FOOTNOTES:

In 1870, Mr. Ranyard[1091]—whose death, December 14, 1894, was a serious loss to astronomy—acting upon an earlier suggestion of Sir William Huggins, collected records of unusual appearances on the disc of Jupiter, with a view to investigate the question of their recurrence at regular intervals. He concluded that the development of the deeper tinges of colour, and of the equatorial "port-hole" markings girdling the globe in regular alternations of bright and dusky, agreed, so far as could be ascertained, with epochs of sun-spot maximum. The further inquiries of Dr. Lohse at Bothkamp in 1873[1092]went to strengthen the coincidence, which had been anticipatedà prioriby Zöllner in 1871.[1093]Moreover, separate and distinct evidence was alleged by Mr. Denning in 1899 of decennial outbreaks of disturbance in north temperate regions.[1094]It may, indeed, be taken for granted that what Hahn terms the universal pulse of the solar system[1095]affects the vicissitudes of Jupiter; but the law of those vicissitudes is far from being so obviously subordinate to the rhythmical flow of central disturbance as are certain terrestrial phenomena. The great planet, being in fact himself a "semi-sun," may be regarded as an originator, no less than a recipient, of agitating influences, the combined effects of which may well appear insubordinate to any obvious law.

It is likely that Saturn is in a still earlier stage of planetary development than Jupiter. He is the lightest for his size of all the planets. In fact, he would float in water. And since his density is shown, by the amount of his equatorial bulging, to increase centrally,[1096]it follows that his superficial materials must be of a specific gravity so low as to be inconsistent, on any probable supposition, with the solid or liquid states. Moreover, the chief arguments in favour of the high temperature of Jupiter, apply, with increased force, to Saturn; so that it may be concluded, without much risk of error, that a large proportion of his bulky globe, 73,000 miles in diameter, is composed of heated vapours, kept in active and agitated circulation by the process of cooling.

His unique set of appendages has, since the middle of the last century, formed the subject of searching and fruitful inquiries, both theoretical and telescopic. The mechanical problem of the stability of Saturn's rings was left by Laplace in a very unsatisfactory condition. Considering them as rotating solid bodies, he pointed out that they could not maintain their position unless their weight were in some way unsymmetrically distributed; but made no attempt to determine the kind or amount of irregularity needed to secure this end. Some observations by Herschel gave astronomers an excuse for taking for granted the fulfilment of the condition thus vaguely postulated; and the question remained in abeyance until once more brought prominently forward by the discovery of the dusky ring in 1850.

The younger Bond led the way, among modern observers, in denying the solidity of the structure. The fluctuations in its aspect were, he asserted in 1851,[1097]inconsistent with such a hypothesis. The fine dark lines of division, frequently detected in both bright rings, and as frequently relapsing into imperceptibility, were due, in his opinion, to the real nobility of their particles, and indicated a fluid formation. Professor Benjamin Peirce of Harvard University immediately followed with a demonstration, on abstract grounds, of their non-solidity.[1098]Streams of some fluid denser than water were, he maintained, the physical reality giving rise to the anomalous appearance first disclosed by Galileo's telescope.

The mechanism of Saturn's rings, proposed as the subject of the Adams Prize, was dealt with by James Clerk Maxwell in 1857. His investigation forms the groundwork of all that is at present known in the matter. Its upshot was to show that neither solid nor fluid rings could continue to exist, and that the only possible composition of the system was by an aggregated multitude of unconnected particles, each revolving independently in a periodcorresponding to its distance from the planet.[1099]This idea of a satellite-formation had been, remarkably enough, several times entertained and lost sight of. It was first put forward by Roberval in the seventeenth century, again by Jacques Cassini in 1715, and with perfect definiteness by Wright of Durham in 1750.[1100]Little heed, however, was taken of these casual anticipations of a truth which reappeared, a virtual novelty, as the legitimate outcome of the most refined modern methods.

The details of telescopic observation accord, on the whole, admirably with this hypothesis. The displacements or disappearance of secondary dividing-lines—the singular striated appearance, first remarked by Short in the eighteenth century, last by Perrotin and Lockyer at Nice, March 18, 1884[1101]—show the effects of waves of disturbance traversing a moving mass of gravitating particles;[1102]the broken and changing line of the planet's shadow on the ring gives evidence of variety in the planes of the orbits described by those particles. The whole ring-system, too, appears to be somewhat elliptical.[1103]

The satellite-theory has derived unlooked-for support from photometric inquiries. Professor Seeliger pointed out in 1888[1104]that the unvarying brilliancy of the outer rings under all angles of illumination, from 0° to 30°, can be explained from no other point of view. Nor does the constitution of the obscure inner ring offer any difficulty. For it is doubtless formed of similar small bodies to those aggregated in the lucid members of the system, only much more thinly strewn, and reflecting, consequently, much less light. It is not, indeed, at first easy to see why these sparser flights should show as a dense dark shading on the body of Saturn. Yet this is invariably the case. The objection has been urged by Professor Hastings of Baltimore. The brightest parts of these appendages, he remarked,[1105]are more lustrous than the globe they encircle; but if the inner ring consists of identical materials, possessing presumably an equal reflective capacity, the mere fact of their scanty distribution would not cause them to show as dark against the same globe. Professor Seeliger, however, replied[1106]that the darkening is due to the never-ending swarms of their separateshadows transiting the planet's disc. Sunlight is not, indeed, wholly excluded. Many rays come and go between the open ranks of the meteorites. For the dusky ring is transparent. The planet it encloses shows through it, as if veiled with a strip of crape. A beautiful illustration of its quality in this respect was derived by Professor Barnard from an eclipse of Japetus, November 1, 1889.[1107]The eighth moon remained steadily visible during its passage through the shadow of the inner ring, but with a progressive loss of lustre in approaching its bright neighbour. There was no breach of continuity. The satellite met no gap, corresponding to that between the dusky ring and the body of Saturn, through which it could shine with undiminished light, but was slowly lost sight of as it plunged into deeper and deeper gloom. The important facts were thus established, that the brilliant and obscure rings merge into each other, and that the latter thins out towards the Saturnian globe.

