1871.] 



President's Address. 



55 



Nutrition; 3. Fever; 4. Celestial Dynamics; 5. The Mechanical Equiva- 

 lent of Heat. 



I may presume that the last named, viz. the Mechanical Equivalent of 

 Heat, though not the sole, is yet the principal ground on which the award 

 of the Copley Medal was made ; and, so far as this may have been the 

 case, the award may perhaps he considered to require some explanation on 

 the part of the President and Council, inasmuch as this is the second of 

 two Copley Medals awarded (and, I believe, in each case rightly awarded) 

 for what may, perhaps, be mainly regarded as one and the same dis- 

 covery, — the later (which is the present award to Dr. Mayer) being for 

 investigations earlier in date than those of our countryman Mr. Joule, to 

 whom the first Medal of the two was awarded. This seeming inconsistency 

 can, I believe, be fully justified. Seeing also the scientific interest of the 

 history of the double investigation, I have thought it desirable to obtain a 

 short but comprehensive notice on the subject from the able hands of our 

 junior Secretary, Professor Stokes ; it is as follows : — 



" In a paper published in 1842, Mayer showed that he clearly conceived 

 " the convertibility of falling force, or of the vis viva, which is its equiva- 

 " lent or representative in visible motion, into heat, which again can dis- 

 " appear as heat by reconversion into work or vis viva, as the case maybe. 

 "He pointed out the mechanical equivalent of heat as a fundamental 

 " datum, like the space through which a body falls in one second, to be 

 " obtained from experiment. He went further. When air is condensed 

 " by the application of pressure, heat, as is well known, is produced. 

 " Taking the heat so produced as the equivalent of the work done in com- 

 " pressing the air, Mayer obtained a numerical value of the mechanical 

 " equivalent of heat which, when corrected by employing a more precise 

 " value of the specific heat of air than that accessible to Mayer, does not 

 " much differ from Joule's result. This was undoubtedly a bold idea, 

 " and the numerical value obtained by Mayer's method is, as we now 

 " know, very nearly correct. Nevertheless it must be observed that an 

 " essential condition in a trustworthy determination is wanting in Mayer's 

 " method; the portion of matter operated on does not go through a cycle of 

 " changes. Mayer reasons as if the production of heat were the sole effect 

 " of the work done in compressing air. But the volume of the air is 

 " changed at the same time, and it is quite impossible to say a priori 

 " whether this change may not involve what is analogous to the statical 

 " compression of a spring, in which a portion/or even a large portion of the 

 " work done in compression may have been expended. In that case the 

 " numerical result given by Mayer's method would have been erroneous, 

 " and might have been even widely erroneous. Hence the practical cor- 

 " rectness of the equivalent obtained by Mayer's method must not lead us 

 " to shut our eyes to the merit of our own countryman Joule, in being the 

 " first to determine the mechanical equivalent of heat by methods which 

 " are unexceptionable, as fulfilling the essential condition that no ultimate 

 " change of state is produced in the matter operated upon," 



