because the angular velocity of the moon is also
very small compared with that of the earth. We may therefore dismiss
as insignificant the contributions from this source of energy to the
sum total.

I have frequently used illustrations derived from machinery, but I
want now to emphasize the profound distinction that exists between the
rotation of the earth and the rotation of a fly-wheel in a machine
shop. They are both, no doubt, energy-holders, but it must be borne in
mind, that as the fly-wheel doles out its energy to supply the wants
of the machines with which it is connected, a restitution of its store
is continually going on by the action of the engine, so that on the
whole the speed of the fly-wheel does not slacken. The earth, however,
must be likened to a fly-wheel which has been disconnected with the
engine. If, therefore, the earth have to supply certain demands on its
accumulation of energy, it can only do so by a diminution of its
hoard, and this involves a sacrifice of some of its speed.

In the earth-moon system there is no engine at hand to restore the
losses of energy which are inevitable when work has to be done. But we
have seen that work is done; we have shown, in fact, that the tides
are at present doing work, and have been doing work for as long a
period in the past as our imagination can extend to. The energy which
this work has necessitated can only have been drawn from the existing
store in the system; that energy consists of two parts--the
moon-energy and the earth's rotation energy. The problem therefore for
us to consider is, which of these two banks the tides have drawn on to
meet their constant expenditure. This is not a question that can be
decided offhand; in fact, if we attempt to decide it in an offhand
manner we shall certainly go wrong. It seems so very plausible to say
that as the moon causes the tides, therefore the energy which these
tides expend should be contributed by the moon. But this is not the
case. It actually happens that though the moon does cause the tides,
yet when those tides consume energy they draw it not from the distant
moon, but from the vast supply which they find ready to their hand,
stored up in the rotation of the earth.

The demonstration of this is not a very simple matter; in fact, it is
so far from being simple that many philosophers, including some
eminent ones too, while admitting that of course the tides must have
drawn their energy from one or other