The meteoric constitution of these appendages was beautifully demonstrated in 1895 by Professor Keeler,[1108]then director of the Alleghany Observatory, Pittsburgh. From spectrographs taken with the slit adjusted to coincidence with the equatorial plane of the system, he determined the comparative radial velocities of its different parts. And these supply a crucial test of Clerk Maxwell's theory. For if the rings were solid, the swiftest rates of rotation should be at their outer edges, corresponding to wider circles described in the same period; while, if they are pulverulent, the inverse relation must hold good. This proved to be actually the case. The motion slowed off outward, in agreement with the diminishing speed of particles travelling freely, each in its own orbit. Keeler's result was promptly confirmed by Campbell,[1109]as well as by Deslandres and Bélopolsky.

A question of singular interest, and one which we cannot refrain from putting to ourselves, is—whether we see in the rings of Saturn a finished structure, destined to play a permanent part in the economy of the system; or whether they represent merely a stage in the process of development out of the chaotic state in which it is impossible to doubt that the materials of all planets were originally merged. M. Otto Struve attempted to give a definite answer to this important query.

A study of early and later records of observations disclosed to him, in 1851, an apparent progressive approach of the inner edge of the bright ring to the planet. The rate of approach he estimated at about fifty-seven English miles a year, or 11,000 miles during the194 years elapsed since the time of Huygens.[1110]Were it to continue, a collapse of the system must be far advanced within three centuries. But was the change real or illusory—a plausible, but deceptive inference from insecure data? M. Struve resolved to put it to the test. A set of elaborately careful micrometrical measures of the dimensions of Saturn's rings, executed by himself at Pulkowa in the autumn of 1851, was provided as a standard of future comparison; and he was enabled to renew them, under closely similar circumstances, in 1882.[1111]But the expected diminution of the space between Saturn's globe and his rings had not taken place. A slight extension in the width of the system, both outward and inward, was indeed, hinted at; and it is worth notice that just such a separation of the rings was indicated by Clerk Maxwell's theory, so that there is anà priorilikelihood of its being in progress. Yet Hall's measures in 1884-87[1112]failed to supply evidence of alteration with time; and Barnard's, executed at Lick in 1894-95,[1113]showed no sensible divergence from them. Hence, much weight cannot be laid upon Huygens's drawings and descriptions, which had been held to prove conclusively a partial filling up, since 1657, of the interval between the ring and the planet.[1114]

The rings of Saturn replace, in Professor G. H. Darwin's view,[1115]an abortive satellite, scattered by tidal action into annular form. For they lie closer to the planet than is consistent with the integrity of a revolving body of reasonable bulk. The limit of possible existence for such a mass was fixed by Roche of Montpellier, in 1848,[1116]at 2·44 mean radii of its primary; while the outer edge of the ring-system is distant 2·38 radii of Saturn from his centre. The virtual discovery of its pulverulent condition dates, then, according to Professor Darwin, from 1848. He conjectures that the appendage will eventually disappear, partly through the dispersal of its constituent particles inward, and their subsidence upon the planet's surface, partly by their dispersal outward, to a region beyond "Roche's limit," where coalescence might proceed unhindered by the strain of unequal attractions. One modest satellite, revolving inside Mimas, would then be all that was left of the singular appurtenances we now contemplate with admiration.

There seems reason to admit that Kirkwood's law of commensurability has had some effect in bringing about the present distribution of the matter composing them. Here the influentialbodies are Saturn's moons, while the divisions and boundaries of the rings represent the spaces where their disturbing action conspires to eliminate revolving particles. Kirkwood, in fact, showed, in 1867,[1117]that a body circulating in the chasm between the bright rings known as "Cassini's division," would have a period nearly commensurable with those offourout of the eight moons; and Meyer of Geneva subsequently calculated all such combinations, with the result of bringing out coincidences between regions of maximum perturbation and the limiting and dividing lines of the system.[1118]This is in itself a strong confirmation of the view that the rings are made up of independently revolving small bodies.

On December 7, 1876, Professor Asaph Hall discovered at Washington a bright equatorial spot on Saturn, which he followed and measured through above sixty rotations, each performed in ten hours fourteen minutes twenty-four seconds.[1119]This, he was careful to add, represented the period, not necessarily of theplanet, but only of the individual spot. The only previous determination of Saturn's axial movement (setting aside some insecure estimates by Schröter) was Herschel's in 1794, giving a period of ten hours sixteen minutes. The substantial accuracy of Hall's result was verified by Mr. Denning in 1891.[1120]In May and June of that year, ten vague bright markings near the equator were watched by Mr. Stanley Williams, who derived from them a rotation period only two seconds shorter than that determined at Washington. Nevertheless, similarly placed spots gave in 1892 and 1893 notably quicker rates;[1121]so that the task of timing the general drift of the Saturnian surface by the displacements of such objects is hampered, to an indefinite extent, by their individual proper motions.

Saturn's outermost satellite, Japetus, is markedly variable—so variable that it sends us, when brightest, just 4-1/2 times as much light as when faintest. Moreover, its fluctuations depend upon its orbital position in such a way as to make it a conspicuous telescopic object when west, a scarcely discernible one when east of the planet. Herschel's inference[1122]of a partially obscured globe turning always the same face towards its primary seems the only admissible one, and is confirmed by Pickering's measurements of the varying intensity of its light. He remarked further that the dusky and brilliant hemispheres must be so posited as to divide the disc, viewed from Saturn, into nearly equal parts; so that this Saturnian moon,even when "full," appears very imperfectly illuminated over one-half of its surface.[1123]

Zöllner estimated the albedo of Saturn at 0·51, Müller at 0·88, a value impossibly high, considering that the spectrum includes no vestige of original emissions. Closely similar to that of Jupiter, it shows the distinctive dark line in the red (wave-length 618), which we may call the "red-star line"; and Janssen, from the summit of Etna in 1867[1124]found traces in it of aqueous absorption. The light from the ring appears to be pure reflected sunshine unmodified by original atmospheric action.[1125]

Uranus, when favourably situated, can easily be seen with the naked eye as a star between the fifth and sixth magnitudes. There is indeed, some reason to suppose that he had been detected as a wandering orb by savage "watchers of the skies" in the Pacific long before he swam into Herschel's ken. Nevertheless, inquiries into his physical habitudes are still in an early stage. They are exceedingly difficult of execution, even with the best and largest modern telescopes; and their results remain clouded with uncertainty.

