of four or five days. It is fortunately true that the moon does
not behave thus; but it has the ability of doing so, and thus the mere
separation between the earth and the moon involves the existence of a
stupendous quantity of energy, capable under certain conditions of
undergoing transformation.

There is also another source of mechanical energy besides that we have
just referred to. A rapidly moving body possesses, in virtue of its
motion, a store of readily available energy, and it is easy to show
that energy of this type is capable of transformation into other
types. Think of a cannon-ball rushing through the air at a speed of a
thousand feet per second; it is capable of wreaking disaster on
anything which it meets, simply because its rapid motion is the
vehicle by which the energy of the gunpowder is transferred from the
gun to where the blow is to be struck. Had the cannon been directed
vertically upwards, then the projectile, leaving the muzzle with the
same initial velocity as before, would soar up and up, with gradually
abating speed, until at last it reached a turning-point, the elevation
of which would depend upon the initial velocity. Poised for a moment
at the summit, the cannon-ball may then be likened to the
clock-weight, for the entire energy which it possessed by its motion
has been transformed into the statical energy of a raised weight. Thus
we see these two forms of energy are mutually interchangeable. The
raised weight if allowed to fall will acquire velocity, or the rapidly
moving weight if directed upwards will acquire altitude.

The quantity of energy which can be conveyed by a rapidly moving body
increases greatly with its speed. For instance, if the speed of the
body be doubled, the energy will be increased fourfold, or, in
general, the energy which a moving body possesses may be said to be
proportional to the square of its speed. Here then we have another
source of the energy present in our earth-moon system; for the moon is
hurrying along in its path with a speed of two-thirds of a mile per
second, or about twice or three times the speed of a cannon-shot.
Hence the fact that the moon is continuously revolving in a circle
shows us that it possesses a store of energy which is nine times as
great as that which a cannon-ball as massive as the moon, and fired
with the ordinary velocity, would receive from the powder which
discharged it.

Thus we see that the moon is endowed with two sources o