CHAPTER VII. 

 BOILER FEEDERS. 



ATMOSPHERIC PRESSURE. 



The atmosphere or air surrounding the earth has 

 a weight or pressure on the surface of the earth of 

 about 15 Ibs. per square inch. This pressure varies 

 at different heights. Three and one-half miles up 

 the pressure is only one-half that at the surface. 

 Down in the mines the pressure is greater than at the 

 surface. The pressure is everywhere pressing on all 

 objects on their lower sides as well as on their upper 

 sides and has a tendency to fill all space. Air is 

 elastic and when relieved of its pressure it will expand; 

 when under pressure it will contract and occupy less 

 space; at one-half the pressure it will occupy twice 

 the space; at twice the pressure it occupies one-half 

 the space. 



As the water in the boiler evaporates or passes off 

 in steam, it must be replaced, and this is done by 

 devices adapted to force water into the boiler. There 

 are two general kinds of boiler feeders in common 

 use, pumps and injectors. 



PUMPS. 



Pumps are either self acting, (independent), or 

 else they are operated by the piston or crosshead of 

 the engine, hence called crosshead pumps. Their 



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