498 Dr. J. R. Mayer on tlie Mechanical Equivalent of Heat, 



We are taught by history that in this case also the most 

 sagacious hypotheses concerning the state and nature of a peculiar 

 "matter" of heat, concerning a "thermal sether," whether at 

 rest or in a state of vibration, concerning "thermal atoms," 

 supposed to exercise their functions in the interstices between 

 the material atoms, or other hypotheses of like nature, have not 

 availed to solve the problem. It is, notwithstanding, of no less 

 wonderfully simple a nature than the laws of the lever, about 

 which the founder of the peripatetic philosophy cudgelled his 

 brains in vain. 



After what has gone before, the reader cannot be in any 

 doubt about what is the course now to be pursued. We must again 

 make quantitative determinations : we must measure and count. 



If we proceed in this direction and measure the quantity of 

 heat developed by mechanical agency, as well as the amount of 

 force used up in producing it, and compare these quantities with 

 each other, we at once find that they stand to each other in the 

 simplest conceivable relation — that is to say, in an invariable 

 direct proportion, and that the proportion also holds when, 

 inversely, mechanical force is again produced by the aid of heat. 



Putting these facts into brief and plain language, we may say, 



Heat and motion are transformable one into the othei\ 



We cannot and ought not, however, to let this suffice us. 

 We require to know how much mechanical force is needed 

 for the production of a given amount of heat, and conversely. 

 In other words, the law of the invariable quantitative rela- 

 tion between motion and heat must be expressed numerically. 



When we appeal hereupon to experiment, we find that raising 

 the temperature of a given weight of water 1 degree of the Cen- 

 tigrade scale corresponds to the elevation of an equal weight 

 to the height of about 1200 [French] feet*. 



This number is the Mechanical Equivalent of Heat. 



The production of heat by friction and other mechanical 

 operations is a fundamental fact of such constant occurrence, 

 that the importance of its establishment on a scientific basis will 

 be recognized by naturalists without any preliminary enumera- 

 tion of its useful applications; and, for the same reason, a few 

 historical remarks touching the circumstances attending the dis- 

 covery of the foregoing fundamental law. will not be out of place 

 here. 



In the summer of 1840, on the occasion of bleeding Euro- 

 peans newly arrived in Java, I made the observation that the 



* [Applying the corrected specific heat of air, M. Regnault finds the 

 equivalent according to the method of Mayer to be 426 kilogram-metres, 

 Mr. Joule's equivalent is 425. — G. C. F.] 



