">Jun.</span> -<br /><span class="smallest">MEMBER OF THE SOCIÉTÉ ASTRONOMIQUE DE FRANCE; -<br />MEMBER OF THE SOCIÉTÉ BELGE D’ASTRONOMIE; -<br />AUTHOR OF ‘ASTRONOMERS OF TO-DAY’</span></p> -<p class="tbcenter"><span class="small">WILLIAM BLACKWOOD AND SONS</span> -<br /><span class="smaller">EDINBURGH AND LONDON -<br />MCMVI -<br /><i>All Rights reserved</i></span></p> -</div> -<div class="pb" id="Page_v">v</div> -<h2><span class="small">PREFACE.</span></h2> -<p>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.</p> -<p>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.’</p> -<div class="verse"> -<p class="t0"><span class="sc">Balerno, Mid-Lothian</span>,</p> -<p class="t2"><i>October 1906</i>.</p> -</div> -<div class="pb" id="Page_vii">vii</div> -<h2 id="toc" class="center">CONTENTS.</h2> -<dl class="toc"> -<dt class="jr"><span class="smaller">PAGE</span></dt> -<dt><a href="#c1">CHAPTER I.</a> 1</dt> -<dt class="center">HERSCHEL THE PIONEER.</dt> -<dd>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</dd> -<dt><a href="#c2">CHAPTER II.</a> 15</dt> -<dt class="center">HERSCHEL THE DISCOVERER.</dt> -<dd>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</dd> -<dt><a href="#c3">CHAPTER III.</a> 43</dt> -<dt class="center">THE SUN.</dt> -<dd>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</dd> -<dt><a href="#c4">CHAPTER IV.</a> 65</dt> -<dt class="center">THE MOON.</dt> -<dd>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</dd> -<dt><a href="#c5">CHAPTER V.</a> 80</dt> -<dt class="center">THE INNER PLANETS.</dt> -<dd>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</dd> -<dt><a href="#c6">CHAPTER VI.</a> 103</dt> -<dt class="center">THE OUTER PLANETS.</dt> -<dd>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</dd> -<dt><a href="#c7">CHAPTER VII.</a> 123</dt> -<dt class="center">COMETS.</dt> -<dd>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</dd> -<dt><a href="#c8">CHAPTER VIII.</a> 138</dt> -<dt class="center">METEORS.</dt> -<dd>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</dd> -<dt><a href="#c9">CHAPTER IX.</a> 150</dt> -<dt class="center">THE STARS.</dt> -<dd>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</dd> -<dt><a href="#c10">CHAPTER X.</a> 169</dt> -<dt class="center">THE LIGHT OF THE STARS.</dt> -<dd>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</dd> -<dt><a href="#c11">CHAPTER XI.</a> 197</dt> -<dt class="center">STELLAR SYSTEMS AND NEBULÆ.</dt> -<dd>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</dd> -<dt><a href="#c12">CHAPTER XII.</a> 214</dt> -<dt class="center">STELLAR DISTRIBUTION AND THE STRUCTURE OF THE UNIVERSE.</dt> -<dd>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</dd> -<dt><a href="#c13">CHAPTER XIII.</a> 227</dt> -<dt class="center">CELESTIAL EVOLUTION.</dt> -<dd>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</dd> -</dl> -<div class="pb" id="Page_1">1</div> -<h1 title=""><span class="smaller">A CENTURY’S PROGRESS IN ASTRONOMY.</span></h1> -<h2 id="c1"><span class="small">CHAPTER I.</span> -<br /><span class="smaller">HERSCHEL THE PIONEER.</span></h2> -<p>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.</p> -<p>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 -<span class="pb" id="Page_2">2</span> -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.”</p> -<p>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 -<span class="pb" id="Page_3">3</span> -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.</p> -<p>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 -<span class="pb" id="Page_4">4</span> -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.</p> -<p>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.”</p> -<p>Herschel, like the true student of nature, held -theories very lightly. They were to him but -roads to the truth. Unlike many scientists, -<span class="pb" id="Page_5">5</span> -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.”</p> -<p>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.”</p> -<div class="pb" id="Page_6">6</div> -<p>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.”</p> -<p>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.</p> -<p>At the garrison school in Hanover, where -the Herschels were educated, William Herschel -showed intense love and aptitude for learning, -<span class="pb" id="Page_7">7</span> -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.</p> -<p>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 -<span class="pb" id="Page_8">8</span> -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.”</p> -<p>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 -<span class="pb" id="Page_9">9</span> -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.</p> -<p>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 -<span class="pb" id="Page_10">10</span> -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.</p> -<p>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. -<span class="pb" id="Page_11">11</span> -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.</p> -<p>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. -<span class="pb" id="Page_12">12</span> -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.”</p> -<p>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 -<span class="pb" id="Page_13">13</span> -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.</p> -<p>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.</p> -<p>The death of her brother was a terrible blow -to Caroline Herschel. Expecting to live only -a twelvemonth, she returned to Hanover to -<span class="pb" id="Page_14">14</span> -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.</p> -<div class="pb" id="Page_15">15</div> -<h2 id="c2"><span class="small">CHAPTER II.</span> -<br /><span class="smaller">HERSCHEL THE DISCOVERER.</span></h2> -<p>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.</p> -<p>“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 -<span class="pb" id="Page_16">16</span> -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.”</p> -<p>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 <i>Alexander Wilson</i> (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 -<span class="pb" id="Page_17">17</span> -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 -<span class="pb" id="Page_18">18</span> -of eleven years, the discovery of which -was afterwards made by Schwabe.</p> -<p>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.</p> -<p>“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 -<span class="pb" id="Page_19">19</span> -waxed and waned in size, the north polar cap -shrinking during the summer of the northern -hemisphere, increasing in winter, and <i>vice versa</i> -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.</p> -<p>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.</p> -<p>In connection with the discovery of the -Asteroids, Herschel showed a very fine spirit. -<span class="pb" id="Page_20">20</span> -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.”</p> -<p>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.</p> -<p>Herschel’s researches regarding Saturn were, -however, much more important than those on -<span class="pb" id="Page_21">21</span> -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.</p> -<p>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 -<span class="pb" id="Page_22">22</span> -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.</p> -<p>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.</p> -<p>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 -<span class="pb" id="Page_23">23</span> -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.</p> -<p>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 -<span class="pb" id="Page_24">24</span> -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.”</p> -<p>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 -<span class="pb" id="Page_25">25</span> -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.</p> -<p>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.</p> -<p>“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 -<span class="pb" id="Page_26">26</span> -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 -<span class="pb" id="Page_27">27</span> -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.</p> -<p>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.”</p> -<p>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 -<span class="pb" id="Page_28">28</span> -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.”</p> -<p>Herschel, on January 10, 1782, submitted to -the Royal Society a catalogue of 269 double -<span class="pb" id="Page_29">29</span> -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.</p> -<p>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; -<span class="pb" id="Page_30">30</span> -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.</p> -<p>In 1782 the French astronomer, <i>Charles -Messier</i> (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 -<span class="pb" id="Page_31">31</span> -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.”</p> -<p>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!</p> -<p>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 -<span class="pb" id="Page_32">32</span> -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.)</p> -<p>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 <i>the -nebulosity about the star is not of a starry</i> -<span class="pb" id="Page_33">33</span> -<i>nature</i>. 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.”</p> -<p>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 -<span class="pb" id="Page_34">34</span> -was a grand conception, and a magnificent attack -on the secrets of nature.</p> -<p>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.</p> -<p>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 -<span class="pb" id="Page_35">35</span> -he found that the nebulæ were aggregated near -the galactic poles; in other words, where nebulæ -are numerous, stars are scarce, and <i>vice versa</i>. -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.”</p> -<p>“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 -<span class="pb" id="Page_36">36</span> -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.</p> -<p>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 -<span class="pb" id="Page_37">37</span> -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.”</p> -<p>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 -<span class="pb" id="Page_38">38</span> -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.”</p> -<p>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 -<span class="pb" id="Page_39">39</span> -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.</p> -<p>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.”</p> -<p>This was the virtual abandonment of the disc -theory. Six years later Herschel announced -that in six cases he had failed to resolve the -<span class="pb" id="Page_40">40</span> -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.</p> -<p>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 -<span class="pb" id="Page_41">41</span> -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.”</p> -<p>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 -<span class="pb" id="Page_42">42</span> -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.</p> -<p>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.”</p> -<div class="pb" id="Page_43">43</div> -<h2 id="c3"><span class="small">CHAPTER III.</span> -<br /><span class="smaller">THE SUN.</span></h2> -<p>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 <i>Samuel Heinrich Schwabe</i> (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 -<span class="pb" id="Page_44">44</span> -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.</p> -<p><i>Rudolf Wolf</i> (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 <i>John -Lamont</i> (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 <i>Edward Sabine</i> -(1788-1883), the English physicist, from a discussion -of an entirely different set of observations, -independently demonstrated the same -<span class="pb" id="Page_45">45</span> -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 <i>Alfred Gautier</i> -(1793-1881) independently made the same discovery, -which had thus been made by four -separate investigators.</p> -<p>In the same year an English amateur astronomer, -<i>Richard Christopher Carrington</i> (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 <i>Friedrich -Wilhelm Gustav Spörer</i> (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 -<span class="pb" id="Page_46">46</span> -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.</p> -<p>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 -<span class="pb" id="Page_47">47</span> -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.</p> -<p>The son of a poor glazier, <i>Joseph von Fraunhofer</i> -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.</p> -<p>Fraunhofer became acquainted with prominent -<span class="pb" id="Page_48">48</span> -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.</p> -<p>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 <i>William -Hyde Wollaston</i> (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. -<span class="pb" id="Page_49">49</span> -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.</p> -<p>Although he ascertained the existence of the -dark lines in the Sun’s spectrum, Fraunhofer -never really found out what they represented. -<span class="pb" id="Page_50">50</span> -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.</p> -<p>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 <i>John -Herschel</i> (1792-1871), <i>William Allen Miller</i> -(1817-1870), Sir <i>David Brewster</i> (1781-1868), and -others. Two famous men of science had partly -discovered the secret. These were Sir <i>George -Stokes</i> (1819-1903), of Cambridge, and <i>Anders -John Angström</i> (1812-1872) of Upsala. Of -Angström’s work, published in 1853, it has -been said that it would “have obtained a high -<span class="pb" id="Page_51">51</span> -celebrity if it had appeared in French, English, -or German, instead of Swedish.”</p> -<p>It was not until 1859 that the principles of -spectrum analysis were fully enunciated by -<i>Gustav Robert Kirchhoff</i> (1824-1887), and his -colleague in the University of Heidelberg, <i>Robert -Wilhelm Bunsen</i> (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 -<span class="pb" id="Page_52">52</span> -these maps have been recently superseded by -the investigations of Sir <i>Joseph Norman Lockyer</i> -(born 1836), and of the American physicist, -<i>Henry Augustus Rowland</i> (1848-1901). Rowland -largely increased our knowledge of the -elements in the solar atmosphere.</p> -<p>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, -<i>Angelo Secchi</i> (1818-1878), and the Englishman, -<i>Warren De la Rue</i> (1815-1889), secured photographs -of the solar prominences. The problem of -1868 was the constitution of these prominences.</p> -<p><i>Pierre Jules César Janssen</i>, 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 -<span class="pb" id="Page_53">53</span> -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.</p> -<p>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. -<span class="pb" id="Page_54">54</span> -These two men were <i>William Huggins</i> (born -1824) and <i>Johann Carl Friedrich Zöllner</i>. 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 <i>Charles Augustus Young</i> (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.</p> -<p>To the Italian school of astronomers, however, -we owe the persistent and systematic study -of the prominences. Among them the three -greatest names are <i>Angelo Secchi</i> (1818-1878), -<span class="pb" id="Page_55">55</span> -<i>Lorenzo Respighi</i> (1824-1889), and <i>Pietro Tacchini</i> -(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.</p> -<p>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, <i>George Ellery Hale</i> (born 1868), -formerly director of the Yerkes Observatory, -<span class="pb" id="Page_56">56</span> -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.</p> -<p>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 -<span class="pb" id="Page_57">57</span> -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.</p> -<p>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, <i>Alexis Hansky</i>, -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.