Professor Buckland on Artesian Wells. 323 



logged porous stratum thus placed between two beds of cte-y, 

 through which no water can pass, may be compared to that 

 of water enclosed in a tick or waterproof case, to form what 

 is called a water-bed. We may, in imagination, extend inde- 

 finitely the size of this bed, containing water instead of 

 feathers ; and if we added to this water sand, or pebbles, or 

 angular stones, the intervals of all these would be occupied 

 by that portion of the fluid which was not displaced by the 

 solid bodies thus immersed in it. Such a tick or bed-case full 

 of stones and water would represent the permanently drenched 

 and water-logged condition of all permeable strata below the 

 level of the lowest springs by which their water can find 

 issue. A sheet of such water-logged stone, or of permanently 

 wet sand, is called by tho French geologists a *' Nappe 

 (TEau ;" it is not a sheet of pure water, but a bed or sheet of 

 sand or stones, whose interstices are filled with water, subject 

 to the laws of hydrostatic pressure. The lowest regions of 

 the chalk and of other porous strata are usually filled with 

 such sheets of water, supplied by rain descending through in- 

 numerable cracks and fissures ; and it was one of the infinite 

 wise provisions we find in the natural world that the same 

 water, which if placed in casks or open tanks, becomes putrid, 

 continues fresh so long as it remains in the cavities and in- 

 terstices of the strata of the earth. 



The greatest number of ordinary wells are dug in shallow 

 beds of gravel resting on the hollow surface of a subjacent 

 bed of clay. Wells sunk to a greater depth through stratified 

 rocks often afibrd larger supplies, but rise rarely to the 

 surface ; and in cases where they do so, they are called 

 Artesian wells, from the circumstance of such artificial over- 

 flowing wells being common in Artois, the ancient Roman 

 province, of Artesium, The deepest well we know of this 

 kind is that just mentioned, at Grenelle, near Paris, about 

 1800 feet deep ; from which the water rises thirty feet 

 above the surface, and at the temperature of 91° Fahren- 

 heit. Less deep, but similar wells abound near London ; 

 and the Board of Woods and Forests was now erecting 

 two large fountains in Trafalgar Square, to be supplied by 

 two contiguous wells, in which it was expected that wateij 



