THE STORY OF THE BACTERIA. 121 



in the lono- run would be the most efficient 



o 



and most thoroughly under control of the 

 authorities, would be the construction of large 

 filter-beds, or filtering apparatus, by means of 

 which the water might be freed from all dan- 

 gerous contaminations under all conditions. It 

 will thus be seen that while the Croton water 

 is not at present a source of actual danger it 

 may at any moment become so, unless some 

 efficient means be taken to protect its sources, 

 or purify it before distribution. 



Conditions similar to that of the Croton are 

 very common in the city water supplies every- 

 where, in this, as in other countries. And the 

 more rapidly are the regions from which the 

 water is derived becoming populated, the more 

 serious does the danger grow. 



Another great source of water supply for 

 large cities and towns is the rivers on whose 

 banks they are built. The water is usually 

 taken at a point some distance above the 

 town, so as to avoid the sewage of the town 

 itself, which, as a rule, is allowed to escape 

 directly into it. But in almost all cases, in 

 thickly settled countries, there are other towns 



