GENERAL CONSIDERATION. >« 



limits, yet the term is useful. It is an instance of the difficulties 

 that are always experienced in making scientific classifications : 

 they must always be more or less artificial, and however elaborate 

 they can never be in complete harmony with the infinite variety of 



nature. 



From the foregoing remarks, it follows that the "spring wells" 

 just described, differ from the typical examples in London and Paris 

 with regard to the situation of the well, relatively to the intake area 

 of its water-supply. Since the clay bands which form their charac- 

 teristic feature are of very limited extent, it follows that the water 

 which they contain is, properly speaking, only "ground- water," 

 derived from the same source as the water of ordinary percolation 

 wells, that is, from purely local rainfall. These wells illustrate 

 examples of local artesian action within a reservoir of water which 

 can scarcely be called an artesian reservoir. But in the case of 

 artesian wells like those of Paris, the porous stratum forming the 

 natural reservoir from which they derive their supply has no com- 

 munication with the surface except at its outcrop many miles away. 



A very important result of this structure is that the supply of the 

 artesian well is independent of local rainfall. 



Special characteristics . 



of the supply from deep Owing to its greater altitude, the collecting 



artesian sources. . ■ 



area is usually favoured with a more abundant 

 rainfall than the locality at which the wells are situated. If it is 

 sufficient to cause a constant overflow from the outcrop of the water- 

 bearing stratum, the head of water will naturally be constant, as 

 the artificial wells usually abstract but a small portion of the water 

 furnished to the water-bearing stratum over the entire extent of 

 its outcrop. Moreover, a fall of the water-level at the fountain head 

 will not cause a corresponding loss of effective head in the well, for, 

 owing to the retardation caused by friction, the rise of water in the 

 well is but a fraction of what it would be otherwise. Any alteration 

 in the level of the fountain head can only affect the outflow at the 

 well in the same diminished proportion. 



( " ) 