</p> -<p>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 <i>Christian Doppler</i> (1803-1853), -Professor of Mathematics at Prague, had expressed -<span class="pb" id="Page_58">58</span> -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.</p> -<p><i>Nils Christopher Dunér</i>, born in 1839 in -Scania, was employed as an assistant at Lund -<span class="pb" id="Page_59">59</span> -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.</p> -<p>In 1873 the Astronomer-Royal of England -commenced at Greenwich Observatory to photograph -the Sun daily. This work has been -carried on there by <i>Edward Walter Maunder</i> -(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 -<span class="pb" id="Page_60">60</span> -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.</p> -<p>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, <i>Hervé Faye</i> -(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 -<span class="pb" id="Page_61">61</span> -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.”</p> -<p>The important question of the distance of the -Sun was thoroughly investigated in 1824 by -<i>Johann Franz Encke</i> (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 <i>Peter Andreas -Hansen</i> (1795-1874), in 1854, and <i>Urban Jean -Joseph Le Verrier</i> (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 -<span class="pb" id="Page_62">62</span> -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 <i>David Gill</i> (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 <i>Johann Gottfried Galle</i> (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 -<span class="pb" id="Page_63">63</span> -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.</p> -<p>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 -<span class="pb" id="Page_64">64</span> -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.”</p> -<p>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.”</p> -<div class="pb" id="Page_65">65</div> -<h2 id="c4"><span class="small">CHAPTER IV.</span> -<br /><span class="smaller">THE MOON.</span></h2> -<p>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, <i>Johann -Hieronymus Schröter</i> (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 -<span class="pb" id="Page_66">66</span> -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.</p> -<p>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 -<span class="pb" id="Page_67">67</span> -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.</p> -<p>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 -<span class="pb" id="Page_68">68</span> -behind him,” says Mr Arthur Mee, “an imperishable -record, and a noble example to observers -of all time.”</p> -<p><i>Wilhelm Gotthelf Lohrmann</i>, 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. -<i>Johann Heinrich von Mädler</i> (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 <i>Wilhelm Beer</i> (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.</p> -<p>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 -<span class="pb" id="Page_69">69</span> -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.</p> -<p>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.</p> -<p>Mädler’s successor in the field of lunar astronomy -was <i>Johann Friedrich Julius Schmidt</i> -(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 -<span class="pb" id="Page_70">70</span> -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.</p> -<p>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 -<span class="pb" id="Page_71">71</span> -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.</p> -<p>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 -<span class="pb" id="Page_72">72</span> -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.</p> -<p>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 -<span class="pb" id="Page_73">73</span> -after the publication of his great chart. He -died at Athens, in his fifty-ninth year, -February 8, 1884.</p> -<p>Schmidt’s announcement of the change in the -appearance of Linné was followed in 1878 by -a statement by <i>Hermann Joseph Klein</i> (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, <i>Thomas William -Webb</i> (1807-1885), while the study was popularised -by <i>James Nasmyth</i> (1808-1890), the -famous engineer, who published, in 1874, in -conjunction with <i>James Carpenter</i> of Greenwich -Observatory, a beautifully-illustrated volume -entitled ‘The Moon.’ This was succeeded, in -1876, by the larger work of <i>Edmund Neison</i> -(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 -<span class="pb" id="Page_74">74</span> -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 <i>Thomas Gwyn Elger</i> (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.</p> -<p>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.</p> -<div class="pb" id="Page_75">75</div> -<p>Lunar photography was introduced as long -ago as 1858 by <i>Lewis Morris Rutherfurd</i> (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 <i>Maurice Loewy</i> (born 1833), and his assistant, -Pusiex, at the Paris Observatory; by <i>Ladislas -Weinek</i> at Prague; by the astronomers of the -Lick Observatory; and by <i>William Henry Pickering</i> -(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 -<span class="pb" id="Page_76">76</span> -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.”</p> -<p>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.”</p> -<p>The first successful attempt to measure the -heating power of moonlight was made in 1846 on -Mount Vesuvius by <i>Melloni</i>, an Italian physicist, -<span class="pb" id="Page_77">77</span> -whose results were confirmed four years later by -<i>Zantedeschi</i>, another Italian. The most important -work in this direction was accomplished -by the present <i>Earl of Rosse</i> (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 <i>Otto Boeddicker</i> (Lord Rosse’s -astronomer), during the total lunar eclipse of -October 4, 1884. Further investigations on this -subject were afterwards made by <i>Samuel Pierpont -Langley</i> (1834-1906), of Alleghany, and by his -assistant, <i>Frank Very</i>.</p> -<p>The motion of the Moon and its perturbations -were made the subject of deep study by the -famous <i>Pierre Simon Laplace</i> (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 -<span class="pb" id="Page_78">78</span> -his death the subject was taken up by <i>Charles -Theodore Damoiseau</i> (1768-1846), and the most -important advance was made by <i>Giovanni -Antonio Amadeo Plana</i> (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 <i>Peter Andreas -Hansen</i> (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 -<i>Charles Eugene Delaunay</i> (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 <i>Simon -Newcomb</i> (born 1835) is of great importance, -particularly in his correction of Hansen’s tables. -<i>John Couch Adams</i> (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 -<span class="pb" id="Page_79">79</span> -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.</p> -<div class="pb" id="Page_80">80</div> -<h2 id="c5"><span class="small">CHAPTER V.</span> -<br /><span class="smaller">THE INNER PLANETS.</span></h2> -<p>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 -<span class="pb" id="Page_81">81</span> -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—<i>James Watson</i> -(1838-1880), the well-known astronomer, and -<i>Lewis Swift</i> (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 <i>Charles Dillon Perrine</i> (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.</p> -<p>The study of the physical appearance of -Mercury was inaugurated by Schröter, who in -<span class="pb" id="Page_82">82</span> -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 <i>Friedrich Wilhelm -Bessel</i> (1784-1846) to 24 hours 53 seconds.</p> -<p>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. <i>Giovanni Virginio Schiaparelli</i> -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.</p> -<p>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, -<span class="pb" id="Page_83">83</span> -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.</p> -<p>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, <i>Percival Lowell</i> (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 -<span class="pb" id="Page_84">84</span> -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 <i>William Frederick -Denning</i> (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 <i>Hermann Carl -Vogel</i> (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.</p> -<p>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; -<span class="pb" id="Page_85">85</span> -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 <i>Francesco Di Vico</i> (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, <i>Thomas Dick</i> (1772-1857), who -suggested daylight observations on Venus to -solve the problem of the rotation.</p> -<p>In 1877 the question was attacked by -Schiaparelli, who commenced a series of observations -<span class="pb" id="Page_86">86</span> -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.</p> -<p>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, <i>Camille</i> -<span class="pb" id="Page_87">87</span> -<i>Flammarion</i> (born 1842), and others, under -less favourable climatic conditions, arrived at -a period of 24 hours. <i>Aristarch Bélopolsky</i> -(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.</p> -<p>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 <i>Franz von Gruithuisen</i>, and in 1878 -by the French astronomer <i>Trouvelot</i> (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 -<span class="pb" id="Page_88">88</span> -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, <i>Paul -Stroobant</i>, 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.</p> -<p>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 <i>Seth Carlo Chandler</i> -(born 1846) at Cambridge, Mass., with his -newly-invented instrument, the “almucantar,” -which indicated an appreciable variation of -latitude. This was confirmed by <i>Friedrich -Küstner</i> (born 1856), now director of the -Observatory at Bonn. The idea now occurred -to Chandler to search through the older records -<span class="pb" id="Page_89">89</span> -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, <i>he</i> -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.</p> -<div class="pb" id="Page_90">90</div> -<p>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 -<i>Friedrich Kaiser</i> (1808-1872) of Leyden, in 1869 -by <i>Richard Anthony Proctor</i> (1837-1888), and -in 1892 by <i>Henricius Gerardus van de Sande -Bakhuyzen</i> (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 <i>William -Rutter Dawes</i> (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.’ -<span class="pb" id="Page_91">91</span> -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 <i>canali</i>. 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 <i>Henri Perrotin</i> (1845-1904), -who also observed their duplication. Since -1886 they have been observed by many astronomers, -including Camille Flammarion in France, -<i>William Frederick Denning</i> (born 1848) in -England, <i>Vincenzo Cerulli</i> (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 -<span class="pb" id="Page_92">92</span> -black spots, to which he gave the name of -“lakes,” in keeping with the view that the dark -regions of the planet were seas.</p> -<p>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 <i>Andrew Ellicott Douglass</i> (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 -<span class="pb" id="Page_93">93</span> -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.</p> -<p>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 <i>John Martin -Schaeberle</i> (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 -<span class="pb" id="Page_94">94</span> -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 -<span class="pb" id="Page_95">95</span> -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.</p> -<p>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 <i>James -Edward Keeler</i> (1857-1900), <i>Edward Emerson -Barnard</i> (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.</p> -<p>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, <i>William Wallace Campbell</i> (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 -<span class="pb" id="Page_96">96</span> -previous observations, and reaffirmed the presence -of water-vapour in the Martian atmosphere.</p> -<p>During the opposition of 1830, Mädler undertook -an extensive search for a Martian satellite, -but was unsuccessful. In 1862 the search was -resumed by <i>Heinrich Louis D’Arrest</i> (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 <i>Asaph -Hall</i>, 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 -<span class="pb" id="Page_97">97</span> -planet, nor is it likely that any further satellites -of Mars are in existence.</p> -<p>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 <i>Johann Elert Bode</i> -(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 <i>Franz Xavier von Zach</i> (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 -<span class="pb" id="Page_98">98</span> -to a separate astronomer. One of them was -reserved for <i>Giuseppe Piazzi</i> (1746-1826), director -of the Observatory of Palermo.</p> -<p>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, -<i>Friedrich Gauss</i> (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 <i>Heinrich Olbers</i> -(1758-1840) at Bremen.</p> -<p>On March 28, 1802, while observing Ceres -<span class="pb" id="Page_99">99</span> -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 <i>Karl Ludwig -Harding</i> (1765-1834), Schröter’s assistant, discovered -Juno, and Olbers himself detected Vesta, -March 29, 1807.</p> -<p>After 1816 the search was relinquished, as -no more planets were discovered. In 1830, -however, a German amateur, <i>Karl Ludwig -Hencke</i> (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, <i>John Russell Hind</i> (1823-1895), the -English astronomer, discovered Iris. Since 1847 -not a year has passed without one or more planets -<span class="pb" id="Page_100">100</span> -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 <i>Christian H. F. Peters</i> (1813-1890), Henri -Perrotin, <i>Paul Henry</i> (1848-1905), <i>Prosper -Henry</i> (1849-1903), James Watson, <i>Robert -Luther</i> (1822-1900), <i>Johann Palisa</i> (born 1848), -and <i>Max Wolf</i> (born 1863).</p> -<p>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 <i>Arnold Schwassmann</i> (born 1870), -<i>Luigi Carnera</i> (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 -<span class="pb" id="Page_101">101</span> -first glance: year by year fresh discoveries are -announced from the Heidelberg Observatory, -until more than five hundred asteroids are now -known.</p> -<p>Waning interest in the ever-increasing family -of asteroids was revived in 1898 by the discovery -by <i>Karl Gustav Witt</i> (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.</p> -<p>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 -<span class="pb" id="Page_102">102</span> -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.</p> -<div class="pb" id="Page_103">103</div> -<h2 id="c6"><span class="small">CHAPTER VI.</span> -<br /><span class="smaller">THE OUTER PLANETS.</span></h2> -<p>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 -<span class="pb" id="Page_104">104</span> -<i>George Philip Bond</i> (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.</p> -<p>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.</p> -<p>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 -<span class="pb" id="Page_105">105</span> -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 <i>Theodor Alexandrovitch Brédikhine</i> -(1831-1904), and at Potsdam by <i>Wilhelm Oswald -Lohse</i> (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.</p> -<p>The great red spot has been observed since -its discovery by Denning at Bristol and <i>George -Hough</i> (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 -<span class="pb" id="Page_106">106</span> -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.”</p> -<p>In 1870 <i>Arthur Cowper Ranyard</i> (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 -<span class="pb" id="Page_107">107</span> -that native emissions from Jupiter’s visible -surface are local and fitful, not permanent and -general.”