the
pressure of a considerable column of water collected in the more
elevated portion of the same stratum. The water will then rush out, just
as the liquid from a large barrel which is tapped, and it will rise to a
height corresponding to the level of its point of departure, or, rather,
to a height which balances the pressure previously exerted by the
confined waters against the roof and sides of the stratum or reservoir
_a_ _a_. In like manner, if there happen to be a natural fissure _c_, a
spring will be produced at the surface on precisely the same principle.

[Illustration: Fig. 20.]

Among the causes of the failure of Artesian wells, we may mention those
numerous rents and faults which abound in some rocks, and the deep
ravines and valleys by which many countries are traversed; for, when
these natural lines of drainage exist, there remains a small quantity
only of water to escape by artificial issues. We are also liable to be
baffled by the great thickness either of porous or impervious strata, or
by the dip of the beds, which may carry off the waters from the
adjoining high lands to some trough in an opposite direction, as when
the borings are made at the foot of an escarpment where the strata
incline inwards, or in a direction opposite to the face of the cliffs.

The mere distance of hills or mountains need not discourage us from
making trials; for the waters which fall on these higher lands readily
penetrate to great depths through highly inclined or vertical strata, or
through the fissures of shattered rocks, and after flowing for a great
distance, must often reascend and be brought up again by other fissures,
so as to approach the surface in the lower country. Here they may be
concealed beneath the covering of undisturbed horizontal beds, which it
may be necessary to pierce in order to reach them. It should be
remembered, that the course of waters flowing under ground bears but a
remote resemblance to that of rivers on the surface, there being, in the
one case, a constant descent from a higher to a lower level from the
source of the stream to the sea; whereas, in the other, the water may at
one time sink far below the level of the ocean, and afterwards rise
again high above it.

Among other curious facts ascertained by aid of the borer, it is proved
that in strata of different ages and compositions, there are often open
passages by which the subterranean waters circulate. Thus, at St. Ouen,
in France, fiv