It will be remembered that Uranus presents the unusual spectacle of a system of satellites travelling nearly at right angles to the plane of the ecliptic. The existence of this anomaly gives a special interest to investigations of his axial movement, which might be presumed, from the analogy of the other planets, to be executed in the same tilted plane. Yet this is far from being certainly the case.

Mr. Buffham in 1870-72 caught traces of bright markings on the Uranian disc, doubtfully suggesting a rotation in about twelve hours in a planenotcoincident with that in which his satellites circulate.[1126]Dusky bands resembling those of Jupiter, but very faint, were barely perceptible to Professor Young at Princeton in 1883. Yet, though almost necessarily inferred to be equatorial, they made a considerable angle with the trend of the satellites' orbits.[1127]More distinctly by the brothers Henry, with the aid of their fine refractor, two gray parallel rulings, separated by a brilliant zone, were discerned every clear night at Paris from January to June, 1884.[1128]What were taken to be the polar regions appeared comparatively dusky. The direction of the equatorial rulings (for so we may safely call them) made an angle of 40° with the satellites' line of travel. Similar observations were made at Nice by MM. Perrotin and Thollon, March to June, 1884, a lucid spot near the equator, in addition, indicatingrotation in a period of about ten hours.[1129]The discrepancy was, however, considerably reduced by Perrotin's study of the planet in 1889 with the new 30-inch equatoreal.[1130]The dark bands, thus viewed to better advantage than in 1884, appeared to deviate no more than 10° from the satellites' orbit-plane. No definitive results, on the other hand, were derived by Professors Holden, Schaeberle, and Keeler from their observations of Uranus in 1889-90 with the potent instrument on Mount Hamilton. Shadings, it is true, were almost always, though faintly, seen; but they appeared under an anomalous, possibly an illusory aspect. They consisted, not of parallel, but of forked bands.[1131]

Measurements of the little sea-green disc which represents to us the massive bulk of Uranus, by Young, Schiaparelli,[1132]Safarik, H. C. Wilson[1133]and Perrotin, prove it to be quite distinctlybulged. The compression at once caught Barnard's trained eye in 1894,[1134]when he undertook at Lick a micrometrical investigation of the system; and he was surprised to perceive that the major axis of the elliptical surface made an angle of about 28° with the line of travel pursued by the satellites. Nothing more can be learned on this curious subject for some years, since the pole of the planet is just now turned nearly towards the earth; but Barnard's conclusion is unlikely to be seriously modified. He fixed the mean diameter of Uranus at 34,900 miles. But this estimate was materially reduced through Dr. See's elimination of irradiative effects by means of daylight measures, executed at Washington in 1901.[1135]

The visual spectrum of this planet was first examined by Father Secchi in 1869, and later, with more advantages for accuracy, by Huggins, Vogel,[1136]and Keeler.[1137]It is a very remarkable one. In lieu of the reflected Fraunhofer lines, imperceptible perhaps through feebleness of light, six broad bands of original absorption appear, one corresponding to the blue-green ray of hydrogen (F), another to the "red-star line" of Jupiter and Saturn, the rest as yet unidentified. The hydrogen band seems much too strong and diffuse to be the mere echo of a solar line, and might accordingly be held to imply the presence of free hydrogen in the Uranian atmosphere. This, however, would be difficult of reconcilement with Keeler's identification of an absorption-group in the yellow with a telluric waterband.

Notwithstanding its high albedo—0·62, according to Zöllner—proof is wanting that any of the light of Uranus is inherent. Mr. Albert Taylor announced, indeed, in 1889, his detection, with Common's giant reflector, of bright flutings in its spectrum;[1138]but Professor Keeler's examination proved them to be merely contrast effects.[1139]Sir William and Lady Huggins, moreover, obtained about the same time a photograph purely solar in character. The spectrum it represented was crossed by numerous Fraunhofer lines, and by no others. It was, then, presumably composed entirely of reflected light.

Judging from the indications of an almost evanescent spectrum, Neptune, as regards physical condition, is the twin of Uranus, as Saturn of Jupiter. Of the circumstances of his rotation we are as good as completely ignorant. Mr. Maxwell Hall, indeed, noticed at Jamaica, in November and December, 1883, certain rhythmical fluctuations of brightness, suggesting revolution on an axis in slightly less than eight hours;[1140]but Professor Pickering reduces the supposed variability to an amount altogether too small for certain perception, and Dr. G. Müller denies its existencein toto. It is true their observations were not precisely contemporaneous with those of Mr. Hall[1141]who believes the partial obscurations recorded by himself to have been of a passing kind, and to have suddenly ceased after a fortnight of prevalence. Their less conspicuous renewal was visible to him in November, 1884, confirming a rotation period of 7·92 hours.

It was ascertained at first by indirect means that the orbit of Neptune's satellite is inclined about 20° to his equator. Mr. Marth[1142]having drawn attention to the rapid shifting of its plane of motion, M. Tisserand and Professor Newcomb[1143]independently published the conclusion that such shifting necessarily results from Neptune's ellipsoidal shape. The movement is of the kind exemplified—although with inverted relations—in the precession of the equinoxes. The pole of the satellite, owing to the pull of Neptune's equatorial protuberance, describes a circle round the pole of his equator in a retrograde direction, and in a period of over five hundred years. The amount of compression indicated for the primary body is, at the outside, 1/85; whence it can be inferred that Neptune possesses a lower rotatory velocity than the other giant planets. Directverification of the trend theoretically inferred for the satellite's movement was obtained by Dr. See in 1899. The Washington 26-inch refractor disclosed to him, under exceptionally favourable conditions, a set of equatorial belts on the disc of Neptune, and they took just the direction prescribed by theory. Their objective reality cannot be doubted, although Barnard was unable, either with the Lick or the Yerkes telescope,[1144]to detect any definite markings on this planet. Its diameter was found by him to be 32,900 miles.

The possibility that Neptune may not be the most remote body circling round the sun has been contemplated ever since he has been known to exist. Within the last few years the position at a given epoch of a planet far beyond his orbital verge has been approximately fixed by two separate investigators.

