lO 



Elements of Water Bacteriology. 



teria and their seasonal fluctuations in a stream of fair 

 quality (the Lahn) and a highly polluted one (theWieseck). 

 In the former case the bacterial numbers are highest wheii 

 rain brings surface pollution; in the latter, when the sewage 

 constantly present is least diluted. 



MONTHLY VARIATIONS OF BACTERIA EST A NORMAL 

 AND A POLLUTED STREAM. 



(KISSKALT, 1906.) 



' Rain or high water due to previous thaw. 



In standing waters all the tendencies which make for 

 the reduction of bacteria are intensified, and when a river 

 passes into a natural or artificial reservoir a more notable 

 reduction in numbers occurs. The following table shows 

 the striking effect produced upon the water of the Potomac 

 River by its successive passage through the three reservoirs 

 of the Washington water supply in the first nine months 

 of 1907. We owe these figures to the courtesy of Mr. F. 

 F. Longley, the engineer in charge of the Washington 

 filter plant. 



