Mr. J. Gill on the Dynamical Theory of Heat. 5 



of heat given to the steam before its admission to the cylinder, 

 and, secondly, the quantity of heat given out by the steam in the 

 condenser ; the difference should obviously be the heat consumed 

 in performing the work. 



The quantity of heat required to evaporate a given weight of 

 water under given conditions is known by Regnault's formulae. 

 Its value is complex; but this consideration does not greatly 

 affect the present inquiry, as we are certain that the steam would 

 give out in condensing all the heat it had previously absorbed for 

 its formation, provided none of it be lost by any intermediate 

 process — such, for instance, as the performance of external work 

 at its expense. 



In these recent experimental researches of M. Hirn, the steam 

 on leaving the boiler was always more or less superheated, with- 

 out change of pressure. This condition is indispensable for ob- 

 taining correct results, because ordinary steam generally carries 

 into the cylinder a certain quantity of water, necessarily variable 

 in amount, and very difficult to measure or estimate; conse- 

 quently it is impossible in these circumstances to value correctly 

 the quantity of heat furnished by the boiler. 



The additional amount of heat required to superheat the steam 

 a given number of degrees is also ascertained by a corresponding 

 formula. We can thus ascertain the number of thermal units 

 in the steam furnished to the cylinder in a given time ; and, 

 theoretically, this same quantity of heat should be found in the 

 condenser, provided that none of it were lost during its passage 

 through the engine by being changed into the work done. 



The quantity of heat passing out from the engine in a given 

 time can be easily ascertained by noting the quantity of water 

 heated a certain number of degrees in the condenser during the 

 same time. The difference between this quantity of heat and 

 the total heat of the steam as it leaves the boiler in the same 

 time should be the equivalent of the work done in that time. 



In all these experiments the pressure in the boiler was main- 

 tained nearly constant, not varying more than one-tenth of an 

 atmosphere; the temperature of the saturated steam was there- 

 fore known from the tables of tension. The temperature of the 

 superheated steam was ascertained by a thermometer in the steam- 

 pipe close to the cylinder, where the pressure was nearly the same 

 as that in the boiler. The temperature of the condenser-water 

 was taken every minute while the engine was in regular action, 

 and was represented in each case by the mean of thirty obser- 

 vations. 



To ascertain with accuracy the respective quantities of the 

 steam and the injection-water employed in given times under 

 the varying circumstances of the experiments was a task of great 



