16 FRAGMENTS OF SCIENCE. 
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 ma- 
terial universe is a constant quantity. 
A body cast upward consumes the actual energy of pro- 
jection, 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 Holzrinne of the woodcutters 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 stone 
avalanches of the Matterhorn and Weisshorn are illustra- 
tions in point. The hammer of the great bell of West- 
minster, when raised before striking, 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 oscillations of a heated body. An 
atom is driven against its neighbor, and recoils. The ulti- 
mate amplitude of the recoil being attained, the motion of 
the atom in that direction is checked, and for an instant 
its energy is all potential. It is then drawn toward its 
neighbor with accelerated speed; thus, by attraction, con- 
verting its potential into dynamic energy. Its motion in 
this direction is also finally checked, and again, for an in- 
stant, its energy is all potential. It once more retreats, 
converting, by repulsion, its potential into dynamic energy, 
till the latter attains a maximum, after which it is again 
Changed into potential energy. Thus, what is true of the 