</p> -<p>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.</p> -<p>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 <i>Andrew -Anslie Common</i> (1841-1903), with his great 5-foot -reflector at Ealing, near London. The new -<span class="pb" id="Page_108">108</span> -satellite was found by Barnard to revolve round -Jupiter in 11 hours 57 minutes at a mean distance -of 112,000 miles.</p> -<p>Although the existence of other satellites of -Jupiter was predicted by Sir <i>Robert Stawell Ball</i> -(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.</p> -<p>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 -<span class="pb" id="Page_109">109</span> -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, <i>William Cranch Bond</i> -(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.</p> -<p>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 <i>Edouard Roche</i> (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, <i>James Clerk-Maxwell</i> (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, -<span class="pb" id="Page_110">110</span> -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 <i>Hugo -Seeliger</i> (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, -<i>Henri Deslandres</i> at Meudon, and Bélopolsky -at Pulkowa.</p> -<p>In 1851 a startling theory regarding Saturn’s -rings was put forward by the famous <i>Otto -Wilhelm von Struve</i> (1819-1905). Comparing his -measurements on the rings made at Pulkowa in -1850 and 1851 with those of other astronomers -<span class="pb" id="Page_111">111</span> -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.</p> -<p>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 <i>Stanley -Williams</i>, an English observer, in the same year. -On June 16, 1903, Barnard, at the Yerkes -Observatory, discovered a bright spot, from -<span class="pb" id="Page_112">112</span> -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 <i>José Comas Sola</i>, -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.</p> -<p>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 -<i>William Lassell</i> (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. -<span class="pb" id="Page_113">113</span> -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.</p> -<p>If little is known of the globe of Saturn, still -less is known regarding Uranus. Dusky bands -<span class="pb" id="Page_114">114</span> -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.</p> -<p>Herschel left our knowledge of the Uranian -satellites in a very uncertain state. The two -<span class="pb" id="Page_115">115</span> -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.</p> -<p>After the discovery of Uranus by Herschel, -mathematical astronomers determined its orbit -and calculated its position in the future. <i>Alexis -Bouvard</i>, 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 -<span class="pb" id="Page_116">116</span> -planet. Bouvard himself was one of the first to -make the suggestion, but died before the planet -was discovered. An English amateur, the Rev. -<i>T. J. Hussey</i>, 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.</p> -<p>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 <i>James Challis</i> (1803-1882), -the director of the Cambridge Observatory, a -paper containing the elements of what was -named by Adams “the new planet.” On -<span class="pb" id="Page_117">117</span> -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 <i>George Biddell -Airy</i> (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, -<span class="pb" id="Page_118">118</span> -and Dawes’ letter was accidentally destroyed by -a housemaid. So Adams’ theory remained in -obscurity.</p> -<p>The question now came under the notice of -<i>François Jean Dominique Arago</i> (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.</p> -<p>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 -<span class="pb" id="Page_119">119</span> -discovered. After much calculation Le Verrier, -in his third memoir (August 31, 1846), assigned -to the planet a position in the constellation -Aquarius.</p> -<p>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.”</p> -<p>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 -<span class="pb" id="Page_120">120</span> -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.”</p> -<p>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.</p> -<p>Only a fortnight after the discovery of Neptune, -the astronomer Lassell observed a satellite -to the distant planet on October 10, 1846. This -<span class="pb" id="Page_121">121</span> -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, <i>Maxwell -Hall</i>, 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.</p> -<p>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.</p> -<div class="pb" id="Page_122">122</div> -<p>Two independent investigators, <i>David Peck -Todd</i> (born 1855) in America and <i>George -Forbes</i> 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.”</p> -<div class="pb" id="Page_123">123</div> -<h2 id="c7"><span class="small">CHAPTER VII.</span> -<br /><span class="smaller">COMETS.</span></h2> -<p>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.</p> -<p>Heinrich Wilhelm Matthias Olbers was born -at Arbergen, a village near Bremen, October 11, -1758. His father was a clergyman who, in -<span class="pb" id="Page_124">124</span> -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.</p> -<p>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, -<span class="pb" id="Page_125">125</span> -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.”</p> -<p>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.</p> -<p>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 -<span class="pb" id="Page_126">126</span> -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.</p> -<p>Some years after Olbers’ famous theory was -given to the world, a great advance was made -<span class="pb" id="Page_127">127</span> -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 <i>J. L. -Pons</i> (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 -<span class="pb" id="Page_128">128</span> -class of celestial bodies, distinguished as comets -of short period.”</p> -<p>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.</p> -<p>The second comet recognised as periodic was -that discovered on February 27, 1826, by an -Austrian officer, <i>Wilhelm von Biela</i> (1782-1856), -and ten days later by the French observer, -<i>Gambart</i> (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 -<span class="pb" id="Page_129">129</span> -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.</p> -<p>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 <i>Didrik Magnus Axel -Möller</i> (1830-1896), director of the Lund Observatory, -who reached a negative conclusion.</p> -<div class="pb" id="Page_130">130</div> -<p>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 <i>Giovanni Battista Donati</i> -(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 -<span class="pb" id="Page_131">131</span> -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.</p> -<p>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 -<i>Friedrich August Theodor Winnecke</i> (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.</p> -<p>The time had now come for a more complete -<span class="pb" id="Page_132">132</span> -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 -<span class="pb" id="Page_133">133</span> -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.</p> -<p>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.”</p> -<p>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 -<span class="pb" id="Page_134">134</span> -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.</p> -<p>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 <i>Ralph Copeland</i> -(1837-1905), Astronomer-Royal of Scotland, who -<span class="pb" id="Page_135">135</span> -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.</p> -<p>The comet which appeared in 1880 was announced -by <i>Benjamin Apthorp Gould</i> (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.</p> -<p>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 -<span class="pb" id="Page_136">136</span> -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 <i>Jean Pierre Octave Callandrean</i> (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.</p> -<p>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 <i>Henry Draper</i> -(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, -<span class="pb" id="Page_137">137</span> -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.</p> -<div class="pb" id="Page_138">138</div> -<h2 id="c8"><span class="small">CHAPTER VIII.</span> -<br /><span class="smaller">METEORS.</span></h2> -<p>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 <i>Ernst Chladni</i> (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.</p> -<p>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 -<span class="pb" id="Page_139">139</span> -watched by <i>Denison Olmsted</i> (1791-1859), Professor -of Mathematics at Yale, and by the American -physicist, <i>A. C. Twining</i> (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.</p> -<p>The falling stars of August were next proved -by <i>Adolphe Quetelet</i> (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, -<span class="pb" id="Page_140">140</span> -for, says Miss Clerke, “If it would be a violation -of probability to attribute to <i>one</i> such agglomeration -a period of an exact year or sub-multiple -of a year, it would be plainly absurd to suppose -the movements of <i>two</i> 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 <i>Daniel -Kirkwood</i> (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 <i>Hubert Anson Newton</i> (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 -<span class="pb" id="Page_141">141</span> -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.</p> -<p>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 -<span class="pb" id="Page_142">142</span> -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 <i>Ernst -Tempel</i> (1821-1889). Peters of Altona had -meanwhile reached the same conclusion; while -<i>Edmund Weiss</i> (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.</p> -<p>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 -<i>Ernst Klinkerfues</i> (1827-1884), Professor at -Göttingen, believing that Biela’s comet itself had -encountered the Earth, telegraphed to <i>Norman -Robert Pogson</i> (1829-1891), Government astronomer -at Madras, to search for the comet in the -<span class="pb" id="Page_143">143</span> -opposite region of the sky. Pogson did observe -a comet, but certainly not Biela’s, although probably -another fragment of the missing body.</p> -<p>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.”</p> -<p>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 <i>falling stars</i>” he says, -“expresses simply and precisely the truth -respecting them. These bodies have the same -<span class="pb" id="Page_144">144</span> -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.”</p> -<p>In 1878 <i>Alexander Stewart Herschel</i> (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 -<span class="pb" id="Page_145">145</span> -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 <i>scientific</i> 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.”</p> -<div class="pb" id="Page_146">146</div> -<p>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.</p> -<p>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, <i>François Felix Tisserand</i> -(1845-1896), to doubt the existence of these -stationary radiants; but the fact of their -<span class="pb" id="Page_147">147</span> -existence cannot be doubted, although no really satisfactory -explanation has been offered.</p> -<p>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.”</p> -<p>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 -<span class="pb" id="Page_148">148</span> -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, <i>Bernhardt Hasselberg</i> (born 1848), who -detected the presence of hydrocarbons, which are -also present in cometary spectra.</p> -<p>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 -<span class="pb" id="Page_149">149</span> -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.</p> -<p>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.</p> -<div class="pb" id="Page_150">150</div> -<h2 id="c9"><span class="small">CHAPTER IX.</span> -<br /><span class="smaller">THE STARS.</span></h2> -<p>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.</p> -<p>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 -<span class="pb" id="Page_151">151</span> -worn out and unfit for use—had revealed -parallaxes in Sirius, Aldebaran, Procyon, and -Vega; Calandrelli, another Italian, and <i>John -Brinkley</i> (1763-1835), Astronomer-Royal of Ireland, -were similarly deluded; and in 1821 it was -shown by <i>Friedrich Georg Wilhelm Struve</i> (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.</p> -<p>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 -<span class="pb" id="Page_152">152</span> -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.</p> -<p>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.</p> -<div class="pb" id="Page_153">153</div> -<p>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.</p> -<p>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 -<span class="pb" id="Page_154">154</span> -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 <i>Thomas -Henderson</i>, the great Scottish astronomer.</p> -<p>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 -<span class="pb" id="Page_155">155</span> -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.</p> -<p>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 <i>Thomas -Maclear</i> (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″.</p> -<p>Other determinations of stellar parallax, some -genuine and others illusory, were made soon after -these successful observations. C. A. F. Peters -<span class="pb" id="Page_156">156</span> -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 <i>Friedrich -Brünnow</i> (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. -<i>Robert Stawell Ball</i>, 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.</p> -<p>But the greatest extension of our knowledge -of stellar distances, in recent years, is due to a -<span class="pb" id="Page_157">157</span> -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, <i>David Gill</i> directed Lord -Lindsay’s private observatory at Dunecht, in -Aberdeenshire, from 1876 to 1879. In the latter -year he succeeded <i>Edward James Stone</i> (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 <i>William -Lewis Elkin</i> (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.</p> -<p>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. -<span class="pb" id="Page_158">158</span> -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 <i>Jacobus Cornelius -Kapteyn</i> (born 1851), who determined by the -absolute method the parallaxes of fifteen northern -stars.</p> -<p>The first application of photography to the -problem was due to the zeal and energy of -<i>Charles Pritchard</i> (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 -<span class="pb" id="Page_159">159</span> -viewed from a distance of a hundred and eighty -miles.”</p> -<p>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, <i>Francis Baily</i> (1774-1844), -and by <i>Giovanni Santini</i> (1786-1877), director -of the observatory at Padua; but Bessel’s successor -in this branch of research was <i>Friedrich -Wilhelm August Argelander</i> (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 -<span class="pb" id="Page_160">160</span> -include 133,659 stars in the southern hemisphere, -by his assistant <i>Eduard Schönfeld</i> (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.</p> -<p>In the southern hemisphere, working at Cordova -in Argentina, was the great American -astronomer, <i>Gould</i>, 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.</p> -<p>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 -<span class="pb" id="Page_161">161</span> -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.</p> -<p>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 <i>Amédée Mouchez</i> -(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 -<span class="pb" id="Page_162">162</span> -the progress of the work, which is now -(1906) well advanced towards completion.