e_ of a lunar atmosphere as its
_disappearance_. But disappearance, of course, implies previous
existence. In like manner it has always been a commonly accepted view
that the moon probably once had enough water to form lakes and seas.

These, it has been calculated, could have been absorbed into the lunar
globe as it cooled off. But Johnstone Stoney's theory offers another
method by which they could have escaped, through evaporation and the
gradual flight of the molecules into open space. Possibly both methods
have been in operation, a portion of the constituents of the former
atmosphere and oceans having entered into chemical combinations in the
lunar crust, and the remainder having vanished in consequence of the
lack of sufficient gravitative force to retain them.

But why, it may be asked, should it be assumed that the moon ever had
things which it does not now possess? Perhaps no entirely satisfactory
reply can be made. Some observers have believed that they detected
unmistakable indications of alluvial deposits on lunar plains, and of
the existence of beaches on the shores of the "seas." Messrs. Loewy and
Puiseux, of the Paris Observatory, whose photographs of the moon are
perhaps the finest yet made, say on this subject:

"There exists, from the point of view of relief, a general similarity
between the 'seas' of the moon and the plateaux which are covered to-day
by terrestrial oceans. In these convex surfaces are more frequent than
concave basins, thrown back usually toward the verge of the depressed
space. In the same way the 'seas' of the moon present, generally at the
edges, rather pronounced depressions. In one case, as in the other, we
observe normal deformations of a shrinking globe shielded from the
erosive action of rain, which tends, on the contrary, in all the
abundantly watered parts of the earth to make the concave surfaces
predominate. The explanation of this structure, such as is admitted at
present by geologists, seems to us equally valid for the moon."[17]

[Footnote 17: Comptes Rendus, June 26, July 3, 1899.]

It might be urged that there is evidence of former volcanic activity on
the moon of such a nature that explosions of steam must have played a
part in the phenomena, and if there was steam, of course there was
water.

But perhaps the most convincing argument tending to show that the moon
once had a supply of water, of which some remnant may yet remain below
the surface of the lunar glob