.SYA" WILLIAM SIEMENS, F.R.S. 37 



I ii accordance with the principles put forth, the power of an 

 miue is expressed by a simple formula: 



in which c is the velocity of the piston in feet per minute ; a the 

 area of the same in square inches ; t the temperature of the 

 steam, or air, on entering the cylinder ; t' its temperature on 

 leaving the same ; v expresses the volumes of the steam, or air, 

 on entering the cylinder, as compared to one volume of water ; 

 r the ratio of expansion, or fraction of stroke at which the supply 

 is shut off; A a constant, denoting the mechanical equivalent, 

 per unit of heat, being (as shown by the diagrams) for steam = 

 400, and for air = 0'267 x 400 = IOC ; p the pressure of the 

 fluid on entering the cylinder (pressure in boiler) in Ibs. per square 

 inch ; and p' the pressure against the working piston (back 

 pressure) in Ibs. per square inch. 



The power required to work air, or feed pumps, has to be 

 deducted from the result of this formula. 



Take, for example, an expansive and condensing steam-engine 

 of 16 inches diameter and 200 feet velocity of piston ; the total 

 pressure of steam in the boiler GO Ibs., cut off at one-fourth part 

 of the stroke ; the vacuum in the cylinder averaging 11 Ibs. 

 (having 4 Ibs. resisting pressure) ; the initial temperature of the 

 steam = 295 ; and the final temperature If = 207 Fahr. ; the 

 volume at 60 Ibs. pressure would be = 460 (see diagram, or any 

 table on the pressure, temperature, and volume of steam). 



The indicated horse-power of this engine will be 



lOZ)) - 36-7, 



The evaporation in the boiler is 2 ^ - = 9'1 cubic feet of 

 water per hour. 



The result agrees with that obtained by the usual method of 

 computing the contents of the expansion curve, and is certainly 

 more accurate and more expeditiously arrived at. 



r A (t t') has no value, 

 For non-expansive engines, the factor - 



because / = i\ and r = 1. 



