nd therefore, as
meteorites are still falling, it would follow that they must still be
constantly ejected from the moon. The volcanoes on the moon are,
however, not now active; observers have long studied its surface, and
they find no reliable traces of volcanic activity at the present day.
It is utterly out of the question, whatever the moon may once have been
able to do, that at the present date she could still continue to launch
forth meteorites. It is just possible that a meteorite expelled from the
moon in remote antiquity, when its volcanoes were active, may, under the
influence of the disturbances of the other bodies of the system, have
its orbit so altered, that at length it comes within reach of the
atmosphere and falls to the earth, but in no circumstances could the
moon send us a meteorite at present. It is therefore reasonable to look
elsewhere in our search for volcanoes fulfilling the conditions of the
problem.

Let us now direct our attention to the planets, and examine the
circumstances in which volcanoes located thereon could eject a meteorite
which should ultimately tumble on the earth. We cannot see the planets
well enough to tell whether they have or ever had any volcanoes; but the
almost universal presence of heat in the large celestial masses seems to
leave us in little doubt that some form of volcanic action might be
found in the planets. We may at once dismiss the giant planets, such as
Jupiter or Saturn: their appearance is very unlike a volcanic surface;
while their great mass would render it necessary to suppose that the
meteorites were expelled with terrific velocity if they should succeed
in escaping from the gravitation of the planet. Applying the rule
already given, a volcano on Jupiter would have to be five or six times
as powerful as the volcano on the earth. To avoid this difficulty, we
naturally turn to the smaller planets of the system; take, for instance,
one of that innumerable host of minor planets, and let us enquire how
far this body is likely to have ejected a missile which should fall upon
the earth. Some of these globes are only a few miles in diameter. There
are bodies in the solar system so small that a very moderate velocity
would be sufficient to project a missile away from them altogether. We
have, indeed, already illustrated this point in discussing the minor
planets. It has been suggested that a volcano placed on one of the minor
planets might be quite powerful enough t