100 



that's it ; 



that from whence it came. The 

 valve is mainly instrumental in 

 this, for if it remained open as 

 at 3, the air would again pass 

 out that way, and no perceptible 

 force could be acquired through 

 the nose, 6. 



Valves may be made to act 

 against each other, as in the case 

 of the pump. By depressing the 

 handle 7, the piston and valve, 8, 

 are lifted up ; this increases the 

 size of the chamber between 

 8 and 9, and water rushing in, 

 under the pressure of the ex- 

 ternal atmosphere, lifts up the 

 valve, 9, which acts in the same 

 manner as the valve of a bellows, 

 3. But when the handle, 10, is 



376. 



elevated, it forces down the pis- 

 ton, 11, the effect is to close the 

 valve, 12, which opens only up- 

 wards, the other valve, 11, flies 

 open, while 12 remains closed, and 

 thus water is raised by the mutual 

 action of the valves. 



A piston is a plug fitted to 

 work tightly up and down a 

 smooth cylinder, as in the case 

 of the pump. The plug of a pis- 

 ton may be fitted with a valve, 8, 

 or be made without it, according 



377. 



to necessity. A valve may have 

 no connection with a piston, as 

 in the case of the bellows. There 

 are various forms of valves, but 

 the principle of them is always 

 the same. 



We will now examine one of 

 the earliest steam engines, 378, 

 keeping in mind the expansive 

 force of steam, as shown by 370, 

 the nature of valves, 375, 376, 

 and the construction of a piston, 

 374 and 376. 



Steam is generated in the 

 boiler, 1, immediately over which 

 there is a pipe leading into the 

 cylinder, 2, the passage from the 

 boiler to the cylinder being pro- 

 tected by a valve, 3, to which a 

 handle is affixed for the purpose 

 of turning the steam off or on. 



