THE BACTERIA IN NATURAL WATERS. g 



librium tends to be established. A good river- water under 

 favorable conditions should thus contain only a few hun- 

 dred bacteria. Heavy rains which introduce wash from 

 the surrounding watershed may, however, at any time 

 upset this condition of equilibrium and surface-waters 

 are apt to show sudden fluctuations in their bacterial con- 

 tent. Particularly in the spring and fall high numbers 

 manifest themselves, and seasonal variations arise, such 

 as are well shown in the appended table. 



NUMBER OF BACTERIA PER CUBIC CENTIMETER IN CERTAIN 

 SURFACE-WATERS. 



* Whipple, 1896. 

 t Frankland, 1894. 



t Massachusetts State Board of Health, 1902. 

 Miquel, 1891. 



Two factors influence this seasonal distribution of bac- 

 teria. First, during the summer months the water flow- i 

 ing in open rivers is largely derived from springs and 

 subterranean sources, while during the autumn and 

 spring months, there is a much greater proportion of 

 " run-off " water contaminated by contact with the sur- 



