482 Mr. Henwood on the Cornish Pwnping-Engifies. 



Let us now examine the operation of a single-acting engine, 

 and the movements of an indicator fixed on it. 



Every thing being at rest, the piston of the engine at the 

 top of the cylinder, and the point of the pencil standing at A, 

 (in the figure below) steam is admitted from the boiler above the 

 piston of the engine; the piston of the indicator is forced up- 

 wards, and the line A B is described by the pencil. The en- 

 gine now begins to move, but so slowly that the steam enters 

 from the boiler more rapidlj' than the piston recedes before 

 it ; its pressure in the cylinder, therefore, still inci'eases, and 

 the piston of the indicator continues to rise: but as the work- 

 ing stroke of the engine commences, the slider moves in the 

 direction GF, and the compound of the two motions generates 

 the line BC. At C the space left by the descent of the pis- 

 ton is exactly filled by the steam, which enters from the boiler 

 in the same time; the indicator-piston, therefore, does not 

 stir; but as the engine moves, the slider still advances in the 

 same direction, (GF,) and the horizontal line Cc is produced. 

 The piston now acquires speed, whilst the steam (in the boiler 

 having expanded) enters the cylinder with diminished velo- 

 city, and is insufficient to fill the enlarging space and still re- 



tain the same density: it therefore expands, and the piston 

 of the indicator descends, whilst the slider still moves in the 

 same direction, and the curve cD is delineated. At D the 

 steam valve, through which the steam from the boiler enters 

 the cylinder, is closed, but the piston of the engine still de- 

 scends by virtue of the elasticity of the steam already 

 introduced, and of the momentum acquired by the moving- 

 parts of the machine. Whilst the steam expands, the indi- 

 cator-piston descends, and as the same horizontal motion of 

 the slider still continues, the parabolic curve DE is made by 

 the pencil. 



The equilibrium valve, which connects the upper part of 

 the cylinder with the lower, is now opened ; and as the steam 

 thus presses equally on both sides of the piston, the working 

 stroke terminates, and the return stroke is made : the motion 

 of the slider is at the same time reversed. 



