rmation of the coal
in the furnace into heat. This heat converts water into steam, whose
motion is communicated by proper machinery into a dynamo, the product of
which is electricity. That electricity is then conveyed along wires, and
work is done by it, by moving trams along the connected tram system, or
it may be converted into heat in the carbon filament in the car itself,
which, if heated enough, will then produce the electric light. So that
starting from the coal, we have several transformations therefrom into
the forms of heat, light, motion, and finally mechanical energy, which
results in Work. The question arises as to what is the law of
equivalence in regard to the transformation of energy. That is, if we
have a certain amount of energy of a given sort, how much of any other
sort can be produced by it? The answer is partly to be found in a
statement made by Joule in 1843, which practically embodies what is
known as the first law of Thermo-dynamics, and is as follows: "When
equal quantities of mechanical effects are produced by any means
whatever, from purely thermal sources, or lost in purely thermal
effects, then equal quantities of heat are put out of existence or are
generated, and for every unit of heat measured by raising a pound of
water one degree F. in temperature, you have to expend 772 foot-pounds
of work." From this law we learn that heat may be used to do work, but
that a certain amount of heat is always used up in the process. It can
also be demonstrated that electric currents can do work, but to generate
the currents a certain amount of work must be done.

This equivalence and transformation prevail in all forms of energy,
whether it be mechanical energy, thermal or heat energy, or electrical
energy.

ART. 54. _Potential Energy._--Energy has been divided into two classes,
which are termed respectively Potential Energy and Kinetic Energy. We
will look at the former first.

_Potential Energy_ may be briefly defined as energy of position.

Thus if we lift a body from the ground, the energy which has been
imparted to it is energy of position, or potential energy. A glacier
high up the mountain possesses potential energy, because of its
position. By the mere fact that it is situated high up the mountain, it
has a capacity for doing work by its descent, and if that descent be
very sudden, the work done will be destructive work, as it may sweep
away all houses and villages in its sudden descent. Th