510 Dr. J. R. Mayer on the Mechanical Equivalent of Heat. 



the force of gravity plays in writings on natural science. The 

 inost arbitrary explanations are given of this word, and then, 

 when facts no longer admit of anything else, a retreat is sought 

 in the Newtonian conception. 



Gravity being called a force, and at the same time the term force 

 being connected, in accordance with the common use of language, 

 with the conception of an object capable of producing motion, 

 leads to the false assumption that a mechanical effect (the pro- 

 duction of motion) can be produced without a corresponding 

 expenditure of a measurable object ; and here is likewise plainly 

 the reason why our author could neither keep clear in his facts 

 nor consistent in his reasoning. If once the production of motion 

 out of nothing is granted, the annihilation of motion must also 

 be admitted as a consequence; and the magnitude of motion 

 must, in accordance with this assumption, be simply propor- 

 tional to the velocity, or =Me^ and the " quantity of motion 

 once in existence" must be =+llc— Mc = 0. But notwith- 

 standing his "inexhaustible forces," the writer referred to 

 expressly declares that motion is indestructible; but, instead 

 of stating his opinion as to what becomes of motion which dis- 

 appears by friction, he says in another place again that it remains 

 " undecided " whether the effect of a force (the amount of motion 

 produced by it) is measured by the first or by the second power 

 of the velocity (that is, whether it is or is not destructible) : he 

 even appears, from repeated expressions, to hold it possible that 

 a given quantity of heat can produce motion in infinitum ! If 

 such were the case, it would certainly be useless to consider the 

 convertibility of these magnitudes : the ground would rather have 

 been won for the contact the or v. 



The polemics of my respected critic, whom I have here intro- 

 duced as the representative and spokesman of prevailing views, 

 and to whom I feel that my sincere thanks are due for his atten- 

 tive examination of my first publication, appear to me to be 

 necessarily without result, inasmuch as the first problem in com- 

 bating 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 variable, 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 consider is, not what sort of thing a 

 "force" is, but to what thing we shall give the name "force." 

 Backwards and forwards talk about gravity is fruitless, since all 

 who understand the matter are agreed as to its nature ; for gra- 

 vity is and remains a differential quotient of the velocity by the 

 time, directly proportional to the attracting mass, and inversely 

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



