ntary force on the opposite side of the
curve. Thus beats the heart of the universe, but without increase or
diminution of its total stock of force.

I have thus far tried to steer clear amid confusion, by fixing the
mind of the reader upon things rather than upon names. But good names
are essential; and here, as yet, we are not provided with such. We
have had the force of gravity and living force--two utterly distinct
things. We have had pulls and tensions; and we might have had the
force of heat, the force of light, the force of magnetism, or the
force of electricity--all of which terms have been employed more or
less loosely by writers on physics. This confusion is happily avoided
by the introduction of the term 'energy,' which embraces both _tension_
and _vis viva_. Energy is possessed by bodies already in motion; it is
then actual, and we agree to call it actual or dynamic energy. It is
our old _vis viva_. On the other hand, energy is possible to bodies not
in motion, but which, in virtue of attraction or repulsion, possess a
power of motion which would realise itself if all hindrances were
removed. Looking, for example, at gravity; a body on the earth's
surface in a position from which it cannot fall to a lower one
possesses no energy. It has neither motion nor power of motion. But
the same body suspended at a height above the earth has a power of
motion, though it may not have exercised it. Energy is possible to
such a body, and we agree to call this potential energy. It consists
of our old tensions. We, moreover, speak of the conservation of
energy, instead of the conservation of force; and say that the sum of
the potential and dynamic energies of the material universe is a
constant quantity.

A body cast upwards consumes the actual energy of projection, and lays
up potential energy. When it reaches its utmost height all its actual
energy is consumed, its potential energy being then a maximum. When
it returns, there is a reconversion of the potential into the actual.
A pendulum at the limit of its swing possesses potential energy; at
the lowest point of its arc its energy is all actual. A patch of snow
resting on a mountain slope has potential energy; loosened, and
shooting down as an avalanche, it possesses dynamic energy. The
pine-trees growing on the Alps have potential energy; but rushing down
the _Holzrinne_ of the woodcutters they possess actual energy. The same
is true of the mountain