THE PRESSURE OF WATER 



335 



FIG. 204. The elevated mountain lake serves as a source of water. 



and steeper the elevation, the greater the force with which the 

 water flows through the valley pipes, and the stronger the dis- 

 charge from the faucets. 



Relatively few cities and towns are so favorably situated as 

 regards water. More often the mountains are too distant, or 

 the elevation is too slight, to be of practical value. Cities 

 situated in plains and remote from mountains are obliged to 

 utilize the streams that flow through the land, forcing the 

 water to the necessary height by means of pumps. Streams 

 that flow through populated regions are apt to be contami- 

 nated, and water from them requires public filtration. Cities 

 using such a water supply thus have the double expense of 

 pumping and of filtration. 



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 compari- 

 son 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 calculated. The greater the 

 quantity of water to be held in the reservoir, the heavier are 



