Page:Popular Science Monthly Volume 24.djvu/505

Rh as well as the comet of 1812, now visible on its first predicted return. It was originally detected on the 20th of July, and was the thirteenth discovered by Pons within ten years. Its appearance at first was that of an irregular nebula without tail or beard, and it was only visible through a telescope. By the 14th of September it was easily seen without optical aid; its tail was over two degrees in length, and the diameter of its nucleus was five or six seconds. It continued visible till October—a period of ten weeks—and was consequently well observed. Cooper's valuable work on "Cometic Orbits" contains eight sets of elements by different computers. Encke distinctly recognized the elliptic form of the orbit, and the elements which he assigned have been generally preferred. They are as follows:

According to Encke, therefore, the next perihelion passage was to have been expected in June, 1883—about three months before the actual discovery of the comet by Mr. W. R. Brooks. A re-discussion of the observations of 1812 had, however, been recently completed by Dr. Schulhof and M. Bossert, whose calculations gave a probable period about seven months longer than that obtained by Encke. The true period is found to be very nearly a mean between these earlier and later estimates.

On its present return the comet was first glimpsed on the night of September 1st, by Mr. William R. Brooks, Director of Red House Observatory, Phelps, New York. He was, however, prevented by clouds from verifying his conjecture of the cometary character of the nebulous speck till the evening of the 3d. Its identity with the comet of 1812 was shown on the 18th of September, by the Rev. Mr. Searles, of New York, and independently on the day following by Professor Lewis Boss, of the Dudley Observatory. The latter designated January 25, 1884, as the date of perihelion passage. Astronomers of the twentieth century will probably witness its next apparition in the summer of 1955.

The comet of 1812 is one of a remarkable group whose periods range between sixty-eight and seventy-six years, all of their aphelia being some distance beyond the orbit of Neptune. It seems, however, to be specially related to the fourth comet of 1846. The latter was discovered by De Vico, at Rome, on February 20th, and independently, by Professor G. P. Bond, February 26th. It remained visible ten weeks, and its elements were calculated by Peirce, Hind,