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In earlier times the phenomena of springs gave rise to much 
discussion. While some considered that they originated in great 
subterranean reservoirs, others asserted that they were dne to 
the percolation of sea-water which flowed upwards by subter- 
ranean cavities, losing its saltness in the passage. More than a 
hundred years ago, Valisnieri partly explained their dependence 
on the fall of rain, and the nature and arrangement of the strata 
through which the water percolates. Since his day, the theory 
of springs lias by degrees come to be well understood, and from 
the time of Smith, by strict attention to geological data, it has 
been possible to estimate with almost absolute certainty the 
results of sinkings in search of underground water in numerous 
localities. 
Rocks are of many degrees of hardness, and variously disposed. 
Thus, for instance, granite and its igneous allies, are but slightly 
porous, and it is only through joints and cracks, generally of no 
great depth, and having little intercommunication, that the sur- 
face water can penetrate ; and thus the subterranean oozings 
are isolated, so that generally no great body of underground 
water is anywhere collected, and numerous feeble springs rise 
here and there to the surface. But it is different with many of 
the stratified rocks, which not only by a multiplicity of joints, 
but also from their extreme porosity, and the sloping disposition 
of the beds, are often perfectly adapted to the conduction, and 
partial retention, of large bodies of water, at depths varying with 
the disposition of the strata. 
The nature of artesian wells is simple. If I take a bent tube 
and pour therein any quantity of water, it will maintain a corre- 
sponding level on either side ; and if I insert another tube shorter 
than the curved arms, (we shall suppose at the lowest point of 
the curve,) then, by virtue of a law of hydrostatic pressure, the 
water will rise in the inserted tube, an equal amount being dis- 
placed in the curved arms on either side. There it will rest. 
But if a constant supply be yielded to one or both of the open- 
ings of the curved reservoir, then the water will overflow at the 
mouth of the central inserted tube, which thus represents the 
boring of an artesian well. 
The strata around Paris are in a general way very similar to 
those forming and surrounding the London basin, (as it is often 
