212 THE WATER PROBLEM OF A LARGE CITY 





of a supply on a lower floor. During the working hours of 

 the day, immense quantities of water are drawn off from in- 

 numerable faucets, and hence the quantity in the pipes de- 

 creases considerably unless the supply station is able to drive 

 water through the vast network of pipes as fast as it is drawn 

 off. Buildings at a distance from the reservoir surfer under 

 such circumstances, because while the diminished pressure is 

 ordinarily powerful enough to supply the lower floors, it is 

 frequently too weak to force a continuous stream to high 

 levels. At night, however, and out of working hours, few 

 faucets are open, less water is drawn off at any one time, and 

 the intricate pipes are constantly full of water under high 

 pressure. At such times, a good flow is 

 obtainable even on the uppermost floors. 



In order to overcome the disadvantage 

 of a decrease in flow during the day, stand- 

 pipes (Fig. 153) are sometimes placed in 

 various sections. These are practically 

 small steel reservoirs full of water and 

 connecting with the city pipes. During 

 " rush" hours, .water passes from these into 

 the communicating pipes and increases 

 the available supply, while during the 

 night, when the faucets are turned off, 

 water accumulates in the standpipe against 

 the next emergency '(Figs. 151 and 154). 

 The service rendered by the standpipe is 

 similar to that of the air cushion discussed 

 in Section 184. 

 FIG. 153.- A standpipe. ^- The Cost of Water. In the gravity 



system, where an elevated lake or stream 

 serves as a natural reservoir, the cost of the city's waterworks 

 is practically limited to the laying of pipes. But when the 



