142 ASTRONOMY. 
the planet’s disk, which by measurements instituted has been found to be 
quite considerable. Consequently by reason of this considerable flattening. 
the velocity of rotation of Uranus must be very great. 
The Comets, Nebula, Groups of Stars. 
60. Fig. 15, pl. 8, represents the comet of 1819, which, suddenly emerging 
from the beams of the sun, appeared to Europe, in the beginning of July, of 
remarkable size. Arago maintained that the light of this comet exhibited 
traces of polarization, which can only be exhibited by reflected light. This 
fact speaks strongly for the theory of Olbers, who maintained that the 
comets are non-luminous bodies, only rendered visible by the reflected 
light of the sun. Comets consist of a usually spherical nebulous envelope, 
with a somewhat brighter nucleus, although occasionally without the latter. 
Sometimes the nucleus is of great size; thus, for instance, that of the great 
comet of 1811 had a diameter of at least 560,000 geographical miles. In 
general the nebulosity does not entirely surround the nucleus, but exists as 
a spherical hull, elongated on the side of the tail, so that the tail appears as 
a continuation of the nebulosity. It seems besides, that this nebulous 
envelope constitutes the chief peculiarity of comets, as many of these 
wandering stars are seen, some without tails, some without nucleus, none 
however without the nebulosity. The tail is generally found on the side 
f opposite to the sun; at times, however, it deviates from this direction, which 
may be a consequence of the resistance of the ether in which the comet 
moves. The length of the tail is very various. Thus for instance the 
length of the tail of the great comet of 1811 (pl. 8, fig. 17) amounted on 
Oct. 12 to above 88 millions of geographical miles. The tails also become 
broader towards their extremity, and are often divided longitudinally in 
their middle by a dark line, so that it seems as if the tail were double. This 
was plainly perceived on the 10th of Sept. 1811 in the great comet, as shown 
in fig. 16. The sun undoubtedly produces the tail, as this is always first 
visible when the comet approaches the sun, becomes larger as the ap- 
proximation increases, and again diminishes with the gradual increase of 
distance between the two bodies. Consequently the tails appear, for the 
most part, to consist of very thin vapor developed by the heat of the sun 
from the nucleus of the comet. The alterations arising from this cause, 
which, according to numerous observations, must often be enormous, and 
may take place even within a few days, doubtless produce the changes 
observed in the size, shape, and brilliancy of comets. When a comet 
becomes visible to the naked eye, it is generally seen but a short time, and 
has a very different course in the heavens from the planets, though it 
follows the usual daily motion of the heavens. Formerly, on account of the 
rarity of comets, their remarkable appearance, and their course, it was 
supposed that they were not true heavenly bodies. Newton, however, first 
showed that they, like the planets, are heavenly bodies belonging to our 
solar system. Like the planets, they describe, according to the same laws, 
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