tc., are all forms of potential energy, or energy which is stored
up in matter. All forms of food have a certain amount of energy in them,
which is used up in the body in building up waste tissue and imparting
energy to the physical frame.

Again, all forms of water-power, whether it be in the form of the
flowing river or the tidal motion of the sea, possess a large amount of
potential energy which may be used up to do mechanical work. They also
possess kinetic energy, or energy of motion. We find illustrations of
the possession of potential energy by rivers and tides, in the fact that
by their fall from a higher to a lower level they may be made to do
mechanical work, as in the case of the turning of the water-wheel by the
fall of the water, which motion is communicated to machinery, and
various forms of work are the result. In Switzerland and America
advantage is being taken of the energy of falling water to generate
electricity, by means of which villages and towns are being supplied
with electric light at a very small cost.

ART. 55. _Kinetic Energy._--Kinetic energy may be defined as energy of
motion, and is the energy which a body possesses in consequence of its
motion. A body in motion thus possesses kinetic energy, which it must
impart to some other body before it can be brought to a state of rest.
The body may be simply an atom, as a vortex atom, but if it be in
motion, as all atoms are, then it must possess kinetic energy, which may
be transferred to another atom by collision, or by some other method. As
has already been pointed out in previous articles, kinetic and potential
energy are complementary to one another, the sum-total of the two
combined always remaining the same in any cycle of work, according to
the principle of the conservation of energy. We get a good example of
this oscillation from kinetic to potential, and _vice versa_, in the
planetary system. When the earth is farthest from the sun, its velocity,
and consequently its kinetic energy, is at its lowest point; but there
the potential energy is at its greatest. As the earth turns round in its
orbit, however, and begins to approach the sun again, its potential
energy decreases, while its kinetic energy increases with its increased
velocity. So that by the time it has reached the nearest part of its
orbit to the sun, its velocity, and consequently its kinetic energy, is
at a maximum, while the potential energy is at a minimum. Then as the
eart