llude to this subject further on.

The energy of the earth which is significant in the earth-moon theory
is due to the earth's rotation upon its axis. We may here again use as
an illustration the action of machinery; and the special contrivance
that I now refer to is the punching-engine that is used in our
ship-building works. In preparing a plate of iron to be riveted to the
side of a ship, a number of holes have to be made all round the margin
of the plate. These holes must be half an inch or more in diameter,
and the plate is sometimes as much as, or more than, half an inch in
thickness. The holes are produced in the metal by forcing a steel
punch through it; and this is accomplished without even heating the
plate so as to soften the iron. It is needless to say that an intense
force must be applied to the punch. On the other hand, the distance
through which the punch has to be moved is comparatively small. The
punch is attached to the end of a powerful lever, the other end of the
lever is raised by a cam, so as to depress the punch to do its work.
An essential part of the machine is a small but heavy fly-wheel
connected by gearing with the cam.

This fly-wheel when rapidly revolving contains within it, in virtue of
its motion, a large store of energy which has gradually accumulated
during the time that the punch is not actually in action. The energy
is no doubt originally supplied from a steam-engine. What we are
especially concerned with is the action of the rapidly rotating wheel
as a reservoir in which a large store of energy can be conveniently
maintained until such time as it is wanted. In the action of punching,
when the steel die comes down upon the surface of the plate, a large
quantity of energy is suddenly demanded to force the punch against the
intense resistance it experiences; the energy for this purpose is
drawn from the store in the fly-wheel, which experiences no doubt a
check in its velocity, to be regained again from the energy of the
engine during the interval which elapses before the punch is called on
to make the next hole.

Another illustration of the fly-wheel on a splendid scale is seen in
our mighty steel works, where ponderous rails are being manufactured.
A white-hot ingot of steel is presented to a pair of powerful rollers,
which grip the steel, and send it through at the other side both
compressed and elongated. Tremendous power is required to meet the
sudden demand on the machine at t