

388 



ORIGIN OF SPRINGS. 



[Ch. XVII. 



lying clay, and the porous superstratum is alternately 

 saturated by the water of the Thames as the tide rises, and 

 then drained again to the distance of several hundred feet 

 from the banks when the tide falls, so that the wells in this 

 tract regularly ebb and flow. 



The transmission of water through a porous medium being 

 so rapid, we may easily understand why springs are thrown 

 out on the side of a hill, where the upper set of strata con- 

 sist of chalk, sand, or other permeable substances, while the 

 subjacent are composed of clay or other retentive soils. The 

 only difficulty, indeed, is to explain why the water does not 

 ooze out everywhere along the line of junction of the two 

 formations, so as to form one continuous land-soak, instead 

 of a few springs only, and these oftentimes far distant from 

 each other. The principal cause of such a concentration of 

 the waters at a few points is, first, the existence of in- 

 equalities in the upper surface of the impermeable stratum, 

 which lead the water, as valleys do on the external surface 

 of a country, into certain low levels and channels, and 

 secondly, the frequency of rents and fissures, which act as 

 natural drains. That the generality of springs owe their 

 supply to the atmosphere is evident from this, that they 

 vary in the different seasons of the year, becoming languid 

 or entirely ceasing to flow after long droughts, and being 

 again replenished after a continuance of rain. Many of 

 them are probably indebted for the constancy and uniformity 

 of their volume to the great extent of the subterranean 

 reservoirs with which they communicate, and the time re- 

 quired for these to empty themselves by percolation. Such 

 a gradual and regulated discharge is exhibited, though in a 

 less perfect degree, in all great lakes, for these are not 

 sensibly affected in their levels by a sudden shower, but are 

 only slightly raised, and their channels of efflux, instead ol 

 being swollen suddenly like the bed of a torrent, carry off the 

 surplus water gradually. 



Much light has been thrown, of late years, on the theory 

 of springs, by the boring of what are called by the French 

 ' Artesian wells/ because the method has long been known 

 and practised in Artois ; and it is now demonstrated that 



C* 



r, 







) 





• 



i 



' 



I 



at 



is 



\ 



K 





