Water and Ice 187 



to the deep underlying collections. Now in 

 the surface water supplies, as from rivers or 

 from lakes, man is his own worst enemy, 

 because the most serious dangers from impure 

 waters arise from its contamination with 

 human waste. 



Many great water supplies, which, under 

 ordinary conditions, are good, are constantly 

 liable to become sources of danger, because 

 the sewage from dwellings is discharged, if 

 not into them, still, so near to them that it 

 may now and then enter, being washed in by 

 rains or in some other way. This, under 

 ordinary conditions, may, if the sewage be 

 largely diluted in the reservoirs or streams, be 

 simply disgusting and filthy, though not posi- 

 tively dangerous. But if, as is at any time 

 liable to happen, typhoid-fever discharges in 

 considerable quantity get into the waste-pipes 

 and so into the water, the danger of the spread 

 o this disease becomes of great importance. 



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 



