_Encke’s Comet._—Until the year 1819 Halley’s Comet was the only one certainly known to be periodical. Then the able deductions of Encke presented us with a veritable “Mercury[43] of comets.” He showed that a small comet, discovered by the unwearying Pons of Marseilles on Nov. 26, 1818, was really identical with three previous comets—viz. 1786 I. (Mechain), 1795 (C. Herschel), and 1805 (Thulis),—and that its period was a very short one of about 3-1/3 years. Its return to perihelion was predicted to occur on May 24, 1822, and this was observed in the southern hemisphere. It returned again on Sept. 16, 1825, and on this occasion the circumstances were more favourable. Since 1825 this object has effected nineteen returns to perihelion, and one of the most singular facts noticed in connection with it is that its period is gradually shortening. In 1795 it was 1212 days, while in 1858 it was 1210. In order to explain this contraction of orbit, it has been necessary to assume the existence of a thin medium in space capable of affording a slight resistance to the tenuous materials of a comet, though not dense enough to appreciably affect the motions of planets. If this closing-up of the orbit and shortening of period continue to operate through a vast interval of time, Encke’s Comet must be ultimately precipitated upon the Sun![44] _Biela’s Comet._—This comet was discovered on Feb. 27, 1826, by Wilhelm von Biela, an Austrian officer, at Josephstadt in Bohemia, and ten days later by Gambart at Marseilles. It was found to be revolving in an orbit of short period, and its elements presented an agreement with those of the comet of 1772 (Montaigne) and 1806 I. (Pons). Identity was inferred, and the next return was fixed for Nov. 27, 1832, when the object reappeared with great punctuality. At the end of 1845 this comet displayed some startling phenomena; for it divided into two portions, apparently quite disconnected, and which travelled side by side, separated by an interval of more than 150,000 miles! The double comet was observed again in 1852, when the interval separating them had, however, increased eightfold, for the dark space between measured 1,250,000 miles. This instance of a divided comet is by no means unique. The great comet of 1882 underwent a process of disruption, by throwing off small masses of nebulosity, which, however, survived the separation only a few days. Brooks’s Comet (1889 V.) was found by Barnard, on Aug. 1, 1889, to be divided into four parts! Two of these had a brief existence; but one of the minor fragments retained a very distinct appearance near the parent mass during the ensuing months of September and October. The phenomena of Biela’s Comet found an excellent counterpart here. [Illustration: Fig. 49. Brooks’s double Comet, Sept. 17, 1889. (10-inch reflector, power 60. W. F. Denning.) ]