ARTESIAN WELLS. 421 



clay, into porous beds of the Plastic-clay formation, or into 

 the Chalk.* 



Important treatises upon the subject of Artesian Wells 

 have lately been published by M. Hericart de Thury and M. 

 Arago in France, and by M. Von Bruckmann in Germany.f 



* One of the first Artesian wells near London was that of Norland House 

 on the N. W. of Holland House, made in 1794, and described in Phil. Trans. 

 London, 1797. The water of this well was derived from sandy strata of the 

 plastic-clay formation, but so much obstruction by sand attends the admis- 

 sion of water to the pipes from this formation, that it is now generally found 

 more convenient to pass lower through these sandy strata, and obtain water 

 from the subjacent chalk. Examples of wells that rise to the surface of the 

 lowest tract of land on the W. of London may be seen in the Artesian foun- 

 tain in front of the Episcopal palace at Fulham, and in the garden of the Hor- 

 ticultural Society. Many such fountains have been made in the Town of 

 Brentford, from which the water rises to the height of a few feet above the 

 surface. 



This height is found to diminish as the number of perpetually flowing 

 fountains increases; and a general application of them would discharge the 

 subjacent water so much more rapidly than it arrives through the inter- 

 stices of the chalk, that fountains of this kind when numerous would cease 

 to overflow, although the water within them would rise and maintain its 

 level nearly at the surface of the land. 



The Section, PL G8 is intended to explain the cause of the rise of water 

 in Artesian Wells in the Basin of London, from permeable strata in the 

 Plastic-clay formation, and subjacent Chalk. The water in all these 

 strata is derived from the rain, which falls on those portions of their surface 

 that are not covered by the London Clay, and is upheld by clay beds of the 

 Gault, beneath the Chalk and Fire-stone. Thus admitted and sustained, it 

 accumulates in the joints and crevices of these strata to the line A, B. at 

 which it overflows by springs, in valleys, such as that represented in our 

 section under C. Below this line, all the permeable strata must be perma- 

 nently filled with a subterranean sheet of water, except where faults and 

 other disturbing causes afford local sources of relief. Where these reliefs 

 do not interfere, the horizontal line A, B, represents the level to which water 

 would rise by hydrostatic pressure, in any perforations through the London 

 Clay, either into sandy beds of the Plastic-clay formation, or into the Chalk; 

 such as those represented at D. E. F. G. H. I. If the Perforation be made 

 at G. or H. where the surface of the country is below the line A. B. t!ic 

 water will rise in a perpetually flowing Artesian fountain, as it does in the 

 valley of the Thames between Brentford and London. 



+ See Hericart de Thury's Considerations sur la cause du Jaillissement 

 des Eaux des puits fores, 1839. 

 VOL. I. — 36 



