330 PUMPS, AND THEIR VALUE TO MAN 



them, together with the livery, demanded a flow of 10 gallons 

 per minute. The ground was full of rock and difficult to pen- 

 etrate, and it required 6 weeks of constant work for two skilled 

 men to drill the opening, lower the suction pipe, and install the 

 pump, the cost being approximately $700. 



The water from such a well is safe and pure except under the 

 conditions represented in Figure 72. If sewage or slops are 

 poured upon the ground in the neighborhood of the well, the 

 liquid will seep through the ground and some may make its 

 way into the pump before it has been purified by the earth. 

 The impure liquid thus contaminates the otherwise pure water 

 and renders it decidedly harmful. For absolute safety the 

 sewage discharge should be at least 75 feet from the well. 

 In large hotels, where there is necessarily a large quantity of 

 sewage, the distance should be much greater. As the sewage 

 seeps through the ground it loses its impurities, but the quantity 

 of earth required to purify it depends upon its abundance ; a 

 small depth of soil cannot take care of an indefinite amount of 

 sewage. Hence, the greater the number of people in a hotel, 

 or the more abundant the sewage, the greater should be the 

 distance between well and sewer. 



By far the best way to avoid contamination is to see to it 

 that the sewage discharges into the ground below the well ; that 

 is, to dig the well in such a location that the sewage drains 

 away from the well. 



Pumps which compress air.- The pumps just considered 

 have their widest application in agricultural districts. From a 

 commercial and industrial standpoint a most important class of 

 pump is that known as the compression type. In these, air 01 

 any other gas is compressed rather than rarefied. 



Air brakes and self-opening and self-closing doors on cars 

 are operated by means of compression pumps. The laying 

 of bridge and pier foundations, in fact all work which must 



