us, by the mere
fact of its elevation, it possesses a power of doing work, which it has
lost when it has descended. Again, work done in winding up the spring of
a clock is stored up in the form of potential energy, and gradually runs
out in the form of motion or kinetic energy.

Potential energy is really the complementary principle of kinetic
energy. That is to say, the amount of potential energy lost by any body,
is equal to the amount of kinetic energy gained by the other body, to
which the energy has been transferred. In the case of a body falling, as
the potential energy diminishes, the kinetic energy increases, but the
total amount of the two combined always remains the same. This is well
illustrated in the case of a swinging pendulum. When a pendulum is at
the highest point of its swing, its velocity or kinetic energy is zero,
but at that point its potential energy is greatest. As it descends, the
potential energy decreases, but the kinetic energy increases. When the
pendulum is at the lowest point its energy is wholly kinetic, the
potential energy being zero at that point, while it has sufficient
kinetic energy to raise it to the highest level again. Throughout the
cycle of these operations, the sum-total of the two energies always
remains the same.

Professor Tait points out, in his _Recent Advances in Physical Science_,
that the available sources of all potential energy may be divided into
four classes--

1st. Fuel.
2nd. Food of Animals.
3rd. Water-power.
4th. Tidal Water-power.

All these are different forms of potential energy. Under the head of
fuel he includes not only wood, coal, but also all forms of matter that
may be used or burnt up by heat, or dissolved by chemical agencies. Thus
zinc and lead, which are used in batteries, are merely forms of fuel.
That potential energy resides in such things as wood and coal is a
matter of common experience. All our coal-fields are stores of energy,
which received their energy when in plant form, ages ago, from the sun,
and this energy is now being used to drive our machinery, to warm our
houses, and to give light to our homes and our cities. It has been
calculated that a pound of coal would give out 14,000 heat units, which
is equal to 11,000,000 foot-pounds of work, which is also equal to the
amount of work a horse can do in five hours. Again, all food, whether it
be the food of animals, as vegetables and plants, or of man, as bread,
meat, e