OF COMETS.
Besides these planets already mention ed, there are some other bodies which re volve round the sun, called comets. They move in very eccentric ellipses, and their periods of revolution are so long, and so uncertainly known, that few are ever ob served twice. They are only seen by us when they are in that part of their orbit which is nearest to the sun; and then they move so fast, that they soon become again invisible to us ; the number of comets is unknown ; numbers of small ones have been discovered by telescopes. Their distances are inconceivably great, and most of them move entirely beyond the planetary orbits, though some have de scended below Mars. Their appearances are very different. Some appear only a faint vapour; others have a nucleus, or solid part, in the middle. When they approach the sun, they put forth the ap pearance of a beard, or tail, of luminous matter, which is sometimes of astonishing length. These tails are always directed from the sun. There are three comets, viz. of 1680, 1744, and 1759, that deserve to have a farther account given of them. The comet of 1680 was remarkable for its near approach to the sun ; so near, that in its perihelion it was not above a sixth part of the diameter of the luminary from the surface thereof. The tail, like that of other comets, increased in length and brightness as it came nearer to the sun ; and grew shorter and fainter, as it went farther from him and from the earth, till that and the comet were too far off to be any longer visible. The comet of 1744 was first seen at Lausanne, in Switzer land, December 13, 1743, N. S. From that time it increased -in brightness and magnitude as it was coming nearer to the sun. Its diameter, when at the distance of the sun from us, measured about one minute, which brings it out equal to three times the diameter of the earth. It came so near Mercury, that if its attraction had been proportionable to its magnitude, it was thought probable it would have dis. turbed the motion of that planet. Mr. Betts, of Oxford, however, from some ob servations made there, and at Lord Mac clesfield's observatory at She rburn, found, that when the comet was at its least dis tance from Mercury, and almost twice as near the sun as that planet was, it was still distant from him a fifth part of the distance of the sun from the earth, and could therefore have no effect upon the planet's motions. He judged the comet
to be at least equal in magnitude to tte earth. He says, that in the evening of January 23, this comet appeared exceed ingly distinct and bright, and the diame ter of its nucleus nearly equal to that of Jupiter. Its tail extended above 16 de grees from its body; and was in length, supposing the sun's parallax 10", no less than 33 millions of miles. Dr. Bevis, in the month of May, 1744, made four ob servations of Mercury, and found the places of that planet, calculated from cor rect tables, differed so little from the places observed, as to chew that the co met had no influence upon Mercury's mo tion. The nucleus, which had before been always round, on th,e 10th of February 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 separa ted the beard from the hair of the comet. The odd phenomena disappeared the next day, and nothing was seen but ir regular obscure spaces like smoke in the middle of the tail: and the head resumed its natural form. February 15, the tail was divided into two branches ; the eastern part about seven or eight degrees long, the western 24. On the 33d the tail be gan to be bent ; it showed no tail till it was as near to the sun as the orbit of Mars; the tail grew longer as it approach ed nearer the sun ; and at its greatest length was computed to equal a third part of the distance of the earth from the sun. The comet of 1759 did not make any con siderable appearance, by reason of the unfavourable situation of the earth all the time its tail might otherwise have been conspicuous ; the comet being then to near the sun to be seen by us ; but de serves our particular consideration, as it was the first that ever had its return fore told. With respect to the real nature and use of the comets in the system, we are entirely unacquainted.