ther great planets.
Of course, it will be understood that when we say the weight of a comet
is inappreciable, we mean with regard to the other bodies of our system.
Perhaps no one now doubts that a great comet must really weigh tons;
though whether those tons are to be reckoned in tens, in hundreds, in
thousands, or in millions, the total seems quite insignificant when
compared with the weight of a body like the earth.

The small mass of comets is also brought before us in a very striking
way when we recall what has been said in the last chapter on the
important subject of the planetary perturbations. We have there treated
of the permanence of our system, and we have shown that this permanence
depends upon certain laws which the planetary motions must invariably
fulfil. The planets move nearly in circles, their orbits are all nearly
in the same plane, and they all move in the same direction. The
permanence of the system would be imperilled if any one of these
conditions was not fulfilled. In that discussion we made no allusion to
the comets. Yet they are members of our system, and they far outnumber
the planets. The comets repudiate these rules of the road which the
planets so rigorously obey. Their orbits are never like circles; they
are, indeed, more usually parabolic, and thus differ as widely as
possible from the circular path. Nor do the planes of the orbits of
comets affect any particular aspect; they are inclined at all sorts of
angles, and the directions in which they move seem to be mere matters of
caprice. All these articles of the planetary convention are violated by
comets, but yet our system lasts; it has lasted for countless ages, and
seems destined to last for ages to come. The comets are attracted by the
planets, and conversely, the comets must attract the planets, and must
perturb their orbits to some extent; but to what extent? If comets moved
in orbits subject to the same general laws which characterise planetary
motion, then our argument would break down. The planets might experience
considerable derangements from cometary attraction, and yet in the lapse
of time those disturbances would neutralise each other, and the
permanence of the system would be unaffected. But the case is very
different when we deal with the actual cometary orbits. If comets could
appreciably disturb planets, those disturbances would not neutralise
each other, and in the lapse of time the system would be wrecked by a
continuou