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 globe, is based upon the probable similarity in
composition of the earth and the moon. This similarity results almost
equally whether we regard the moon as having originated in a ring of
matter left off from the contracting mass that became the earth, or
whether we accept the suggestion of Prof. G.H. Darwin, that the moon is
the veritable offspring of the earth, brought into being by the
assistance of the tidal influence of the sun. The latter hypothesis is
the more picturesque of the two, and, at present, is probably the more
generally favored. It depends upon the theory of tidal friction, which
was referred to in Chapter III, as offering an explanation of the manner
in which the rotation of the planet Mercury has been slowed down until
its rotary period coincides with that of its revolution.
The gist of the hypothesis in question is that at a very early period in
its history, when the earth was probably yet in a fluid condition, it
rotated with extreme rapidity on its axis, and was, at the same time,
greatly agitated by the tidal attraction of the sun, and finally huge
masses were detached from the earth which, ultimately uniting, became
the moon.
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