34:0 THE MECHANICAL EQUIVALENT OF HEAT. 



power of the velocity (that is, whether it is or is not de- 

 structible) : he even appears, from repeated expressions, to 

 hold it possible that a given quantity of heat can produce mo- 

 tion ad infinitum ! If such were the case, it would certainly 

 be useless to consider the convertibility of these magni- 

 tudes : the ground would rather have been won for the contact 

 theory. 



The polemics of my respected critic, whom I have here 

 introduced as the representative and spokesman of prevailing 

 views, and to whom I feel that my sincere thanks are due for 

 his attentive examination of my first publication, appear to 

 me to be necessarily without result, inasmuch as the first 

 problem in combating my assertions, which all revolve about 

 the one point of an invariable quantitative relation between 

 heat and motion, must be to find out that this relation is va- 

 riable, and in what cases. Formal controversy without a 

 material basis is only beating the air ; and as to what relates 

 specially to the questions about force, the first point to con- 

 sider is, not what sort of thing a "force" is, but to what 

 thing we shall give the name " force." Backwards and for- 

 wards talk about gravity is fruitless, since all who understand 

 the matter are agreed as to its nature ; for gravity is and re- 

 mains a differential quotient of the velocity by the time, di- 

 rectly proportional to the attracting mass, and inversely pro- 

 portional to the square of the distance : on this point a final 

 decision was come to long ago. But whether it is expedient 

 to call this magnitude a force is quite another question. 



Since, whenever an innovation of essential importance is 

 proposed, the public is so ready to misapprehend, I will here 

 state once more, as clearly as I can, my reasons for saving 

 that " the force of gravity " is an improper expression. 



It is an unassailable truth that the production of every 

 falling motion is connected with a corresponding expenditure 

 of a measurable magnitude. This magnitude, if it is to be 

 made an object of scientific investigation (and why should it 



