in the Cornish Pumping Engines. 43 



attached to the piston, and the other to the top of the cylin- 

 der : the upper extremity of the piston-rod is provided -with 

 a receptacle for a pencil. A tapered stopcock is fixed on the 

 lower end of the cyUnder, and is introduced into the grease- 

 hole or other aperture in the cylinder-cover of any engine on 

 which the indicator is placed. A light frame of wood, about 

 eighteen inches long and four inches wide, is fastened to the 

 top of the indicator-cylinder, and in it a small board slides 

 horizontally in grooves. 



During the working-stroke of the engine, a du-ect motion 

 is given to the slider by means of a string which passes over 

 a pulley, and is connected with the radius-rod of the parallel 

 motion. Its return is eifected by the action of a counterpoise 

 suspended over a similar small wheel. On this moveable 

 board a piece of paper is firmly secured, and a pencil is placed 

 on the top of the piston-rod of the indicator. 



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, 

 steam is admitted from the boiler above the piston of the en- 

 gine ; the piston of the indicator is forced upwards, and the 

 line AB is described by the pencil. The engine now begins 

 to move, but so slowly that the steam enters from the boiler 

 more rapidly than the piston recedes before it ; its pressure in 

 the cylinder, therefore, still increases, and the piston of the 

 indicator continues to rise : but as the working-stroke of the 

 engine commences, the slider moves in the du-ection GF, and 

 the compound of the two motions generates the line BC, At 

 C the space left by the descent of the piston 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 en- 

 gine moves, the slider still advances in the same direction 