</p> -<p>A unique star catalogue is in course of preparation -by the Scottish astronomer, <i>William -Peck</i> (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.</p> -<p>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 -<span class="pb" id="Page_163">163</span> -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 <i>Ludwig -Struve</i> (born 1858), son of Otto Struve; but -these must be reserved for a later chapter.</p> -<p>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 -<span class="pb" id="Page_164">164</span> -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.</p> -<p>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.</p> -<p>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 -<span class="pb" id="Page_165">165</span> -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.”</p> -<p>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 -<span class="pb" id="Page_166">166</span> -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.</p> -<p>In 1887 H. C. Vogel, working at Potsdam -Astrophysical Observatory, applied photography -to the measurement of radial motion. Assisted -by <i>Julius Scheiner</i> (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.</p> -<div class="pb" id="Page_167">167</div> -<p>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 <i>Thomas -Galloway</i>, and in 1859 and 1863 by Airy and -<i>Edwin Dunkin</i> (1821-1898), assistant at Greenwich -Observatory.</p> -<p>Meanwhile, in 1886, <i>Arthur Auwers</i>, 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 <i>Oscar Stumpe</i>, of Bonn, and <i>Lewis</i> -<span class="pb" id="Page_168">168</span> -<i>Boss</i> (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.</p> -<p>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.”</p> -<div class="pb" id="Page_169">169</div> -<h2 id="c10"><span class="small">CHAPTER X.</span> -<br /><span class="smaller">THE LIGHT OF THE STARS.</span></h2> -<p>“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 -<span class="pb" id="Page_170">170</span> -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.</p> -<p>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. -<span class="pb" id="Page_171">171</span> -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.</p> -<p>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 -<span class="pb" id="Page_172">172</span> -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.</p> -<p>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.</p> -<div class="pb" id="Page_173">173</div> -<p>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.</p> -<p>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<i>a</i>,—represented -by Sirius and Vega,—the metallic -lines are “very faint and fine,” and the hydrogen -lines conspicuous. In I<i>b</i> no hydrogen lines are -<span class="pb" id="Page_174">174</span> -visible, while in I<i>c</i> 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<i>b</i> 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<i>a</i> being represented by the Sun, Capella, and -other well-known stars, while II<i>b</i> 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.</p> -<p>Vogel’s classification of spectra is generally -adopted by astronomers, although others have -been proposed by Lockyer and by <i>Edward Charles -Pickering</i> (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. -<span class="pb" id="Page_175">175</span> -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.</p> -<p>The first spectroscopic star-catalogue was -published in 1883 by Vogel, assisted by <i>Gustav -Müller</i> (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 <i>Nicolaus Thege von Konkoly</i> -(born 1842) at O’Gyalla in Hungary.</p> -<p>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 -<span class="pb" id="Page_176">176</span> -designated his classes by the capital letters of -the alphabet.</p> -<p>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.</p> -<p>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.</p> -<p>Born in Boston in 1846, Edward Charles -Pickering was appointed in 1865 Instructor of -<span class="pb" id="Page_177">177</span> -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.</p> -<p>The study of stellar photometry glides into -that of stellar variation. At the beginning of -<span class="pb" id="Page_178">178</span> -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, -<i>John Goodricke</i> (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.</p> -<p>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 -<span class="pb" id="Page_179">179</span> -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 <i>a</i> and <i>b</i> the comparison stars, and <i>v</i> -the variable, <i>a</i> being brighter than <i>b</i>, and if <i>v</i> -is judged to be midway in brightness between <i>a</i> -and <i>b</i>, we write <i>a</i>5<i>v</i>5<i>b</i>. If <i>v</i> is slightly nearer to -<i>b</i>, we write <i>a</i>6<i>v</i>4<i>b</i>. We may also write <i>a</i>3<i>v</i>7<i>b</i>, -or <i>a</i>7<i>v</i>3<i>b</i>, the sum of the steps being always -ten.”</p> -<p>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, -<span class="pb" id="Page_180">180</span> -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.</p> -<p>In 1874 a very important, although not so -obvious, service to variable-star astronomy was -rendered by the Danish observer, <i>Hans Carl -Fredrik Christian Schjellerup</i> (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.</p> -<p>The study of variable stars has been pursued -in recent years by many astronomers, both by -<span class="pb" id="Page_181">181</span> -means of photography and by the visual method. -The most important names in the visual discovery -of variables are Gustav Müller and <i>Paul -Friedrich Ferdinand Kempf</i> (born 1856) of Potsdam; -<i>Alexander William Roberts</i> of Lovedale, -South Africa; Seth Carlo Chandler of Boston; -Nils Christopher Dunér at Upsala; and <i>John -Ellard Gore</i> (born 1845) in Dublin.</p> -<p>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.</p> -<p>Recently photography has come much to the -front in the discovery of variable stars. Pickering -at Harvard, and Wolf at Heidelberg, have -<span class="pb" id="Page_182">182</span> -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.</p> -<p>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, -<span class="pb" id="Page_183">183</span> -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.</p> -<p>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 -<span class="pb" id="Page_184">184</span> -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 -<span class="pb" id="Page_185">185</span> -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.</p> -<p>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 -<span class="pb" id="Page_186">186</span> -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.</p> -<p>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.</p> -<p>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 -<span class="pb" id="Page_187">187</span> -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.</p> -<p>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.”</p> -<p>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 -<span class="pb" id="Page_188">188</span> -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.</p> -<p>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.</p> -<p>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 -<span class="pb" id="Page_189">189</span> -at Tuam, in Ireland, by the Irish astronomer, -<i>John Birmingham</i>. 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.</p> -<p>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 <i>Alfred -Cornu</i> (1841-1902), and some days later at -<span class="pb" id="Page_190">190</span> -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.</p> -<p>A new star appeared in the centre of the -great nebula in Andromeda in August 1885. -The first announcement of the discovery was by -<i>Karl Ernst Albrecht Hartwig</i> (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.</p> -<p>The discovery of the next temporary star was -announced February 1, 1892, by the Rev. <i>Thomas</i> -<span class="pb" id="Page_191">191</span> -<i>D. Anderson</i>, 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.</p> -<p>After March 9, 1892, the new star steadily -faded, falling to the sixteenth magnitude on -April 26. But on August 17 <i>Edward Singelton -Holden</i> (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 -<span class="pb" id="Page_192">192</span> -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.</p> -<p>After 1892 several new stars appeared, and -were detected on photographic plates by <i>Mrs -Fleming</i> (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.</p> -<p>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 -<span class="pb" id="Page_193">193</span> -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.</p> -<p>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 -<span class="pb" id="Page_194">194</span> -slowly changing into that of a gaseous nebula. -In August and September 1901 the nebular -spectrum became more apparent.</p> -<p>In August 1901 Wolf at Heidelberg discovered -a faint trace of nebula near the nova. On September -20 this nebula was photographed by -<i>George Ritchey</i> 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.</p> -<p>On March 16, 1903, <i>Herbert Hall Turner</i> -(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 -<span class="pb" id="Page_195">195</span> -Mrs Fleming on the Harvard photographs, situated -in Aquila.</p> -<p>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, <i>Jacobus Halm</i>, of the Royal -Observatory, Edinburgh, extended and developed -Seeliger’s theory, showing also that the necessary -<span class="pb" id="Page_196">196</span> -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.</p> -<div class="pb" id="Page_197">197</div> -<h2 id="c11"><span class="small">CHAPTER XI.</span> -<br /><span class="smaller">STELLAR SYSTEMS AND NEBULÆ.</span></h2> -<p>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 -<span class="pb" id="Page_198">198</span> -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.</p> -<p>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 -<i>James South</i> (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.</p> -<div class="pb" id="Page_199">199</div> -<p>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.</p> -<p>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 <i>Felix -Savary</i> (1797-1841), who computed the orbit of -ξ Ursæ Majoris, and found the period to be -<span class="pb" id="Page_200">200</span> -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.</p> -<p>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, -<span class="pb" id="Page_201">201</span> -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.</p> -<p>Various other astronomers have devoted themselves -to the observation of double stars, among -them <i>Ercole Dembowski</i> (1815-1881), of Milan; -<i>Karl Hermann Struve</i> (born 1854), son of -Otto Struve; <i>William Doberck</i> (born 1845); -<i>William J. Hussey</i> (born 1864), now director of -the Detroit Observatory; Camille Flammarion; -N. C. Dunér; G. V. Schiaparelli; <i>Thomas -Jefferson Jackson See</i> (born 1866). But the -greatest living <i>discoverer</i> is <i>Sherburne Wesley -Burnham</i> (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 -<span class="pb" id="Page_202">202</span> -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.</p> -<p>The computation of double-star orbits has been -undertaken by various astronomers, among them -Mädler, Klinkerfues, Dunér, Flammarion, Seeliger, -See, Gore, Burnham, <i>Robert Grant Aitken</i> (born -1864) of the Lick Observatory, and <i>Giovanni -Celoria</i> (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.</p> -<p>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 <i>Truman -Henry Safford</i> (1836-1901), which indicated the -position of the satellite. Close to this position -it was accidentally discovered by <i>Alvan Clark</i> -<span class="pb" id="Page_203">203</span> -(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.</p> -<p>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 -<span class="pb" id="Page_204">204</span> -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.</p> -<p>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 -<span class="pb" id="Page_205">205</span> -the Lowell Observatory, and by <i>Edwin Brant -Frost</i> (born 1866), now director of the Yerkes -Observatory, and his assistant, <i>Walter Adams</i>. -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 <i>Hugh Frank Newall</i> -(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.</p> -<p>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 -<span class="pb" id="Page_206">206</span> -system is not of the prevailing type of stellar -systems.”</p> -<p>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.</p> -<p>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 -<i>John Louis Emil Dreyer</i> (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.</p> -<div class="pb" id="Page_207">207</div> -<p>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 <i>William Parsons</i>, third <i>Earl of Rosse</i> -(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, -<i>John Pringle Nichol</i> (1804-1859), and some other -investigators, held to the theory of Herschel.</p> -<p>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 -<span class="pb" id="Page_208">208</span> -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.</p> -<p>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 -<span class="pb" id="Page_209">209</span> -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.</p> -<p>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.</p> -<p>In 1885 nebular photography was commenced -by <i>Isaac Roberts</i> (1829-1904), the English -<span class="pb" id="Page_210">210</span> -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 <i>William James Lockyer</i>: “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.”</p> -<p>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.</p> -<div class="pb" id="Page_211">211</div> -<p>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. -<span class="pb" id="Page_212">212</span> -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 <i>one hundred and twenty -thousand</i> 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.</p> -<p>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 -<span class="pb" id="Page_213">213</span> -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.</p> -<p>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.</p> -<div class="pb" id="Page_214">214</div> -<h2 id="c12"><span class="small">CHAPTER XII.</span> -<br /><span class="smaller">STELLAR DISTRIBUTION AND THE STRUCTURE OF THE UNIVERSE.</span></h2> -<p>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.</p> -<p>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 -<span class="pb" id="Page_215">215</span> -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 -<span class="pb" id="Page_216">216</span> -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 <i>Robert Grant</i> (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.</p> -<p>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 -<span class="pb" id="Page_217">217</span> -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.</p> -<p>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 -<span class="pb" id="Page_218">218</span> -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.</p> -<p>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 <i>William Whewell</i> -(1794-1866) in 1853, and by <i>Herbert Spencer</i> -(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 -<span class="pb" id="Page_219">219</span> -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.”</p> -<p>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 -<span class="pb" id="Page_220">220</span> -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.”</p> -<p>Proctor’s discovery of the excess of bright stars -on the Galaxy was confirmed by <i>Jean Charles -Houzeau</i> (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 -<span class="pb" id="Page_221">221</span> -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, -<i>Charles Easton</i>, finds a connection between the -distribution of ninth-magnitude stars and the -luminous and obscure spots in the Galaxy.