be
preserved intact. Note, how far we were justified in speaking of this
condition as a final one. It is final so far as the lunar tides are
concerned; and were the system to be screened from all outer
interference, this accommodation between the earth and the moon would
be eternal.

There is indeed another way of demonstrating that a condition of the
system in which the day has assumed equality with the month must
necessarily be one of dynamical equilibrium. We have shown that the
energy which the tides demand is derived not from the mere fact that
there are high tides and low tides, but from the circumstance that
these tides do rise and fall; that in falling and rising they do
produce currents; and it is these currents which generate the friction
by which the earth's velocity is slowly abated, its energy wasted, and
no doubt ultimately dissipated as heat. If therefore we can make the
ebbing and the flowing of the tides to cease, then our argument will
disappear. Thus suppose, for the sake of illustration, that at a
moment when the tides happened to be at high water in the Thames, such
a change took place in the behaviour of the moon that the water always
remained full in the Thames, and at every other spot on the earth
remained fixed at the exact height which it possessed at this
particular moment. There would be no more tidal friction, and
therefore the system would cease to course through that series of
changes which the existence of tidal friction necessitates.

But if the tide is always to be full in the Thames, then the moon must
be always in the same position with respect to the meridian, that is,
the moon must always be fixed in the heavens over London. In fact, the
moon must then revolve around the earth just as fast as London
does--the month must have the same length as the day. The earth must
then show the same face constantly to the moon, just as the moon
always does show the same face towards the earth; the two globes will
in fact revolve as if they were connected with invisible bonds, which
united them into a single rigid body.

We need therefore feel no surprise at the cessation of the progress of
tidal evolution when the month and the day are equal, for then the
movement of moon-raised tides has ceased. No doubt the same may be
said of the state at the beginning of the history, when the day and
the month had the brief and equal duration of a few hours. While the
equality of the two periods lasted t