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 Ura.n.u.s, as Saturn of Jupiter. Of the circ.u.mstances 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. Muller denies its existence _in 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 pa.s.sing 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 792 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. Direct verification 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...o...b..tal 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 Deichmuller'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 about groping 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 Ura.n.u.s. They gave him a virtually identical situation for the new planet with that derived from the cl.u.s.tering of cometary aphelia.[1149] Yet its a.s.signed 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:

[Footnote 965: _Phil. Trans._, vol. lxxiv., p. 260.]

[Footnote 966: _Novae Observationes_, p. 105.]

[Footnote 967: _Phil. Trans._, vol. i., p. 243.]

[Footnote 968: _Mem. de l'Ac._, 1720, p. 146.]

[Footnote 969: _Phil. Trans._, vol. lxxiv., p. 273.]

[Footnote 970: A large work, ent.i.tled _Areographische Fragmente_, in which Schroter embodied the results of his labours on Mars, 1785-1803, narrowly escaped the conflagration of 1813, and was published at Leyden in 1881.]

[Footnote 971: _Beitrage_, p. 124.]

[Footnote 972: _Mem. R. A. Soc._, vol. x.x.xii., p. 183.]

[Footnote 973: _Astr. Nach._, No. 1,468.]

[Footnote 974: _Observatory_, vol. viii., p. 437.]

[Footnote 975: _Month. Not._, vols. xxviii., p. 37; xxix., p. 232; x.x.xiii., p. 552.]

[Footnote 976: Flammarion, _L'Astronomie_, t. i., p. 266.]

[Footnote 977: Smyth, _Cel. Cycle_, vol. i., p. 148 (1st ed.).]

[Footnote 978: _Phil. Trans._, vol. cxxi., p. 417.]

[Footnote 979: _Month. Not._, vol. xxv., p. 227.]

[Footnote 980: _Phil. Mag._, vol. x.x.xiv., p. 75.]

[Footnote 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.]

[Footnote 982: _Am. Jour. of Sc._, vol. xxviii., p. 163.]

[Footnote 983: Burton, _Trans. Roy. Dublin Soc._, vol. i., 1880, p.

169.]

[Footnote 984: _Month. Not._, vol. xxvii., p. 179; _Astroph. Journ._, vol. i., p. 193.]

[Footnote 985: _Untersuchungen uber die Spectra der Planeten_, p. 20; _Astroph. Journ._, vol. i., p. 203.]

[Footnote 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.]

[Footnote 987: _Ibid._, vol. v., p. 328.]

[Footnote 988: _Ibid._, vols. i., p. 311; iii., p. 254.]

[Footnote 989: C. Christiansen, _Beiblatter_, 1886, p. 532.]

[Footnote 990: _Astr. and Astrophysics_, vol. xi., p. 671.]

[Footnote 991: Flammarion, _La Planete Mars_, p. 574.]

[Footnote 992: _Memoires Couronnes_, t. x.x.xix.]

[Footnote 993: Lockyer, _Nature_, vol. xlvi., p. 447.]

[Footnote 994: _Mem. Spettr. Italiani_, t. xi., p. 28.]

[Footnote 995: _Bull. Astr._, t. iii., p. 324.]

[Footnote 996: _Journ. Brit. Astr. a.s.s._, vol. i., p. 88.]

[Footnote 997: _Publ. Pac. Astr. Soc._, vol. ii., p. 299; Percival Lowell, _Mars_, 1896; _Annals of the Lowell Observatory_, vol. ii., 1900.]

[Footnote 998: _Old and New Astr._, p. 545.]

[Footnote 999: _L'Astronomie_, t. xi., p. 445.]

[Footnote 1000: _La Planete Mars_, p. 588.]

[Footnote 1001: _Month. Notices_, vol. lvi., p. 166.]

[Footnote 1002: _L'Astronomie_, t. viii.]