s accumulation of irregularities. The facts, however, show that
the system has lived, and is living, notwithstanding comets; and hence
we are forced to the conclusion that their masses must be insignificant
in comparison with those of the great planetary bodies.

These considerations exhibit the laws of universal gravitation and their
relations to the permanence of our system in a very striking light. If
we include the comets, we may say that the solar system includes many
thousands of bodies, in orbits of all sizes, shapes, and positions, only
agreeing in the fact that the sun occupies a focus common to all. The
majority of these bodies are imponderable in comparison with planets,
and their orbits are placed anyhow, so that, although they may suffer
much from the perturbations of the other bodies, they can in no case
inflict any appreciable disturbance. There are, however, a few great
planets capable of producing vast disturbances; and if their orbits were
not properly adjusted, chaos would sooner or later be the result. By the
mutual adaptations of their orbits to a nearly circular form, to a
nearly coincident plane, and to a uniformity of direction, a permanent
truce has been effected among the great planets. They cannot now
permanently disorganise each other, while the slight mass of the comets
renders them incompetent to do so. The stability of the great planets is
thus assured; but it is to be observed that there is no guarantee of
stability for comets. Their eccentric and irregular paths may undergo
the most enormous derangements; indeed, the history of astronomy
contains many instances of the vicissitudes to which a cometary career
is exposed.

Great comets appear in the heavens in the most diverse circumstances.
There is no part of the sky, no constellation or region, which is not
liable to occasional visits from these mysterious bodies. There is no
season of the year, no hour of the day or of the night when comets may
not be seen above the horizon. In like manner, the size and aspect of
the comets are of every character, from the dim spot just visible to an
eye fortified by a mighty telescope, up to a gigantic and brilliant
object, with a tail stretching across the heavens for a distance which
is as far as from the horizon to the zenith. So also the direction of
the tail of the comet seems at first to admit of every possible
position: it may stand straight up in the heavens, as if the comet were
about to plun