WHY THE WATER SUPPLY VARIES 



337 



vessel, the pressure gradually sinks, and the flow drops corre- 

 spondingly. 



In seasons of prolonged 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 the outflowing water is 

 lessened. 



Why the water supply is not uniform in all parts of the city. 

 In the preceding section, we saw that the flow from a faucet 

 depends upon the height of the reserve water above the tap. 

 Houses on a level with the main supply pipes (Figs. 204 and 206) 

 have a strong flow, because the water is under the pressure of a 

 column A ; houses situated on elevation B have less flow, 



FIG. 206. Water pressure varies in different parts of a water system. 



because the water is under the pressure of a shorter column B \ 

 and houses at a considerable elevation C have a less rapid flow 

 corresponding to the diminished depth (C). 



Not only does the flow vary with the elevation of the house, 

 but it varies with the location of the faucet within the house. 

 Unless the reservoir is very high, or the pumps very power- 

 ful, the flow on the upper floors is noticeably less than that in 

 the cellar, and in the upper stories of some high buildings the 

 flow is scarcely more than a feeble trickle. 



During a long journey, considerable force is expended against 

 friction, and the flow at a distance from the reservoir falls 

 to but a fraction of its original strength. For this reason, 



