S7X WILLIAM SIEMENS, F.R.S. 33 



certainly derive but little advantage from, the preceding numerical 

 statement, the result of abstract calculation, unless it can be proved 

 by simple demonstration, and in such a manner that the essential 

 difference between the actually and the theoretically perfect engine 

 is clearly pointed out. 



The author proposes to accomplish this by means of a diagram 

 (Fi.u. 1, Plate t>), which is, in effect, the expansion curve of 

 saturated steam indefinitely prolonged. 



The vertical lines and figures at the bottom signify the pres- 

 sures of saturated steam in Ibs. per square inch ; the horizontal 

 lines and figures on the sides denote the volume of steam compared 

 with the volume of water from which it is produced ; and the 

 horizontal lines, with figures on the curve, express the tempera- 

 tures of the steam corresponding to the pressure and volume of the 

 same. 



The outer curve, a a , is that usually employed in calcu- 

 lating the power of expansion engines, being the expression of 

 Watt's law " that the total heat in steam is the same at all 

 pressures." 



The inner curve, b 1 7>, is the corrected one, in accordance with 

 the recent discoveries of Regnault, " on the total heat of steam," * 

 and may be termed " the curve of equal heat." 



The fields between the horizontal dotted lines represent the 

 power given out by the steam, in losing equal decrements of 10 

 of temperature in its expansion, and it is important to observe, 

 that the areas of these fields are nearly alike, between the limits to 

 which the pressure and temperature of steam are experimentally 

 known, increasing only slightly, and in a uniform ratio inversely 

 as the temperatures. 



This gradual increase may be ascribed to the fact, that the 

 curve in question is one of equal heat, whereas it has been 

 shown, that in expanding behind a working piston, the steam 

 must lose heat in the dynamical proportion to the power given 

 out. 



Messrs. Rankine and Clausius have first drawn attention to 

 this circumstance, and proved that the expansion of steam, behind 

 a piston, must be attended by partial condensation. 



* Vide the Publications of the Cavendish Society, Vol. I. 

 VOL. I. D 