</p> -<p>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.</p> -<p>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. -<span class="pb" id="Page_222">222</span> -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.</p> -<p>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 -<span class="pb" id="Page_223">223</span> -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.”</p> -<p>Positive evidence that this is the case was -obtained by Giovanni Celoria, now director of -<span class="pb" id="Page_224">224</span> -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.</p> -<p>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 -<span class="pb" id="Page_225">225</span> -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.</p> -<p>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.</p> -<div class="pb" id="Page_226">226</div> -<p>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 -<i>visible</i> 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.”</p> -<div class="pb" id="Page_227">227</div> -<h2 id="c13"><span class="small">CHAPTER XIII.</span> -<br /><span class="smaller">CELESTIAL EVOLUTION.</span></h2> -<p>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. <i>Pierre Simon Laplace</i> 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.</p> -<p>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 -<span class="pb" id="Page_228">228</span> -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.</p> -<p>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 -<span class="pb" id="Page_229">229</span> -by the famous German physicist, <i>Hermann -Ludwig Ferdinand von Helmholtz</i> (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, <i>Julius -Robert Mayer</i>, 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 -<i>William Thomson</i>, now <i>Lord Kelvin</i> (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.</p> -<p>Several objections to Laplace’s nebular theory -<span class="pb" id="Page_230">230</span> -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.</p> -<p>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.”</p> -<div class="pb" id="Page_231">231</div> -<p>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.”</p> -<p>Lockyer divided the stars into seven groups, -according to temperature, the order of evolution -being from red stars through a division of second-type -<span class="pb" id="Page_232">232</span> -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.</p> -<p>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 -<span class="pb" id="Page_233">233</span> -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.</p> -<p>Distinctly supplementary to the nebular theory -are the remarkable researches, commenced in -1879, by Sir <i>George Howard Darwin</i> (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.</p> -<p>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 -<span class="pb" id="Page_234">234</span> -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.</p> -<p>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.</p> -<p>The fact that the Earth and Moon were almost -in contact suggests that they were probably in -<span class="pb" id="Page_235">235</span> -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.</p> -<p>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.” -<span class="pb" id="Page_236">236</span> -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.</p> -<p>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.</p> -<p>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 -<span class="pb" id="Page_237">237</span> -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.”</p> -<p>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.”</p> -<p>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 -<span class="pb" id="Page_238">238</span> -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.’”</p> -<div class="pb" id="Page_239">239</div> -<h2 id="c14"><span class="small">INDEX.</span></h2> -<p class="center"><a class="ab" href="#index_A">A</a> <a class="ab" href="#index_B">B</a> <a class="ab" href="#index_C">C</a> <a class="ab" href="#index_D">D</a> <a class="ab" href="#index_E">E</a> <a class="ab" href="#index_F">F</a> <a class="ab" href="#index_G">G</a> <a class="ab" href="#index_H">H</a> <a class="ab" href="#index_I">I</a> <a class="ab" href="#index_J">J</a> <a class="ab" href="#index_K">K</a> <a class="ab" href="#index_L">L</a> <a class="ab" href="#index_M">M</a> <a class="ab" href="#index_N">N</a> <a class="ab" href="#index_O">O</a> <a class="ab" href="#index_P">P</a> <a class="ab" href="#index_Q">Q</a> <a class="ab" href="#index_R">R</a> <a class="ab" href="#index_S">S</a> <a class="ab" href="#index_T">T</a> <a class="ab" href="#index_U">U</a> <a class="ab" href="#index_V">V</a> <a class="ab" href="#index_W">W</a> <span class="ab">X</span> <a class="ab" href="#index_Y">Y</a> <a class="ab" href="#index_Z">Z</a></p> -<dl class="index"> -<dt class="center b" id="index_A">A</dt> -<dt>Absolute parallax, <a href="#Page_158">158</a>.</dt> -<dt>Adams, J. C., <a href="#Page_78">78</a>, <a href="#Page_116">116</a>, <a href="#Page_117">117</a>, <a href="#Page_118">118</a>, <a href="#Page_119">119</a>, <a href="#Page_120">120</a>, <a href="#Page_140">140</a>.</dt> -<dt>Adams, W., <a href="#Page_205">205</a>.</dt> -<dt>Aerolites, <a href="#Page_147">147</a>, <a href="#Page_148">148</a>, <a href="#Page_149">149</a>.</dt> -<dt>Airy, Sir G. B., <a href="#Page_27">27</a>, <a href="#Page_104">104</a>, <a href="#Page_117">117</a>, <a href="#Page_120">120</a>.</dt> -<dt>Aitken, R. G., <a href="#Page_202">202</a>.</dt> -<dt>Alcyone (η Tauri), <a href="#Page_217">217</a>.</dt> -<dt>Aldebaran (α Tauri), <a href="#Page_151">151</a>, <a href="#Page_166">166</a>, <a href="#Page_170">170</a>, <a href="#Page_172">172</a>.</dt> -<dt>Algol (β Persei), <a href="#Page_178">178</a>, <a href="#Page_182">182</a>, <a href="#Page_183">183</a>, <a href="#Page_184">184</a>, <a href="#Page_193">193</a>, <a href="#Page_204">204</a>.</dt> -<dt>Al-Sufi, <a href="#Page_180">180</a>, <a href="#Page_183">183</a>.</dt> -<dt>Altair (α Aquilæ), <a href="#Page_170">170</a>.</dt> -<dt>Anderson, T. D., <a href="#Page_191">191</a>, <a href="#Page_192">192</a>.</dt> -<dt>Andromeda nebula, <a href="#Page_180">180</a>, <a href="#Page_208">208</a>.</dt> -<dt>Andromedæ (γ), <a href="#Page_201">201</a>.</dt> -<dt>Andromedæ (Nova), <a href="#Page_180">180</a>.</dt> -<dt>Andromedid meteors, <a href="#Page_142">142</a>, <a href="#Page_149">149</a>.</dt> -<dt>Angström, A. J., <a href="#Page_50">50</a>, <a href="#Page_51">51</a>.</dt> -<dt>Antares (α Scorpii), <a href="#Page_171">171</a>.</dt> -<dt>Aquila, <a href="#Page_195">195</a>.</dt> -<dt>Aquilæ (η), <a href="#Page_185">185</a>, <a href="#Page_186">186</a>.</dt> -<dt>Arago, F. J. D., <a href="#Page_6">6</a>, <a href="#Page_11">11</a>, <a href="#Page_31">31</a>, <a href="#Page_37">37</a>, <a href="#Page_40">40</a>, <a href="#Page_118">118</a>, <a href="#Page_120">120</a>, <a href="#Page_129">129</a>.</dt> -<dt>Arcturus (α Bootis), <a href="#Page_165">165</a>, <a href="#Page_170">170</a>.</dt> -<dt>Arequipa Observatory, <a href="#Page_75">75</a>.</dt> -<dt>Argelander, F. W. A., <a href="#Page_27">27</a>, <a href="#Page_159">159</a>, <a href="#Page_167">167</a>, <a href="#Page_178">178</a>, <a href="#Page_179">179</a>, <a href="#Page_180">180</a>, <a href="#Page_218">218</a>.</dt> -<dt>Argo Navis, <a href="#Page_221">221</a>.</dt> -<dt>Argus (η), <a href="#Page_187">187</a>, <a href="#Page_188">188</a>.</dt> -<dt>Armagh Observatory, <a href="#Page_206">206</a>.</dt> -<dt>Asteroids, <a href="#Page_19">19</a>, <a href="#Page_62">62</a>, <a href="#Page_97">97</a>-102.</dt> -<dt>Astronomer-Royal of Scotland, <a href="#Page_134">134</a>, <a href="#Page_155">155</a>, <a href="#Page_191">191</a>;</dt> -<dd>of England, <a href="#Page_59">59</a>, <a href="#Page_17">17</a>;</dd> -<dd>of Ireland, <a href="#Page_151">151</a>, <a href="#Page_156">156</a>.</dd> -<dt>Astronomy of the invisible, <a href="#Page_203">203</a>.</dt> -<dt>Aurigæ (Nova), <a href="#Page_191">191</a>, <a href="#Page_192">192</a>, <a href="#Page_195">195</a>.</dt> -<dt>Auwers, A., <a href="#Page_167">167</a>, <a href="#Page_188">188</a>, <a href="#Page_203">203</a>.</dt> -</dl> -<dl class="index"> -<dt class="center b" id="index_B">B</dt> -<dt>Babinet, <a href="#Page_230">230</a>.</dt> -<dt>Baily, F., <a href="#Page_159">159</a>.</dt> -<dt>Bakhuyzen, H. G., <a href="#Page_91">91</a>.</dt> -<dt>Ball, Sir R. S., <a href="#Page_23">23</a>, <a href="#Page_34">34</a>, <a href="#Page_108">108</a>, <a href="#Page_141">141</a>, <a href="#Page_149">149</a>, <a href="#Page_156">156</a>, <a href="#Page_158">158</a>, <a href="#Page_230">230</a>.</dt> -<dt>Barnard, E. E., <a href="#Page_19">19</a>, <a href="#Page_95">95</a>, <a href="#Page_107">107</a>, <a href="#Page_108">108</a>, <a href="#Page_110">110</a>, <a href="#Page_111">111</a>, <a href="#Page_113">113</a>, <a href="#Page_136">136</a>, <a href="#Page_191">191</a>, <a href="#Page_211">211</a>.</dt> -<dt>Beer, W., <a href="#Page_68">68</a>, <a href="#Page_69">69</a>, <a href="#Page_90">90</a>.</dt> -<dt>Bellatrix (γ Orionis), <a href="#Page_209">209</a>.</dt> -<dt>Bélopolsky, A., <a href="#Page_87">87</a>, <a href="#Page_110">110</a>, <a href="#Page_166">166</a>, <a href="#Page_185">185</a>, <a href="#Page_186">186</a>, <a href="#Page_204">204</a>, <a href="#Page_205">205</a>.</dt> -<dt>Berlin Observatory, <a href="#Page_119">119</a>, <a href="#Page_120">120</a>.</dt> -<dt>Bessel, F. W., <a href="#Page_24">24</a>, <a href="#Page_82">82</a>, <a href="#Page_116">116</a>, <a href="#Page_151">151</a>, <a href="#Page_152">152</a>, <a href="#Page_153">153</a>, <a href="#Page_154">154</a>, <a href="#Page_159">159</a>, <a href="#Page_167">167</a>, <a href="#Page_202">202</a>, <a href="#Page_203">203</a>.</dt> -<dt>Betelgeux (α Orionis), <a href="#Page_165">165</a>, <a href="#Page_171">171</a>, <a href="#Page_172">172</a>, <a href="#Page_182">182</a>, <a href="#Page_187">187</a>.</dt> -<dt>Biela, W., <a href="#Page_128">128</a>.</dt> -<dt>Biela’s comet, <a href="#Page_128">128</a>, <a href="#Page_129">129</a>, <a href="#Page_142">142</a>, <a href="#Page_143">143</a>, <a href="#Page_146">146</a>, <a href="#Page_149">149</a>.</dt> -<dt>Birmingham, J., <a href="#Page_189">189</a>.</dt> -<dt>Bode, J. E., <a href="#Page_97">97</a>, <a href="#Page_98">98</a>, <a href="#Page_152">152</a>.</dt> -<dt>Bode’s Law, <a href="#Page_97">97</a>.</dt> -<dt>Boeddicker, O., <a href="#Page_77">77</a>.</dt> -<dt>Bond, G. P., <a href="#Page_103">103</a>, <a href="#Page_109">109</a>, <a href="#Page_130">130</a>, <a href="#Page_136">136</a>, <a href="#Page_207">207</a>.</dt> -<dt class="pb" id="Page_240">240</dt> -<dt>Bond, W. C., <a href="#Page_109">109</a>, <a href="#Page_112">112</a>, <a href="#Page_120">120</a>.</dt> -<dt>‘Bonn Durchmusterung,’ <a href="#Page_159">159</a>, <a href="#Page_160">160</a>, <a href="#Page_218">218</a>.</dt> -<dt>Bonn Observatory, <a href="#Page_88">88</a>, <a href="#Page_97">97</a>, <a href="#Page_160">160</a>.</dt> -<dt>Böotis (ε), <a href="#Page_30">30</a>.</dt> -<dt>Borisiak, <a href="#Page_192">192</a>.</dt> -<dt>Boss, L., <a href="#Page_168">168</a>.</dt> -<dt>Bouvard, A., <a href="#Page_115">115</a>, <a href="#Page_116">116</a>.</dt> -<dt>Bradley, J., <a href="#Page_159">159</a>, <a href="#Page_167">167</a>.</dt> -<dt>Brédikhine, T. A., <a href="#Page_105">105</a>, <a href="#Page_131">131</a>, <a href="#Page_132">132</a>, <a href="#Page_133">133</a>, <a href="#Page_134">134</a>, <a href="#Page_135">135</a>.</dt> -<dt>Brewster, Sir D., <a href="#Page_50">50</a>, <a href="#Page_101">101</a>, <a href="#Page_178">178</a>.</dt> -<dt>Brinkley, J., <a href="#Page_151">151</a>.</dt> -<dt>Brünnow, F., <a href="#Page_156">156</a>.</dt> -<dt>Bruno, G., <a href="#Page_35">35</a>.</dt> -<dt>Buffon, <a href="#Page_103">103</a>.</dt> -<dt>Bunsen, R. W., <a href="#Page_51">51</a>.</dt> -<dt>Burchell, <a href="#Page_188">188</a>.</dt> -<dt>Burnham, S. W., <a href="#Page_201">201</a>, <a href="#Page_202">202</a>, <a href="#Page_212">212</a>.</dt> -</dl> -<dl class="index"> -<dt class="center b" id="index_C">C</dt> -<dt>Callandreau, O., <a href="#Page_136">136</a>.</dt> -<dt>Callandrelli, <a href="#Page_151">151</a>.</dt> -<dt>Cambridge Observatory, <a href="#Page_116">116</a>.</dt> -<dt>Campbell, T. (Poet), <a href="#Page_2">2</a>.</dt> -<dt>Campbell, W. W., <a href="#Page_24">24</a>, <a href="#Page_107">107</a>, <a href="#Page_110">110</a>, <a href="#Page_166">166</a>, <a href="#Page_168">168</a>, <a href="#Page_187">187</a>, <a href="#Page_191">191</a>, <a href="#Page_193">193</a>, <a href="#Page_204">204</a>, <a href="#Page_205">205</a>.</dt> -<dt>Canals of Mars, <a href="#Page_91">91</a>, <a href="#Page_92">92</a>, <a href="#Page_93">93</a>, <a href="#Page_94">94</a>, <a href="#Page_95">95</a>.</dt> -<dt>Cancri (ζ), <a href="#Page_206">206</a>.</dt> -<dt>Cancri (S), <a href="#Page_180">180</a>.</dt> -<dt>Canis Major, <a href="#Page_188">188</a>.</dt> -<dt>‘Cape Durchmusterung,’ <a href="#Page_161">161</a>, <a href="#Page_162">162</a>.</dt> -<dt>Cape Observatory, <a href="#Page_155">155</a>, <a href="#Page_157">157</a>.</dt> -<dt>Capella (α Aurigæ), <a href="#Page_170">170</a>, <a href="#Page_176">176</a>, <a href="#Page_193">193</a>, <a href="#Page_205">205</a>.</dt> -<dt>Carnera, L., <a href="#Page_100">100</a>.</dt> -<dt>Carpenter, J., <a href="#Page_73">73</a>.</dt> -<dt>Carrington, R. C., <a href="#Page_45">45</a>, <a href="#Page_46">46</a>, <a href="#Page_59">59</a>.</dt> -<dt>Cassini, D., <a href="#Page_21">21</a>.</dt> -<dt>Cassiopeia, <a href="#Page_221">221</a>.</dt> -<dt>Castor (α Geminorum), <a href="#Page_30">30</a>, <a href="#Page_200">200</a>, <a href="#Page_205">205</a>.</dt> -<dt>Celoria, G., <a href="#Page_202">202</a>, <a href="#Page_218">218</a>, <a href="#Page_221">221</a>, <a href="#Page_223">223</a>, <a href="#Page_224">224</a>.</dt> -<dt>Centauri (α), <a href="#Page_155">155</a>, <a href="#Page_188">188</a>, <a href="#Page_225">225</a>.</dt> -<dt>Centaurus, <a href="#Page_221">221</a>.</dt> -<dt>Cephei (δ), <a href="#Page_178">178</a>, <a href="#Page_182">182</a>, <a href="#Page_185">185</a>, <a href="#Page_186">186</a>.</dt> -<dt>Cepheus, <a href="#Page_221">221</a>.</dt> -<dt>Ceres, <a href="#Page_19">19</a>, <a href="#Page_98">98</a>, <a href="#Page_101">101</a>.</dt> -<dt>Cerulli, V., <a href="#Page_86">86</a>, <a href="#Page_91">91</a>, <a href="#Page_94">94</a>.</dt> -<dt>Chacornac, <a href="#Page_161">161</a>.</dt> -<dt>Challis, J., <a href="#Page_116">116</a>, <a href="#Page_119">119</a>, <a href="#Page_120">120</a>.</dt> -<dt>Chambers, G. F., <a href="#Page_31">31</a>.</dt> -<dt>Chandler, S. C., <a href="#Page_88">88</a>, <a href="#Page_89">89</a>, <a href="#Page_181">181</a>, <a href="#Page_184">184</a>.</dt> -<dt>Chladni, E., <a href="#Page_138">138</a>.</dt> -<dt>Chromosphere, solar, <a href="#Page_55">55</a>, <a href="#Page_56">56</a>.</dt> -<dt>Clark, A., <a href="#Page_202">202</a>.</dt> -<dt>Clerke, Miss A. M., <a href="#Page_3">3</a>, <a href="#Page_5">5</a>, <a href="#Page_8">8</a>, <a href="#Page_12">12</a>, <a href="#Page_13">13</a>, <a href="#Page_15">15</a>, <a href="#Page_25">25</a>, <a href="#Page_26">26</a>, <a href="#Page_34">34</a>, <a href="#Page_42">42</a>, <a href="#Page_58">58</a>, <a href="#Page_75">75</a>, <a href="#Page_86">86</a>, <a href="#Page_92">92</a>, <a href="#Page_105">105</a>, <a href="#Page_109">109</a>, <a href="#Page_124">124</a>, <a href="#Page_125">125</a>, <a href="#Page_131">131</a>, <a href="#Page_132">132</a>, <a href="#Page_133">133</a>, <a href="#Page_140">140</a>, <a href="#Page_142">142</a>, <a href="#Page_169">169</a>, <a href="#Page_186">186</a>, <a href="#Page_187">187</a>, <a href="#Page_189">189</a>.</dt> -<dt>Clerk-Maxwell, J., <a href="#Page_109">109</a>, <a href="#Page_110">110</a>.</dt> -<dt>Coggia’s comet, <a href="#Page_131">131</a>, <a href="#Page_132">132</a>, <a href="#Page_133">133</a>.</dt> -<dt>Comet families, <a href="#Page_135">135</a>.</dt> -<dt>Comets, <a href="#Page_24">24</a>, <a href="#Page_123">123</a>-137, <a href="#Page_141">141</a>, <a href="#Page_142">142</a>, <a href="#Page_143">143</a>, <a href="#Page_144">144</a>, <a href="#Page_146">146</a>, <a href="#Page_149">149</a>, <a href="#Page_152">152</a>.