. With this proviso, however, it matters not what
may happen to the earth or moon, or what influence one of them may
exert upon the other, no matter what tides may be raised, no matter
even if the earth fly into fragments, the whole quantity of spin of
all those fragments would, if added to the spin of the moon, yield the
same unalterable total. We are here in possession of a most valuable
dynamical principle. We are not concerned with any special theory as
to the action of the tides; it is sufficient for us that in some way
or other the tides have been caused by the moon, and that being so,
the principle of the conservation of spin will apply.

Were the earth and the moon both rigid bodies, then there could be of
course no tides on the earth, it being rigid and devoid of ocean. The
rotation of the earth on its axis would therefore be absolutely
without change, and therefore the necessary condition of the
conservation of spin would be very simply attained by the fact that
neither of the constituent parts changed. The earth, however, not
being entirely rigid, and being subject to tides, this simple state of
things cannot continue; there must be some change in progress.

I have already shown that the fact of the ebbing and the flowing of
the tide necessitates an expenditure of energy, and we saw that this
energy must come either from that stored up in the earth by its
rotation, or from that possessed by the moon in virtue of its distance
and revolution. The law of the conservation of spin will enable us to
decide at once as to whence the tides get their energy. Suppose they
took it from the moon, the moon would then lose in energy, and
consequently come nearer the earth. The quantity of spin contributed
by the moon would therefore be lessened, and accordingly the spin to
be made up by the earth would be increased. That means, of course,
that the velocity of the earth rotating on its axis must be increased,
and this again would necessitate an increase in the earth's rotational
energy. It can be shown, too, that to keep the total spin right, the
energy of the earth would have to gain more than the moon would have
lost by revolving in a smaller orbit. Thus we find that the total
quantity of energy in the system would be increased. This would lead
to the absurd result that the action of the tides manufactured energy
in our system. Of course, such a doctrine cannot be true; it would
amount to a perpetual motion! We might as w