35@ {Larls of Comets 
form of their orbits,” and adds: “ supposing them to beencircied 
with an atmosphere, like the planets, would not the incidence 
of the sun’s rays upon them, in their very elliptical orbits, 
produce some such effect ?”’ 
The following extract from the Edinburgh Encyclopedia, 
article Comet, is probably as much to the purpose as any 
thing which can be said on a subject respectirig which we 
can have no precise knowledge. 
*¢ Various opinions have been entertained by astronomers, 
respecting the tailsof comets. They were supposed by Appian, 
Cardan, and Tycho Brahe, to be the light of the sun transmit- 
ted through the nucleus of the comet, which they believed to 
be transparent like a lens. Kepler thought that the impul- 
sion of the solar rays drove away the denser parts of the 
comet’s atmosphere, and thus formed the tail. Descartes as- 
cribes the tail to the refraction of light by the nucleus. New- 
ton maintained that it is a thin vapour, raised by the heat of 
the sun from the comet. Euler asserts that the tail is occa~ 
sioned by the impulsion of the solar rays driving off the at- 
mosphere of the comet; and that the curvature observed in 
the tail, is the joint effect of this impulsive force, and the gra- 
vitation of the atmospheric particles to the solid nucleus. 
Mairan imagines that comets’ tails are portions of the sun’s 
atmosphere. Dr. Hamilton, of Dublin, supposes them to be 
streams of electric matter; and Biot supposes, with Newton, 
that the tails are vapours produced by the excessive heat of 
the sun; and also that the comets are solid bodies before they 
reach their perihelion; but that they are afterwards either 
partly or totally converted into vapour by the intensity of the 
solar heat. 
‘¢ Of all the theories, that of Euler seems to be the most phi- 
losophical. Since the comets are composed chiefly of nebu- 
lous matter, and have very large atmospheres, the external 
atmospheric strata must be drawn towards the comet by very 
slight powers of attraction, and will therefore yield to the 
smallest impulse. From the great density of the planets, on 
the contrary, and the small size of their atmospheres, the ex- 
ternal strata are attracted towards them with a very great 
force, and therefore cannot yield, like those of the comets, to 
a slight impulse. Here we see the reason why the comets 
have tails, while none of the planetary bodies exhibit such a 
phenomenon. Whatever opinion may be entertained of this 
explanation, it must, at least, be admitted that, if light is a 
