best, its career as a living world must have been brief. If the water
and air were gradually absorbed, as some have conjectured, by its
cooling interior rocks, its surface might, nevertheless, have retained
them for long ages; but if, as others think, their disappearance was
due to the escape of their gaseous molecules in consequence of the
inability of the relatively small lunar gravitation to retain them,
then the final catastrophe must have been as swift as it was
inevitable. Accepting Darwin's hypothesis, that the moon was separated
from the earth by tidal action while both were yet plastic or
nebulous, we may reasonably conclude that it began its career with a
good supply of both water and air, but did not possess sufficient mass
to hold them permanently. Yet it may have retained them long enough
for life to develop in many forms upon its surface; in fact, there are
so many indications that air and water have not always been lacking to
the lunar world that we are driven to invent theories to explain both
their former presence and their present absence.

But whatever the former condition of the moon may have been, its
existing appearance gives it a resistless fascination, and it bears so
clearly the story of a vast catastrophe sculptured on its rocky face
that the thoughtful observer cannot look upon it without a feeling of
awe. The gigantic character of the lunar features impresses the
beholder not less than the universality of the play of destructive
forces which they attest. Let us make a few comparisons. Take the
lunar crater called ``Tycho'', which is a typical example of its kind.
In the telescope Tycho appears as a perfect ring surrounding a
circular depression, in the center of which rises a group of
mountains. Its superficial resemblance to some terrestrial volcanic
craters is very striking. Vesuvius, seen from a point vertically
above, would no doubt look something like that (the resemblance would