by
powerful telescopes, they become as diffused and transparent as the
fainter cometic substance. Comets are properly atmospheres without
contained spheres; enormous clouds rushing along in space, and bathed
with its sunshine, for they have no light excepting sunlight. They
become brighter and brighter as they get deeper within the solar
glare, and dimmer and paler as they float outwards from the same. The
light of the comet only differs from the light of a cloud that is
drifted across the cerulean sky of noon, in the fact, that it is
reflected from the inside as well as the outside of the vaporous
substance. The material illuminated reflects light, and is permeated
by light, at once. In this respect it resembles air as much as
cloud--the blueness of the sky is the sunlit air seen through the
lower and inner strata of itself. In the same way, the whiteness of
the comet is sunlit vapour seen through portions of itself. The
sunbeams pass as readily through the entire thickness of the cometic
substance as they do through our own highly permeable atmosphere.

The belief in the comet's surpassing thinness and lightness is not a
mere speculative opinion. It rests upon incontrovertible proof. In
1770, Lexell's Comet passed within six times the moon's distance of
the earth, and was considerably retarded in its motion by the
terrestrial attraction. If its mass had been of equal amount with the
earth's mass, its attraction would have influenced the earth's
movement in a like degree in return, and the earth would have been so
held back in its orbitual progress in consequence, that the year would
have been lengthened to the extent of three hours. The year was not,
however, lengthened on that occasion by so much as the least
perceptible fraction of a second; hence it can be shewn, that the
comet must have been composed of some substance many thousand times
lighter than the terrestrial substance. Newton was of opinion, that a
few ounces of matter would be sufficient for the construction of the
largest comet's tail.

Light as the comet's substance is, it is not, however, light enough to
escape the grasp of the sun's gravitating attraction. When the mass of
thin vapour is rushing through the obscurity of starlit space, so far
from the sun that the solar sphere looks but the brightest of the
stellar host, it feels the influence of the solar mass, remote as it
is, and is constrained to bend its course towards it. Onwards the thin
vapo