Professor George Forbes of Edinburgh adopted in 1880 a novel plan of search for unknown members of the solar system, the first idea of which was thrown out by M. Flammarion in November, 1879.[1145]It depends upon the movements of comets. It is well known that those of moderately short periods are, for a reason already explained, connected with the larger planets in such a way that the cometary aphelia fall near some planetary orbit. Jupiter claims a large retinue of such partial dependents, Neptune owns six, and there are two considerable groups, the farthest distances of which from the sun lie respectively near 100 and 300 times that of the earth. At each of these vast intervals, one involving a period of 1,000, the other of 5,000 years, Professor Forbes maintains that an unseen planet circulates. He even computed elements for the nearer of the two, and fixed its place on the celestial sphere;[1146]but the photographic searches made for it by Dr. Roberts at Crowborough and by Mr. Wilson at Daramona proved unavailing. Undeterred by Deichmüller's discouraging opinion that cometary orbits extending beyond the recognised bounds of the solar system are too imperfectly known to serve as the basis of trustworthy conclusions,[1147]the Edinburgh Professor returned to the attack in 1901.[1148]He now sought to prove that the lost comet of 1556 actually returned in 1844, but with elements so transformed by ultra-Neptunian perturbations as to have escaped immediate identification. If so, the "wanted" planet has just entered the sign Libra, and, being larger than Jupiter, should be possible to find.

Almost simultaneously with Forbes, Professor Todd set aboutgroping for the same object by the help of a totally different set of indications. Adams's approved method commended itself to him; but the hypothetical divagations of Neptune having scarcely yet had time to develop, he was thrown back upon the "residual errors" of Uranus. They gave him a virtually identical situation for the new planet with that derived from the clustering of cometary aphelia.[1149]Yet its assigned distance was little more than half that of the nearer of Professor Forbes's remote pair, and it completed a revolution in 375 instead of 1,000 years. The agreement in them between the positions determined, on separate grounds, for the ultra-Neptunian traveller was merely an odd coincidence; nor can we be certain, until it is seen, that we have really got into touch with it.

