Besides the comet of Halley, several other of the great comets recorded in history have been surmised with more or less probability to return periodically, and therefore to move in elongated ellipses around the sun. Such is the great comet of 1680, whose period is estimated at 575 years, and which has been considered, with at least a high _prima facie_ probability, to be identical with a magnificent comet observed at Constantinople and in Palestine, and referred by contemporary historians, both European and Chinese, to the year A. D. 1106; with that of A. D. 531, which was seen at noonday close to the sun; with the comet of 43 B. C., already spoken of as having appeared after the death of Cæsar, and which was also observed in the daytime; and finally with two other comets, mention of which occurs in the Sibylline Oracles, and in a passage of Homer, and which are referred, as well as the obscurity of chronology and the indications themselves will allow, to the years 618 and 1194 B. C. It is to the assumed near approach of this comet to the earth, about the time of the Deluge, that Whiston ascribed that overwhelming tide-wave to whose agency his wild fancy ascribed that great catastrophe—a speculation, it is needless to remark, purely visionary. These coincidences of time are certainly remarkable, especially when it is considered how very rare are the appearances of comets of this class. Professor Encke, however, has discussed, with all possible care, the observations recorded of the comet of 1680, taking into consideration the perturbations of the planets (which are of trifling importance, by reason of the great inclination of its orbit to the ecliptic), and his calculations show that no elliptic orbit, with such a period as 575 years, is competent to represent them within any probable or even possible limits of error, the most probable period assigned by them being 8814 Julian years. Independent of this consideration, there are circumstances recorded of the comet of A. D. 1106 incompatible with its motion in any orbit identical with that of the comet of 1680, so that the idea of referring all these phenomena to one and the same comet, however seducing, must be relinquished. Another great comet, whose return about the year 1848 had been considered by more than one eminent authority in this department of astronomy highly probable, is that of 1556, to the terror of whose aspect some historians have attributed the abdication of the Emperor Charles V. This comet is supposed to be identical with that of 1264, mentioned by many historians as a great comet, and observed also in China.