ile _potential_ energy
is reserve, or stored, energy. All moving bodies have kinetic
energy, and all stationary bodies which have within them the
_capability_ of causing motion possess potential energy. A bent bow,
a piece of stretched rubber, a suspended weight, the water above a
mill dam, all have the capability of causing motion and all have
potential energy. Examples of kinetic energy are found in the
movements of machinery, in steam and electricity, in winds, and in
currents of water. Kinetic is the active, and potential the
inactive, form of energy.

70 As the atoms of hydrogen and oxygen that make up the molecules of
water separate, they unite with atoms of their own kind--the hydrogen
with hydrogen and the oxygen with oxygen atoms. Since these
combinations are weaker than those of the water molecules, energy is
required to bring about the change. But when hydrogen burns in the
oxygen, the change is from a weaker to a stronger combination. The
stored-up energy is then given up or becomes active.

71 In the evaporation of water, the energy of the sun is stored with
reference to the force of gravity. In evaporating, water rises as a
gas, or vapor, above the earth's surface, but on condensing into a
liquid, it falls as rain. It then finds its way through streams back
to the ocean. All water above the sea level is in such a position
that gravity can act on it to cause motion, and it possesses, on
this account, potential or stored-up energy. It is because of this
energy that rapids and waterfalls are such important sources of
power.

72 Energy, like matter, can neither be created nor destroyed. It can,
however, be transferred from one body to another and transformed
from one form to another form. Whenever work is done, energy is
transferred from the body doing the work, to the body upon which the
work is done. During this process there may, or may not, be a
transformation of energy. In turning a grindstone, kinetic energy is
passed to the stone and used without transformation, but in winding
a clock, the kinetic energy from the hand is transformed into
potential energy in the clock spring. Then as the clock runs down
this is retransformed into kinetic energy, causing the movements of
the wheels.

Not only