</dt> -<dt>Common, A. A., <a href="#Page_107">107</a>, <a href="#Page_209">209</a>.</dt> -<dt>Copeland, R., <a href="#Page_134">134</a>, <a href="#Page_135">135</a>, <a href="#Page_190">190</a>, <a href="#Page_208">208</a>.</dt> -<dt>Cornu, A., <a href="#Page_189">189</a>.</dt> -<dt>Corona Borealis, <a href="#Page_188">188</a>.</dt> -<dt>Corona, solar, <a href="#Page_55">55</a>, <a href="#Page_57">57</a>, <a href="#Page_64">64</a>.</dt> -<dt>Coronæ (Nova), <a href="#Page_188">188</a>, <a href="#Page_189">189</a>.</dt> -<dt>Crossley, E., <a href="#Page_211">211</a>.</dt> -<dt>Crux, <a href="#Page_221">221</a>.</dt> -<dt>Cygni (61), <a href="#Page_152">152</a>, <a href="#Page_158">158</a>.</dt> -<dt>Cygni (Y), <a href="#Page_184">184</a>, <a href="#Page_185">185</a>.</dt> -<dt>Cygni (Nova), <a href="#Page_189">189</a>, <a href="#Page_190">190</a>.</dt> -<dt>Cygnus, <a href="#Page_152">152</a>, <a href="#Page_189">189</a>, <a href="#Page_221">221</a>.</dt> -</dl> -<dl class="index"> -<dt class="center b" id="index_D">D</dt> -<dt>Damoiseau, <a href="#Page_78">78</a>.</dt> -<dt>D’Arrest, H. L., <a href="#Page_96">96</a>, <a href="#Page_119">119</a>, <a href="#Page_142">142</a>, <a href="#Page_212">212</a>.</dt> -<dt>Dartmouth Observatory, <a href="#Page_56">56</a>.</dt> -<dt>Darwin, Sir G. H., <a href="#Page_233">233</a>, <a href="#Page_234">234</a>, <a href="#Page_235">235</a>, <a href="#Page_236">236</a>.</dt> -<dt>Dawes, W. R., <a href="#Page_90">90</a>, <a href="#Page_117">117</a>.</dt> -<dt>De la Rue, W., <a href="#Page_52">52</a>, <a href="#Page_75">75</a>.</dt> -<dt>Delaunay, C. E., <a href="#Page_78">78</a>, <a href="#Page_79">79</a>.</dt> -<dt>Dembowski, E., <a href="#Page_201">201</a>.</dt> -<dt>Deneb (α Cygni), <a href="#Page_165">165</a>.</dt> -<dt>Denning, W. F., <a href="#Page_84">84</a>, <a href="#Page_85">85</a>, <a href="#Page_91">91</a>, <a href="#Page_95">95</a>, <a href="#Page_105">105</a>, <a href="#Page_111">111</a>, <a href="#Page_112">112</a>, <a href="#Page_144">144</a>, <a href="#Page_145">145</a>, <a href="#Page_146">146</a>.</dt> -<dt>Deslandres, H., <a href="#Page_110">110</a>.</dt> -<dt>Dick, T., <a href="#Page_85">85</a>, <a href="#Page_238">238</a>.</dt> -<dt>Disc-theory, <a href="#Page_32">32</a>, <a href="#Page_36">36</a>, <a href="#Page_38">38</a>, <a href="#Page_39">39</a>, <a href="#Page_214">214</a>, <a href="#Page_218">218</a>.</dt> -<dt>Di Vico, F., <a href="#Page_85">85</a>, <a href="#Page_86">86</a>, <a href="#Page_170">170</a>.</dt> -<dt class="pb" id="Page_241">241</dt> -<dt>Doberck, W., <a href="#Page_201">201</a>.</dt> -<dt>Donati, G. B., <a href="#Page_130">130</a>, <a href="#Page_131">131</a>, <a href="#Page_169">169</a>.</dt> -<dt>Donati’s comet, <a href="#Page_130">130</a>, <a href="#Page_133">133</a>, <a href="#Page_136">136</a>.</dt> -<dt>Doppler, C., <a href="#Page_57">57</a>, <a href="#Page_58">58</a>.</dt> -<dt>Doppler’s Principle, <a href="#Page_58">58</a>, <a href="#Page_59">59</a>, <a href="#Page_87">87</a>, <a href="#Page_110">110</a>, <a href="#Page_165">165</a>, <a href="#Page_168">168</a>, <a href="#Page_203">203</a>.</dt> -<dt>Douglass, A. E., <a href="#Page_92">92</a>, <a href="#Page_107">107</a>.</dt> -<dt>Draconis (λ), <a href="#Page_182">182</a>.</dt> -<dt>Draper, H., <a href="#Page_136">136</a>, <a href="#Page_172">172</a>, <a href="#Page_175">175</a>.</dt> -<dt>Dreyer, J. L. E., <a href="#Page_206">206</a>.</dt> -<dt>Dunecht Observatory, <a href="#Page_157">157</a>.</dt> -<dt>Dunér, N. C., <a href="#Page_58">58</a>, <a href="#Page_59">59</a>, <a href="#Page_174">174</a>, <a href="#Page_175">175</a>, <a href="#Page_181">181</a>, <a href="#Page_184">184</a>, <a href="#Page_185">185</a>, <a href="#Page_201">201</a>, <a href="#Page_202">202</a>, <a href="#Page_233">233</a>.</dt> -<dt>Dunkin, E., <a href="#Page_27">27</a>, <a href="#Page_167">167</a>.</dt> -<dt>Dunsink Observatory, <a href="#Page_156">156</a>.</dt> -</dl> -<dl class="index"> -<dt class="center b" id="index_E">E</dt> -<dt>Earth, <a href="#Page_76">76</a>, <a href="#Page_97">97</a>, <a href="#Page_103">103</a>, <a href="#Page_104">104</a>, <a href="#Page_147">147</a>, <a href="#Page_148">148</a>, <a href="#Page_149">149</a>, <a href="#Page_153">153</a>, <a href="#Page_154">154</a>, <a href="#Page_156">156</a>, <a href="#Page_236">236</a>.</dt> -<dt>Earth-Moon system, <a href="#Page_234">234</a>, <a href="#Page_235">235</a>.</dt> -<dt>Easton, C., <a href="#Page_221">221</a>.</dt> -<dt>Eclipses, lunar, <a href="#Page_77">77</a>.</dt> -<dt>Eclipses, solar, <a href="#Page_56">56</a>, <a href="#Page_57">57</a>, <a href="#Page_80">80</a>, <a href="#Page_81">81</a>.</dt> -<dt>Edinburgh (Royal) Observatory, <a href="#Page_195">195</a>.</dt> -<dt>Electrical repulsion theory, <a href="#Page_126">126</a>.</dt> -<dt>Elger, T. G., <a href="#Page_74">74</a>.</dt> -<dt>Elkin, W. L., <a href="#Page_157">157</a>.</dt> -<dt>Encke, J. F., <a href="#Page_30">30</a>, <a href="#Page_61">61</a>, <a href="#Page_119">119</a>, <a href="#Page_127">127</a>, <a href="#Page_128">128</a>, <a href="#Page_216">216</a>.</dt> -<dt>Encke’s comet, <a href="#Page_127">127</a>, <a href="#Page_128">128</a>, <a href="#Page_137">137</a>.</dt> -<dt>Erman, <a href="#Page_140">140</a>.</dt> -<dt>Eros, <a href="#Page_62">62</a>, <a href="#Page_101">101</a>.</dt> -<dt>Ertborn, <a href="#Page_85">85</a>.</dt> -<dt>Euler, L., <a href="#Page_88">88</a>, <a href="#Page_89">89</a>.</dt> -<dt>Evolution, planetary, <a href="#Page_228">228</a>, <a href="#Page_229">229</a>, <a href="#Page_230">230</a>, <a href="#Page_231">231</a>.</dt> -<dt>Evolution, stellar, <a href="#Page_33">33</a>, <a href="#Page_34">34</a>, <a href="#Page_231">231</a>, <a href="#Page_232">232</a>.</dt> -</dl> -<dl class="index"> -<dt class="center b" id="index_F">F</dt> -<dt>Faye, H., <a href="#Page_60">60</a>, <a href="#Page_129">129</a>, <a href="#Page_230">230</a>.</dt> -<dt>Faye’s comet, <a href="#Page_129">129</a>, <a href="#Page_137">137</a>.</dt> -<dt>Ferguson, J., <a href="#Page_9">9</a>, <a href="#Page_178">178</a>.</dt> -<dt>Flammarion, C., <a href="#Page_87">87</a>, <a href="#Page_91">91</a>, <a href="#Page_95">95</a>, <a href="#Page_121">121</a>, <a href="#Page_147">147</a>, <a href="#Page_164">164</a>, <a href="#Page_187">187</a>, <a href="#Page_195">195</a>, <a href="#Page_201">201</a>, <a href="#Page_202">202</a>, <a href="#Page_226">226</a>.</dt> -<dt>Flamsteed, J., <a href="#Page_5">5</a>.</dt> -<dt>Fleming, Mrs, <a href="#Page_192">192</a>, <a href="#Page_195">195</a>.</dt> -<dt>Forbes, G., <a href="#Page_122">122</a>.</dt> -<dt>Fraunhofer, J. <a href="#Page_47">47</a>, <a href="#Page_48">48</a>, <a href="#Page_49">49</a>, <a href="#Page_50">50</a>, <a href="#Page_3">3</a>, <a href="#Page_151">151</a>, <a href="#Page_153">153</a>, <a href="#Page_169">169</a>.</dt> -<dt>Fraunhofer lines, <a href="#Page_48">48</a>, <a href="#Page_49">49</a>, <a href="#Page_50">50</a>, <a href="#Page_51">51</a>, <a href="#Page_169">169</a>, <a href="#Page_172">172</a>.</dt> -<dt>Frost, E. B., <a href="#Page_205">205</a>.</dt> -</dl> -<dl class="index"> -<dt class="center b" id="index_G">G</dt> -<dt>Galactic poles, <a href="#Page_35">35</a>, <a href="#Page_224">224</a>.</dt> -<dt>Galaxies, external, <a href="#Page_32">32</a>, <a href="#Page_218">218</a>, <a href="#Page_225">225</a>, <a href="#Page_226">226</a>.</dt> -<dt>Galaxy, or Milky Way, <a href="#Page_32">32</a>, <a href="#Page_36">36</a>-42, <a href="#Page_186">186</a>, <a href="#Page_211">211</a>, <a href="#Page_215">215</a>, <a href="#Page_216">216</a>, <a href="#Page_217">217</a>, <a href="#Page_219">219</a>, <a href="#Page_220">220</a>, <a href="#Page_221">221</a>, <a href="#Page_224">224</a>, <a href="#Page_225">225</a>.</dt> -<dt>Galileo, <a href="#Page_44">44</a>, <a href="#Page_107">107</a>.</dt> -<dt>Galle, J. G., <a href="#Page_62">62</a>, <a href="#Page_108">108</a>, <a href="#Page_109">109</a>, <a href="#Page_119">119</a>, <a href="#Page_142">142</a>.</dt> -<dt>Galloway, T., <a href="#Page_167">167</a>.</dt> -<dt>Gambart, <a href="#Page_128">128</a>.</dt> -<dt>Gauss, C. F., <a href="#Page_27">27</a>, <a href="#Page_98">98</a>, <a href="#Page_167">167</a>.</dt> -<dt>Gautier, A., <a href="#Page_45">45</a>.</dt> -<dt>Gemini, <a href="#Page_11">11</a>, <a href="#Page_194">194</a>.</dt> -<dt>Geminorum (Nova), <a href="#Page_194">194</a>.</dt> -<dt>Geminorum (ζ), <a href="#Page_180">180</a>, <a href="#Page_182">182</a>, <a href="#Page_185">185</a>.</dt> -<dt>George III., <a href="#Page_11">11</a>, <a href="#Page_23">23</a>.</dt> -<dt>Gill, Sir D., <a href="#Page_62">62</a>, <a href="#Page_136">136</a>, <a href="#Page_155">155</a>, <a href="#Page_157">157</a>, <a href="#Page_160">160</a>, <a href="#Page_161">161</a>, <a href="#Page_221">221</a>.</dt> -<dt>Glasgow Observatory, <a href="#Page_216">216</a>.</dt> -<dt>Goodricke, J., <a href="#Page_178">178</a>, <a href="#Page_183">183</a>.</dt> -<dt>Gore, J. E., <a href="#Page_24">24</a>, <a href="#Page_38">38</a>, <a href="#Page_179">179</a>, <a href="#Page_181">181</a>, <a href="#Page_182">182</a>, <a href="#Page_183">183</a>, <a href="#Page_192">192</a>, <a href="#Page_202">202</a>, <a href="#Page_215">215</a>, <a href="#Page_220">220</a>, <a href="#Page_225">225</a>, <a href="#Page_226">226</a>.</dt> -<dt>Gould, B. A., <a href="#Page_135">135</a>, <a href="#Page_160">160</a>, <a href="#Page_163">163</a>, <a href="#Page_180">180</a>, <a href="#Page_220">220</a>, <a href="#Page_221">221</a>.</dt> -<dt>Grant, R., <a href="#Page_216">216</a>.</dt> -<dt>Gravitation, law of, <a href="#Page_29">29</a>.</dt> -<dt>Greenwich Observatory, <a href="#Page_59">59</a>, <a href="#Page_117">117</a>.</dt> -<dt>Grimmler, <a href="#Page_192">192</a>.</dt> -<dt>Groombridge (1830), <a href="#Page_156">156</a>, <a href="#Page_162">162</a>, <a href="#Page_223">223</a>.</dt> -<dt>Groombridge (1618), <a href="#Page_156">156</a>.</dt> -<dt>Gruithuisen, <a href="#Page_87">87</a>.</dt> -</dl> -<dl class="index"> -<dt class="center b" id="index_H">H</dt> -<dt>Hale, G. E., <a href="#Page_55">55</a>, <a href="#Page_57">57</a>, <a href="#Page_233">233</a>.</dt> -<dt>Hall, A., <a href="#Page_96">96</a>, <a href="#Page_111">111</a>, <a href="#Page_112">112</a>, <a href="#Page_156">156</a>, <a href="#Page_190">190</a>.</dt> -<dt>Hall, Maxwell, <a href="#Page_121">121</a>.</dt> -<dt>Halley, E., <a href="#Page_138">138</a>.</dt> -<dt>Halley’s comet, <a href="#Page_123">123</a>, <a href="#Page_130">130</a>, <a href="#Page_152">152</a>.</dt> -<dt>Halm, J., <a href="#Page_195">195</a>, <a href="#Page_196">196</a>.</dt> -<dt>Hansen, P. A., <a href="#Page_61">61</a>, <a href="#Page_78">78</a>, <a href="#Page_79">79</a>.</dt> -<dt>Hansky, A., <a href="#Page_57">57</a>.</dt> -<dt>Harding, K. L., <a href="#Page_99">99</a>, <a href="#Page_153">153</a>, <a href="#Page_220">220</a>.</dt> -<dt>Hartwig, E., <a href="#Page_190">190</a>.</dt> -<dt class="pb" id="Page_242">242</dt> -<dt>Harvard Observatory, <a href="#Page_174">174</a>, <a href="#Page_175">175</a>, <a href="#Page_191">191</a>.</dt> -<dt>Hasselberg, B., <a href="#Page_148">148</a>, <a href="#Page_190">190</a>.</dt> -<dt>Heis, E., <a href="#Page_179">179</a>, <a href="#Page_220">220</a>.</dt> -<dt>Heliometer, <a href="#Page_153">153</a>, <a href="#Page_157">157</a>.</dt> -<dt>Helium stars, <a href="#Page_174">174</a>.</dt> -<dt>Helmholtz, H., <a href="#Page_61">61</a>, <a href="#Page_229">229</a>.</dt> -<dt>Hencke, K. L., <a href="#Page_99">99</a>.</dt> -<dt>Henderson, T., <a href="#Page_154">154</a>, <a href="#Page_155">155</a>.</dt> -<dt>Henry, Paul and Prosper, <a href="#Page_100">100</a>, <a href="#Page_114">114</a>, <a href="#Page_210">210</a>.</dt> -<dt>Hercules, <a href="#Page_167">167</a>.</dt> -<dt>Herculis (α), <a href="#Page_182">182</a>.</dt> -<dt>Herculis (λ), <a href="#Page_26">26</a>.</dt> -<dt>Herschel, William, <a href="#Page_1">1</a>-42, <a href="#Page_43">43</a>, <a href="#Page_60">60</a>, <a href="#Page_63">63</a>, <a href="#Page_65">65</a>, <a href="#Page_69">69</a>, <a href="#Page_74">74</a>, <a href="#Page_77">77</a>, <a href="#Page_85">85</a>, <a href="#Page_90">90</a>, <a href="#Page_99">99</a>, <a href="#Page_103">103</a>, <a href="#Page_109">109</a>, <a href="#Page_111">111</a>, <a href="#Page_112">112</a>, <a href="#Page_115">115</a>, <a href="#Page_123">123</a>, <a href="#Page_150">150</a>, <a href="#Page_162">162</a>, <a href="#Page_167">167</a>, <a href="#Page_176">176</a>, <a href="#Page_196">196</a>, <a href="#Page_197">197</a>, <a href="#Page_207">207</a>, <a href="#Page_214">214</a>, <a href="#Page_216">216</a>, <a href="#Page_218">218</a>, <a href="#Page_224">224</a>, <a href="#Page_227">227</a>.</dt> -<dt>Herschel, A., <a href="#Page_144">144</a>.</dt> -<dt>Herschel, Caroline, <a href="#Page_6">6</a>, <a href="#Page_8">8</a>, <a href="#Page_9">9</a>, <a href="#Page_12">12</a>, <a href="#Page_13">13</a>, <a href="#Page_14">14</a>, <a href="#Page_30">30</a>, <a href="#Page_35">35</a>, <a href="#Page_127">127</a>, <a href="#Page_198">198</a>.</dt> -<dt>Herschel, Sir J., <a href="#Page_4">4</a>, <a href="#Page_17">17</a>, <a href="#Page_27">27</a>, <a href="#Page_30">30</a>, <a href="#Page_37">37</a>, <a href="#Page_50">50</a>, <a href="#Page_112">112</a>, <a href="#Page_113">113</a>, <a href="#Page_120">120</a>, <a href="#Page_130">130</a>, <a href="#Page_144">144</a>, <a href="#Page_167">167</a>, <a href="#Page_187">187</a>, <a href="#Page_188">188</a>, <a href="#Page_197">197</a>, <a href="#Page_198">198</a>, <a href="#Page_199">199</a>, <a href="#Page_200">200</a>, <a href="#Page_214">214</a>, <a href="#Page_215">215</a>.</dt> -<dt>Hind, J. R., <a href="#Page_99">99</a>, <a href="#Page_129">129</a>, <a href="#Page_135">135</a>, <a href="#Page_180">180</a>, <a href="#Page_188">188</a>.</dt> -<dt>Hoek, <a href="#Page_135">135</a>.</dt> -<dt>Holden, E. S., <a href="#Page_191">191</a>.</dt> -<dt>Hough, G., <a href="#Page_105">105</a>.</dt> -<dt>Houzeau, J. C., <a href="#Page_220">220</a>.</dt> -<dt>Huggins, Lady, <a href="#Page_172">172</a>.</dt> -<dt>Huggins, Sir W., <a href="#Page_54">54</a>, <a href="#Page_57">57</a>, <a href="#Page_74">74</a>, <a href="#Page_95">95</a>, <a href="#Page_106">106</a>, <a href="#Page_114">114</a>, <a href="#Page_131">131</a>, <a href="#Page_136">136</a>, <a href="#Page_165">165</a>, <a href="#Page_170">170</a>, <a href="#Page_171">171</a>, <a href="#Page_172">172</a>, <a href="#Page_173">173</a>, <a href="#Page_189">189</a>, <a href="#Page_190">190</a>, <a href="#Page_191">191</a>, <a href="#Page_193">193</a>, <a href="#Page_195">195</a>, <a href="#Page_207">207</a>, <a href="#Page_208">208</a>, <a href="#Page_212">212</a>, <a href="#Page_231">231</a>.</dt> -<dt>Humboldt, A., <a href="#Page_44">44</a>, <a href="#Page_139">139</a>.</dt> -<dt>Hussey, W. J., <a href="#Page_116">116</a>, <a href="#Page_201">201</a>.</dt> -</dl> -<dl class="index"> -<dt class="center b" id="index_I">I</dt> -<dt>Innes, R., <a href="#Page_188">188</a>.</dt> -<dt>Intra-Mercurial planet, <a href="#Page_80">80</a>, <a href="#Page_81">81</a>.</dt> -<dt>Italian spectroscopists, <a href="#Page_54">54</a>, <a href="#Page_55">55</a>.</dt> -</dl> -<dl class="index"> -<dt class="center b" id="index_J">J</dt> -<dt>Janssen, P. J. C., <a href="#Page_52">52</a>, <a href="#Page_53">53</a>, <a href="#Page_54">54</a>, <a href="#Page_57">57</a>, <a href="#Page_59">59</a>, <a href="#Page_112">112</a>, <a href="#Page_209">209</a>.</dt> -<dt>Juno, <a href="#Page_19">19</a>, <a href="#Page_99">99</a>, <a href="#Page_101">101</a>.</dt> -<dt>Jupiter, <a href="#Page_20">20</a>, <a href="#Page_75">75</a>, <a href="#Page_97">97</a>, <a href="#Page_101">101</a>-108, <a href="#Page_112">112</a>, <a href="#Page_114">114</a>, <a href="#Page_121">121</a>, <a href="#Page_122">122</a>, <a href="#Page_135">135</a>, <a href="#Page_144">144</a>, <a href="#Page_146">146</a>.</dt> -<dt>Juvisy Observatory, <a href="#Page_164">164</a>.</dt> -</dl> -<dl class="index"> -<dt class="center b" id="index_K">K</dt> -<dt>Kaestner, <a href="#Page_65">65</a>, <a href="#Page_124">124</a>.</dt> -<dt>Kaiser, F., <a href="#Page_90">90</a>.</dt> -<dt>Kant, I., <a href="#Page_34">34</a>, <a href="#Page_35">35</a>, <a href="#Page_101">101</a>, <a href="#Page_103">103</a>.