298 ON THE HEAT PRODUCED BY THE COMPRESSION OF A GAS. 
In the second member of this equation p’ denotes the pressure of the air 
through the second spiral, which would be the atmospheric pressure, or exces- 
sively near it, if, as in Joun’s third experiment, mentioned above (described by 
the author in p. 378 of the volume* containing his paper), the air leaving the 
second spiral be measured by means of a pneumatic trough: p denotes the pres- 
sure in the first spiral, which ought to be constant, and must be carefully mea- 
sured; « denotes the volume of air which leaves the apparatus in any time; 
and H denotes the quantity of heat emitted in the same time. The experiment 
might be continued for any length of time, and each one of these four quantities 
might be determined with great accuracy, so that probably very accurate direct 
results of observations might be obtained. If so, no way of experimenting could 
be better adapted than this to the determination of Carnor’s function, for differ- 
ent temperatures, in terms of JouLr’s mechanical equivalent of heat. 
* Phil. Mag., vol. xxvi. 

