a trifle larger than it appears at other times. By this I
mean that it really is seen larger, because it is closer to us. But you
have no doubt often noticed that when the moon is near the horizon it
seems to be very large indeed. This apparent increase of size is,
however, an illusion, owing to our unconsciously comparing it with the
apparent size of terrestrial objects.

"The surface of the moon shows evidence of very violent volcanic action
having occurred in every part of it, and astronomers in the past were
much puzzled to account for the excessive volcanic energy which was
indicated by what they saw, as such a small globe as the moon would not,
in the ordinary course of events, have ever possessed sufficient heat to
have developed such violent action. A theory of later years has,
however, provided a reasonable explanation. It is that the moon was at
one time a part of the same mass as the earth, which became separated
from it before the earth had quite cooled down and solidified into its
present form, and was then gradually driven farther and farther away
from the earth by natural forces. It was therefore originally as hot as
the rest of the mass which formed the earth, but being formed into a
smaller globe of much less gravity--only one-sixth of that of the
earth--volcanic action of the same intensity as that on the earth would
have a much more far-reaching effect. A force which on the earth would
project volcanic lava and scoriae a distance of three miles would, on the
moon, project it a distance of eighteen miles. This accounts for the
very high mountains we see on the moon, some of which are comparatively,
for the size of the globe, much higher than those on the earth. It also
accounts for the vast size of the lunar craters, ring-plains, and
ring-mountains.

"These latter are formations quite unknown upon our earth, but on the
moon they are numbered by hundreds of all sizes, from a few miles up to
one hundred and fifty miles in diameter. They are large plains, roughly
circular in shape, and surrounded by mountains; in a few cases the ring
is in some parts a double range of mountains. Sometimes the plain (or
'floor,' as it is termed) is many thousands of feet below the general
level of the lunar surface; in a few cases it is raised considerably
above it, and in one or two instances, instead of being flat, the floor
is convex. Some of the mountain rings are comparatively low, but in
other cases the mountains ar