564 ARTESIAN WELLS. 
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 
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 admission 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 fountain in front of the Episcopal palace 
at Fulham, and in the garden of the Horticultural 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 interstices 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, PI. 68 is intended to explain the cause of the rise 
of water in Artesian Wells in the Basin of London, from perme- 
able 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 the 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 permanently filled with a subter- 
ranean 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 
