332 Mr H. Meikle on the Specific Heat 



stopper one-eighth round, so as to leave its apertures complete- 

 ly open ; and, on observing the gauge, it just sprung to the same 

 height as before. Repeated trials satisfied me, that, with such 

 small variations of density, it would require considerable haste 

 to turn the stopper too quickly. In both of the cases just men- 

 tioned, the range through which the stopper turned was limited 

 by a catch. But in the experiments to be afterwards noticed, 

 I generally used a hghter fluid than water. 



It is evident, that, instead of injecting air, as I usually did, to 

 increase the pressure above that of the atmosphere, it would 

 come to the same thing, if we first close the large vessel at a 

 temperature a few degrees below that at which we wish to oper- 

 ate, and then raise it to the temperature which is to remain con- 

 stant during the experiment. This consideration affords, per- 

 haps, the simplest means of explaining the rationale, or use of 

 this sort of experiments. For, let the pressure of the air when 

 just shut in, be in equilibrio with the atmosphere, but suppose 

 that the temperature of the apparatus is next raised, so as to in- 

 crease the pressure and depress the liquor in the gauge b 

 inches, which we may call b degrees ; then, if, whilst this 

 augmented temperature of the apparatus remains constant, the . 

 stopper be turned one-fourth round, as above described, the 

 equilibrium with the atmosphere will be for a moment restored, 

 the communication with it again cut off, and the included air cool- 

 ed by the dilatation, but it will soon absorb heat, and recover 

 the former temperature, as will be indicated by a second depres- 

 sion of c inches or degrees. This is obviously the change of 

 temperature due to the excess of the quantity of heat, which 

 would raise the temperature b degrees, under a constant pres- 

 sure, above what raises it b degrees under a constant volume. 



From this it would follow, that the quantity of heat which 

 raises the temperature b degrees under a constant volume, would 

 only raise it b — c degrees under a constant pressure ; or, that 

 the specific heat in the first case is to that in the second as b — c 

 to b. 



Strictly speaking, neither the volume during the first increase 

 of temperature nor that during the second is constant, because 

 the depression of the liquor in the gauge tube makes a little 

 noore room for the air. This, to be sure, could be obviated by 



