vity.

*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
weight puts it in a position where gravity can cause it to move (Fig. 81).
As a consequence _the raising of bodies above the earth's surface is a
means of storing energy_--the energy remaining stored until the bodies
fall. As they fall, the stored-up (potential) energy becomes kinetic and
can be made to do work.

2. _Storing of Energy through Elasticity._--Energy is stored also by doing
work in opposition to elasticity, as in bending a bow or in winding a
clock spring. The bending, twisting, stretching, or compressing of elastic
substances puts them in a condition of _strain_ which causes them to exert
a pressure (called elastic force) that tends to restore them to their
former condition. Energy stored by this means becomes active as the
distorted or compressed substance returns to its former shape or volume.

These simple methods of storing energy will serve to illustrate the
general principles upon which such storage depends:

1. To store energy, energy must be expended, or work done.

2. The work must be against some force, such as gravity or elasticity,
which can undo the work, i.e., bring about an effect opposite to that of
the work.

3. The stored energy becomes active (kinetic) as the force through which
the energy was stored undoes the work, or puts the substance upon which
the work was done into its former condition (gravity causing bodies to
fall, etc.).

These principles are further illustrated by the

*Storing of Energy through Chemical Means.*--A good example of storing
energy by chemical means is that of decomposing water with electricity. If
a current of electricity is passed through acidulated water in a suitable
apparatus (Fig. 82), the water separates into its component gases, oxygen
and hydrogen. These gases now have power (energy) which they did not
possess before they were separated. The hydrogen will burn in the oxygen,
giving heat; and if the two gases are mixed in the right proportions and
then ignited, they explode with violence. This energy was derived from the
electricity. It was stored by _decomposing_ the water.

[Fig. 82]

Fig. 82--*S