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 

 avc 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 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 

 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 wood-cutters they possess actual 

 energy. The same is true of the mountains themselves. 

 As long as ihe 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 



