THE STEAM ENGINE. 



pipe from the boiler enters, and below the valve c is another opening, 

 at which enters the exhausting pipe. In like manner, above the 

 valve s' in the lower steam box enters a steam pipe leading from 

 the boiler, and below the valve c' enters an exhausting pipe. It 

 is evident, therefore, that steam can always be admitted above the 

 piston by opening the valve s, and below it by opening the valve 

 s' ; and, in like manner, steam can be withdrawn from the cylinder 

 above the piston, by opening the valve c, and from below it by 

 opening the valve c'. 



Supposing the piston p to be at the top of the cylinder, and the 

 cylinder below the piston to be filled with pure steam, let the 

 valves s and c' be opened, the valves c and s' being closed, as 

 represented in fig. 15. Steam from the boiler will, therefore, flow 

 in through the open valve s, and will press the piston downwards, 

 while the steam that has filled the cylinder below the piston will 

 pass through the open valve c' into the exhausting pipe. The 

 piston will, therefore, be pressed downwards by the action of the 

 steam above it. Having arrived at the bottom of the cylinder, 

 let the valves s and c' be both closed, and 

 the valves s' and c be opened, as represented 

 in fig. 16. Steam will now be admitted 

 through the open valve s' and through the 

 passage B' below the piston, while the steam 

 which has just driven the piston down- 

 wards, filling the cylinder above the piston, 

 will be drawn off through the open valve c, 

 and the exhausting pipe, leaving in the 

 cylinder above the piston a vacuum. The 

 piston will, therefore, be pressed upwards 

 by the action of the steam below it, and will 

 ascend with the same force as that with 

 which it had descended. 



The alternate action of the piston up- 

 wards and downwards may evidently be 

 continued by opening and closing the valves alternately in 

 pairs. Whenever the piston is at the top of the cylinder, as 

 represented in fig. 15, the valves s and c', that is, the upper 

 steam valve and the lower exhausting valve are opened ; and the 

 valves c and s', that is, the upper exhausting valve and the lower 

 steam valve, are closed ; and when the piston has arrived at the 

 bottom of the cylinder, as represented in fig. 16, the valves c and s', 

 that is, the upper exhausting valve and the lower steam valve, 

 are opened, and the valves s and c', that is, the upper steam valve 

 and the lower exhausting valve, are closed. 

 If these valves, as has been here supposed, be opened and closed 

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