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THE CONSTITUTION OF NATURE. 27

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 embraces our old
tensions. We, moreover, speak of' the conservation of en-
ergy instead of the conservation of force; and say that the
sum of the potential and dynamic energies of the material
universe 1s a constant quantity.

A body cast upward 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 re-
turns, 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 Holzrimze of the wood-cutters they possess actual
energy. The same is true of the mountains themselves.
As long as the rocks which compose them can fall to a
lower level, they possess potential energy, which is con-
verted into actual when the frost ruptures their cohesion
and hands them over to the action of gravity. The hammer
of the great bell Of Westminster, when raiSed before strik-
ing, possesses potential energy ; when it falls, the energy
becomes dynamic; and after the stroke, we have the
rhythmic play _of potential and dynamic in the vibrations
of the bell. The same holds good for the molecular oscilla-
tions of a heated body. An atom is pressed against its
neighbor, and recoils. But the ultimate amplitude of the
recoil is soon attained, the motion of the atom in that
direction is checked, and for an instant its energy is all

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