TEMPERATURE OF ASCENDING CURRENTS VON BEZOLD 339 



initial pressure 



760 mm 



760 + 1376 



of mercury; corresponding to this we have 



£ 2 = 18\6 

 p\ 14.6 



and if t t is o°C. then t 2 = — 5. 6°. In this case therefore the cooling 

 scarcely amounts to 6° C. 



Since we can assume that the piece of iron has the same mass as 

 the quantity of air used in the other experiments, therefore in all 

 these three cases we have allowed equal masses to rise through 

 equal altitudes; and yet in one case no cooling takes place; in the 

 second case a cooling of about 50 C; and in the third case one of 

 scarcely 6° C. Thus the cooling is not to be attributed to the work 

 done in lifting but exclusively to the work done by expansion. 



By these considerations it must have been put beyond all doubt 

 that in the ascent of masses of air in the atmosphere the work of 

 lifting does not come into consideration during steady motion and 

 only to a very slight degree during the process of the establishment 

 of such motions. 



In all investigations hitherto made relative to ascending and de- 

 scending currents the steady motion has been assumed or implied, 

 therefore nothing need be said as to the work done in lifting. 



Now that we consider the error of basing the theory of cooling of 

 ascending masses of air on the work of lifting to have been fully 

 demonstrated, we have still to solve the question how it was possible 

 from this assumption to arrive at the same numerical values as by 

 the exclusive consideration of the work of expansion. 



This most surprising fact is easily explained by the consideration 

 of the following well-known formulae. 4 



For any given change of condition of the unit mass of dry air 

 let the quantity of heat communicated to it be Q, the specific pres- 

 sure p, the specific volume v, the specific heat under constant pres- 

 sure c p , the reciprocal of the mechanical equivalent of heat A, we 

 have then the equation 



dQ = c p dt - Avdp 



4 See J. Hann. Zeit. Oesterreich. Gesell. f. Met. 1874. IX, p. 321. Or 

 the translation published in the "Short Memoirs" Annual Report Smithsonian 

 Institution 1877. 



