theory of which we have already spoken. The
latter solution seems, indeed, the most reasonable of the two, for the
force of gravity at the lunar surface appears too weak to hold down any
known gases. This argument seems also to dispose of the question of
absence of water; for Dr. George Johnstone Stoney, in a careful
investigation of the subject, has shown that the liquid in question,
when in the form of vapour, will escape from a planet if its mass is
less than _one-fourth_ that of our earth. And the mass of the moon is
very much less than this; indeed only the _one-eightieth_, as we have
already stated.

In consequence of this lack of atmosphere, the condition of things upon
the moon will be in marked contrast to what we experience upon the
earth. The atmosphere here performs a double service in shielding us
from the direct rays of the sun, and in bottling the heat as a
glass-house does. On the moon, however, the sun beats down in the
day-time with a merciless force; but its rays are reflected away from
the surface as quickly as they are received, and so the cold of the
lunar night is excessive. It has been calculated that the day
temperature on the moon may, indeed, be as high as our boiling-point,
while the night temperature may be more than twice as low as the
greatest cold known in our arctic regions.

That a certain amount of solar heat is reflected to us from the moon is
shown by the sharp drop in temperature which certain heat-measuring
instruments record when the moon becomes obscured in a lunar eclipse.
The solar heat which is thus reflected to us by the moon is, however, on
the whole extremely small; more light and heat, indeed, reach us
_direct_ from the sun in half a minute than we get by _reflection_ from
the moon during the entire course of the year.

With regard to the origin of the lunar craters there has been much
discussion. Some have considered them to be evidence of violent volcanic
action in the dim past; others, again, as the result of the impact of
meteorites upon the lunar surface, when the moon was still in a plastic
condition; while a third theory holds that they were formed by the
bursting of huge bubbles during the escape into space of gases from the
interior. The question is, indeed, a very difficult one. Though
volcanic action, such as would result in craters of the size of
Ptolemaeus, is hard for us to picture, and though the lone peaks which
adorn the centres of many craters have no