44 INIr Henwood on the Expansive Action of Steam 



(GF), and the horizontal line C c is produced. The piston 

 now acquires speed, whilst the steam (in the hoiler having 

 expanded) entei's the cylinder with diminished velocity, and is 

 insufficient to fill the enlarging space and still retain 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 fD 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 descends 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 indicator-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 retm*n stroke is made : the motion 

 of the slider is at the same time reversed. 



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

 top of the cylinder 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 Aveight of the pump- 

 I'ods alone ; the pressure of the steam contained in the cylin- 

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

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

 liorizontal. 



But shortly before the termination of the return stroke, the 

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

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

 til at 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 

 lonunences another working-stroke. This compression of the 

 steam contained in the upper part of the cylinder, forces the 

 in licator-piston upward ; and the resultant of this gradual 



