wonderful line of craters and crateriform valleys of a magnitude
stupendous even for the moon. Another similar line follows the western
edge. Three or four ``seas'' are thrust between these mountainous
belts. By the effects of ``libration'' parts of the opposite
hemisphere of the moon which is turned away from the earth are from
time to time brought into view, and their aspect indicates that that
hemisphere resembles in its surface features the one which faces the
earth. There are many things about the craters which seem to give some
warrant for the hypothesis which has been particularly urged by Mr G.
K. Gilbert, that they were formed by the impact of meteors; but there
are also many things which militate against that idea, and, upon the
whole, the volcanic theory of their origin is to be preferred.

The enormous size of the lunar volcanoes is not so difficult to
account for when we remember how slight is the force of lunar gravity
as compared with that of the earth. With equal size and density,
bodies on the moon weigh only one-sixth as much as on the earth.
Impelled by the same force, a projectile that would go ten miles on
the earth would go sixty miles on the moon. A lunar giant thirty-five
feet tall would weigh no more than an ordinary son of Adam weighs on
his greater planet. To shoot a body from the earth so that it would
not drop back again, we should have to start it with a velocity of
seven miles per second; a mile and a half per second would serve on
the moon. It is by no means difficult to believe, then, that a lunar
volcano might form a crater ring eight or ten times broader than the
greatest to be found on the earth, especially when we reflect that in
addition to the relatively slight force of gravity, the materials of
the lunar crust are probably lighter than those of our terrestrial
rocks.