00 THE PROGRESS OF SCIENCE Tt 



and this energy may be said to be energy of position, 

 because it depends upon the relative position of 

 the earth and the stone. The stone is solicited to 

 move but cannot, so long as the muscular strength 

 of the holder 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 falling. Hence the energy 

 which was exerted, 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 Thus the energy, withdrawn 

 from the general stock to raise the stone, is re- 

 turned when it falls, and there is no change in the 

 total amount. Energy, as a whole, is conserved. 

 Taking this as a very broad and general state- 

 ment of the essential facts of the case, the raising 

 of the stone is intelligible enough, as a case of 

 the communication of motion from one body to 

 another. But the potential energy of the raised 

 stone is not so easily intelligible. To all appear- 

 ance, there is nothing either pushing or pulling it 

 towards the earth, or the earth towards it ; and 

 yet it is quite certain that the stone tends to move 



