not exactly synonymous terms. Thus electricity, heat, and light are
forms of energy, and are convertible into one another, in the same way
that the forces were convertible. Thus we get transformations of energy
in the same way that we had transformations of force, and conservation
of energy in the same way that we had conservation of force.

Even the term Energy, however, is being replaced in the present times by
something more definite and simple, and instead of the term Energy, we
shall find, in the development of this phase of natural phenomena, that
that term is being replaced by the simple idea of motion, or modes of
motion, and that all forms of energy, as light, heat, magnetism, and
electricity, and even Gravitation itself, are due to motion of some kind
or other. We will, however, lead up to this truth by looking briefly at
the term Energy, and see what it implies and embodies.

Energy, therefore, is that property which a body possesses, by which it
is capable of doing work. Thus our ideas of work give us our conception
of energy. For example, when a weight is lifted, work is done, and a
certain amount of energy is expended in the process. Further, the amount
of work done is proportionate to the weight lifted, and the height to
which the body is raised. Work is done against resistance, so that
whenever resistance is overcome, then work is the result. For example,
suppose one pound is lifted one foot high, in opposition to the force of
gravity, then work is done, and this amount of work is known as a
foot-pound.

If a body weighs ten pounds, and is lifted ten feet, the work done is
equal to ten pounds multiplied by ten feet (10 x 10 equals 100), so that
one hundred times the amount of work has been done in comparison with
the lifting of the one pound one foot high.

As all weight is essentially a gravitational measure, depending upon the
intensity of gravity at the place, then, whenever a body is raised or
lifted, the work so done is done against the gravity of the earth.

Work is also done, as Newton points out in the first and second laws,
whenever we apply force to any body, either stationary or already in
motion. The results of all observation and experiments prove, that
whenever we have two bodies upon which work is being done, the amount of
work is determined by the amount of energy transferred from one body to
the other, and that the actual amount of energy gained by one is equal
to the amount of energ