</dt> -<dt>Kapteyn, J. C., <a href="#Page_27">27</a>, <a href="#Page_158">158</a>, <a href="#Page_161">161</a>, <a href="#Page_162">162</a>, <a href="#Page_168">168</a>, <a href="#Page_221">221</a>, <a href="#Page_222">222</a>.</dt> -<dt>Keeler, J. E., <a href="#Page_95">95</a>, <a href="#Page_114">114</a>, <a href="#Page_185">185</a>, <a href="#Page_208">208</a>, <a href="#Page_211">211</a>, <a href="#Page_212">212</a>, <a href="#Page_213">213</a>, <a href="#Page_231">231</a>.</dt> -<dt>Kelvin, Lord, <a href="#Page_229">229</a>.</dt> -<dt>Kempf, P., <a href="#Page_181">181</a>.</dt> -<dt>Kepler, J., <a href="#Page_5">5</a>, <a href="#Page_35">35</a>, <a href="#Page_137">137</a>.</dt> -<dt>Kirchoff, G. R., <a href="#Page_61">61</a>, <a href="#Page_169">169</a>, <a href="#Page_172">172</a>.</dt> -<dt>Kirkwood, D., <a href="#Page_140">140</a>, <a href="#Page_230">230</a>.</dt> -<dt>Klein, H. J., <a href="#Page_73">73</a>.</dt> -<dt>Klinkerfues, E., <a href="#Page_142">142</a>, <a href="#Page_202">202</a>.</dt> -<dt>Konkoly, N., <a href="#Page_175">175</a>.</dt> -<dt>Küstner, F., <a href="#Page_88">88</a>.</dt> -</dl> -<dl class="index"> -<dt class="center b" id="index_L">L</dt> -<dt>Lalande, <a href="#Page_23">23</a>, <a href="#Page_152">152</a>.</dt> -<dt>Lambert, J. H., <a href="#Page_34">34</a>.</dt> -<dt>Lamont, J., <a href="#Page_44">44</a>.</dt> -<dt>Langley, S. P., <a href="#Page_77">77</a>.</dt> -<dt>Laplace, P. S., <a href="#Page_20">20</a>, <a href="#Page_33">33</a>, <a href="#Page_34">34</a>, <a href="#Page_77">77</a>, <a href="#Page_109">109</a>, <a href="#Page_148">148</a>, <a href="#Page_152">152</a>, <a href="#Page_195">195</a>, <a href="#Page_227">227</a>, <a href="#Page_228">228</a>, <a href="#Page_229">229</a>.</dt> -<dt>Lassell, W., <a href="#Page_112">112</a>, <a href="#Page_115">115</a>, <a href="#Page_117">117</a>, <a href="#Page_120">120</a>.</dt> -<dt>Latitude, variation of, <a href="#Page_88">88</a>, <a href="#Page_89">89</a>.</dt> -<dt>Leipzig Observatory, <a href="#Page_173">173</a>.</dt> -<dt>Leonid meteors, <a href="#Page_139">139</a>, <a href="#Page_140">140</a>, <a href="#Page_142">142</a>.</dt> -<dt>Leonis (β), <a href="#Page_183">183</a>.</dt> -<dt>Lescarbault, <a href="#Page_80">80</a>.</dt> -<dt>Le Verrier, U. J. J., <a href="#Page_61">61</a>, <a href="#Page_80">80</a>, <a href="#Page_81">81</a>, <a href="#Page_118">118</a>, <a href="#Page_119">119</a>, <a href="#Page_120">120</a>, <a href="#Page_142">142</a>, <a href="#Page_164">164</a>.</dt> -<dt>Leyden Observatory, <a href="#Page_91">91</a>.</dt> -<dt>Libræ (δ), <a href="#Page_180">180</a>.</dt> -<dt>Lick Observatory, <a href="#Page_93">93</a>, <a href="#Page_107">107</a>, <a href="#Page_166">166</a>, <a href="#Page_168">168</a>, <a href="#Page_191">191</a>, <a href="#Page_213">213</a>.</dt> -<dt>Light, extinction of, <a href="#Page_40">40</a>, <a href="#Page_215">215</a>, <a href="#Page_216">216</a>, <a href="#Page_224">224</a>, <a href="#Page_225">225</a>.</dt> -<dt>Lindsay, Lord, <a href="#Page_157">157</a>.</dt> -<dt>Linné, <a href="#Page_71">71</a>, <a href="#Page_72">72</a>.</dt> -<dt>Lockyer, Sir J. N., <a href="#Page_52">52</a>, <a href="#Page_53">53</a>, <a href="#Page_54">54</a>, <a href="#Page_55">55</a>, <a href="#Page_58">58</a>, <a href="#Page_149">149</a>, <a href="#Page_174">174</a>, <a href="#Page_191">191</a>, <a href="#Page_193">193</a>, <a href="#Page_195">195</a>, <a href="#Page_208">208</a>, <a href="#Page_209">209</a>, <a href="#Page_231">231</a>, <a href="#Page_232">232</a>.</dt> -<dt>Lockyer, W. J. S., <a href="#Page_210">210</a>.</dt> -<dt>Loewy, M., <a href="#Page_75">75</a>.</dt> -<dt>Lohrmann, W. G., <a href="#Page_68">68</a>, <a href="#Page_71">71</a>.</dt> -<dt>Lohse, W. O., <a href="#Page_88">88</a>, <a href="#Page_105">105</a>.</dt> -<dt>Loomis, <a href="#Page_188">188</a>.</dt> -<dt class="pb" id="Page_243">243</dt> -<dt>Lowell, P., <a href="#Page_83">83</a>, <a href="#Page_84">84</a>, <a href="#Page_86">86</a>, <a href="#Page_87">87</a>, <a href="#Page_91">91</a>, <a href="#Page_92">92</a>, <a href="#Page_93">93</a>, <a href="#Page_94">94</a>, <a href="#Page_122">122</a>, <a href="#Page_236">236</a>, <a href="#Page_237">237</a>.</dt> -<dt>Lowell Observatory, <a href="#Page_92">92</a>, <a href="#Page_94">94</a>, <a href="#Page_106">106</a>, <a href="#Page_114">114</a>.</dt> -<dt>Lund Observatory, <a href="#Page_59">59</a>.</dt> -<dt>Lupus, <a href="#Page_221">221</a>.</dt> -<dt>Luther, R., <a href="#Page_100">100</a>.</dt> -<dt>Lyra, <a href="#Page_221">221</a>.</dt> -<dt>Lyra (β), <a href="#Page_178">178</a>, <a href="#Page_182">182</a>, <a href="#Page_185">185</a>.</dt> -<dt>Lyrid meteors, <a href="#Page_122">122</a>.</dt> -</dl> -<dl class="index"> -<dt class="center b" id="index_M">M</dt> -<dt>Maclaurin, C., <a href="#Page_9">9</a>.</dt> -<dt>Maclear, Sir T., <a href="#Page_155">155</a>.</dt> -<dt>Mädler, J. H., <a href="#Page_27">27</a>, <a href="#Page_68">68</a>, <a href="#Page_69">69</a>, <a href="#Page_71">71</a>, <a href="#Page_96">96</a>, <a href="#Page_104">104</a>, <a href="#Page_202">202</a>, <a href="#Page_203">203</a>, <a href="#Page_216">216</a>, <a href="#Page_217">217</a>, <a href="#Page_218">218</a>.</dt> -<dt>Magellanic clouds, <a href="#Page_219">219</a>.</dt> -<dt>Magnetism, <a href="#Page_44">44</a>, <a href="#Page_60">60</a>.</dt> -<dt>Mars, <a href="#Page_18">18</a>, <a href="#Page_19">19</a>, <a href="#Page_90">90</a>-97, <a href="#Page_101">101</a>, <a href="#Page_144">144</a>, <a href="#Page_236">236</a>.</dt> -<dt>Maunder, E. W., <a href="#Page_59">59</a>, <a href="#Page_60">60</a>, <a href="#Page_94">94</a>, <a href="#Page_95">95</a>, <a href="#Page_134">134</a>, <a href="#Page_145">145</a>, <a href="#Page_166">166</a>, <a href="#Page_224">224</a>.</dt> -<dt>Mascari, A., <a href="#Page_86">86</a>.</dt> -<dt>Mayer, C., <a href="#Page_164">164</a>.</dt> -<dt>Mayer, J. R., <a href="#Page_229">229</a>.</dt> -<dt>Mazapil meteorite, <a href="#Page_149">149</a>.</dt> -<dt>Méchain, <a href="#Page_127">127</a>.</dt> -<dt>Mee, A., <a href="#Page_68">68</a>.</dt> -<dt>Melloni, <a href="#Page_76">76</a>.</dt> -<dt>Mercury, <a href="#Page_18">18</a>, <a href="#Page_80">80</a>, <a href="#Page_81">81</a>-84, <a href="#Page_97">97</a>, <a href="#Page_236">236</a>.</dt> -<dt>Messier, C., <a href="#Page_30">30</a>.</dt> -<dt>Meteorites, <a href="#Page_147">147</a>, <a href="#Page_148">148</a>, <a href="#Page_149">149</a>, <a href="#Page_229">229</a>, <a href="#Page_231">231</a>.</dt> -<dt>‘Meteoritic Hypothesis,’ <a href="#Page_231">231</a>, <a href="#Page_232">232</a>.</dt> -<dt>Meteors, <a href="#Page_138">138</a>-149.</dt> -<dt>Meudon Observatory, <a href="#Page_59">59</a>.</dt> -<dt>Milan Observatory, <a href="#Page_82">82</a>, <a href="#Page_202">202</a>, <a href="#Page_224">224</a>.</dt> -<dt>Milky Way. See Galaxy.</dt> -<dt>Miller, W. A., <a href="#Page_50">50</a>, <a href="#Page_172">172</a>.</dt> -<dt>Mira Ceti, <a href="#Page_11">11</a>, <a href="#Page_182">182</a>, <a href="#Page_186">186</a>, <a href="#Page_187">187</a>.</dt> -<dt>Mitchel, O. M., <a href="#Page_31">31</a>.</dt> -<dt>Mizar (ζ Ursæ Majoris), <a href="#Page_204">204</a>.</dt> -<dt>Möller, A., <a href="#Page_129">129</a>.</dt> -<dt>Moon, the, <a href="#Page_10">10</a>, <a href="#Page_24">24</a>, <a href="#Page_65">65</a>-79, <a href="#Page_90">90</a>, <a href="#Page_95">95</a>, <a href="#Page_148">148</a>, <a href="#Page_228">228</a>.</dt> -<dt>Moscow Observatory, <a href="#Page_132">132</a>.</dt> -<dt>Mouchez, A., <a href="#Page_161">161</a>.</dt> -<dt>Müller, G., <a href="#Page_175">175</a>, <a href="#Page_181">181</a>.</dt> -<dt>Munich Observatory, <a href="#Page_44">44</a>, <a href="#Page_224">224</a>.</dt> -</dl> -<dl class="index"> -<dt class="center b" id="index_N">N</dt> -<dt>Napoleon, <a href="#Page_67">67</a>, <a href="#Page_127">127</a>.</dt> -<dt>Nasmyth, J., <a href="#Page_73">73</a>, <a href="#Page_103">103</a>.</dt> -<dt>Nebulæ, <a href="#Page_30">30</a>, <a href="#Page_31">31</a>, <a href="#Page_207">207</a>-213, <a href="#Page_228">228</a>.</dt> -<dt>Nebular Hypothesis, <a href="#Page_33">33</a>, <a href="#Page_195">195</a>, <a href="#Page_227">227</a>, <a href="#Page_228">228</a>, <a href="#Page_229">229</a>, <a href="#Page_230">230</a>, <a href="#Page_233">233</a>.</dt> -<dt>Neison (Nevill), E., <a href="#Page_73">73</a>.</dt> -<dt>Neisten, <a href="#Page_86">86</a>, <a href="#Page_105">105</a>.</dt> -<dt>Neptune, <a href="#Page_120">120</a>, <a href="#Page_121">121</a>, <a href="#Page_135">135</a>, <a href="#Page_229">229</a>, <a href="#Page_230">230</a>.</dt> -<dt>Newall, H. F., <a href="#Page_205">205</a>.</dt> -<dt>Newcomb, S., <a href="#Page_27">27</a>, <a href="#Page_64">64</a>, <a href="#Page_78">78</a>, <a href="#Page_89">89</a>, <a href="#Page_94">94</a>, <a href="#Page_162">162</a>, <a href="#Page_168">168</a>, <a href="#Page_220">220</a>, <a href="#Page_222">222</a>, <a href="#Page_223">223</a>, <a href="#Page_224">224</a>, <a href="#Page_232">232</a>.</dt> -<dt>Newton, H. A., <a href="#Page_140">140</a>, <a href="#Page_141">141</a>.</dt> -<dt>Newton, Sir I., <a href="#Page_2">2</a>, <a href="#Page_17">17</a>, <a href="#Page_29">29</a>, <a href="#Page_77">77</a>.</dt> -<dt>Nichol, J. P., <a href="#Page_31">31</a>, <a href="#Page_207">207</a>.</dt> -<dt>Nordvig, L., <a href="#Page_192">192</a>.</dt> -</dl> -<dl class="index"> -<dt class="center b" id="index_O">O</dt> -<dt>Olbers, H. W. M., <a href="#Page_19">19</a>, <a href="#Page_20">20</a>, <a href="#Page_69">69</a>, <a href="#Page_98">98</a>, <a href="#Page_99">99</a>, <a href="#Page_123">123</a>, <a href="#Page_124">124</a>, <a href="#Page_125">125</a>, <a href="#Page_126">126</a>, <a href="#Page_127">127</a>, <a href="#Page_129">129</a>, <a href="#Page_130">130</a>, <a href="#Page_139">139</a>, <a href="#Page_148">148</a>, <a href="#Page_152">152</a>, <a href="#Page_153">153</a>.</dt> -<dt>Olbers’ comet, <a href="#Page_125">125</a>.</dt> -<dt>Olmsted, D., <a href="#Page_138">138</a>, <a href="#Page_207">207</a>.</dt> -<dt>Ophiuchi (α), <a href="#Page_183">183</a>.</dt> -<dt>Orion, <a href="#Page_221">221</a>.</dt> -<dt>Orion nebula, <a href="#Page_10">10</a>, <a href="#Page_33">33</a>, <a href="#Page_207">207</a>, <a href="#Page_208">208</a>, <a href="#Page_209">209</a>, <a href="#Page_213">213</a>.</dt> -<dt>Orion stars, <a href="#Page_174">174</a>, <a href="#Page_193">193</a>, <a href="#Page_209">209</a>, <a href="#Page_232">232</a>.</dt> -<dt>Orionis (κ), <a href="#Page_209">209</a>.</dt> -<dt>Orionis (η), <a href="#Page_209">209</a>.</dt> -<dt>Orionis (θ), <a href="#Page_209">209</a>.</dt> -<dt>Orionis (U), <a href="#Page_181">181</a>, <a href="#Page_182">182</a>, <a href="#Page_186">186</a>, <a href="#Page_187">187</a>.</dt> -</dl> -<dl class="index"> -<dt class="center b" id="index_P">P</dt> -<dt>Palisa, J., <a href="#Page_100">100</a>.</dt> -<dt>Pallas, <a href="#Page_19">19</a>, <a href="#Page_99">99</a>, <a href="#Page_101">101</a>.</dt> -<dt>Parallax, solar, <a href="#Page_61">61</a>, <a href="#Page_62">62</a>, <a href="#Page_63">63</a>, <a href="#Page_101">101</a>.</dt> -<dt>Parallax, stellar, <a href="#Page_150">150</a>-158, <a href="#Page_190">190</a>.</dt> -<dt>Paris Congresses, <a href="#Page_161">161</a>.</dt> -<dt>Paris Observatory, <a href="#Page_78">78</a>, <a href="#Page_118">118</a>, <a href="#Page_171">171</a>.</dt> -<dt>Perrine, C. D., <a href="#Page_81">81</a>, <a href="#Page_108">108</a>, <a href="#Page_194">194</a>.</dt> -<dt>Perrine’s comet, <a href="#Page_136">136</a>.</dt> -<dt>Perrotin, H., <a href="#Page_86">86</a>, <a href="#Page_91">91</a>, <a href="#Page_100">100</a>, <a href="#Page_114">114</a>.</dt> -<dt>Peck, W., <a href="#Page_162">162</a>.</dt> -<dt>Persei (Nova), <a href="#Page_192">192</a>, <a href="#Page_193">193</a>, <a href="#Page_194">194</a>, <a href="#Page_195">195</a>.</dt> -<dt>Perseid meteors, <a href="#Page_122">122</a>, <a href="#Page_141">141</a>.</dt> -<dt>Perseus, <a href="#Page_192">192</a>, <a href="#Page_221">221</a>.</dt> -<dt>Peters, C. H. F., <a href="#Page_100">100</a>.</dt> -<dt>Peters, C. A. F., <a href="#Page_142">142</a>, <a href="#Page_153">153</a>, <a href="#Page_155">155</a>, <a href="#Page_202">202</a>.</dt> -<dt>Photography, astronomical, <a href="#Page_54">54</a>, <a href="#Page_56">56</a>, <a href="#Page_57">57</a>, <a href="#Page_59">59</a>, <a href="#Page_75">75</a>, <a href="#Page_81">81</a>, <a href="#Page_94">94</a>, <a href="#Page_108">108</a>, <a href="#Page_113">113</a>, <a href="#Page_136">136</a>, <a href="#Page_158">158</a>, <a href="#Page_160">160</a>, <a href="#Page_161">161</a>, <a href="#Page_172">172</a>, <a href="#Page_175">175</a>, <a href="#Page_192">192</a>, <a href="#Page_193">193</a>, <a href="#Page_194">194</a>, <a href="#Page_203">203</a>, <a href="#Page_208">208</a>, <a href="#Page_209">209</a>, <a href="#Page_210">210</a>, <a href="#Page_211">211</a>, <a href="#Page_212">212</a>.</dt> -<dt class="pb" id="Page_244">244</dt> -<dt>Photometry, <a href="#Page_176">176</a>, <a href="#Page_177">177</a>.</dt> -<dt>Piazzi, G., <a href="#Page_19">19</a>, <a href="#Page_20">20</a>, <a href="#Page_98">98</a>, <a href="#Page_150">150</a>.</dt> -<dt>Pickering, E. C., <a href="#Page_174">174</a>, <a href="#Page_175">175</a>, <a href="#Page_176">176</a>, <a href="#Page_177">177</a>, <a href="#Page_181">181</a>, <a href="#Page_182">182</a>, <a href="#Page_183">183</a>, <a href="#Page_185">185</a>, <a href="#Page_193">193</a>, <a href="#Page_194">194</a>, <a href="#Page_203">203</a>, <a href="#Page_204">204</a>, <a href="#Page_220">220</a>, <a href="#Page_221">221</a>.</dt> -<dt>Pickering, W. H., <a href="#Page_75">75</a>, <a href="#Page_76">76</a>, <a href="#Page_81">81</a>, <a href="#Page_91">91</a>, <a href="#Page_92">92</a>, <a href="#Page_93">93</a>, <a href="#Page_107">107</a>, <a href="#Page_113">113</a>, <a href="#Page_209">209</a>.</dt> -<dt>Plana, G., <a href="#Page_78">78</a>, <a href="#Page_79">79</a>.</dt> -<dt>Pleiades, <a href="#Page_124">124</a>, <a href="#Page_210">210</a>, <a href="#Page_217">217</a>.</dt> -<dt>Pogson, N. R., <a href="#Page_142">142</a>, <a href="#Page_180">180</a>.</dt> -<dt>Pole Star, <a href="#Page_205">205</a>.</dt> -<dt>Pollux, <a href="#Page_165">165</a>, <a href="#Page_170">170</a>.</dt> -<dt>Pons, J. L., <a href="#Page_127">127</a>.</dt> -<dt>Pontécoulant, <a href="#Page_79">79</a>.</dt> -<dt>Potsdam Observatory, <a href="#Page_46">46</a>, <a href="#Page_173">173</a>, <a href="#Page_176">176</a>.</dt> -<dt>Pritchard, C., <a href="#Page_158">158</a>, <a href="#Page_177">177</a>.</dt> -<dt>Proctor, R. A., <a href="#Page_4">4</a>, <a href="#Page_20">20</a>, <a href="#Page_38">38</a>, <a href="#Page_41">41</a>, <a href="#Page_90">90</a>, <a href="#Page_91">91</a>, <a href="#Page_104">104</a>, <a href="#Page_148">148</a>, <a href="#Page_163">163</a>, <a href="#Page_164">164</a>, <a href="#Page_218">218</a>, <a href="#Page_219">219</a>, <a href="#Page_220">220</a>, <a href="#Page_231">231</a>.</dt> -<dt>Procyon (α Canis Minoris), <a href="#Page_151">151</a>, <a href="#Page_203">203</a>.</dt> -<dt>Prominences, solar, <a href="#Page_52">52</a>, <a href="#Page_53">53</a>, <a href="#Page_55">55</a>, <a href="#Page_64">64</a>.</dt> -<dt>Puiseux, P., <a href="#Page_75">75</a>.</dt> -<dt>Pulkowa Observatory, <a href="#Page_200">200</a>.</dt> -</dl> -<dl class="index"> -<dt class="center b" id="index_Q">Q</dt> -<dt>Quetelet, A., <a href="#Page_139">139</a>.</dt> -</dl> -<dl class="index"> -<dt class="center b" id="index_R">R</dt> -<dt>Radiant points, meteoric, <a href="#Page_139">139</a>, <a href="#Page_144">144</a>, <a href="#Page_145">145</a>, <a href="#Page_146">146</a>.</dt> -<dt>Ranyard, A. C., <a href="#Page_106">106</a>, <a href="#Page_146">146</a>.</dt> -<dt>Red spot on Jupiter, <a href="#Page_105">105</a>, <a href="#Page_106">106</a>.</dt> -<dt>Regulus (α Leonis), <a href="#Page_164">164</a>, <a href="#Page_165">165</a>.</dt> -<dt>Relative parallax, <a href="#Page_157">157</a>.</dt> -<dt>Réseau, Photospherique, <a href="#Page_59">59</a>.</dt> -<dt>Resisting medium, <a href="#Page_128">128</a>.</dt> -<dt>Respighi, L., <a href="#Page_55">55</a>.</dt> -<dt>Reversing layer, <a href="#Page_56">56</a>, <a href="#Page_57">57</a>.</dt> -<dt>Ricco, A., <a href="#Page_87">87</a>, <a href="#Page_105">105</a>.</dt> -<dt>Rigel (β Orionis), <a href="#Page_165">165</a>, <a href="#Page_209">209</a>.</dt> -<dt>Ritchey, G., <a href="#Page_194">194</a>.