FOOTNOTES:[965]Phil. Trans., vol. lxxiv., p. 260.[966]Novæ Observationes, p. 105.[967]Phil. Trans., vol. i., p. 243.[968]Mém. de l'Ac., 1720, p. 146.[969]Phil. Trans., vol. lxxiv., p. 273.[970]A large work, entitledAreographische Fragmente, in which Schröter embodied the results of his labours on Mars, 1785-1803, narrowly escaped the conflagration of 1813, and was published at Leyden in 1881.[971]Beiträge, p. 124.[972]Mem. R. A. Soc., vol. xxxii., p. 183.[973]Astr. Nach., No. 1,468.[974]Observatory, vol. viii., p. 437.[975]Month. Not., vols. xxviii., p. 37; xxix., p. 232; xxxiii., p. 552.[976]Flammarion,L'Astronomie, t. i., p. 266.[977]Smyth,Cel. Cycle, vol. i., p. 148 (1st ed.).[978]Phil. Trans., vol. cxxi., p. 417.[979]Month. Not., vol. xxv., p. 227.[980]Phil. Mag., vol. xxxiv., p. 75.[981]Proctor,Quart. Jour. of Science, vol. x., p. 185; Maunder,Sunday Mag., January, February, March, 1882; Campbell,Publ. Astr. Pac. Soc., vol. vi., p. 273.[982]Am. Jour. of Sc., vol. xxviii., p. 163.[983]Burton,Trans. Roy. Dublin Soc., vol. i., 1880, p. 169.[984]Month. Not., vol. xxvii., p. 179;Astroph. Journ., vol. i., p. 193.[985]Untersuchungen über die Spectra der Planeten, p. 20;Astroph. Journ., vol. i., p. 203.[986]Publ. Astr. Pac. Soc., vols. vi., p. 228; ix., p. 109;Astr. and Astroph., vol. xiii., p. 752;Astroph. Jour., vol. ii., p. 28.[987]Ibid., vol. v., p. 328.[988]Ibid., vols. i., p. 311; iii., p. 254.[989]C. Christiansen,Beiblätter, 1886, p. 532.[990]Astr. and Astrophysics, vol. xi., p. 671.[991]Flammarion,La Planète Mars, p. 574.[992]Mémoires Couronnés, t. xxxix.[993]Lockyer,Nature, vol. xlvi., p. 447.[994]Mem. Spettr. Italiani, t. xi., p. 28.[995]Bull. Astr., t. iii., p. 324.[996]Journ. Brit. Astr. Ass., vol. i., p. 88.[997]Publ. Pac. Astr. Soc., vol. ii., p. 299; Percival Lowell,Mars, 1896;Annals of the Lowell Observatory, vol. ii., 1900.[998]Old and New Astr., p. 545.[999]L'Astronomie, t. xi., p. 445.[1000]La Planète Mars, p. 588.[1001]Month. Notices, vol. lvi., p. 166.[1002]L'Astronomie, t. viii.[1003]Astr. Nach., No. 3,271;Astr. and Astrophysics, vol. xiii., p. 716.[1004]Month. Not., vol. xxxviii., p. 41;Mem. Roy. Astr. Soc., vol. xliv., p. 123.[1005]Astr. and Astrophysics, vol. xi., p. 668.[1006]Ibid., p. 850.[1007]Comptes Rendus, t. cxv., p. 379.[1008]Astr. Jour., No. 384;Publ. Astr. Pac. Soc., vol. vi., p. 109.Cf. Observatoryvol. xvii., pp. 295-336.[1009]See Mr. Wentworth Erck's remarks inTrans. Roy. Dublin Soc., vol. i., p. 29.[1010]Month. Not., vol. xxxviii., p. 206.[1011]Annals Harvard Coll. Obs., vol. xi., pt. ii., p. 217.[1012]Young,Gen. Astr., p. 366.[1013]Campbell,Publ. Pac. Astr. Soc., vol. vi., p. 270.[1014]Astr. Nach., No. 3,319.[1015]Witch of Atlas, stanza iii. I am indebted to Dr. Garnett for the reference.[1016]Recommended by Chandler,Astr. Jour., No. 452.[1017]Harvard Circulars, Nos. 36, 37, 51.[1018]Astr. Nach., No. 3,687.[1019]Montangerand,Comptes Rendus, March 11, 1901.[1020]Pickering,Astroph. Jour., vol. xiii., p. 277.[1021]Harvard Circular, No. 58.[1022]Astr. Nach., No. 752.[1023]L. Niesten,Annuaire, Bruxelles, 1881, p. 269.[1024]According to Svedstrup (Astr. Nach., Nos. 2,240-41), the inclination to the ecliptic of the "mean asteroid's" orbit is = 6°.[1025]Smiths.Report, 1876, p. 358;The Asteroids(Kirkwood), p. 42, 1888.[1026]Tisserand,Annuaire, Paris, 1891, p. B. 15; Newcomb,Astr. Jour., No. 477; Backlund,Bull. Astr., t. xvii., p. 81; Parmentier,Bull. Soc. Astr. de France, March, 1896; Observatory, vol. xviii., p. 207.[1027]Berberich,Astr. Nach., No. 3,088.[1028]Bull. Astr., t. xviii., p. 39.[1029]The Asteroids, p. 48;Publ. Astr. Pac. Soc., vols. ii., p. 48; iii., p. 95.[1030]Comptes Rendus, t. xxxvii., p. 797.[1031]Bull. Astr., t. v., p. 180.[1032]Annuaire, Bruxelles, 1881, p. 243.[1033]Johns Hopkins Un. Circular, January, 1895;Observatory, vol. xviii., p. 127.[1034]Harvard Annals, vol. xi., part ii., p. 294.[1035]Astr. Nach., Nos. 2,724-5.[1036]Month. Not., vol. lxi., p. 69.[1037]Astroph. Jour., vol. vii., p. 25.[1038]Spectra der Planeten, p. 24.[1039]Tome i., p. 93.[1040]Berlinische Monatsschrift, 1785, p. 211.[1041]Month. Not., vol. xiii., p. 40.[1042]Mem. Am. Ac., vol. viii., p. 221.[1043]Photom. Unters., p. 303.[1044]Astr. Nach., No. 1,851.[1045]Mém. de l'Ac., t. x., p. 514.[1046]Ibid., 1692, p. 7.[1047]Month. Not., vol. xliv., p. 63.[1048]Photom. Unters., pp. 165, 273;Potsdam Publ., No. 30.[1049]Vogel,Sp. der Planeten, p. 33,note.[1050]Proc. Roy. Soc., vol. xviii., p. 250.[1051]Month. Not., vol. xl., p. 433.[1052]Sitzungsberichte, Berlin, 1895, ii., p. 15.[1053]The anomalous shadow-effects recorded by Webb (Cel. Objects, p. 170, 4th ed.) are obviously of atmospheric and optical origin.[1054]Engelmann,Ueber die Helligkeitsverhältnisse der Jupiterstrabanten, p. 59.[1055]Month. Not., vol. xxviii., p. 11.[1056]Observatory, vol. vii., p. 175.[1057]Month. Not., vol. xlviii., p. 43.[1058]Publ. Astr. Pac. Soc., vol. ii., p. 296.[1059]Pickering failed to obtain any photometric evidence of their variability.Harvard Annals, vol. xi., p. 245.[1060]Astr. and Astroph., vol. xii., pp. 194, 481.[1061]Annals Lowell Obs., vol. ii., pt. i.[1062]Astr. Nach., Nos. 2,995, 3,206;Month. Not., vols. li., p. 556; liv., p. 134. Barnard remains convinced that the oval forms attributed to Jupiter's satellites are illusory effects of their markings.Astr. Nach., Nos. 3,206, 3,453;Astr. and Astroph., vol. xiii., p. 272.[1063]Publ. Astr. Pac. Soc., vol. iii., p. 355.[1064]Astr. Nach., No. 1,017.[1065]Publ. Astr. Pac. Soc., vol. iii., p. 359.[1066]Astr. Nach., No. 3,432.[1067]Astr. Jour., Nos. 275, 325, 367, 472;Observatory, vol. xv., p. 425.[1068]Tisserand,Comptes Rendus, October 8, 1894; Cohn,Astr. Nach., No. 3,404.[1069]Bull. Ac. R. Bruxelles, t. xlviii., p. 607.[1070]Astr. Nach., No. 2,294.[1071]Ibid., No. 2,284.[1072]Denning,Month. Not., vol. xliv., pp. 64, 66;Nature, vol. xxv., p. 226.[1073]Sidereal Mess., December, 1886, p. 289.[1074]Astr. Nach., Nos. 2,280, 2,282.[1075]Month. Not., vol. xlvi., p. 117.[1076]Proc. Roy. Soc. N. S. Wales, vol. xiv., p. 68.[1077]Phil. Trans., vol. i., p. 143.[1078]For indications relative to the early history of the red spot, see Holden,Publ. Astr. Pac. Soc., vol. ii., p. 77; Noble,Month. Not., vol. xlvii., p. 515; A. S. Williams,Observatory, vol. xiii., p. 338.[1079]Astr. and Astrophysics, vol. xi., p. 192.[1080]Month. Not., vol. l., p. 520.[1081]Observatory, vol. xiii., pp. 297, 326.[1082]Trans. R. Dublin Soc., vol. iv., p. 271, 1889.[1083]Publ. Astr. Pac. Soc., vol. ii., p. 289.[1084]Astr. and Astrophysics, vol. xi., p. 686.[1085]Denning,Knowledge, vol. xxiii., p. 200;Observatory, vol. xxiv., p. 312;Pop. Astr., vol. ix., p. 448;Nature, vol. lv., p. 89.[1086]Williams,Observatory, vol. xxiii., p. 282.[1087]Month. Not., vol. lvi., p. 143.[1088]Bélopolsky,Astr. Nach., No. 3,326.[1089]Publ. Astr. Pac. Soc., vol. iv., p. 176.[1090]Bull. Astr., 1900, p. 70.[1091]Month. Not., vol. xxxi., p. 34.[1092]Beobachtungen, Heft ii., p. 99.[1093]Ber. Sächs. Ges. der Wiss., 1871, p. 553.[1094]Month. Not., vol. lix., p. 76.[1095]Beziehungen der Sonnenfleckenperiode, p. 175.[1096]A. Hall,Astr. Nach., No. 2,269.[1097]Astr. Jour.(Gould's), vol. ii., p. 17.[1098]Ibid., p. 5.[1099]On the Stability of the Motion of Saturn's Rings, p. 67.[1100]Mém. de l'Ac., 1715, p. 47; Montucla,Hist. des Math., t. iv., p. 19;An Original Theory of the Universe, p. 115.[1101]Comptes Rendus, t. xcviii., p. 718.[1102]Proctor,Saturn and its System(1865), p. 125.[1103]Perrotin,Comptes Rendus, t. cvi., p. 1716.[1104]Abhandl. Akad. der Wiss., Munich, Bd. xvi., p. 407.[1105]Smiths. Report, 1880 (Holden).[1106]Quoted by Dr. E. Anding,Astr. Nach., No. 2,881.[1107]Astr. and Astrophysics, vol. xi., p. 119;Month. Not., vol. l., p. 108.[1108]Astroph. Jour., vol. i., p. 416.[1109]Ibid., vol. ii., p. 127.[1110]Mém. de l'Ac. Imp.(St. Petersb.), t. vii., 1853, p. 464.[1111]Astr. Nach., No. 2,498.[1112]Washington Observations, App. ii., p. 22[1113]Month. Not., vol. lvi., p. 163.[1114]T. Lewis,Observatory, vol. xviii., p. 379.[1115]Harper's Magazine, June, 1889.[1116]Mém. de l'Acad. de Montpellier, t. viii., p. 296, 1873.[1117]Meteoric Astronomy, chap. xii. He carried the subject somewhat farther in 1871. SeeObservatory, vol. vi., p. 335.[1118]Astr. Nach., No. 2,527.[1119]Amer. Jour. of Sc., vol. xiv., p. 325.[1120]Observatory, vol. xiv., p. 369.[1121]Month. Not., vol. liv., p. 297.[1122]Phil. Trans., vol. lxxxii., p. 14.[1123]Smiths. Report, 1880.[1124]Comptes Rendus, t. lxiv., p. 1304.[1125]Huggins,Proc. R. Soc., vol. xlvi., p. 231; Keeler,Astr. Nach., No. 2,927; Vogel,Astroph. Jour., vol. i., p. 278.[1126]Month. Not., vol. xxxiii., p. 164.[1127]Astr. Nach., No 2,545.[1128]Comptes Rendus, t. xcviii., p. 1419.[1129]Comptes Rendus, t. xcviii., pp. 718, 967.[1130]V. J. S. Astr. Ges., Jahrg. xxiv., p. 267.[1131]Publ. Astr. Pac. Soc., vol. iii., p. 287.[1132]Astr. Nach., No. 2,526.[1133]Ibid., No. 2,730.[1134]Astr. Jour., Nos. 370, 374.[1135]Astr. Nach., No. 3,768.[1136]Ann. der Phys., Bd. clviii., p. 470;Astroph. Jour., vol. i., p. 280.[1137]Astr. Nach., No. 2,927.[1138]Month. Not., vol. xlix., p. 405.[1139]Astr. Nach., No. 2,927; Scheiner'sSpectralanalyse, p. 221.[1140]Month. Not., vol. xliv., p. 257.[1141]Observatory, vol. vii., pp. 134, 221, 264.[1142]Month. Not., vol. xlvi., p. 507.[1143]Comptes Rendus, t. cvii., p. 804;Astr. and Astroph., vol. xiii., p. 291;Astr. Jour., No. 186.[1144]Astr. Jour., Nos. 342, 436, 508.[1145]Astr. Pop., p. 661;La Nature, January 3, 1880.[1146]Proc. Roy. Soc. Edinb., vols. x., p. 429; xi., p. 89.[1147]Vierteljahrsschrift. Astr. Ges., Jahrg. xxi., p. 206.[1148]Proc. Roy. Soc. Edinb., vol. xxiii., p. 370;Nature, vol. lxiv., p. 524.[1149]Amer. Jour. of Science, vol. xx., p. 225.

