22 FKAGMENTS OF SCIENCE. 



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 re- 

 moved. 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 sus- 

 pended at a height above the earth has a power of mo- 

 tion, 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 uni- 

 verse 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 con- 

 sumed, its potential energy being then a maximum. 

 When it returns, there is a reconversion of the poten- 

 tial 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 pos- 

 sesses dynamic energy. The pine-trees growing on the 

 Alps have potential energy; but rushing down the 

 Holzrinne of the woodcutters they possess actual en- 

 ergy. 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 cohe- 

 sion 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 



