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ASTRONOMY. 
OF COMETS. 
Besides these planets already mentioned, 
there are some other bodies which revolve 
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 observed 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 tele- 
scopes. Their distances are inconceivably 
great, and most of them move, entirely be- 
yond the planetary orbits; though some 
have descended below Mars. Their ap- 
pearances 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 appear- 
ance 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 far- 
ther 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 Switzerland De- 
cember 13, 1743, N. S. From that time it 
increased in brightness and magnitude as it 
was coming nearer to the sun. Its dia- 
meter, when at the distance of the sun 
from us, measured about one minute, which 
brings it out equal to three times the diame- 
ter of the earth. It came so near Mercu- 
ry? that if its attraction had been propor- 
tionable to its magnitude, it was thought 
probable it would have disturbed the mo- 
tion of that planet. Mr. Betts of Oxford, 
however, from some observations made 
there, and at Lord Macclesfield’s observa- 
tory at Sherburn, found, that when the 
comet was at its least distance from Mercury, 
and almost twice as near the sun as that pla- 
net 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 
the earth. He says, that in the evening of 
January 23, this comet appeared exceed- 
ingly distinct and bright, and the diameter 
of its nucleus nearly equal to that of Jupi- 
ter. Its tail extended above 16 degrees 
from its body ; and was in length, suppos- 
ing the sun's parallax 10”, no less than 33 
millions of miles. Dr. Bevis, in the month 
of May, 1744, made four observations of 
Mercury, and found the places of that pla- 
net, calculated from correct tables, dif- 
fered so little from the places observed, as 
to shew that the comet had no influence 
upon Mercury’s motion. The nucleus, 
which had before been always round, on 
the 10th of February appeared oblong, in 
the direction of the tail, and seemed divid- 
ed 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. The odd phenomena disap- 
peared the next day, and nothing was seen 
but irregular obscure spaces like smoke in 
the middle of the tail ; and the head re- 
sumed 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 23d, the tail 
began 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 approached nearer 
the sun ; and at its greatest length was 
computed to equal a third part of the dis- 
tance of the earth from the sun. The comet 
of 1759 did not make any considerable ap- 
pearance by reason of the unfavourable si- 
tuation of the earth all the time its tail 
might otherwise have been conspicuous ; 
the comet being then too near the sun to 
be seen by us ; but deserves our particular 
consideration, as it was the first that ever 
had its return foretold. 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 conse- 
quence of their being scattered in so con- 
fused a manner, and our not being able to 
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