MATTER AND ENERGY. 55 



rest, as solar energy may be laid away in coal. The 

 actinic rays of the sun, which are kinetic or active, 

 elaborated carbon in plants, and this became coal, 

 with an affinity, under kindling, for oxygen, consti- 

 tuting combustion and the liberation of a force that 

 had long been potential, but now, in a restored kinetic 

 or active state, may drive the wheels of a locomotive, 

 or do other work under the guidance of man's skilled 

 hand. 



When a wound-up top is spun, the released activ- 

 ity is kinetic, though the swiftly revolving toy is seem- 

 ingly still. But in this display of energies double 

 kinesis is called into play that imparted in the wind- 

 ing up process, and that spent in running down, grav- 

 ity acting all the time, but not with sufficient force to 

 overthrow the body acted upon. 



The kinetic energy of the savage arm in bending 

 a bow stores up a force which, when suddenly released, 

 again becomes kinetic in speeding the arrow. The 

 force is held potential by the string while the bow is 

 bent. A clock wound up possesses potential energy, 

 and parts with it kinetically when expending the en- 

 ergy in measuring time. Water stored in a pond em- 

 braces potential energy which may be liberated with 

 devastating effect. Vapor in globules of moisture, 

 wafted to the tops of the Alleghenies, is by the cold 

 of the region condensed into rain, which runs in rills 

 and brooks, expending energy in turning wheels, and 

 at length is imprisoned by a barrier, by the Cone- 

 maugh dam. There it rests in a placid lake until the 

 barrier yields, releasing kinetic force enough to over- 

 whelm Johnstown. But a part of the story of trans- 

 mutating energies remains to be told. The sun's heat 

 unlocked the gravitative force of water by converting 



