MECHANICAL ENERGY AND HEAT 



189 



indicated in the diagram, if steam from the boiler is admitted 

 through the pipe st into the steam chest S, it will pass through 

 the channel ch into the cylinder. Here it will exert pres- 

 sure upon the piston and drive it to the other end of the 

 cylinder. Steam which may have remained in the opposite 

 end of the cylinder from the previous stroke escapes from 

 in front of the moving piston through the channel ch' under 

 the slide valve to the exhaust pipe e, which leads into the 



FIG. 95. The working parts of a steam engine 



W, flywheel ; C, cylinder; S, steam chest ; at, steam pipe from the boiler ; ch, ch', 



passages for steam between steam chest and cylinder; s, slide valve; p, piston; 



r, piston rod ; e, exhaust pipe 



open air or into a condenser. A working engine is so 

 arranged that when the piston reaches the position indi- 

 cated by the dotted lines, the slide valve s is moved into 

 the position indicated by the dotted lines. This, it should 

 be noticed, prevents the steam from passing into the right- 

 hand end of the cylinder, while at the same time it admits 

 it to the opposite end. The pressure of the steam on the 

 other side of the piston drives it back toward the end of 

 the cylinder from which it started. When the piston again 

 reaches the end of the cylinder, the slide valve returns to 

 its original position, and this action is repeated for each 

 stroke of the piston. 



