nger any
necessary identity between the period of rotation and that of
revolution. A gleam of hope is thus projected over the astronomy of the
distant future. We know that the time of revolution of the moon is
increasing, and so long as the tidal governor could act, the time of
rotation must increase sympathetically. We have now surmised a state of
things in which the control is absent. There will then be nothing to
prevent the rotation remaining as at present, while the period of
revolution is increasing. The privilege of seeing the other side of the
moon, which has been withheld from all previous astronomers, may thus in
the distant future be granted to their successors.

The tides which the moon raises in the earth act as a brake on the
rotation of the earth. They now constantly tend to bring the period of
rotation of the earth to coincide with the period of revolution of the
moon. As the moon revolves once in twenty-seven days, the earth is at
present going too fast, and consequently the tidal control at the
present moment endeavours to retard the rotation of the earth. The
rotation of the moon long since succumbed to tidal control, but that was
because the moon was comparatively small and the tidal power of the
earth was enormous. But this is the opposite case. The earth is large
and more massive than the moon, the tides raised by the moon are but
small and weak, and the earth has not yet completely succumbed to the
tidal action. But the tides are constant, they never for an instant
relax the effort to control, and they are gradually tending to render
the day and the month coincident, though the progress is a very slow
one.

The theory of the tides leads us to look forward to a remote state of
things, in which the moon revolves around the earth in a period equal to
the day, so that the two bodies shall constantly bend the same face to
each other, provided the tidal control be still able to guide the moon's
rotation. So far as the mutual action of the earth and the moon is
concerned, such an arrangement possesses all the attributes of
permanence. If, however, we venture to project our view to a still more
remote future, we can discern an external cause which must prevent this
mutual accommodation between the earth and the moon from being eternal.
The tides raised by the moon on the earth are so much greater than those
raised by the sun, that we have, in the course of our previous
reasoning, held little account of