the Specific Heat of Air under constant Volume. 433 



The calculation of a mean from these individual series is faci- 

 litated by the circumstance that the first observation was in each 

 case made about the same time (two seconds) after the com- 

 mencement of the stroke, and that thenceforward the intervals 

 of time were almost equal. Hence it is sufficient if we take the 

 arithmetical means both of the almost equal times t and of the 

 corresponding values of y. 



Thus we find 







y- 





t. 







Difference. 









Observed. 



Calculated. 





seconds. 



millims. 



millims. 



millim. 



2-07 



7-62 



774 



-012 



3-87 



5-66 



5o2 



+ 014 



5-75 



3-85 



3-88 



-0-03 



8-12 



2-46 



2-49 



-003 



10-87 



1-52 



1-48 



+ 004 



19-52 



0-46 



0-29 



+0-17 



39-2 



012 



007 



+0-05 



The calculated values are obtained thus. Putting the quan- 

 tity of heat added to the mass of air in each minute proportional 

 to the difference in temperature from the surrounding medium, 

 or, what is the same thing, the alteration in pressure propor- 

 tional to the difference y of the momentary from the final pres- 

 sure, we have 



lt~ Xy > 



lognaty: 



lognatC— At. 



We introduce in the calculation for A and C, 

 = 11-41, A=0-1877. 



The calculated values, as we see, agree well with observation. 

 The expression is valid only from the moment at which the 

 stopcock was closed, which was the case at O7o second. We 

 get for this time from the formula ?/ = 9"912. 



In order to calculate accurately the amount of heat absorbed 

 from the beginning of the stroke to that time, it would be neces- 

 sary to have an exact knowledge of the course of the piston ; 

 but the correction may be approximately calculated in the fol- 

 lowing manner : — At the time 075 we get the change in pres- 

 sure due to change in temperature 



|= -1-860. 



at 



At the time it was =0. Hence as the mean from to 0*75 