</dt> -<dt>Roberts, A. W., <a href="#Page_181">181</a>.</dt> -<dt>Roberts, I., <a href="#Page_209">209</a>, <a href="#Page_210">210</a>.</dt> -<dt>Roche, E., <a href="#Page_109">109</a>.</dt> -<dt>Roman College Observatory, <a href="#Page_85">85</a>.</dt> -<dt>Rosse, third Earl of, <a href="#Page_141">141</a>, <a href="#Page_156">156</a>, <a href="#Page_207">207</a>, <a href="#Page_208">208</a>, <a href="#Page_218">218</a>.</dt> -<dt>Rosse, fourth Earl of, <a href="#Page_77">77</a>.</dt> -<dt>Rotation of the Sun, <a href="#Page_58">58</a>, <a href="#Page_59">59</a>;</dt> -<dd>of the planets, <a href="#Page_82">82</a>, <a href="#Page_83">83</a>, <a href="#Page_84">84</a>, <a href="#Page_85">85</a>, <a href="#Page_86">86</a>, <a href="#Page_87">87</a>, <a href="#Page_104">104</a>, <a href="#Page_111">111</a>, <a href="#Page_112">112</a>.</dd> -<dt>Rowland, H. A., <a href="#Page_52">52</a>.</dt> -<dt>Rutherfurd, L. M., <a href="#Page_75">75</a>, <a href="#Page_169">169</a>.</dt> -</dl> -<dl class="index"> -<dt class="center b" id="index_S">S</dt> -<dt>Sabine, Sir E., <a href="#Page_44">44</a>.</dt> -<dt>Safford, T. H., <a href="#Page_202">202</a>.</dt> -<dt>Santini, G., <a href="#Page_159">159</a>.</dt> -<dt>Savary, F., <a href="#Page_30">30</a>, <a href="#Page_199">199</a>.</dt> -<dt>Satellites, <a href="#Page_96">96</a>, <a href="#Page_107">107</a>, <a href="#Page_108">108</a>, <a href="#Page_112">112</a>, <a href="#Page_113">113</a>, <a href="#Page_115">115</a>, <a href="#Page_120">120</a>, <a href="#Page_121">121</a>, <a href="#Page_236">236</a>.</dt> -<dt>Saturn, <a href="#Page_20">20</a>, <a href="#Page_21">21</a>, <a href="#Page_22">22</a>, <a href="#Page_97">97</a>, <a href="#Page_103">103</a>, <a href="#Page_108">108</a>-113, <a href="#Page_121">121</a>, <a href="#Page_135">135</a>.</dt> -<dt>Schaeberle, J. M., <a href="#Page_93">93</a>, <a href="#Page_107">107</a>, <a href="#Page_191">191</a>, <a href="#Page_203">203</a>.</dt> -<dt>Scheiner, C., <a href="#Page_44">44</a>.</dt> -<dt>Scheiner, J., <a href="#Page_166">166</a>, <a href="#Page_174">174</a>, <a href="#Page_176">176</a>.</dt> -<dt>Schiaparelli, G. V., <a href="#Page_82">82</a>, <a href="#Page_83">83</a>, <a href="#Page_84">84</a>, <a href="#Page_85">85</a>, <a href="#Page_86">86</a>, <a href="#Page_87">87</a>, <a href="#Page_91">91</a>, <a href="#Page_92">92</a>, <a href="#Page_114">114</a>, <a href="#Page_141">141</a>, <a href="#Page_143">143</a>, <a href="#Page_149">149</a>, <a href="#Page_201">201</a>, <a href="#Page_220">220</a>, <a href="#Page_224">224</a>, <a href="#Page_231">231</a>, <a href="#Page_235">235</a>.</dt> -<dt>Schjellerup, H., <a href="#Page_180">180</a>.</dt> -<dt>Schmidt, J. F. J., <a href="#Page_69">69</a>, <a href="#Page_70">70</a>, <a href="#Page_71">71</a>, <a href="#Page_72">72</a>, <a href="#Page_73">73</a>, <a href="#Page_104">104</a>, <a href="#Page_179">179</a>, <a href="#Page_180">180</a>, <a href="#Page_189">189</a>.</dt> -<dt>Schönfeld, E., <a href="#Page_160">160</a>, <a href="#Page_179">179</a>, <a href="#Page_180">180</a>, <a href="#Page_188">188</a>, <a href="#Page_189">189</a>.</dt> -<dt>Schröter, J. H., <a href="#Page_16">16</a>, <a href="#Page_65">65</a>, <a href="#Page_66">66</a>, <a href="#Page_67">67</a>, <a href="#Page_68">68</a>, <a href="#Page_69">69</a>, <a href="#Page_70">70</a>, <a href="#Page_74">74</a>, <a href="#Page_81">81</a>, <a href="#Page_82">82</a>, <a href="#Page_84">84</a>, <a href="#Page_85">85</a>, <a href="#Page_86">86</a>, <a href="#Page_87">87</a>, <a href="#Page_97">97</a>, <a href="#Page_99">99</a>, <a href="#Page_153">153</a>.</dt> -<dt>Schwabe, S. H., <a href="#Page_18">18</a>, <a href="#Page_43">43</a>, <a href="#Page_44">44</a>, <a href="#Page_46">46</a>, <a href="#Page_55">55</a>.</dt> -<dt>Schwassman, A., <a href="#Page_100">100</a>.</dt> -<dt>Secchi, A., <a href="#Page_52">52</a>, <a href="#Page_54">54</a>, <a href="#Page_55">55</a>, <a href="#Page_60">60</a>, <a href="#Page_72">72</a>, <a href="#Page_90">90</a>, <a href="#Page_114">114</a>, <a href="#Page_141">141</a>, <a href="#Page_170">170</a>, <a href="#Page_171">171</a>, <a href="#Page_173">173</a>.</dt> -<dt>Secchi’s types of stellar spectra, <a href="#Page_170">170</a>, <a href="#Page_171">171</a>, <a href="#Page_173">173</a>, <a href="#Page_174">174</a>, <a href="#Page_175">175</a>, <a href="#Page_189">189</a>, <a href="#Page_232">232</a>.</dt> -<dt>See, T. J. J., <a href="#Page_201">201</a>, <a href="#Page_202">202</a>, <a href="#Page_236">236</a>.</dt> -<dt>Seeliger, H., <a href="#Page_110">110</a>, <a href="#Page_195">195</a>, <a href="#Page_196">196</a>, <a href="#Page_202">202</a>, <a href="#Page_206">206</a>, <a href="#Page_224">224</a>, <a href="#Page_225">225</a>.</dt> -<dt>Serviss, G. P., <a href="#Page_158">158</a>.</dt> -<dt>Sirius (α Canis Majoris), <a href="#Page_151">151</a>, <a href="#Page_170">170</a>, <a href="#Page_173">173</a>, <a href="#Page_188">188</a>, <a href="#Page_202">202</a>, <a href="#Page_225">225</a>.</dt> -<dt>Slipher, V. M., <a href="#Page_106">106</a>, <a href="#Page_114">114</a>, <a href="#Page_204">204</a>.</dt> -<dt>Sime, J., <a href="#Page_27">27</a>.</dt> -<dt class="pb" id="Page_245">245</dt> -<dt>Sola, J. C., <a href="#Page_112">112</a>.</dt> -<dt>Solar cluster, <a href="#Page_221">221</a>, <a href="#Page_222">222</a>.</dt> -<dt>Solar system, motion of, <a href="#Page_26">26</a>, <a href="#Page_27">27</a>, <a href="#Page_167">167</a>, <a href="#Page_168">168</a>.</dt> -<dt>South, Sir J., <a href="#Page_198">198</a>.</dt> -<dt>Spectroscopic binaries, <a href="#Page_203">203</a>, <a href="#Page_204">204</a>, <a href="#Page_205">205</a>.</dt> -<dt>Spencer, H., <a href="#Page_218">218</a>.</dt> -<dt>Spica (α Virginis), <a href="#Page_204">204</a>.</dt> -<dt>Spörer, F. W. G., <a href="#Page_45">45</a>, <a href="#Page_46">46</a>, <a href="#Page_54">54</a>, <a href="#Page_59">59</a>.</dt> -<dt>Star-catalogues, <a href="#Page_159">159</a>, <a href="#Page_160">160</a>, <a href="#Page_161">161</a>, <a href="#Page_162">162</a>.</dt> -<dt>Star-clusters, <a href="#Page_30">30</a>, <a href="#Page_31">31</a>, <a href="#Page_32">32</a>, <a href="#Page_206">206</a>, <a href="#Page_210">210</a>.</dt> -<dt>Star-drift, <a href="#Page_164">164</a>.</dt> -<dt>Star-gauging, <a href="#Page_36">36</a>, <a href="#Page_40">40</a>, <a href="#Page_41">41</a>, <a href="#Page_224">224</a>.</dt> -<dt>Stars, distance of, <a href="#Page_150">150</a>-158.</dt> -<dt>Stars, distribution of, <a href="#Page_35">35</a>, <a href="#Page_39">39</a>, <a href="#Page_40">40</a>, <a href="#Page_198">198</a>-214.</dt> -<dt>Stars, double, <a href="#Page_28">28</a>, <a href="#Page_29">29</a>, <a href="#Page_30">30</a>, <a href="#Page_197">197</a>-206.</dt> -<dt>Stars, gaseous, <a href="#Page_171">171</a>, <a href="#Page_174">174</a>.</dt> -<dt>Stars, proper motion of, <a href="#Page_162">162</a>, <a href="#Page_163">163</a>, <a href="#Page_164">164</a>, <a href="#Page_165">165</a>.</dt> -<dt>Stars, radial motion of, <a href="#Page_165">165</a>, <a href="#Page_166">166</a>.</dt> -<dt>Stars, temporary, <a href="#Page_156">156</a>, <a href="#Page_182">182</a>, <a href="#Page_188">188</a>-196.</dt> -<dt>Stars, triple and multiple, <a href="#Page_206">206</a>.</dt> -<dt>Stars, variable, <a href="#Page_177">177</a>-188.</dt> -<dt>Stellar spectra, <a href="#Page_169">169</a>-176, <a href="#Page_187">187</a>, <a href="#Page_189">189</a>, <a href="#Page_190">190</a>, <a href="#Page_191">191</a>, <a href="#Page_193">193</a>, <a href="#Page_194">194</a>.</dt> -<dt>Stellar universe, <a href="#Page_35">35</a>-42, <a href="#Page_214">214</a>, <a href="#Page_215">215</a>-226.</dt> -<dt>Stereo-comparator, <a href="#Page_100">100</a>, <a href="#Page_101">101</a>.</dt> -<dt>Stokes, Sir G., <a href="#Page_50">50</a>.</dt> -<dt>Stone, E. J., <a href="#Page_157">157</a>, <a href="#Page_160">160</a>.</dt> -<dt>Stroobant, P., <a href="#Page_88">88</a>.</dt> -<dt>Struve, F. G. W., <a href="#Page_3">3</a>, <a href="#Page_37">37</a>, <a href="#Page_38">38</a>, <a href="#Page_40">40</a>, <a href="#Page_42">42</a>, <a href="#Page_128">128</a>, <a href="#Page_151">151</a>, <a href="#Page_153">153</a>, <a href="#Page_200">200</a>, <a href="#Page_214">214</a>, <a href="#Page_215">215</a>, <a href="#Page_216">216</a>, <a href="#Page_218">218</a>.</dt> -<dt>Struve, H., <a href="#Page_201">201</a>.</dt> -<dt>Struve, L., <a href="#Page_27">27</a>, <a href="#Page_163">163</a>, <a href="#Page_167">167</a>.</dt> -<dt>Struve, O. W., <a href="#Page_27">27</a>, <a href="#Page_110">110</a>, <a href="#Page_115">115</a>, <a href="#Page_120">120</a>, <a href="#Page_153">153</a>, <a href="#Page_156">156</a>, <a href="#Page_163">163</a>, <a href="#Page_200">200</a>, <a href="#Page_201">201</a>.</dt> -<dt>Stumpe, O., <a href="#Page_167">167</a>.</dt> -<dt>Sun, <a href="#Page_15">15</a>, <a href="#Page_16">16</a>, <a href="#Page_17">17</a>, <a href="#Page_40">40</a>, <a href="#Page_43">43</a>-64, <a href="#Page_65">65</a>, <a href="#Page_80">80</a>, <a href="#Page_81">81</a>, <a href="#Page_105">105</a>, <a href="#Page_125">125</a>, <a href="#Page_128">128</a>, <a href="#Page_170">170</a>, <a href="#Page_222">222</a>, <a href="#Page_228">228</a>, <a href="#Page_229">229</a>, <a href="#Page_230">230</a>, <a href="#Page_237">237</a>.</dt> -<dt>Swift, L., <a href="#Page_81">81</a>.</dt> -<dt>Swift’s comet, <a href="#Page_136">136</a>.</dt> -</dl> -<dl class="index"> -<dt class="center b" id="index_T">T</dt> -<dt>Tacchini, P., <a href="#Page_55">55</a>, <a href="#Page_86">86</a>, <a href="#Page_87">87</a>.</dt> -<dt>Taurus, <a href="#Page_217">217</a>, <a href="#Page_221">221</a>.</dt> -<dt>Tempel, E., <a href="#Page_210">210</a>.</dt> -<dt>Tennyson, <a href="#Page_96">96</a>.</dt> -<dt>Tidal friction, <a href="#Page_79">79</a>, <a href="#Page_87">87</a>, <a href="#Page_233">233</a>, <a href="#Page_234">234</a>, <a href="#Page_235">235</a>, <a href="#Page_236">236</a>.</dt> -<dt>Tisserand, F. F., <a href="#Page_146">146</a>.</dt> -<dt>Todd, D. P., <a href="#Page_122">122</a>.</dt> -<dt>Trans-Neptunian planet, <a href="#Page_121">121</a>, <a href="#Page_122">122</a>.</dt> -<dt>Trouvelot, E., <a href="#Page_86">86</a>, <a href="#Page_87">87</a>.</dt> -<dt>Tschermak, <a href="#Page_148">148</a>.</dt> -<dt>Tulse Hill Observatory, <a href="#Page_171">171</a>.</dt> -<dt>Turner, H. H., <a href="#Page_194">194</a>, <a href="#Page_224">224</a>.</dt> -<dt>Twining, A. C., <a href="#Page_139">139</a>.</dt> -</dl> -<dl class="index"> -<dt class="center b" id="index_U">U</dt> -<dt>Upsala Observatory, <a href="#Page_59">59</a>.</dt> -<dt>Uranometria Argentina, <a href="#Page_160">160</a>.</dt> -<dt>Uranus, <a href="#Page_11">11</a>, <a href="#Page_20">20</a>, <a href="#Page_22">22</a>, <a href="#Page_23">23</a>, <a href="#Page_97">97</a>, <a href="#Page_113">113</a>, <a href="#Page_114">114</a>, <a href="#Page_115">115</a>, <a href="#Page_118">118</a>, <a href="#Page_121">121</a>, <a href="#Page_135">135</a>, <a href="#Page_141">141</a>, <a href="#Page_230">230</a>.</dt> -<dt>Ursa Major, <a href="#Page_162">162</a>, <a href="#Page_164">164</a>.</dt> -<dt>Ursæ Majoris (δ), <a href="#Page_182">182</a>.</dt> -<dt>Ursæ Majoris (ξ), <a href="#Page_199">199</a>.</dt> -</dl> -<dl class="index"> -<dt class="center b" id="index_V">V</dt> -<dt>Venus, <a href="#Page_18">18</a>, <a href="#Page_84">84</a>-88, <a href="#Page_97">97</a>, <a href="#Page_235">235</a>, <a href="#Page_236">236</a>.</dt> -<dt>Venus, transits of, <a href="#Page_61">61</a>, <a href="#Page_62">62</a>, <a href="#Page_87">87</a>.</dt> -<dt>Vega (α Lyræ), <a href="#Page_151">151</a>, <a href="#Page_165">165</a>, <a href="#Page_170">170</a>, <a href="#Page_172">172</a>, <a href="#Page_173">173</a>.</dt> -<dt>Very, F. W., <a href="#Page_77">77</a>.</dt> -<dt>Vesta, <a href="#Page_19">19</a>, <a href="#Page_99">99</a>, <a href="#Page_101">101</a>, <a href="#Page_102">102</a>.</dt> -<dt>Vogel, H. C., <a href="#Page_84">84</a>, <a href="#Page_88">88</a>, <a href="#Page_95">95</a>, <a href="#Page_102">102</a>, <a href="#Page_106">106</a>, <a href="#Page_114">114</a>, <a href="#Page_131">131</a>, <a href="#Page_148">148</a>, <a href="#Page_166">166</a>, <a href="#Page_173">173</a>, <a href="#Page_174">174</a>, <a href="#Page_175">175</a>, <a href="#Page_183">183</a>, <a href="#Page_184">184</a>, <a href="#Page_185">185</a>, <a href="#Page_190">190</a>, <a href="#Page_191">191</a>, <a href="#Page_193">193</a>, <a href="#Page_195">195</a>, <a href="#Page_204">204</a>, <a href="#Page_209">209</a>, <a href="#Page_232">232</a>, <a href="#Page_233">233</a>, <a href="#Page_237">237</a>.</dt> -<dt>Vulcan, <a href="#Page_81">81</a>.</dt> -</dl> -<dl class="index"> -<dt class="center b" id="index_W">W</dt> -<dt>Washington Observatory, <a href="#Page_96">96</a>, <a href="#Page_223">223</a>.</dt> -<dt>Watson, J. C., <a href="#Page_81">81</a>, <a href="#Page_100">100</a>.</dt> -<dt>Webb, T. W., <a href="#Page_72">72</a>, <a href="#Page_73">73</a>, <a href="#Page_104">104</a>.</dt> -<dt>Weinek, L., <a href="#Page_75">75</a>.</dt> -<dt>Weiss, E., <a href="#Page_142">142</a>.</dt> -<dt>Well’s comet, <a href="#Page_134">134</a>.</dt> -<dt>Whewell, W., <a href="#Page_218">218</a>.</dt> -<dt>Williams, A. S., <a href="#Page_110">110</a>, <a href="#Page_193">193</a>.</dt> -<dt>Wilson, A., <a href="#Page_16">16</a>.</dt> -<dt>Winlock, J., <a href="#Page_177">177</a>.</dt> -<dt>Winnecke, F. A. T., <a href="#Page_131">131</a>.</dt> -<dt>Witt, K. G., <a href="#Page_101">101</a>.</dt> -<dt class="pb" id="Page_246">246</dt> -<dt>Wolf, Max, <a href="#Page_100">100</a>, <a href="#Page_181">181</a>, <a href="#Page_191">191</a>, <a href="#Page_194">194</a>, <a href="#Page_211">211</a>.</dt> -<dt>Wolf, R., <a href="#Page_44">44</a>, <a href="#Page_45">45</a>, <a href="#Page_188">188</a>.</dt> -<dt>Wolf and Rayet, <a href="#Page_171">171</a>.</dt> -<dt>Wolf-Rayet stars, <a href="#Page_171">171</a>, <a href="#Page_174">174</a>.</dt> -<dt>Wollaston, W. H., <a href="#Page_48">48</a>.</dt> -<dt>Wright, T., <a href="#Page_34">34</a>, <a href="#Page_110">110</a>.</dt> -</dl> -<dl class="index"> -<dt class="center b" id="index_Y">Y</dt> -<dt>Yale Observatory, <a href="#Page_157">157</a>.</dt> -<dt>Yerkes Observatory, <a href="#Page_55">55</a>, <a href="#Page_111">111</a>, <a href="#Page_202">202</a>.</dt> -<dt>Young, C. A., <a href="#Page_54">54</a>, <a href="#Page_56">56</a>, <a href="#Page_57">57</a>, <a href="#Page_58">58</a>, <a href="#Page_60">60</a>, <a href="#Page_87">87</a>, <a href="#Page_114">114</a>, <a href="#Page_190">190</a>.</dt> -</dl> -<dl class="index"> -<dt class="center b" id="index_Z">Z</dt> -<dt>Zach, F. X., <a href="#Page_97">97</a>, <a href="#Page_98">98</a>, <a href="#Page_152">152</a>.</dt> -<dt>Zantedeschi, <a href="#Page_77">77</a>.</dt> -<dt>Zenger, <a href="#Page_85">85</a>, <a href="#Page_88">88</a>.</dt> -<dt>Zöllner, J. C. F., <a href="#Page_54">54</a>, <a href="#Page_58">58</a>, <a href="#Page_60">60</a>, <a href="#Page_84">84</a>, <a href="#Page_103">103</a>, <a href="#Page_132">132</a>, <a href="#Page_232">232</a>.</dt> -</dl> -<h3 id="c15">CORRIGENDA.</h3> -<dl class="undent"><dt>P. 30, l. 5, <i>for</i> “objects” <i>read</i> “orbits.”</dt> -<dt>P. 36, l. 13, <i>for</i> “unable” <i>read</i> “able.”</dt> -<dt>P. 61, l. 17, <i>for</i> “8″.371” <i>read</i> “8″.571.”</dt> -<dt>P. 63, l. 21, <i>for</i> “bases” <i>read</i> “gases.”</dt> -<dt>P. 100, l. 16, <i>for</i> “Schwussmann” <i>read</i> “Schwassmann.”</dt> -<dt>P. 167, l. 28, <i>for</i> “Strumpe” <i>read</i> “Stumpe.”</dt> -<dt>P. 184, l. 11, <i>for</i> “star-variables” <i>read</i> “variable stars.”</dt> -<dt>P. 199, l. 23, <i>for</i> “2102” <i>read</i> “1202.”</dt></dl> -<p class="center smaller">THE END.</p> -<p class="center smallest">PRINTED BY WILLIAM BLACKWOOD AND SONS.</p> -<h2>Transcriber’s Notes</h2> -<ul> -<li>Silently corrected a few typos, and incorporated the corrigenda into the text.</li> -<li>Retained publication information from the printed edition: this eBook is public-domain in the country of publication.</li> -<li>In the text versions only, text in italics is delimited by _underscores_.</li> -</ul> - - - - - - - -<pre> - - - - - -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-h.htm or 63615-h.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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