THE CONSTITUTION OF NATURE 27 



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 con- 

 sists 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 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 po- 

 tential 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 en- 

 ergy; at the lowest point of its arc its energy is all actual. 

 A patch of snow resting on a mountain slope has poten- 

 tial 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 converted into ac- 

 tual when the frost ruptures their cohesion and hands 

 them over to the action of gravity. The stone avalanches 

 of the Matterhorn and Weisshorn are illustrations in point. 

 The hammer of the great bell of Westminster, 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 mo- 

 lecular oscillations of a heated body. An atom is driven 

 against its neighbor, and recoils. The ultimate amplitude 



