s motion shows
could not reasonably be explained as a mere fortuitous circumstance;
nor need we hesitate to admit that a physical explanation is required
when we find a most satisfactory one ready for our acceptance, as was
originally pointed out by Helmholtz.

There can be no doubt whatever that the constancy of the moon's face
is the work of ancient tides, which have long since ceased to act. We
have shown that if the moon's rotation had once been too rapid to
permit of the same face being always directed towards us, the tides
would operate as a check by which the velocity of that rotation would
be abated. On the other hand, if the moon rotated so slowly that its
other face would be exposed to us in the course of the revolution, the
tides would then be dragged violently over its surface in the
direction of its rotation; their tendency would thus be to accelerate
the speed until the angular velocity of rotation was equal to that of
revolution. Thus the tides would act as a controlling agent of the
utmost stringency to hurry the moon round when it was not turning fast
enough, and to arrest the motion when going too fast. Peace there
would be none for the moon until it yielded absolute compliance to the
tyranny of the tides, and adjusted its period of rotation with exact
identity to its period of revolution. Doubtless this adjustment was
made countless ages ago, and since that period the tides have acted so
as to preserve the adjustment, as long as any part of the moon was in
a state sufficiently soft or fluid to respond to tidal impression. The
present state of the moon is a monument to which we may confidently
appeal in support of our contention as to the great power of the tides
during the ages which have passed; it will serve as an illustration of
the future which is reserved for our earth in ages yet to come, when
our globe shall have also succumbed to tidal influence.

It is owing to the smallness of the moon relatively to the earth that
the tidal process has reached a much more advanced stage in the moon
than it has on the earth; but the moon is incessant in its efforts to
bring the earth into the same condition which it has itself been
forced to assume. Thus again we look forward to an epoch in the
inconceivably remote future when tidal thraldom shall be supreme, and
when the earth shall turn the same face to the moon, as the moon now
turns the same face to the earth.

In the critical state of things thus loom