So, even without calling in imaginary
inhabitants to lend it interest, the comparative inability of the moon
to arrest bodies in motion becomes a fact of much significance. It has
led to the theory that meteorites may have originally been shot out of
the moon's great volcanoes, when those volcanoes were active, and may
have circulated about the sun until various perturbations 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.
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