ASTRONOMY. 



OF COMETS. 



Besides these planets already mention- 

 ed, there are some other bodies which re- 

 volve round the sun, called comets. They 

 move in verv 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 

 \vhich is nearest tothe 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 issometimes 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 Sherburn, 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 tl,~ 

 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 shew that the co- 

 met had no influence upon Mercury's mo- 

 tion. The nucleus, which had before been 

 always round, on the 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. 



OF THE FIXED STARS. 



The fixed stars are so called, because 

 they are observed not to change their 

 places in the heavens, as the planets do. 

 They appear of an infinite variety of sizes; 

 yet, for convenience, it is usual to class 

 them into six or seven magnitudes ; thus, 

 they are called stars of the first, second, 

 &c. magnitude. To the naked eye they 

 appear innumerable, but this is only the 

 consequence of their being scattered in so 



