I 



in the Cornish Pumping Engines. 45 



elevation, and of the continued retrograde motion of the 

 slider, is the small curved line GA, the pencil at the end of 

 the stroke retm-ning to and standing at A. 



It is evident that the form of the portion ABCcD, vi^hich is 

 produced during the admission of steam from the boiler on 

 the piston, must depend on the load of the engine, its size, 

 the dimensions of the steam-valve, the pressure of steam in 

 the boiler, and the capacity of the boiler itself, and that it 

 will, therefore, vary as these particulars may differ. 



The only case in which I have been able to submit the re- 

 sults thus obtained with the indicator to a direct comparison 

 with the quantity of water evaporated in the boilers was at 

 Huel Towan, where 847.5 cubic feet of water were converted 

 into steam. This would give 342,858 feet of steam of a pres- 

 sure of 64.1 lb. on the square inch, (or 49.1 lb. on the inch 

 above the atmosphere), the mean pressure in the boiler during 

 the experiment, or 2,153,647 cubic feet of the pressiu-e of 

 10.21b. on the inch.* The capacity of the cylinder-nozles 

 and other parts of the engine which required to be filled with 

 steam from the boiler at every stroke, was 355.57 cubic feet,t 

 and the number of strokes made during the observations 7881. 

 Therefore, if it were indispensable for the steam on the pis- 

 ton, at the termination of the working-stroke, to be of elasti- 

 city sufficient to sustain the load of the engine, 2,802,247 cu- 

 bic feet (of a pressure of 10.2 lb. on the inch) would have 

 been reqvdsite ; whereas but 2,153,647 cubic feet only could 

 be obtained from the quantity of water evaporated. Conse- 

 quently, but the 0.768th of the contents of the cylinder, &c., 

 covdd, on an average, have been filled with steam of that 

 force ; and the remaining 0.232 of the stroke, must therefore 

 have been performed by virtue of the momentum acquired by 

 the machine in the early part of the working-stroke. 



II. The duty performed rvith a given quantity of fuel. — The 

 experiments with an object to determining the duty perform- 

 ed with a known quantity of fuel, were made on Wilson '.>-, 



* 10.2 lb. was the load of the engine per square inch of the area of the 

 piston, 

 t Brewster's Edinburgh Journal of Science, 0. S, IX. p. 160. 



