1830.] 
On Artesian or Overflowing fVells. 
265 
which falls on the upper part of the limestone strata, and sinks down through the 
various canals which they contain : in a word, that they represent the shorter legs 
of a syphon, the longer of which is buried in the rock. M. Gamier is so convinced 
of the truth of this principle, that he only advises the boring of wells in the valleys 
of those districts whose elevations contain the outgoing of a cavernous limestone. 
Besides, upon a review of (he appearances observed in Artesian wells, it is evi- 
dently sufficient, that an inclined stratum of a fissured or porous limestone he 
included between two water-proof beds of clay, one of which sets a limit to the 
sinking of the water downwards, and the other keeps it back from above. The 
existence of such a cover is evinced by all boring works: a water-proof stratum 
of clay must always be penetrated, before reaching the spring-water. But also, it 
can easily be conceived, that the undermost layer is never wanting ; and although, 
for the most part, some thinner strata of limestone supply its place, yet the strata, 
which conduct the water, always contain it in crevices, Which are. much more 
numerous on the surface than in the centre of the beds : thus, there is a demon- 
stration as in a boring-work at Blengel, that, even in the limestone itself, beds 
of clay occur. From these circumstances, it is easily explained how we can never 
hope to sink Artesian wells in granite, gneiss, porphyry, serpentine, &c. Even 
in schistose mountains, it would not be advisable to sink these wells, because, 
even if found, it would be very easily impregnated with sulphuretted hydrogen, 
from the abundance of pyrites occurring in these strata, and thus be unfitled for 
many uses. Limestone, on the contrary, which is very insoluble, experience 
teaches us, yields a very pure water. 
Other districts, where water has been bored for, shew a similar geognostic con- 
stitution to the Pas-de-Calais. M. Gamier notices this, with regard to Boston, in 
America, and Sheerness 3 , in England. London, where many sugar-works, dis- 
tilleries, and breweries, have, fora long time, been principally supplied with water 
from Artesian wells, lies in the middle of a basin-shaped hollow, the fundamental 
rock of which is a limestone belonging to the chalk formation; which also forms 
the heights iu the vicinity, and which is covered, though at times not immediately, 
by a water-proof clay. The wells, which are not sunk to this, the London clay, 
give abundance of clear, but mostly very hard, water; while those which pene- 
trate through the L mdon clay, into the subjacent plastic clay , a formation imme- 
diately covering the chalk, and consisting of alternating beds of sand, clay, and 
boulders, yields a very soft and pure water 4 , which, on piercing this clay, often 
ascends with such violence, that the workmen have scarcely time to escape 6 . 
Here the plastic clay seems to be either the conducting medium, or the reservoir 
of the water yielded by the chalk. Paris is known to be situate in a district whose • 
geognostic relations are almost identical with those of London, and, therefore, we 
cannot wonder that there, as well as in many other parts of the north and east 
of France, Artesian wells may everywhere be sunk ; nor can we doubt of the 
extension of this very useful discovery 6 . The soil of Vienna seems also to be 
well adapted for the purpose, as partly appears from a geognostic description of 
Prevost 7 , and partly from the details given by Popowitch 8 of one of these springs 
s Very pure and clear water was here found at a depth of 550 feet, under the clay, 
in a chalky limestone, which sprung at first 344 feet high, then suuk, and now remains 
120 feet under the surface of the ground. 
4 It contains some carbonate ot soda, about 4 grains per quart. — Journal of Science , 
vol. xiv. p. 145. - . 
5 Conybeareand Phillips, Outlines of the Geology of England , &c. pt. i. p 34. 
« Most of those which have been bored in the town and its immediate vicinity, 
remain under the surface of the ground, although they are often several feet above 
the surface of the Seine and the common wells. Among a considerable number of 
those which are enumerated by M. Hericart de Thury, in the Annal de V Industrie, t. 
ii. p. 58, there are several from which the water is projected, at least at first, with 
great force, and uot without danger, far above the heads of the workmen. This, for 
instance, was the case with one, which, in the year 1780, was bored in the Vauxhall 
Gardens, and the level of the water of which has ever since been as high as the sur- 
face. This water couies from a depth of forty yards ; hut on accouut of the stouey 
character of the soil, and the consequent expense, they are usually only about half as 
deep : and this may,, perhaps, be one of the causes that permanent spring-wells have 
not yet been sunk. 
7 Journ. de Physique , t. 91. p. 347, & t. 92, p. 428. 
8 Observations of the Physico-Economical Society of the Palatinate for 177 0, pt. 2. 
p. 169. 
