he critical moment. To obtain this
power an engine of stupendous proportions is sometimes attached
directly to the rollers, but more frequently an engine of rather less
horse-power will be used, the might of this engine being applied to
giving rapid rotation to an immense fly-wheel, which may thus be
regarded as a reservoir full of energy. The rolling mills then obtain
from this store in the fly-wheel whatever energy is necessary for
their gigantic task.

These illustrations will suffice to show how a rapidly rotating body
may contain energy in virtue of its rotation, just as a cannon-ball
contains energy in virtue of its speed of translation, or as a
clock-weight has energy in virtue of the fact that it has some
distance to fall before it reaches the earth. The rotating body need
not necessarily have the shape of a wheel--it may be globular in form;
nor need the axes of rotation be fixed in bearings, like those of the
fly-wheel; nor of course is there any limit to the dimensions which
the rotating body may assume. Our earth is, in fact, a vast rotating
body 8000 miles in diameter, and turning round upon its axis once
every twenty-three hours and fifty-six minutes. Viewed in this way,
the earth is to be regarded as a gigantic fly-wheel containing a
quantity of energy great in correspondence with the earth's mass. The
amount of energy which can be stored by rotation also depends upon the
square of the velocity with which the body turns round; thus if our
earth turned round in half the time which it does at present, that is,
if the day was twelve hours instead of twenty-four hours, the energy
contained in virtue of that rotation would be four times its present
amount.

Reverting now to the earth-moon system, the energy which that system
contains consists essentially of two parts--the moon-energy, whose
composite character I have already explained, and the earth-energy,
which has its origin solely in the rotation of the earth on its axis.
It is necessary to observe that these are essentially distinct--there
is no necessary relation between the speed of the earth's rotation and
the distance of the moon, such as there is between the distance of the
moon and the speed with which it revolves in its orbit.

For completeness, it ought to be added that there is also some energy
due to the moon's rotation on its axis, but this is very small for
two reasons: first, because the moon is small compared with the
earth, and second,