. 
198 MR W. J. M. RANKINE ON THE POWER AND ECONOMY 
Duty of unity of weight of steam,— 
SSA AY OS 9 iam lap tl iad 
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,— 
Wal 
ii Ne 
In fact, it is clear that if any five quantities out of the following 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. 
P,, the maximum pressure 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. 
1, the length of stroke. 
n, the number of strokes in unity of time. 
The other quantities, E, V,, Z, are functions of those seven. 

(61.) 
Comparison of the Theory with Mr WicksTEED’s Experiments. 
(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 particular 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 ex- 
pansion successively. The number of strokes, and the consumption of steam 
during each trial, having been accurately registered, Mr WicksTEED gives a table 
shewing the weight of steam consumed per stroke for each of the five rates of 
expansion. 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 expressed 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. 

