Mr. W. J. M. Rankine on the Mechanical Action of Heat. 243 



ticular cases, which is the most frequent cause of failure in the 

 attempt to apply theoretical principles to practice. 



The engine was worked under a uniform load at five different 

 rates of expansion successively. The number of strokes and the 

 consumption of steam during each trial having been accurately 

 registered, Mr. Wicksteed gives a table showing the weight of 

 steam consumed per stroke for each of the five rates of expan- 

 sion. I shall now compute the weight of steam per stroke 

 theoretically, and compare the results. 



Throughout these calculations I shall uniformly use the foot 

 as the unit of length, the avoirdupois pound as that of weight, 

 and the hour as that of time. Pressures are consequently ex- 

 pressed in pounds per square foot for the purpose of calculation ; 

 although in the table of experiments I have reduced them to 

 pounds per square inch, as being the more familiar denomination. 



The data respecting the dimensions and load of the engine, 

 which are constant throughout the experiments, are the fol- 

 lowing : — 



Area of piston A = 34"854 square feet. 



Stroke /= 10 feet. 



Cubic space traversed by piston-1 ^a/=348-54 cubic feet. 



during one down stroke . J 

 Clearance and valve-boxes .... 18*00 



Sum 366-54 



Therefore, c=005 



R= useful load of piston . =1597" lbs. per square foot. 

 F = additional resistance . = 266'6 



R 4- F=total mean pressured _ , ofo.a 

 on piston .... J ~ 



The mode of calculation is the following : — 



V 

 Mr. Wicksteed states the fraction j of the stroke performed 



at full pressure in each experiment. From this the ratio of ex- 

 pansion s is computed by equation {b), giving in this case 



1 = 0-95 y-h 005. 

 s I 



The value of Z corresponding to s is then found by means 

 of the third column of Table II. ; that column being selected 

 because the initial pressures were all below four atmospheres. 

 This affords the means of determining the initial pressure of the 

 Bteam by equation (59), viz. 



P,= -|- (R + F) = l 863-6 1-. 



