Julius Cæsar was murdered. Its nucleus was computed to be about ten times as large as the moon. Its tail extended over a space of seventy degrees in extent.

This is the comet, to the near approach of which to the earth Mr. Whiston attributed the universal deluge in the time of Noah. His opinion was, that the earth, passing through the atmosphere of the comet, attracted from it a great part of the water of the flood; that the nearness of the comet raised a great tide in the subterranean waters; that this could not be done without making fissures or cracks in the outer crust of the earth; that through these fissures the subterraneous waters were forced; that along with the water much slime or mud would rise, which, after the subsiding of the water partly into the fissures and partly into the lower parts of the earth to form the sea, would cover over to a considerable depth the antediluvian earth; and thus he accounts for trees and bones of animals being found at very great depths in the earth. The same comet, he supposed, when coming near the earth, after being heated to an immense degree in its perihelion, would be the instrumental cause of that great catastrophe, the general conflagration. Modern geological researches, however, render all such hypotheses utterly untenable.

2. Another comet which has obtained a certain degree of celebrity is that which appeared in 1682, and is usually distinguished by the name of Halley's comet. This comet appeared with considerable splendour, and exhibited a tail thirty degrees in length. On calculating its elements from its perihelion passage, Dr. Halley was led to conclude that it was identical with the great comets which appeared in 1456, 1531, and 1607, whose elements he had also ascertained. The intervals between these periods being about seventy-five or sev enty-six years, he was led to conclude that this was the period of the revolution of the comet, and ventured to predict that it would again return about the latter part of the year 1758. As this was the first comet whose return had been predicted, when the time of its expected appearance approached, astronomers became anxious to ascertain whether the attraction of the larger planets, Jupiter and Saturn, might not interfere with its orbitual motion, and prevent it from arriving at its perihelion so soon as the time predicted. Clairaut, an eminent French mathematician, after many intricate and laborious calculations in reference to the subject, concluded that the attraction of

Saturn would lengthen the period 100 days, and the action of Jupiter 518, making in all 618 days, by which the expected return would happen later than if no such influence had taken place; so that, instead of the period being 74 years, 323 days, it ought to be 76 years, 211 days; and as the comet passed its perihelion on September 14, 1682, it ought to reach the same point on April 13, 1759. These calculations were read beford the Academy of Sciences, on the 14th of November, 1758; but Clairaut gave notice that, being pressed for time, he had neglected in his calculations small values, which collectively might amount to about thirty days in the seventy-six years. These predictions were accordingly verified; for the comet appeared about the end of December, 1758, and arrived at its perihelion on the 13th of March, 1759, only thirty days before the time fixed by the calculations of Clairaut, who, upon repeating the process by which he had arrived at the result, reduced this error to nineteen days. The same comet again made its appearance, according to prediction, in 1835, of which a particular account will be given in the sequel.

3. Another remarkable comet made its appearance in 1744, which excited a considerable degree of attention. It was first seen at Lausanne, in Switzerland, December 13, 1743; from that period it increased in brightness and magnitude as it approached nearer the sun. On the evening of January 23, 1744, it appeared exceedingly bright and distinct, and the diameter of its nucleus was nearly equal to that of Jupiter. Its tail then extended above 16 degrees from its body, and was supposed to be about 23 millions of miles in length. On the 11th of February, the nucleus, which had before been always round, appeared oblong in the direction of the tail, and seemed divided into two parts by a black stroke in the middle. One of the parts had a sort of beard, brighter than the tail; this beard was surrounded by two unequal dark strokes, that separated the beard from the hair of the comet: these odd phenomena disappeared the next day, and nothing was seen but irregular obscure spaces like smoke in the middle of the tail, and the head resumed its natural form. On the 15th of February the tail was divided into two branches, the eastern about 8 degrees long, the western 24. On the 23d the tail began to be bent. It showed no tail till it was as near the sun as the orbit of Mars, and it increased in length as it approached nearer that luminary. At its greatest length it was

computed to equal a third part of the distance of the earth from the sun.* This was one of the most brilliant comets Chat had appeared since that of 1680. Its tail was visible for a long time after its body was hid under the horizon: it extended 20 or 30 degrees above the horizon, two hours before sunrise.

