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



and the excess of heat supposed to exist in the steam as it leaves 

 the boiler above the quantity of heat which it contains as it leaves 

 the cylinder, which latter quantity is found in the condenser — in 

 other words, that the work performed is proportional to the heat 

 which disappears from the steam in its passage through the 

 engine. After careful examination of the phenomena, I think it 

 will be found that this statement, though on the whole undoubt- 

 edly true, is not yet clearly established by direct experimental 

 proof. 



The quantity of work performed was ascertained by a friction- 

 brake ; but in these experiments it was of comparatively small 

 importance to ascertain the exact quantity of work performed in 

 each case, the object being rather to effect a constant amount of 

 work with a variable consumption of steam. The tension of the 

 steam in the boiler, its degree of superheat, and the quantity and 

 temperature of the water injected into the condenser being all 

 maintained constant, the external work performed by the engine 

 may be modified in two distinct ways. 



(1) This work may be diminished or increased by opening 

 more or less the steam admission-cock. 



(2) This cock being kept full open, and consequently a free 

 passage of steam through it, the amount of work may be increased 

 or diminished by cutting off the steam earlier or later during the 

 stroke. Therefore, reciprocally, the work may be maintained 

 constant if, while the amount of cut-off is diminished, the steam- 

 admission is also diminished by " wire-drawing." 



By carefully clothing the cylinder, the loss of heat from exterior 

 cooling may be reduced to a very small quantity. The propor- 

 tion of work lost in friction and other resistances cannot be 

 avoided to the same extent, nor is it easy to calculate it with even 

 approximate exactness. But if the amount of work done by the 

 engine is maintained constant while the consumption of steam is 

 varied, as above mentioned, it may be supposed that the amount 

 of work lost in friction &c. of the moving parts of the machine 

 is also constant (or nearly so) ; so that the total quantity of ex- 

 ternal work performed by the steam may be supposed to be equal 

 in each case. And as the consumption of steam to produce this 

 constant amount of work may be caused to vary considerably 

 (more than one-third in these experiments) by varying the mode 

 of its admission to the cylinder, if still we find in each case that 

 the disappearance of heat is the same in quantity, notwithstand- 

 ing the variation in the quantity of steam consumed, it may be 

 fairly deduced that the loss of heat in the steam is in all cases 

 proportional to the amount of work performed. 



To ascertain the amount of heat which disappears (or is changed 

 into work) in each case, it is requisite, first, to know the quantity 



