i6 



SMALL WATER SUPPLIES. 



pervious stratum lying over a bed of clay will cause 

 a spring, for instance in fig. 3. The underground 

 water may be tapped at the fault. Again, take the 

 case of fig. 4, which shows clearly the beds of perme- 

 able and impermeable strata in a state which often 

 occurs in practice. Springs will issue at the points 



day 

 FIG. 3 . 



shown along the line of water level. It will also be 

 seen that if a well is sunk at certain points a flow of 

 water will result at the surface, while in others 

 pumping will be required. The first instance con- 

 stitutes an artesian well. In the foregoing cases 

 we have assumed the saturation surface to be level. 

 This is by no means always the case, especially where 

 there is a river in the district. It will then be in- 



Oay 



FIG. 4. 



clined towards the river, and will be along a line 

 joining the mean water level of that river and the 

 top of the nearest outcrop of clay as shown by fig. 

 5. It must not be assumed, however, that the surface 

 of saturation will be invariably a straight line. In 

 fact in most eases it is depressed, while in extreme 



