WHY WATER SUPPLY IS NOT UNIFORM 



209 



Since the pressure borne by i square foot of surface is 

 62.5 pounds, the pressure supported by I square inch of sur- 

 face is -jj^ of 62.5 pounds, or .43 pound, nearly \ pound. 

 Suppose a vessel held water to the depth of 10 feet, then 

 upon every square inch of the bottom of that vessel there 

 would be a pressure of 4.34 pounds. If a one-inch tap 

 were inserted in the bottom of the vessel so that the water 

 flowed out, it would gush forth with a force of 4.34 pounds. 

 If the water were 20 feet deep, the force of the outflowing 

 water would be twice as strong, because the pressure would 

 be doubled. But the 

 flow would not remain 

 constant, because as 

 the water leaves the 

 outlet, less and less of 

 it remains in the vessel, 

 and hence the pressure 

 gradually sinks and the 

 flow drops correspond- 

 ingly.' 



In seasons of pro- 

 longed drought, the 

 streams which feed a 

 city reservoir are apt 

 to contain less than the 

 usual amount of water, 

 hence the level of the 

 water supply sinks, the 

 pressure at the outlet 

 falls, and the force of 



FlG. 150. The pressure at an outlet decreases as 

 the level of the water supply sinks. 



the outflowing water is lessened (Fig. 150). 



196. Why the Water Supply is not uniform in All Parts of 

 the City. In the preceding Section, we saw that the flow 



CL. GEN. SCI. 14 



