16 FRAGMENTS OF SCIENCE. 



of motion. But the same body suspended at a height 

 above the earth lias 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. AVe, 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 Matterhoru 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 



