210 THE WATER PROBLEM OF A LARGE CITY 



from a faucet depends upon the height of the reserve water 

 above the tap. Houses on a level with the main supply pipes 

 (Figs. 148 and 151) have a strong flow because the water is 

 under the pressure of a column A ; houses situated on eleva- 

 tion B have less flow, because the water is under the pres- 

 sure 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 



FlG. 151. Water pressure varies in different parts of a water system. 



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

 the flow is scarcely more than a feeble trickle. 



When the respective flows at A, B, and C (Fig. 151) are 

 measured, they are found to be far lower than the pressures 

 which columns of water of the heights A, B, and C have been 

 shown by actual demonstration to exert. This is because 

 water, in flowing from place to place, expends force in over- 

 coming the friction of the pipes and the resistance of the air. 

 The greater the distance traversed by the water in its journey 

 from reservoir to faucet, the greater the waste force and the 

 less the final flow. 



In practice, large mains lead from the reservoir to the city, 

 smaller mains convey the water to the various sections of the 

 city, and service pipes lead to the individual house taps. 



