208 THE WATER PROBLEM OF A LARGE CITY 



filtration. Cities using such a water supply thus have the 

 double expense of pumping and filtration. 



195. The Pressure of Water. No practical business man 

 would erect a turbine or paddle wheel without calculating in 

 advance the value of his water power. The paddle wheel 

 might be so heavy that the stream could not turn it, or so 

 frail in comparison with the water force that the stream would 

 destroy it. In just as careful a manner, the size and the 

 strength of municipal reservoirs and pumps must be calcu- 

 lated. The greater the quantity of water to be held in the 

 reservoir, the heavier are the walls required ; the greater the 

 elevation of the houses, the stronger must be the pumps and 

 the engines which run them. 



In order to understand how these calculations are made, 

 we must study the physical characteristics of water just as we 



studied the physical characteristics 

 of air. 



When we measure water, we find 

 that i cubic foot of it weighs about 

 62.5 pounds; this is equivalent to 

 saying that water I foot deep presses 

 on the bottom of the containing 

 vessel with a force of 62.5 pounds 

 to the square foot. If the water is 

 2 feet deep, the load supported by 

 the vessel is doubled, and the pres- 

 sure on each square foot of the 

 bottom of the vessel will be 125 

 FIG. 149. Water i foot deep pounds, and if the water is 10 feet 



dee P> the load b rne ^ each SC l Uare 



foot will be 62 5 pounds. The deeper 

 the water, the greater will be the weight sustained by the con- 

 fining vessel and the greater the pressure exerted by the water. 



