Dr. Schroeder van der Kolk on Gases. 135 



not for the reason that Kegnault's determinations were not suffi- 

 ciently accurate, but because the calculation of these experiments 

 leaves much to be desired. Begnault, as is well known, deduced 

 his formulae from individual observations, instead of connecting 

 them by means of the calculus of probabilities with all trust- 

 worthy observations. 



§ V. Calculation of the Mechanical Equivalent of Heat. 

 This value was calculated from the well-known general formula* 



T 



Now the formula p = kr gives 



dk , dk 7 

 vdp = kdr + t -j- dp + r -j- or, 



from which is obtained 



_ dk 

 dr dp 



dr 



and, further, 

 hence 



pdv = kdr -h rdk ; 



^L = l(k + r—) 

 dr p\ dry' 



By substituting both values, the formula becomes 



J \ k -Pdp) 



J is here to be calculated, since all the rest is known. 



1. In my former paper the velocity of sound was taken at 



332-77 metres, from which we obtain ^ =1-4128. But this 



value must undergo a slight correction, since air contains 

 aqueous vapour, and in the investigation it has been assumed 



that the values of ^ are the same for air and for aqueous vapour; 



* Zeuner, Grundziige der Warmetheorie, p. 177- W. Thomson Phil. 

 Mas;. S. 4. vol. iv. p. 168. 



6 L2 



