384 MAN'S IMPROVEMENT OF HIS ENVIRONMENT 



protected will contain no bacteria. Water taken from protected 

 streams into which no sewage flows will have but few bacteria, and 

 these will be destroyed if exposed to the action of the sun and the 

 constant aeration (mixing with oxygen) which the surface water 

 receives in a large lake or reservoir. But water taken from a river 



ORIGINAL CASES IN 



NORTH OCLMSrORO 





FCSULTtNG CA^r. -^ 



I8»l. 



The city of Lowell in 1891 took its water without filter i7ig, i.e. from the Merrimack 



River at the point shown on the map. 

 Typhoid fever broke out in North Chelmsford and about two weeks later cases 



began to appear in Lowell until a great epidemic occurred. Explain this 



outbreak. Each black dot is a case of typhoid. 



into which the sewage of other towns and cities flows must be 

 filtered before it is fit for use. 



Typhoid fever germs live in the food tube, hence the excreta of 

 a typhoid patient will contain large numbers of germs. In a city 

 with a system of sewage such germs might eventually pass from 

 the sewers into a river. Many cities take their water supply 

 directly from rivers, sometimes not far below another large town. 

 Such cities must take many germs into their water supply. Many 

 cities, as Cleveland and Buffalo, take their water from lakes into 

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

 delphia, take their drinking water directly from rivers into which 



