of the oceans which form our tides present a
picture widely different from the tides by which the moon was once
agitated. The tides on the moon were vastly greater than those of the
earth. They were greater because the weight of the earth is greater than
that of the moon, so that the earth was able to produce much more
powerful tides in the moon than the moon has ever been able to raise on
the earth.

That the moon should bend the same face to the earth depends immediately
upon the condition that the moon shall rotate on its axis in precisely
the same period as that which it requires to revolve around the earth.
The tides are a regulating power of unremitting efficiency to ensure
that this condition shall be observed. If the moon rotated more slowly
than it ought, then the great lava tides would drag the moon round
faster and faster until it attained the desired velocity; and then, but
not till then, they would give the moon peace. Or if the moon were to
rotate faster on its axis than in its orbit, again the tides would come
furiously into play; but this time they would be engaged in retarding
the moon's rotation, until they had reduced the speed of the moon to one
rotation for each revolution.

Can the moon ever escape from the thraldom of the tides? This is not
very easy to answer, but it seems perhaps not impossible that the moon
may, at some future time, be freed from tidal control. It is, indeed,
obvious that the tides, even at present, have not the extremely
stringent control over the moon which they once exercised. We now see no
ocean on the moon, nor do the volcanoes show any trace of molten lava.
There can hardly be tides _on_ the moon, but there may be tides _in_ the
moon. It may be that the interior of the moon is still hot enough to
retain an appreciable degree of fluidity, and if so, the tidal control
would still retain the moon in its grip; but the time will probably
come, if it have not come already, when the moon will be cold to the
centre--cold as the temperature of space. If the materials of the moon
were what a mathematician would call absolutely rigid, there can be no
doubt that the tides could no longer exist, and the moon would be
emancipated from tidal control. It seems impossible to predicate how far
the moon can ever conform to the circumstances of an actual rigid body,
but it may be conceivable that at some future time the tidal control
shall have practically ceased. There would then be no lo