Mr. Henwooil on the Cornish Pumping-Engines. 483 



But when this valve is opened, the pipe which connects tlie 

 top of the cyHnder with the bottom, and consequently a larger 

 space, is open to the steam, and as the slider remains for the 

 instant stationary, the indicator-piston descends through the 

 small vertical line EF. 



The return stroke is effected by the weight of the pump- 

 rods alone ; the pressure of the steam contained in the cylin- 

 der, therefore, remains unaltered, the indicator-piston is un- 

 moved, and the line FG, described by the pencil, is perfecdy 

 horizontal. 



But shortly before the termination of the return stroke, the 

 equilibrium valve is closed, and the steam in the cylinder not 

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

 that portion of it which is contained between the upper sur- 

 face of the piston and the cylinder-cover is compressed be- 

 tween them by the ascent of the former, until it is of force 

 enough to support that weight; the return stroke is thus ter- 

 minated, and the engine stops an instant or two before it 

 commences another working stroke. This compression of 

 the steam contained in the upper part of the cylinder forces 

 the indicator-piston upward, and the resultant of this gradual 

 elevation, and of the continued retrograde motion of the 

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

 the stroke returning to and standing at A. 



It is evident that the form of the portion ABCcD, which 

 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, tiie 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 part D~E will deviate from a true parabola only when 

 the steam in the cylinder is heated by being surrounded by a 

 steam-case, or jacket, or by flues containing warm air, or 

 cooled by the influence of the circumambient medium ; con- 

 sequently it will be generally pretty much alike in all cases. 

 The same reasons and influences are equally applicable to the 

 small and nearly vertical line EF, and to the longer horizon- 

 tal one FG. 



But, theoretically speaking, the curve GA is of more im- 

 jiortance than any other portion of the figure ; because it 

 clearly shows what proportion of the working stroke is per- 

 Ibrined by the beneficial influence of working expansively. 



For were the steam from the boiler admitted on the piston 



(hiring the whole oi" the working stroke, or the pressure of 



the steam (if worked expansively) suflicient to support the 



load at the termination thereof^ then the line FG, described 



2 I 2 



