driven off has been supplied by the rifleman? Certainly not;
the energy is, of course, due to the gunpowder, and all the rifleman did
was to provide the means by which the energy stored up in the powder
could be liberated. To a certain extent we may compare this with the
tidal problem; the tides raised by the moon are the originating cause
whereby a certain store of energy is drawn upon and applied to do such
work as the tides are competent to perform. This store of energy,
strange to say, does not lie in the moon; it is in the earth itself.
Indeed, it is extremely remarkable that the moon actually gains energy
from the tides by itself absorbing some of the store which exists in the
earth. This is not put forward as an obvious result; it depends upon a
refined dynamical theorem.

We must clearly understand the nature of this mighty store of energy
from which the tides draw their power, and on which the moon is
permitted to make large and incessant drafts. Let us see in what sense
the earth is said to possess a store of energy. We know that the earth
rotates on its axis once every day. It is this rotation which is the
source of the energy. Let us compare the rotation of the earth with the
rotation of the fly-wheel belonging to a steam-engine. The rotation of
the fly-wheel is really a reservoir, into which the engine pours energy
at each stroke of the piston. The various machines in the mill worked by
the engine merely draw upon the store of energy accumulated in the
fly-wheel. The earth may be likened to a gigantic fly-wheel detached
from the engine, though still connected with the machines in the mill.
From its stupendous dimensions and from its rapid velocity, that great
fly-wheel possesses an enormous store of energy, which must be expended
before the fly-wheel comes to rest. Hence it is that, though the tides
are caused by the moon, yet the energy they require is obtained by
simply appropriating some of the vast supply available from the rotation
of the earth.

There is, however, a distinction of a very fundamental character between
the earth and the fly-wheel of an engine. As the energy is withdrawn
from the fly-wheel and consumed by the various machines in the mill, it
is continually replaced by fresh energy, which flows in from the
exertions of the steam-engine, and thus the velocity of the fly-wheel is
maintained. But the earth is a fly-wheel without the engine. When the
tides draw upon the store of energy and e