256 



"WELLS S NATURAL PniLOSOPHT. 

 Fig. 215. 



581. Steam-boilers, which, although necessary to the generation of tho 

 power, are quite independent of the engine, are constructed of thick sheets 

 of iron or copper, strongly riveted together. 



K, by -which a rotary movement is communicated to the »rheel, Y ; from this the poorer 

 iuay be applied by other wheels, or by bands and pulleys, to effect different operations. 



At the left of the cylinder is an arrancrcment of valves a^d pipes, by which the steam it 

 allowed to act alternately above and below the piston. Aftpr the steam has completed its 

 action by forcing the piston to the extremity of the cylinder, it is necessary that it should 

 be withdrawn, and a vacuum formed in its place. In order tf^ accomplish this, the steam, 

 after having acted, is caused to pass into the cylinder, O, whi?h contains cold water, and 

 is termed the condenser. Here it is condensed, and a vacuum formed in the cylinder 

 above or below the piston, as the case may be. 



As the cold water of the condenser becomes quickly heated bv the condensed steam 

 withdrawn from the cylinder, it becomes necessary to constantly withdraw the hot water 

 and replace it by cold water, in order that the condensation of the steam nay take placa 

 as rapidly as possible. This is effected by means of two pumps ; the one, F M. which is 

 called the " air-pump," which withdraws the hot water from the condenser, and with it 

 aay air that may be present either in the cylinder or the condenser^ the other, H R. 



