228 GENERAL SCIENCE 



necessary to precipitate solid matter and kill bacteria 

 and germs is put into the water just before it pours from 

 the pump delivery pipes into the settling basin. From 

 the settling basin it flows over the filter beds, which are 

 about 20 feet long and 18 feet wide. Each filter bed 

 will remove the solid matter and dead germs from about 

 1,000,000 gallons in 24 hours. These filter beds are 

 washed once every 24 hours by forcing water through 

 them in the opposite direction from that in which the 

 filtering water flows through. 



156. Methods of Supplying Water. Cities have vari- 

 ous methods of supplying sufficient water for their needs, 

 depending upon their location with regard to streams, 

 lakes, and mountains. There are three systems in use. 



(A) Gravity System. Cities that are located near 

 mountains have the water piped from lakes or streams 

 that have an elevation much greater than that of the cities. 

 In this way great pressure is secured for the entire city, 

 and the water will flow with considerable force from any 

 faucet. The water flows to the city because of its own 

 weight. Denver, Colorado, uses the gravity system by 

 having the water piped from the mountain streams. 

 Los Angeles, California, is using this system in part. 

 The water from the mountains is usually pure enough 

 so that it does not need to be filtered, and the expense 

 of both pumping and filtering is avoided. 



(B) The Pumping System is used extensively by small 

 cities along rivers and in those parts of the country in 

 which there are no elevations or hills upon which to place 

 reservoirs or stand pipes. In some Western cities the 

 water is filtered and then pumped through the main pipe 

 lines into the houses. In certain cities along the Ohio 

 River the water is pumped directly into the houses with- 



