22 FRAGMENTS OF SCIENCE. 



motion nor power of motion. But the same body sus- 

 pended 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, more- 

 over, 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 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 

 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 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 illustrations in point. The hammer of the great 

 bell of Westminster, when raised before striking, pos- 

 sesses potential energy ; when it falls, the energy 

 becomes dynamic ; and after the stroke, we have tne 

 rhythmic play of potential and dynamic in the vibra- 

 tions of the bell. The same holds good for the molecular 

 oscillations of a heated body. An atom is driveo 



