OR, PLAIN 



quite invisible, and will force the 

 piston, 9, from the bottom to the 

 top. Cease the application of 

 heat, the steam will become 

 visible vapour, and will again 

 form water. As the steam cools 



374. 



and condenses, and ceases to 

 exert its force against the pressure 

 of the atmosphere, the latter will 

 act upon the piston from without, 

 and will drive it again to the 

 bottom. 



This is the principle of the 

 first steam engine. The water 

 exerts no influence over the 

 piston until heat is applied; 

 and directly heat is withdrawn 

 its influence ceases. Although, 

 therefore, we speak of 6 6 the 

 force of steam" it is really the 

 force of heat which is employed : 

 heat drives the particles of water 

 apart, causing the fluid body to 

 expand into a volume ; water is 

 the medium, heat the active agent. 



TEACHING. 99 



The machinery of the steam 

 engine constitutes the means by~~ 

 which what is commonly called 

 steam power is applied to the 

 production of force and motion. 



There are two essential parts 

 of a steam engine, the action of 

 which it is necessary to compre- 

 hend, before the working of the 

 engine jean be understood. The 

 first of these is the valve; the 

 second the piston. A valve is a 

 cover to an aperture, opening only 

 in one direction, and resisting the 

 passage of anything in an opposite 

 direction. The bellows affords an 

 example of a common form of 

 valve. The handle, 1, being 

 raised increases the size of the 

 interior, and air pressing in to fill 

 the space, lifts up the valve, 3, 

 which works upon a hinge ; the 

 handle, 4, then being put down, 

 compresses the air in the bellows, 

 which presses upon the upper 







375. 



surface of the valve, 5, but as it 

 " opens only in one direction, and 

 resists the passage of anything in 

 an opposite direction,' " the air 

 cannot pass out that way, but 

 is expelled through the nose, 6 ; 

 thus the current of atmosphere is 

 sent in a different direction to 



