184 



ELEMENTS OF GENERAL SCIENCE 



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 open air or into a 

 condenser. A working engine is so arranged that when the 

 piston reaches the position indicated by the dotted lines, the 

 slide valve * is moved into the position indicated by the dotted 

 lines. This, it should be noticed, hinders the steam from pass- 

 ing 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 



eft 



FIG. 91. The working parts of a steam engine 



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



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



r, piston rod ; e, exhaust pipe 



the end of the cy Under from which it started. When the 

 piston again reaches the end of the cylinder, the slide valve 

 returns to the original position, and this action is repeated 

 for each stroke of the piston. 



The moving piston may be connected to machinery which 

 is to be operated. The work which is done by the piston upon 

 the machinery is derived from the steam as it expands in the 

 cylinder. The steam which leaves the exhaust outlet, having 

 parted with some of its energy to the piston, is no longer as 

 hot as it was when it entered the cylinder. 



