tions have brought
them down upon the earth. A body shot radially from the surface of the
moon would need to have a velocity of only about a mile and a half in a
second in order to escape from the moon's control, and we can believe
that a lunar volcano when in action could have imparted such a velocity,
all the more readily because with modern gunpowders we have been able to
give to projectiles a speed one half as great as that needed for
liberation from lunar gravity.

Another consequence of the small gravitative power of the moon bears
upon the all-important question of atmosphere. According to the theory
of Dr. Johnstone Stoney, heretofore referred to, oxygen, nitrogen, and
water vapor would all gradually escape from the moon, if originally
placed upon it, because, by the kinetic theory, the maximum velocities
of their molecules are greater than a mile and a half per second. The
escape would not occur instantly, nor all at once, for it would be only
the molecules at the upper surface of the atmosphere which were moving
with their greatest velocity, and in a direction radial to the center of
the moon, that would get away; but in the course of time this gradual
leakage would result in the escape of all of those gases.[16]

[Footnote 16: The discovery of free hydrogen in the earth's atmosphere,
by Professor Dewar, 1901, bears upon the theory of the escape of gases
from a planet, and may modify the view above expressed. Since hydrogen
is theoretically incapable of being permanently retained in the free
state by the earth, its presence in the atmosphere indicates either that
there is an influx from space or that it emanates from the earth's
crust. In a similar way it may be assumed that atmospheric gases can be
given off from the crust of the moon, thus, to a greater or less extent,
supplying the place of the molecules that escape.]

After it had been found that, to ordinary tests, the moon offered no
evidence of the possession of an atmosphere, and before Dr. Stoney's
theory was broached, it was supposed by many that the moon had lost its
original supply of air by absorption into its interior. The oxygen was
supposed to have entered into combination with the cooling rocks and
minerals, thus being withdrawn from the atmosphere, and the nitrogen was
imagined to have disappeared also within the lunar crust. For it seems
to have always been tacitly assumed that the phenomenon to be accounted
for was not so much the _absenc