o start the meteorites on a
long ramble through space until the chapter of accidents brought them
into collision with the earth. There is but little difficulty in
granting that there might be such volcanoes, and that they might be
sufficiently powerful to drive bodies from the surface of the planet;
but we must remember that the missiles are to fall on the earth, and
dynamical considerations are involved which merit our close attention.
To concentrate our ideas, we shall consider one of the minor planets,
and for this purpose let us take Ceres. If a meteorite is to fall upon
the earth, it must pass through the narrow ring, some 8,000 miles wide,
which marks the earth's path; it will not suffice for the missile to
pass through the ecliptic on the inside or on the outside of the ring,
it must be actually through this narrow strip, and then if the earth
happens to be there at the same moment the meteorite will fall. The
first condition to be secured is, therefore, that the path of the
meteorite shall traverse this narrow ring. This is to be effected by
projection from some point in the orbit of Ceres. But it can be shown on
purely dynamical grounds that although the volcanic energy sufficient to
remove the projectile from Ceres may be of no great account, yet if that
projectile is to cross the earth's track, the dynamical requirements of
the case demand a volcano on Ceres at the very least of three-mile
power. We have thus gained but little by the suggestion of a minor
planet, for we have not found that a moderate volcanic power would be
adequate. But there is another difficulty in the case of Ceres, inasmuch
as the ring on the ecliptic is very narrow in comparison with the other
dimensions of the problem. Ceres is a long way off, and it would require
very great accuracy in volcanic practice on Ceres to project a missile
so that it should just traverse this ring and fall neither inside nor
outside, neither above nor below. There must be a great many misses for
every hit. We have attempted to make the calculation by the aid of the
theory of probabilities, and we find that the chances against this
occurrence are about 50,000 to 1, so that out of every 50,000
projectiles hurled from a point in the orbit of Ceres only a single one
can be expected to satisfy even the first of the conditions necessary if
it is ever to tumble on our globe. It is thus evident that there are two
objections to Ceres (and the same may be said of the ot