lcanic source on some celestial body." Let
us attempt to pursue this reasoning and discuss the problem, which may
be thus stated:--Assuming that at least some of the meteorites have been
ejected from volcanoes, on what body or bodies in the universe must
these volcanoes be situated? This is really a question for astronomers
and mathematicians. Once the mineralogists assure us that these bodies
are volcanic, the question becomes one of calculation and of the balance
of probabilities.

The first step in the enquiry is to realise distinctly the dynamical
conditions of the problem. Conceive a volcano to be located on a planet.
The volcano is supposed to be in a state of eruption, and in one of its
mighty throes projects a missile aloft: this missile will ascend, it
will stop, and fall down again. Such is the case at present in the
eruptions of terrestrial volcanoes. Cotopaxi has been known to hurl
prodigious stones to a vast height, but these stones assuredly return to
earth. The gravitation of the earth has gradually overcome the velocity
produced by the explosion, and down the body falls. But let us suppose
that the eruption is still more violent, and that the stones are
projected from the planet to a still greater height above its surface.
Suppose, for instance, that the stone should be shot up to a height
equal to the planet's radius, the attraction of gravitation will then be
reduced to one-fourth of what it was at the surface, and hence the
planet will find greater difficulty in pulling back the stone. Not only
is the distance through which the stone has to be pulled back increased
as the height increases, but the efficiency of gravitation is weakened,
so that in a twofold way the difficulty of recalling the stone is
increased. We have already more than once alluded to this subject, and
we have shown that there is a certain critical velocity appropriate to
each planet, and depending on its mass and its radius. If the missile be
projected upwards with a velocity equal to or greater than this, then it
will ascend never to return. We all recollect Jules Verne's voyage to
the moon, in which he described the Columbiad, an imaginary cannon,
capable of shooting out a projectile with a velocity of six or seven
miles a second. This is the critical velocity for the earth. If we could
imagine the air removed, then a cannon of seven-mile power would project
a body upwards which would never fall down.

The great difficulty about