[965]Phil. Trans., vol. lxxiv., p. 260.

[966]Novæ Observationes, p. 105.

[967]Phil. Trans., vol. i., p. 243.

[968]Mém. de l'Ac., 1720, p. 146.

[969]Phil. Trans., vol. lxxiv., p. 273.

[970]A large work, entitledAreographische Fragmente, in which Schröter embodied the results of his labours on Mars, 1785-1803, narrowly escaped the conflagration of 1813, and was published at Leyden in 1881.

[971]Beiträge, p. 124.

[972]Mem. R. A. Soc., vol. xxxii., p. 183.

[973]Astr. Nach., No. 1,468.

[974]Observatory, vol. viii., p. 437.

[975]Month. Not., vols. xxviii., p. 37; xxix., p. 232; xxxiii., p. 552.

[976]Flammarion,L'Astronomie, t. i., p. 266.

[977]Smyth,Cel. Cycle, vol. i., p. 148 (1st ed.).

[978]Phil. Trans., vol. cxxi., p. 417.

[979]Month. Not., vol. xxv., p. 227.

[980]Phil. Mag., vol. xxxiv., p. 75.

[981]Proctor,Quart. Jour. of Science, vol. x., p. 185; Maunder,Sunday Mag., January, February, March, 1882; Campbell,Publ. Astr. Pac. Soc., vol. vi., p. 273.

[982]Am. Jour. of Sc., vol. xxviii., p. 163.

[983]Burton,Trans. Roy. Dublin Soc., vol. i., 1880, p. 169.

[984]Month. Not., vol. xxvii., p. 179;Astroph. Journ., vol. i., p. 193.

[985]Untersuchungen über die Spectra der Planeten, p. 20;Astroph. Journ., vol. i., p. 203.

[986]Publ. Astr. Pac. Soc., vols. vi., p. 228; ix., p. 109;Astr. and Astroph., vol. xiii., p. 752;Astroph. Jour., vol. ii., p. 28.

[987]Ibid., vol. v., p. 328.

[988]Ibid., vols. i., p. 311; iii., p. 254.

[989]C. Christiansen,Beiblätter, 1886, p. 532.

[990]Astr. and Astrophysics, vol. xi., p. 671.

[991]Flammarion,La Planète Mars, p. 574.

[992]Mémoires Couronnés, t. xxxix.

[993]Lockyer,Nature, vol. xlvi., p. 447.

