What was thus proposed as theory has been since confirmed as undoubted facts. When the hypothesis was originally advanced, the data required for its mathematical demonstration were entirely wanting. The evidence, however, by which it was sustained was sufficient to give it a high degree of probability. The existence of a divellent force by which comets near their perihelia have been separated into parts is clearly shown by the following facts. Whether this force, as suggested by Schiaparelli, is simply the unequal attraction of the sun on different parts of the nebulous mass, or whether, in accordance with the views of other astronomers, it is to be regarded as a cosmical force of repulsion, is a question left for future discussion. HISTORICAL FACTS. 1. Seneca informs us that Ephoras, a Greek writer of the fourth century before Christ had recorded the singular fact of a comet's separation into two distinct parts.[13] This statement was deemed incredible by the Roman philosopher, inasmuch as the occurrence was then without a parallel. More recent observations of similar phenomena leave no room to question the historian's veracity. [13] "Quæst. Nat.," lib. vii., cap. xvi. 2. The head of the great comet of A.D. 389, according to the writers of that period, was "composed of several small stars." (Hind's "Comets," p. 103.) 3. On June 27, A.D. 416, two comets appeared in the constellation Hercules, and pursued nearly the same apparent path. Probably at a former epoch the pair had constituted a single comet.[14] [14] Chambers' "Descr. Astr.," p. 374. 4. On August 4, 813, "a comet was seen which resembled two moons joined together." They subsequently separated, the fragments assuming different forms.[15] [15] Ibid., p. 383. 5. The Chinese annals record the appearance of three comets--one large and two smaller ones--at the same time, in the year 896 of our era. "They traveled together for three days. The little ones disappeared first, and then the large one."[16] The bodies were probably fragments of a large comet which, on approaching the sun, had been separated into parts a short time previous to the date of their discovery. [16] Ibid., p. 388. 6. _The third comet of 1618._--The great comet of 1618 exhibited decided symptoms of disintegration. When first observed (on November 30), its appearance was that of a lucid and nearly spherical mass. On the eighth day the process of division was distinctly noticed, and on the 20th of December it resembled a cluster of small stars.[17] [17] Hevelius, "Cometographia," p. 341. See also Grant's "Hist. of Phys. Astr.," p. 302. 7. _The comet of 1661._--The elements of the comets of 1532 and 1661 have a remarkable resemblance, and previous to the year 1790 astronomers regarded the bodies as identical. The similarity of the elements is seen at a glance in the following table: Comet of 1532. Comet of 1661.