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

 Equation of effect and power in unity of time, 



Useful effect E=RA/w=WV,{PiZ-r}. . . (57) 

 Weight of steam expended in unity of time, 



W=^ (58) 



From those two fundamental equations the following are 

 deduced : — 



Ratio of mean load on piston to maximum pressure, 



5i^=7 (B9) 



Duty of unity of weight of steam, 



|,=Vi(P,Z-F); (60) 



which, being multiplied by the number of units of weight of 

 steam produced by a given weight of fuel, gives the duty of that 

 weight of fuel. 



Weight of steam expended per stroke, 



^ = # '. . (61) 



n ViS 



In fact, it is clear that if any five quantities out of the follow- 

 ing seven be given, the other two may be determined by means 

 of the equations — 



R + F, the mean load on unit of area of piston. 



Pp the maximum pressui'e of steam in the cylinder. 



s, the ratio of expansion. 



W, the weight of steam produced in unity of time. 



A, the area of the piston. 



/, the length of stroke. 



n, the number of strokes in unity of time. 



The other quantities, E, Vj, Z, are functions of those seven. 



Comparison of the Theory with Mr. Wicksteed's Expeiiments. 



(36.) In order to test the practical value of this theory, I 

 shall compare its results with those of the experiments which 

 were made by Mr. Wicksteed on the large Cornish pumping- 

 engine, built under the direction of that eminent engineer by 

 Messrs. Harvey and West, for the East London Water-works at 

 Old Ford, and which were published in 1841. The dimensions 

 and structure of the engine, and the details of the experiments, 

 are stated with such minuteness and precision, that there is 

 none of that uncertainty respecting the circumstances of par- 



