338 THE WATER PROBLEM OF A LARGE CITY 



buildings, situated near the main supply have a much stronger 

 flow (Fig. 207) than those on the same level but remote from 

 the supply. Artificial reservoirs and standpipes are usually 

 constructed on the near outskirts of a town in order that the 





FIG. 207. The more distant the fountain, the weaker the flow. 



frictional force lost in transmission may be reduced to a 

 minimum. 



Why water does not always flow from a faucet. Most of 

 us have at times been annoyed by the inability to secure water 

 on an upper story, because of the drawing off of water on a 

 lower floor. During the working hours of the day, immense 

 quantities of water are drawn off from innumerable faucets, 

 and the pressure in the pipes decreases considerably. Build- 

 ings at a distance from the reservoir suffer under such cir- 

 cumstances, because while the diminished pressure is ordi- 

 narily 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 pipes are 

 constantly full of water under high pressure. At such times, 

 a ejood flow is obtainable even on the uppermost floors. 



The cost of water. In the gravity system, where an 



