it to revolve around the sun in a period of
seven years, instead of twenty-seven, as had previously been the case.
Also, in 1779, the comet known as Lexell's passed quite close to
Jupiter, and its orbit was so changed by that planet's attraction that
it has never been seen since. The density of comets must, as a rule, be
very much less than the one-thousandth part of that of the air at the
surface of our globe; for, if the density of the comet were even so
small as this, its mass would _not_ be inappreciable.

If comets are really undoubted members of the solar system, the
circumstances in which they were evolved must have been different from
those which produced the planets and satellites. The axial rotations of
both the latter, and also their revolutions, take place in one certain
direction;[24] their orbits, too, are ellipses which do not differ much
from circles, and which, furthermore, are situated fairly in the one
plane. Comets, on the other hand, do not necessarily travel round the
sun in the same fixed direction as the planets. Their orbits, besides,
are exceedingly elliptic; and, far from keeping to one plane, or even
near it, they approach the sun from all directions.

Broadly speaking, comets may be divided into two distinct classes, or
"families." In the first class, the same orbit appears to be shared in
common by a series of comets which travel along it, one following the
other. The comets which appeared in the years 1668, 1843, 1880, 1882,
and 1887 are instances of a number of different bodies pursuing the same
path around the sun. The members of a comet family of this kind are
observed to have similar characteristics. The idea is that such comets
are merely portions of one much larger cometary body, which became
broken up by the gravitational action of other bodies in the system, or
through violent encounter with the sun's surroundings.

The second class is composed of comets which are supposed to have been
seized by the gravitative action of certain planets, and thus forced to
revolve in short ellipses around the sun, well within the limits of the
solar system. These comets are, in consequence, spoken of as "captures."
They move around the sun in the same direction as the planets do.
Jupiter has a fairly large comet family of this kind attached to him. As
a result of his overpowering gravitation, it is imagined that during the
ages he must have attracted a large number of these bodies on his own
acc