. The problem is thus not a simple one, for there are several
disturbing causes, and for none of them are the data enough to base a
quantitative estimate upon; but one certain agent in lengthening the
day, and almost certainly the main agent, is to be found in the tides.

Remember that the tidal humps were produced as the prolateness of a
sphere whirled round and round a fixed centre, like a football whirled
by a string. These humps are pulled at by the moon, and the earth
rotates on its axis against this pull. Hence it tends to be constantly,
though very slightly, dragged back.

In so far as the tidal wave is allowed to oscillate freely, it will
swing with barely any maintaining force, giving back at one
quarter-swing what it has received at the previous quarter; but in so
far as it encounters friction, which it does in all channels where
there is an actual ebb and flow of the water, it has to receive more
than it gives back, and the balance of energy has to be made up to it,
or the tides would cease. The energy of the tides is, in fact,
continually being dissipated by friction, and all the energy so
dissipated is taken from the rotation of the earth. If tidal energy were
utilized by engineers, the machines driven would be really driven at the
expense of the earth's rotation: it would be a mode of harnessing the
earth and using the moon as fixed point or fulcrum; the moon pulling at
the tidal protuberance, and holding it still as the earth rotates, is
the mechanism whereby the energy is extracted, the handle whereby the
friction brake is applied.

Winds and ocean currents have no such effect (as Mr. Fronde in
_Oceania_ supposes they have), because they are all accompanied by
a precisely equal counter-current somewhere else, and no internal
rearrangement of fluid can affect the motion of a mass as a whole;
but the tides are in different case, being produced, not by
internal inequalities of temperature, but by a straightforward pull
from an external body.

The ultimate effect of tidal friction and dissipation of energy will,
therefore, be to gradually retard the earth till it does not rotate with
reference to the moon, _i.e._ till it rotates once while the moon
revolves once; in other words, to make the day and the month equal. The
same cause must have been in operation, but with eighty-fold greater
intensity, on the moon. It has ceased now, because the rotation has
stopped, but if