4. In the month of June, 1770, Messier discovered a comet, the motions of which appeared to be involved in a considerable degree of mystery. The comet continued visible for a long time. Lexell ascertained, from observation, that it described an ellipse around the sun, of which the greater axis was only three times the diameter of the earth's orbit, which corresponds with a revolution of five and a half years. It was therefore expected that it would again frequently make its appearance; but it has never since been visible, although it made a pretty brilliant appearance in 1770. The National Institute of France, not many years ago, requested M. Burckhardt to repeat all the calculations with the utmost care; and the result of his labour has been a complete confirmation of the elements obtained by Lexell. What has become of this comet it is difficult to conjecture. Its aphelion, or greatest distance from the sun, was reckoned to be not far beyond the orbit of Jupiter, and that it approached as near to the earth as the moon, and ought to have appeared twelve times since the year 1770. M. Arago attempts to solve the difficulty by affirming that its orbit was then totally different from that which it has since pursued; that its passage to the point of perihelion in 1776, when it was expected, took place by day, and, before the following return, the form of the orbit was so altered that, had the comet been visible from the earth, it would not have been recognised; that, before 1767, during the whole progress of its revolutions, its shortest distance from the sun was 199,000,000 leagues, and that, after 1779, the minimum distance became 131,000,000 leagues, which was still too far removed for the comet to be perceptible from the earth. Sir David Brewster attempts to account for its disappearance by supposing that it must have been attracted by one of the planets whose orbit it crossed, and must have imparted to it its nebulous mass; and that it is probable the comet passed near Ceres and Pallas, and imparted to them those immense

* Memoirs of the Academy of Sciences for 1744.

atmospheres which distinguish them from all the other planets. Whether any of these opinions be tenable and sufficient to solve the difficulty, is left entirely with the reader to determine.

5. Another comet, which has engaged the particular attention of astronomers during the last twenty years, is distinguished from all preceding comets by the shortness of its periodic revolution. It is usually denominated Encke's comet, so called from Professor Encke, of Berlin, who first ascertained its periodical return. It was discovered at Marseilles, on the 26th November, 1818, by M. Pons, and its parabolic elements were presented to the Board of Longitude, at Paris, by M. Bouvard, on the 13th of January, 1819. It was immediately remarked that the result of Bouvard's calculations was too similar to the elements of a comet which appeared in 1805, not to consider that and the one of 1818 as the same body; and M. Encke soon after established, by incontestable calculations, that this comet took only about 1200 days, or three years and three tenths, to travel through the whole extent of its elliptic orbit. This was considered as a very extraordinary result, as an opinion had previously prevailed that the period of the revolution of a comet must necessarily be long. It now appears that this comet was first seen by Messier and Mechain in 1786; afterward by Miss Herschel in 1795; and its subsequent returns were observed by different astronomers in 1805 and 1819, all of whom, at those periods, supposed that the four comets were four different bodies The elements of this comet, and the short period of its revolution, are now incontrovertibly established; for its reappearance in the southern hemisphere in June, 1822, took place very nearly in the positions previously calculated. The agreement was not less remarkable in 1825; and in 1828, the third period of its announced return, it occupied the places assigned to it by Encke the year preceding. It likewise appeared in 1832, 1835, and 1838.

This comet is very small; its light is feeble; it has no tail; it is invisible to the naked eye, except in very favourable circumstances, but may be seen with a small magnifying power. It revolves in an elliptical orbit of considerable eccentricity, having an inclination to the plane of the ecliptic of 13 degrees. On comparing the intervals between the successive perihelion passages of this comet, a singular fact

has been elicited, namely, that its periods are continually diminishing, and its mean distance from the sun shortening by slow but regular degrees. This is supposed by M. Encke to be produced by a resistance experienced by the comet from a very rare ethereal medium pervading the regions through which it moves; since such resistance, by diminishing also its actual velocity, would diminish also its centrifugal force, and thus give the sun more power over it to draw it nearer. It is therefore the opinion of Sir J. Herschel, that "it will probably fall ultimately into the sun, should it not first be dissipated altogether, a thing no way improbable, when the lightness of its materials is considered, and which seems authorized by the observed fact of its having been less and less conspicuous at each reappearance." The acceleration of this comet is about two days in each revolution; and the frequent opportunities of observation which will occur, in consequence of the shortness of its period, may lead to new and interesting conclusions in relation to the nature of these bodies.

6. Besides the above, another periodical comet has lately been discovered, which is distinguished by the name of Biela's, and sometimes Gambart's comet. This comet was perceived at Johanisberg, on the 27th Feb., 1826, by M. Biela; and by M. Gambart, at Marseilles, ten days afterward. Gambart, without delay, calculated its parabolic elements from his own observations, and by inspecting a general table of comets, he recognised that it was not its first appearance, but that it had been already observed in 1789 and 1795. Messrs. Clausen and Gambart undertook the computation of the comet's revolution, and found, each of them nearly at the same time, that the new comet made its entire revolution round the sun in a period of about seven years. It was afterward found, more accurately, to be 2460 days, or nearly 6 years. M. Damoiseau calculated the perturbations of this comet, and predicted that it would cross the plane of the earth's orbit on the 29th of October, 1832, a little before midnight, at a point about 18,480 miles within the orbit of the earth. According to this prediction, the comet actually made its appearance in 1832, about the time now specified Its next appearance was calculated to happen in 1839; and it was reckoned that it would arrive at its perihelion on the 23d July of that year.

The predicted appearance of this comet in 1832 seems to have produced considerable alarm, particularly in France.