484) Mr. Henwood 07i the Cornish Pumping-Engiiies. 



by the return stroke, would be prolonged horizontally until 

 it intersected an extension of the vertical line AB &i x', at 

 which point the pencil would rest at the end of the return 

 stroke, and the instant the equilibrium valve closed the engine 

 would stop. But it has been seen that the engine continues 

 to move, and that the indicator-piston rises and generates the 

 curve GA, after that valve is closed: which circumstances 

 clearly demonstrate that the steam included between the cy- 

 linder-cover and the upper surface of the piston, is meanwhile 

 undergoing compression ; and that its elasticity both at the 

 conclusion of the working stroke, and at the closing of the 

 equilibrium valve, was insufficient to sustain the load. And 

 it follows, that the portion of the working stroke which has 

 been performed after the steam has expanded so much as to 

 be unequal to supporting the burden, must have been accom- 

 plished by the momentum acquired in the early part of the 

 stroke. When the pencil rests at A, the force of the steam 

 balances the load of the engine ; for the piston is never per- 

 mitted to rise so far as to touch the cylinder-cover. If, there- 

 fore, from A a line be drawn pai'allel to FG, until it cuts the 

 parabolic curve DE, the point of intersection, ;;:, will be at that 

 part of the stroke where the (simple) elasticity of the steam 

 and the load of the engine are exactly in equilibrio ; and the 

 portion .:rE, (described after the steam has so far expanded as 

 to be insufficient to support the burden,) will denote the 

 amount of benefit obtained by working expansively. 



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 34-2,858 feet of steam of a 

 pressure of 64'*1 lbs. on the square inch, (or 49-1 lbs. on the 

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

 during the experiment, or 2,153,647 cubic feet of the press- 

 ure of 10*2 lbs. 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 feetf, and the number of strokes made during the ob- 

 servations 7881. Therefore, if it were indispensable for the 

 steam on the piston, at the termination of the working stroke, 

 to be of elasticity sufficient to sustain the load of the engine, 



* 10'2 lbs. was the load of the engine per square incii of the area of the 

 piston. 



t Brewster's Edinburgh Journal of Science, O. S. IX. p. 160} from this, 

 however, the dimensions of the piston-rod, probably about 2 cubic feet, 

 should be deducted. 



