304 IMPROVEMENT OF MAN'S ENVIRONMENT 



their sewage flows. Others, as Albany, Pittsburgh, and Phila- 

 delphia, take their drinking water directly from rivers into which 

 has flowed sewage from cities above them on the river. A few 

 fortunate cities, such as Los Angeles and New York, bring their 

 water supplies from protected areas far up in the mountains. In 

 the case of cities where the water supply is polluted by sewage, 

 filtration and chlorination are necessary. The water is passed 

 through settling basins and sand filters which remove about 98 



Atnerican Museum of Natural History. 



Sand filter beds at Lawrence, Mass. 



per cent of the germs. Final treatment wdth hypochlorite of lime 

 or liquid chlorine in very small amounts kills most of the remain- 

 ing bacteria. In cities which drain their sewage into rivers and 

 lakes, the problem of sewage disposal is large, and many cities 

 now have means of disposing of their sewage in some manner 

 which renders it harmless to their neighbors. 



Railroads may be responsible for carrying and spreading disease 

 germs. It is said that a certain outbreak of typhoid in Scranton, 

 Pa., was due to the fact that the excreta from a typhoid patient 

 traveling in a sleeping car was washed by rain into a reservoir near 

 which the train passed. Railroads, therefore, in one sense, are like 

 great open sewers. 



Sewage Disposal. — Sewage disposal is an important sanitary 

 problem for every city. Some cities, like New York, pour their 



