Dynamical Theory of Heat. 379 



earth, and where no motion is involved, is converted, when the 

 weight is liberated, into the force of motion. 



But we see, proceeds Mayer, in numberless cases motion 

 destroyed without producing other motion, or without raising 

 any weight: now a force once in existence cannot become nil; 

 hence the question arises, under what other form does the 

 vanished motion make its appearance ? Here experiment alone 

 can help us. We must choose materials which, while they 

 enable us to destroy motion, will themselves undergo the least 

 alteration possible. (The precision with which Mayer here 

 defines the conditions of experiment is worthy of notice; he 

 intends the motion to produce heat, and therefore he avoids 

 changes of aggregation.) When we rub together two plates of 

 metal, we see that motion disappears, but we also see heat 

 make its appearance ; and the question now is, is the motion 

 the cause of the heat ? In the numberless cases in which motion 

 is destroyed and heat generated, has the motion no other effect 

 than the production of heat ? has the heat no other cause than 

 the motion ? 



Thus clearly and distinctly does this original thinker set his 

 problem before him, and he arrives at the conclusion that, with- 

 out a causal connexion between motion and heat, we can neither 

 account for the disappearance of the one nor the appearance of 

 the other. He finds that by stirring water vigorously he can 

 warm it. In what follows, Mayer's power of generalization comes 

 conspicuously into play. He remarks that heat makes its appear- 

 ance when the particles of a body approach each other j conden- 

 sation generates heat. But what is true for the ultimate parti- 

 cles is also true for larger masses. The falling of a body to the 

 earth is actually a diminution of the earth's volume, and must 

 certainly stand in connexion with the heat generated by the fall. 

 This heat is proportional to the mass of the body and the distance 

 through which it has fallen. It is also proportional to the mass 

 multiplied by the square of its velocity*. In water-mills the 

 motion produced by the diminution of the earth's volume by the 

 fall of the water generates continually a considerable amount of 

 heat. Conversely, the steam-engine serves to decompose the 

 heat into motion, or the lifting of weights. The locomotive 

 with its train may be compared to a distilling apparatus. The 

 heat beneath the boiler transforms itself into motion, and this 

 motion again deposits itself as heat in the axles and the wheels. 

 (It would be impossible to find a happier comparison than this.) 



* I would invite the reader to compare this outline of Mayer's paper 

 with the first five columns of Prof. Thomson's article in ' Good Words ' 

 (October 1862), and thus to prove for himself whether I am right or 

 wrong when I state that these five columns deal substantially with the 

 work of Maver. 



