. Fr 
oS. Remarks on the Tails of Comets. 
be astonishing. If the tail of a comet were a brilliant object, we 
might expect to witness from its radiance, at least a degree of the 
light imparted by the moon, and by the planets, it having frequent- 
ly the magnitude of many moons and planets ; but no such effect is 
witnessed. Yet in those regions far beyond the atmosphere of 
the earth, a vast combination of faintly illuminated particles, it is 
rational to conclude, would be distinctly visible, even though 
an individual point might be beyond telescopic power. 
“Another objection,” says the writer, “to this theory is, that if 
the rays of the sun are refracted by the vapor of the comets, so 
as to form a luminous train, the same thing should occur to the 
planets, at least to the two inferior planets.” So many objections 
at once present themselves to this view, that it has occurred to me 
to give the summary one, that some comets have no tails. Like 
causes, it is to be admitted, should produce like effects ; but not 
under unlike circumstances. I have not attempted to explain 
why some comets have no tails, a subject far more difficult than 
the one proposed. In the first place, why did the writer say, “at 
least, the two inferior planets.” It is evident that one at least 
of the many manifest points of difference between the circum 
stances of a planet and those of acomet had presented itsel : 
his mind. © That the planets, in common with the earth, re ac: 
companied by atmospheres, I have no doubt. That of the earth 
is exceedingly limited,—that of the moon still more so; nor is 
there conclusive testimony that the atmosphere of any planet 
bears any considerable proportion to its diameter. ‘The atmos 
phere of the earth capable of reflecting the sun’s light, does not 
exceed the one hundred and sixtieth part of the earth’s radii, and 
a portion of this is sufficiently dense to absorb a measure of the 
sun’s light, and the want of combination in the few more vivid 
rays which escape material absorption, even assuming that the 
chemical properties of the atmosphere are identical with those of 
a comet’s envelope, would render them invisible. In the second 
place, though comets in all cases are accompanied with a shining 
envelope, in appearance analogous to an atmosphere, yet its rela- 
tive position bears no resemblance whatever to the atmosphere of 
the earth, nor to those phenomena which indicate the existence of 
atmospheres in the planets. The envelope of a comet which has 
a tail, is visible only on the side of the comet next to the sul, 
and detached entirely from the nucleus, (compared by some wri- 
