d then the body is said to possess potential
energy. Thus, a bent spring and a raised weight are said to possess
potential energy. In either case, _an energized body receives its energy
by pressure, and has ability to produce pressure on another body_.
Whether or not it does work on another body depends on the rigidity of
the body it acts upon. In any case, it is simply a mechanical
action--body A pushes upon body B (Fig. 1). There is no need to assume
anything more mysterious than mechanical action. Whether body B moves
this way or that depends upon the direction of the push, the point of
its application. Whether the body be a mass as large as the earth or as
small as a molecule, makes no difference in that particular. Suppose,
then, that _a_ (Fig. 2) spends its energy on _b_, _b_ on _c_, _c_ on
_d_, and so on. The energy of _a_ gives translatory motion to _b_, _b_
sets _c_ vibrating, and _c_ makes _d_ spin on some axis. Each of these
has had energy spent on it, and each has some form of energy different
from the other, but no new factor has been introduced between _a_ and
_d_, and the only factor that has gone from _a_ to _d_ has been
motion--motion that has had its direction and quality changed, but not
its nature. If we agree that energy is neither created nor annihilated,
by any physical process, and if we assume that _a_ gave to _b_ all its
energy, that is, all its motion; that _b_ likewise gave its all to _c_,
and so on; then the succession of phenomena from _a_ to _d_ has been
simply the transference of a definite amount of motion, and therefore of
energy, from the one to the other; for _motion has been the only
variable factor_. If, furthermore, we should agree to call the
translatory motion [alpha], the vibratory motion [beta], the
rotary [gamma], then we should have had a conversion of [alpha]
into [beta], of [beta] into [gamma]. If we should consider
the amount of transfer motion instead of the kind of motion, we should
have to say that the [alpha] energy had been transformed into
[beta] and the [beta] into [gamma].

[Illustration: FIG. 1.]

[Illustration: FIG. 2.]

What a given amount of energy will do depends only upon its _form_, that
is, the kind of motion that embodies it.

The energy spent upon a stone thrown into the air, giving it translatory
motion, would, if spent upon a tuning fork, make it sound, but not move
it from its place; while if spent upon a top, would enable the latter to
stand upon