100 Bacteria in Relation to Country Life 



the entire water-shed is fenced in and men and animals 

 are excluded. In other instances, a careful record is 

 kept of the dwellings on the water-shed, and the cases 

 of typhoid occurring there are isolated as far as possible. 

 In still other instances, such inspections are either not 

 made at all, or made in a very incompetent, or perfunc- 

 tory manner, to the ultimate detriment of the com- 

 munity. It is scarcely necessary, therefore, to empha- 

 size here the importance of the topographical inspection 

 of the area whence a city's water-supply is derived. It 

 is necessary to determine whether sources of pollution 

 exist on the water-shed, and the extent to which the 

 drinking-water may become contaminated from such 

 sources. This much established, the aid of chemical 

 and bacteriological methods may be invoked for the 

 measure of the pollution that has already taken place, 

 and, to some extent also, for the measure of pollution 

 that may take place. 



The chemical examination seeks to discover in the 

 water certain substances derived from human wastes. 

 For instance, sewage contains a much larger proportion 

 of common salt than does pure water, since salt is always 

 used in the kitchen. If the salt content of the drinking- 

 water is found to^be greater than that of water known to 

 be pure and derived from the same locality, it may be 

 safely assumed that pollution has occured. Great care 

 must be exercised, however, in the interpretation of 

 the results, for the amount of salt present in the surface- 

 and well-water of different regions is variable. In some 

 localities, deposits of rock-salt exist, and their well- 

 waters contain, therefore, considerable quantities of 



