TO AKTESIAltf WELLS. 



11 



represents deposits of the tertiary system, suppose the 

 valley of the Thames. 



FIG. 1. Artesian "Wells. 



The bed, a a, is an impervious stratum of London clay ; 

 b ft, a porous deposit of gravel or sand, resting on the chalk, 

 c c ; the whole forming a basin-like structure. The water 

 which falls on the chalk-hills, c c, flows either into the chalk, 

 or the porous bed above it, b b and being forced upwards 

 by fresh accessions, would rise to the top, were it not pre- 

 vented by the clay above, a a. The engineer, by boring 

 through the bed, a a, or the chalk, c c, gives exit to the sub- 

 terranean water which rises to the surface and flows con- 

 tinually by means of hydrostatic pressure. The beds above 

 the chalk were formerly considered to contain the water ; it is 

 now, however, ascertained that it is derived from the chalk 

 strata beneath, and that the gault is the great retentive 

 bed of clay that supports the superincumbent sheet of water 

 that niters through the beds of porous chalk and firestone. 

 By these borings through the clay beds of low, level districts 

 in England, France, and Germany, copious streams of water 

 are procured from depths, and under conditions, which 

 would either preclude the sinking a well altogether, or 

 the expense would be such as to prohibit the enterprise. 

 The plan has been adopted with eminent success in the 

 vicinity of London ; but the most important work of the 

 kind is in the Plaine de Grrenelle, near Paris ; this Artesian 

 well yields 516^ gallons of water per minute, the temperature 

 of which is 81 7' Fahr., the depth of the well is 1794 feet ; 

 the shaft passes down into the gault. The Artesian Wells in 

 Trafalgar Square descend into the upper chalk to a depth of 

 393 feet. 