[994]Mem. Spettr. Italiani, t. xi., p. 28.

[995]Bull. Astr., t. iii., p. 324.

[996]Journ. Brit. Astr. Ass., vol. i., p. 88.

[997]Publ. Pac. Astr. Soc., vol. ii., p. 299; Percival Lowell,Mars, 1896;Annals of the Lowell Observatory, vol. ii., 1900.

[998]Old and New Astr., p. 545.

[999]L'Astronomie, t. xi., p. 445.

[1000]La Planète Mars, p. 588.

[1001]Month. Notices, vol. lvi., p. 166.

[1002]L'Astronomie, t. viii.

[1003]Astr. Nach., No. 3,271;Astr. and Astrophysics, vol. xiii., p. 716.

[1004]Month. Not., vol. xxxviii., p. 41;Mem. Roy. Astr. Soc., vol. xliv., p. 123.

[1005]Astr. and Astrophysics, vol. xi., p. 668.

[1006]Ibid., p. 850.

[1007]Comptes Rendus, t. cxv., p. 379.

[1008]Astr. Jour., No. 384;Publ. Astr. Pac. Soc., vol. vi., p. 109.Cf. Observatoryvol. xvii., pp. 295-336.

[1009]See Mr. Wentworth Erck's remarks inTrans. Roy. Dublin Soc., vol. i., p. 29.

[1010]Month. Not., vol. xxxviii., p. 206.

[1011]Annals Harvard Coll. Obs., vol. xi., pt. ii., p. 217.

[1012]Young,Gen. Astr., p. 366.

[1013]Campbell,Publ. Pac. Astr. Soc., vol. vi., p. 270.

[1014]Astr. Nach., No. 3,319.

[1015]Witch of Atlas, stanza iii. I am indebted to Dr. Garnett for the reference.

[1016]Recommended by Chandler,Astr. Jour., No. 452.

[1017]Harvard Circulars, Nos. 36, 37, 51.

[1018]Astr. Nach., No. 3,687.

[1019]Montangerand,Comptes Rendus, March 11, 1901.

[1020]Pickering,Astroph. Jour., vol. xiii., p. 277.

[1021]Harvard Circular, No. 58.

[1022]Astr. Nach., No. 752.

[1023]L. Niesten,Annuaire, Bruxelles, 1881, p. 269.

[1024]According to Svedstrup (Astr. Nach., Nos. 2,240-41), the inclination to the ecliptic of the "mean asteroid's" orbit is = 6°.

[1025]Smiths.Report, 1876, p. 358;The Asteroids(Kirkwood), p. 42, 1888.

[1026]Tisserand,Annuaire, Paris, 1891, p. B. 15; Newcomb,Astr. Jour., No. 477; Backlund,Bull. Astr., t. xvii., p. 81; Parmentier,Bull. Soc. Astr. de France, March, 1896; Observatory, vol. xviii., p. 207.

[1027]Berberich,Astr. Nach., No. 3,088.

[1028]Bull. Astr., t. xviii., p. 39.

[1029]The Asteroids, p. 48;Publ. Astr. Pac. Soc., vols. ii., p. 48; iii., p. 95.

[1030]Comptes Rendus, t. xxxvii., p. 797.

[1031]Bull. Astr., t. v., p. 180.

[1032]Annuaire, Bruxelles, 1881, p. 243.

[1033]Johns Hopkins Un. Circular, January, 1895;Observatory, vol. xviii., p. 127.

[1034]Harvard Annals, vol. xi., part ii., p. 294.

[1035]Astr. Nach., Nos. 2,724-5.

[1036]Month. Not., vol. lxi., p. 69.

[1037]Astroph. Jour., vol. vii., p. 25.

[1038]Spectra der Planeten, p. 24.

[1039]Tome i., p. 93.

[1040]Berlinische Monatsschrift, 1785, p. 211.

[1041]Month. Not., vol. xiii., p. 40.

[1042]Mem. Am. Ac., vol. viii., p. 221.

[1043]Photom. Unters., p. 303.

[1044]Astr. Nach., No. 1,851.

[1045]Mém. de l'Ac., t. x., p. 514.

[1046]Ibid., 1692, p. 7.

[1047]Month. Not., vol. xliv., p. 63.

[1048]Photom. Unters., pp. 165, 273;Potsdam Publ., No. 30.

[1049]Vogel,Sp. der Planeten, p. 33,note.

[1050]Proc. Roy. Soc., vol. xviii., p. 250.

[1051]Month. Not., vol. xl., p. 433.

[1052]Sitzungsberichte, Berlin, 1895, ii., p. 15.

[1053]The anomalous shadow-effects recorded by Webb (Cel. Objects, p. 170, 4th ed.) are obviously of atmospheric and optical origin.

[1054]Engelmann,Ueber die Helligkeitsverhältnisse der Jupiterstrabanten, p. 59.

[1055]Month. Not., vol. xxviii., p. 11.

[1056]Observatory, vol. vii., p. 175.

[1057]Month. Not., vol. xlviii., p. 43.

[1058]Publ. Astr. Pac. Soc., vol. ii., p. 296.

[1059]Pickering failed to obtain any photometric evidence of their variability.Harvard Annals, vol. xi., p. 245.

[1060]Astr. and Astroph., vol. xii., pp. 194, 481.

[1061]Annals Lowell Obs., vol. ii., pt. i.

[1062]Astr. Nach., Nos. 2,995, 3,206;Month. Not., vols. li., p. 556; liv., p. 134. Barnard remains convinced that the oval forms attributed to Jupiter's satellites are illusory effects of their markings.Astr. Nach., Nos. 3,206, 3,453;Astr. and Astroph., vol. xiii., p. 272.

[1063]Publ. Astr. Pac. Soc., vol. iii., p. 355.

[1064]Astr. Nach., No. 1,017.

[1065]Publ. Astr. Pac. Soc., vol. iii., p. 359.

[1066]Astr. Nach., No. 3,432.

[1067]Astr. Jour., Nos. 275, 325, 367, 472;Observatory, vol. xv., p. 425.

[1068]Tisserand,Comptes Rendus, October 8, 1894; Cohn,Astr. Nach., No. 3,404.

[1069]Bull. Ac. R. Bruxelles, t. xlviii., p. 607.

