78 THE ADVANCE OF SCIENCE 



tion of the earth and the stone. The 

 stone is solicited to move but cannot, so 

 long as the muscular strength of the hold- 

 er prevents the solicitation from taking 

 effect. The stone, therefore, has potential 

 energy, which becomes kinetic if it is let 

 go, and the amount of that kinetic energy 

 which will be developed before it strikes 

 the earth depends on its position — on the 

 fact that it is, say, six feet off the earth, 

 neither more nor less. Moreover, it can 

 be proved that the raiser of the stone had 

 to exert as much energy in order to place 

 it in its position, as it will develop in fall- 

 ing. Hence the energy which was exert- 

 ed, and apparently exhausted, in raising 

 the stone, is potentially in the stone, in 

 its raised position, and will manifest itself 

 when the stone is set free. Thus the en- 

 ergy, withdrawn from the general stock 

 to raise the stone, is returned when it 

 falls, and there is no change in the total 

 amount. Energy, as a whole, is conserved. 



