14 SMALL WATER SUPPLIES. 



river will not affect the water from the shallow well, 

 provided this well is not so near the river as to allow 

 of a percolation therefrom, which might be the case 

 if excessive pumping of that well was going on. In 

 fact the water is fairly good except in periods of long 

 continued drought, when the water is very liable to 

 contamination from surface drainage. To avoid this 

 such wells are lined down to a point below the lowest 

 level of the saturation surface. In passing it may be 

 well to say exactly what the saturation surface is, as 

 frequent use has been made of this term. Water fall- 

 ing on the earth seeks by the action of gravity lower 

 levels, always along lines offering the least resistance. 

 Through soil it is vertical, but at great depths it meets 

 the natural body of subterranean water. Its path 

 downwards is here checked and deflected at an angle, 

 steep in compact rocks and flat in permeable rock 

 and gravel ; where these hydraulic laws cause the 

 waters to have a definite surface slope there is the 

 surface of saturation. A plane generally inclined to- 

 wards the sea ; over compact rocks, this surface is not 

 very far below the surface of the ground. In fact it 

 may at times be above it, and we have water-logged 

 ground ; or again the water may issue in the form of 

 a hillside rivulet or rill, and the water so obtained is 

 not always very good for domestic supply except 

 lias formation, where the water has only been able 

 to penetrate a short distance vertically into the sub- 

 soil and superficial strata. 



So much then for river water. However im- 

 portant the question may be to town supplies, for 

 domestic supplies river water takes a back place. 

 Later it will be described how it may be obtained 