[1070]Astr. Nach., No. 2,294.

[1071]Ibid., No. 2,284.

[1072]Denning,Month. Not., vol. xliv., pp. 64, 66;Nature, vol. xxv., p. 226.

[1073]Sidereal Mess., December, 1886, p. 289.

[1074]Astr. Nach., Nos. 2,280, 2,282.

[1075]Month. Not., vol. xlvi., p. 117.

[1076]Proc. Roy. Soc. N. S. Wales, vol. xiv., p. 68.

[1077]Phil. Trans., vol. i., p. 143.

[1078]For indications relative to the early history of the red spot, see Holden,Publ. Astr. Pac. Soc., vol. ii., p. 77; Noble,Month. Not., vol. xlvii., p. 515; A. S. Williams,Observatory, vol. xiii., p. 338.

[1079]Astr. and Astrophysics, vol. xi., p. 192.

[1080]Month. Not., vol. l., p. 520.

[1081]Observatory, vol. xiii., pp. 297, 326.

[1082]Trans. R. Dublin Soc., vol. iv., p. 271, 1889.

[1083]Publ. Astr. Pac. Soc., vol. ii., p. 289.

[1084]Astr. and Astrophysics, vol. xi., p. 686.

[1085]Denning,Knowledge, vol. xxiii., p. 200;Observatory, vol. xxiv., p. 312;Pop. Astr., vol. ix., p. 448;Nature, vol. lv., p. 89.

[1086]Williams,Observatory, vol. xxiii., p. 282.

[1087]Month. Not., vol. lvi., p. 143.

[1088]Bélopolsky,Astr. Nach., No. 3,326.

[1089]Publ. Astr. Pac. Soc., vol. iv., p. 176.

[1090]Bull. Astr., 1900, p. 70.

[1091]Month. Not., vol. xxxi., p. 34.

[1092]Beobachtungen, Heft ii., p. 99.

[1093]Ber. Sächs. Ges. der Wiss., 1871, p. 553.

[1094]Month. Not., vol. lix., p. 76.

[1095]Beziehungen der Sonnenfleckenperiode, p. 175.

[1096]A. Hall,Astr. Nach., No. 2,269.

[1097]Astr. Jour.(Gould's), vol. ii., p. 17.

[1098]Ibid., p. 5.

[1099]On the Stability of the Motion of Saturn's Rings, p. 67.

[1100]Mém. de l'Ac., 1715, p. 47; Montucla,Hist. des Math., t. iv., p. 19;An Original Theory of the Universe, p. 115.

[1101]Comptes Rendus, t. xcviii., p. 718.

[1102]Proctor,Saturn and its System(1865), p. 125.

[1103]Perrotin,Comptes Rendus, t. cvi., p. 1716.

[1104]Abhandl. Akad. der Wiss., Munich, Bd. xvi., p. 407.

[1105]Smiths. Report, 1880 (Holden).

[1106]Quoted by Dr. E. Anding,Astr. Nach., No. 2,881.

[1107]Astr. and Astrophysics, vol. xi., p. 119;Month. Not., vol. l., p. 108.

[1108]Astroph. Jour., vol. i., p. 416.

[1109]Ibid., vol. ii., p. 127.

[1110]Mém. de l'Ac. Imp.(St. Petersb.), t. vii., 1853, p. 464.

[1111]Astr. Nach., No. 2,498.

[1112]Washington Observations, App. ii., p. 22

[1113]Month. Not., vol. lvi., p. 163.

[1114]T. Lewis,Observatory, vol. xviii., p. 379.

[1115]Harper's Magazine, June, 1889.

[1116]Mém. de l'Acad. de Montpellier, t. viii., p. 296, 1873.

[1117]Meteoric Astronomy, chap. xii. He carried the subject somewhat farther in 1871. SeeObservatory, vol. vi., p. 335.

[1118]Astr. Nach., No. 2,527.

[1119]Amer. Jour. of Sc., vol. xiv., p. 325.

[1120]Observatory, vol. xiv., p. 369.

[1121]Month. Not., vol. liv., p. 297.

[1122]Phil. Trans., vol. lxxxii., p. 14.

[1123]Smiths. Report, 1880.

[1124]Comptes Rendus, t. lxiv., p. 1304.

[1125]Huggins,Proc. R. Soc., vol. xlvi., p. 231; Keeler,Astr. Nach., No. 2,927; Vogel,Astroph. Jour., vol. i., p. 278.

[1126]Month. Not., vol. xxxiii., p. 164.

[1127]Astr. Nach., No 2,545.

[1128]Comptes Rendus, t. xcviii., p. 1419.

[1129]Comptes Rendus, t. xcviii., pp. 718, 967.

[1130]V. J. S. Astr. Ges., Jahrg. xxiv., p. 267.

[1131]Publ. Astr. Pac. Soc., vol. iii., p. 287.

[1132]Astr. Nach., No. 2,526.

[1133]Ibid., No. 2,730.

[1134]Astr. Jour., Nos. 370, 374.

[1135]Astr. Nach., No. 3,768.

[1136]Ann. der Phys., Bd. clviii., p. 470;Astroph. Jour., vol. i., p. 280.

[1137]Astr. Nach., No. 2,927.

[1138]Month. Not., vol. xlix., p. 405.

[1139]Astr. Nach., No. 2,927; Scheiner'sSpectralanalyse, p. 221.

[1140]Month. Not., vol. xliv., p. 257.

[1141]Observatory, vol. vii., pp. 134, 221, 264.

[1142]Month. Not., vol. xlvi., p. 507.

[1143]Comptes Rendus, t. cvii., p. 804;Astr. and Astroph., vol. xiii., p. 291;Astr. Jour., No. 186.

[1144]Astr. Jour., Nos. 342, 436, 508.

[1145]Astr. Pop., p. 661;La Nature, January 3, 1880.

[1146]Proc. Roy. Soc. Edinb., vols. x., p. 429; xi., p. 89.

[1147]Vierteljahrsschrift. Astr. Ges., Jahrg. xxi., p. 206.

[1148]Proc. Roy. Soc. Edinb., vol. xxiii., p. 370;Nature, vol. lxiv., p. 524.

[1149]Amer. Jour. of Science, vol. xx., p. 225.

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