22 



HABIT AND INTELLIGENCE. 



[chap. 



instance, to construct a machine that can do work without 

 parting with energy ; and when the energy is all parted 

 with, any machine whatever must necessarily cease to do 

 any more work unless a fresh supply of energy is brought 

 in from without. It is impossible to make a water-mill 

 work without a constantly renewed supply of water, or to 

 make a steam-engine work without a constantly renewed 

 supply of fuel. Every one who understands mechanics 

 knows that any such inexhaustible source of energy is 

 impossible by means of merely mechanical arrangements ; 

 but it is equally true, though not perhaps quite so evident, 

 that it is impossible by means of any arrangement of 

 thermal, electric, or chemical forces. 



N T E. 



FORCE AND KNERGY, 



Faraday's 

 question, 



as to the 

 law of 

 conserva- 

 tion. 



Answer. 



In the present note I purpose to add some further explanations 

 and illustrations of the relation between Force and Energy, in 

 order to assist the formation of clear conceptions on a subject 

 that lies at the very root of physical science, and yet is often 

 insufficiently understood. 



Some years ago, the greatest purely experimental philosopher 

 then living, and perhaps the greatest that has ever lived — I 

 mean, of course, Michael Faraday — surprised not only scientific 

 men but thinking and reading men generally, with the question : 

 How can the law of the Conservation of Force, according to 

 which force is invariable, be reconciled with the law that the 

 force of gravity varies inversely as the square of the distance 1 



This question really perplexed Faraday's mind. But the 

 difficulty arises from a purely verbal confusion. Speak of the 

 Conservation of Energy instead of the Conservation of Force, 

 and the apparent contradiction disappears. There is no contra- 

 diction between the law of the conservation of energy, and the 

 law that gravitative /orce varies in the ratio of the inverse square 

 of the distance. 



