PUBLIC HYGIENE AND SANITATION 409 



and all wells should be filled up. A driven pipe is much 

 safer than a large, open well. Its point should be driven 

 at least twenty feet below the water level, and there should 

 be no cesspools or barnyards near it. 



Usually water for a city is derived from an outside source 

 and is distributed by means of underground pipes. River 

 water is often used, but most rivers are infected by sewage 

 from other towns or from houses or camps along its banks. 

 The drainage from the excretions of a typhoid fever case, 

 reaching a river, has caused epidemics of typhoid fever in 

 other places in which the river was the source of the water 

 supply. 



713. Purification of water. River water should be 

 purified before it is used. The simplest method of purifi- 

 cation is to store the water in a reservoir and allow the 

 bacteria and other solid particles to settle to the bottom. 

 This does not remove all the bacteria, though it greatly 

 improves the water; but reservoir water may be almost 

 completely freed from bacteria and other solid substances 

 by means of good filter beds. 



The ordinary filters used in houses have little effect except to remove 

 very large particles of dirt. For efficient filtration water must pass 

 through the filter slowly and under little pressure. Filter beds for the 

 purification of city water consist of underground beds of clean sand 

 about four feet deep. A new bed does not work properly, for the 

 spaces between the particles of sand allow the bacteria to pass through. 

 In the course of a few days, however, a kind of vegetable organism re- 

 sembling mold grows in the upper layer of sand and covers the sand 

 grains with a gelatinous coating. This coating entangles about ninety- 

 nine per cent of the solid matters which may be floating in the water, 

 and allows only clear water to pass through. 



A filter bed an acre in extent will purify about three million gallons 

 of water daily. If a small quantity of alum is added to the reservoir 

 water, a soft, flaky substance is formed which becomes entangled in the 



