214 THE WATER PROBLEM OF A LARGE CITY 



the town with the necessary amount of water. When, how- 

 ever, we consider the actual height to which the water is 

 raised above the pumping station, and the extra pumping 

 which must be done in order to balance the frictional loss, 

 it is easy to understand that in actual practice a much more 

 powerful engine would be needed. The larger the piston 

 and the faster it works, the greater is the quantity of water 

 raised at each stroke, and the stronger must be the engine 

 which operates the pump. 



In many large cities there is no one single pumping station 

 from which supplies run to all parts of the city, but several 

 pumping stations are scattered throughout the city, and each 

 of them supplies a restricted territory. 



199. The Bursting of Dams and Reservoirs. The construc- 

 tion of a safe reservoir is one of the most important problems 

 of engineers. In October, 1911, a town in Pennsylvania was 

 virtually wiped out of existence because of the bursting of a 

 dam whose structure was of insufficient strength to resist the 

 strain of the vast quantity of water held by it. A similar 

 breakage was the cause of the fatal Johnstown flood in 1889, 

 which destroyed no less than seven towns, and in which approxi- 

 mately 2000 persons are said to have lost their lives. 



Water presses not only on the bottom of a vessel, but upon 

 the sides as well ; a bucket leaks whether the hole is in its 

 side or its bottom, showing that water presses not only down- 

 ward but outward. Usually a leak in a dam or reservoir 

 occurs near the bottom. Weak spots at the top are rare and 

 easily repaired, but a leak near the bottom is usually fatal, 

 and in the case of a large reservoir the outflowing water 

 carries death and destruction to everything in its path. 



If the leak is near the surface, as at a (Fig. 155), the water 

 issues as a feeble stream, because the pressure against the 

 sides at that level is due solely to the relatively small height 



