360 THE STEAM-ENGINE. 



(a.) As in the case of water-power the production of mechanical 

 kinetic energy involves the fall of water from a higher to a lower 

 level, so in the case of steam-power the production of visible 

 energy involves the fall of heat from a higher to a lower temper- 

 ature. 



571. Single- Acting Engine. In a single-acting steam- 

 engine, the piston is pushed one way by the tension of the steam. 

 The steam is then condensed and the piston driven back by atmos- 

 pheric pressure. Such engines have gone out of use and have only 

 an historical interest. 



572. Double - Acting Engine. In a double- 

 acting steam-engine, the steam is admitted to the cylinder 

 alternately above and below the piston. This alternate 

 admission of the steam is accomplished by means of a 

 sliding-valve. The sliding- valve is placed in a steam-chest,, 

 8 9 which is fastened to the side of the cylinder C. 



FIG. 266. 



(a.) In the figure, the steam-chest is represented as being placed 

 at a distance from the cylinder; this is merely for the purpose 

 of making plain the communicating passages to and from the 

 chest. Steam from the boiler enters at M, passes through A to the 



