PROPERTIES OF WATER AND SOURCES OF SUPPLY. I 3 



very much better to collect all this water into a tank 

 and treat it properly. This will be treated in the 

 chapter on storage. At the same time rain water is 

 not palatable. 



The second method of supply is that from rivers, 

 the purity of which must be ascertained as advised 

 before. Provided then that it is pronounced pure, we 

 may see how it can be collected. Most towns are sup- 

 plied from rivers, by impounding the water they bring 

 down at the upper reaches where there are more or 

 less mountain streams free from pollution. A river 

 so far as it concerns water supply is formed by the 

 rainfall of the district, preserving the level of the 

 saturation surface above the natural hollow that 

 forms its bed, and the erosion due to scour of the 

 water therein ; hence it will be observed that water 

 will reach a river in different ways, viz., by surface 

 flow and' rain, percolation of land water and the 

 natural tributaries of the river. On the whole the 

 quality of river water for small domestic supplies is 

 very much inferior to well water, being charged with 

 animal, vegetable, and mineral impurities, and the size 

 of the scheme does not usually warrant the laying 

 down of expensive plant for its purification. The 

 theory put forward that rivers during their course 

 have a self-purifying property has been proved (so 

 far as short rivers in Britain are concerned) to be a 

 fallacy, even over a considerable series of falls, al- 

 though this action will allow of the escape of CO 2 . 

 Shallow well water is also usually of a quality much 

 the same as river water, which is intercepted by 

 these wells on its way along the saturation surface 

 towards the rivers, but of course the pollution of the 



