either is, as already explained, to
destroy the rotation of the body on which the tide is raised, as regards
the tide-raising body, bringing it to turn always the same face towards
its disturber. This, we can see, has been completely brought about in
the case of the moon. There is, however, a secondary or reactive effect.
Action is always mutual. Precisely as much as the moon pulls the
terrestrial tidal wave backward, the tidal wave pulls the moon forward.
But pulling a body forward in its orbit implies the enlargement of that
orbit; in other words, the moon is, as a consequence of tidal friction,
very slowly receding from the earth. This will go on (other
circumstances remaining unchanged) until the lengthening day overtakes
the more tardily lengthening month, when each will be of about 1,400
hours.[1174] A position of what we may call tidal equilibrium between
earth and moon will (apart from disturbance by other bodies) then be
attained.

If, however, it be true that, in the time to come, the moon will be much
farther from us, it follows that in the time past she was much nearer to
us than she now is. Tracing back her history by the aid of Professor
Darwin's clue, we at length find her revolving in a period of somewhere
between two and four hours, almost in contact with an earth rotating
just at the same rate. This was before tidal friction had begun its work
of grinding down axial velocity and expanding orbital range. But the
position was not one of stable equilibrium. The slightest inequality
must have set on foot a series of uncompensated changes. If the moon had
whirled the least iota faster than the earth spun she must have been
precipitated upon it. Her actual existence shows that the trembling
balance inclined the other way. By a second or two to begin with, the
month exceeded the day; the tidal wave crept ahead of the moon; tidal
friction came into play, and our satellite started on its long spiral
journey outward from the parent globe. This must have occurred, it is
computed, _at least_ fifty-four million years ago.

That this kind of tidal reactive effect played its part in bringing the
moon into its present position, and is still, to some slight extent, at
work in changing it, there can be no doubt whatever. An irresistible
conjecture carried the explorer of its rigidly deducible consequences
one step beyond them. The moon's time of revolution, when so near the
earth as barely to escape contact with it, m