2. _Heat Power._—The body keeps itself warm and is able to communicate
warmth to its surroundings.

3. _Nervous Power._—Through the nervous system the body exercises the
power of control over its different parts.

As motion, heat, and nervous power the body uses most of its energy.

*The Source of Bodily Energy.*—As already indicated, the energy of the
body is supplied through the food and the oxygen. These contain energy in
the potential form, which becomes kinetic (active) through their uniting
with each other in the body. Somewhat as the power of the steam engine is
derived from the combustion of fuel in the furnaces, the energy of the
body is supplied through the oxidations at the cells. How the food and
oxygen come to possess energy is seen by a study of the general methods by
which energy is stored up and used.

[Fig. 81]


Fig. 81—*Simple device* for storing energy through gravity.


*Simple Methods of Storing Energy.*—Energy is stored by converting the
kinetic into the potential form. Two of the simplest ways of doing this
are the following:

1. _Storing of Energy through Gravity._—On account of the attraction
between the earth and all bodies upon the earth, the mere lifting of a