[Footnote 222: He died January 21, 1892, in his 71st year.] [Footnote 223: Ledger, _The Sun, its Planets and their Satellites_, p. 414.] [Footnote 224: Presented by the Misses Lassell, after their father's death, to the Greenwich Observatory.] [Footnote 225: _Astr. Jour._, No. 508.] [Footnote 226: _Report of U.S. Naval Observatory for 1900_, p. 15.] [Footnote 227: Grant, _Hist. of Astr._, p. 271.] [Footnote 228: _Month. Not._, vol. ix., p. 91.] [Footnote 229: _Month. Not._, vol. xi., p. 21.] [Footnote 230: _Astr. Nach._, No. 756 (May 2, 1851).] [Footnote 231: _Phil. Trans._, vol. i., p. 246. See H. T. Vivian, _Engl. Mech._, April 20, 1894.] [Footnote 232: Secchi, _Month. Not._, vol. xiii., p. 248.] [Footnote 233: Hind, _ibid._, vol. xv., p. 32.] [Footnote 234: Lynn, _Observatory_, Oct. 1, 1883; Hadley, _Phil. Trans._, vol. xxxii., p. 385.] [Footnote 235: Proctor, _Saturn and its System_, p. 64.] [Footnote 236: _Phil. Trans._, vol. lxxvii., p. 125.] [Footnote 237: _Month. Not._, vol. xi., p. 248.] [Footnote 238: _Ibid._, vol. xxxv., pp. 16-22.] [Footnote 239: _Ibid._, p. 26.] [Footnote 240: _Ibid._, vol. xli., p. 190.] CHAPTER V _COMETS_ Newton showed that the bodies known as "comets," or _hirsute_ stars, obey the law of gravitation; but it was by no means certain that the individual of the species observed by him in 1680 formed a permanent member of the solar system. The velocity, in fact, of its rush round the sun was quite possibly sufficient to carry it off for ever into the depths of space, there to wander, a celestial casual, from star to star. With another comet, however, which appeared two years later, the case was different. Edmund Halley, who afterwards succeeded Flamsteed as Astronomer Royal, calculated the elements of its orbit on Newton's principles, and found them to resemble so closely those similarly arrived at for comets observed by Peter Apian in 1531, and by Kepler in 1607, as almost to compel the inference that all three were apparitions of a single body. This implied its revolution in a period of about seventy-six years, and Halley accordingly fixed its return for 1758-9. So fully alive was he to the importance of the announcement that he appealed to a "candid posterity," in the event of its verification, to acknowledge that the discovery was due to an Englishman. The prediction was one of the test-questions put by Science to Nature, on the replies to which largely depend both the development of knowledge and the conviction of its reality. In the present instance, the answer afforded may be said to have laid the foundation of this branch of astronomy. Halley's comet punctually reappeared on Christmas Day, 1758, and effected its perihelion passage on the 12th of March following, thus proving beyond dispute that some at least of these erratic bodies are domesticated within our system, and strictly conform, if not to its