atmosphere is clearly shown by the perfectly sharp and sudden way in
which the light of a star disappears when it goes behind the moon and
the clear lines of the edge of the satellite in a solar eclipse. The
same evidence shows that there is no vapour of water; moreover, a
careful search which the writer has made shows that the surface has
none of those continuous down grades which mark the work of water
flowing over the land. Nearly all of the surface consists of shallow
or deep pits, such as could not have been formed by water action. We
therefore have not only to conclude that the moon is waterless, but
that it has been in this condition ever since the part that is turned
toward us was shaped.

As the moon, except for the slight movement termed its "libration,"
always turns the same face to us, so that we see in all only about
four sevenths of its surface, it has naturally been conjectured that
the unseen side, which is probably some miles lower than that turned
toward us, might have a different character from that which we behold.
There are reasons why this is improbable. In the first place, we see
on the extreme border of the moon, when the libration turns one side
the farthest around toward the earth, the edge of a number of the
great walled pits such as are so plenty on the visible area; it is
fair to assume that these rings are completed in the invisible realm.
On this basis we can partly map about a third of the hidden side.
Furthermore, there are certain bands of light which, though appearing
on the visible side, evidently converge to some points on the other.
It is reasonable to suppose that, as all other bands radiate from
walled pits, these also start from such topographic features. In this
way certain likenesses of the hidden area to that which is visible is
established, thus making it probable that the whole surface of the
satellite has the same character.

Clearly as the greater part of the moon is revealed to us--so clearly,
indeed, that it is possible to map any elevation of its surface that
attains the height of five hundred feet--the interpretation of its
features in the light of geology is a matter of very great
difficulty. The main points seem to be tolerably clear; they are as
follows: The surface of the moon as we see it is that which was formed
when that body, passing from the state of fluidity from heat, formed a
solid crust. The pits which we observe on its surface are the
depressions which