CITY WATER SYSTEMS 491 



PIPE, is connected with the city water mains. As the water 

 is pumped into the main it backs up into the standpipe until 

 the latter is full. The height of the standpipe determines the 

 water pressure. 



The SECOND METHOD of maintaining pressure on the city 

 system, now generally followed, is to pump the water directly 

 into the city main which is provided with a RELIEF VALVE, a 

 sort of safety valve (see Art. 158) at the pumping station. 

 This valve is so adjusted that it opens when the pressure has 

 reached the desired amount. If it is desired to maintain a 

 pressure of 100 Ib. per sq. in., the valve is so set as to open as 

 soon as the water pressure has reached that point. 



574. Head of Water and Resulting Pressure. The height 

 of the surface of the water above the faucets or taps, is called 

 the HEAD OF WATER. What head of water is required to pro- 

 duce a certain pressure? The pressure of a liquid in a vessel, 

 open at the top, is caused by the weight of the liquid. Each 

 cubic foot of water weighs 62% Ib., but a cubic foot is 1728 

 cu. in Now suppose we had a pipe having a cross-section, 

 or area, of just 1 sq. in., and 144 ft. or 1728 in/ long. If we 

 were to stand this pipe up vertically, that is, on end, plug the 

 lower end and fill it with water, it would hold exactly 1 cu. ft. 

 of the liquid. The entire weight of the water, 62% Ib., would 

 then rest upon the plug closing the lower end. A column of 

 water 144 ft. high, then, produces a downward pressure of 62% 

 Ib. to the sq. in. A column of water 1 ft. high will produce a 

 pressure of H44 of 62% ^. or aDou t .43 Ib. A column of water 

 about 2.3 ft. high produces a pressure of 1 Ib. to the sq. in. 

 Calculate this. 



575. Pressure the Same in All Directions. We have seen 

 that if the head of water is 144 ft., the resulting downward 

 pressure is 62% Ib. to the sq. in. We must also note the 

 fact that this pressure is exerted in every direction, downward, 

 upward, and sidewise. If the water in the standpipe stands 

 144 ft. above the level of the city mains, then the pressure 

 upon those mains will everywhere be 62% Ib. to the sq. in. 



