the words, gravel and clay, as generic for pervious and impervious
material.

[Illustration: Fig. 1.]

"Our drawing is an attempt to combine plan and section, which will
probably be sufficiently illustrative. From A to T is the overlap,
which is, in fact, a dam holding up the water in the gravel. In
this dam there is a weak place at S, through which water issues
permanently (a superficial spring), and runs over the surface from
S to O. This issue has a tendency to lower the water in the gravel
to the line M _m_. But when continued rains overpower this issue,
the water in the gravel rises to the line A _a_, and meeting with
no impediment at the point A, it flows over the surface between A
and S. In addition to these more decided outlets, the water is
probably constantly squeezing, in a slow way, through the whole
dam. Elkington undertakes to drain the surface from A to O. He cuts
a drain from O to B, and then he puts down a bore-hole, an Artesian
well, from B to Z. His hole enters the tail of the gravel; the
water contained therein rises up it: and the tendency of this new
outlet is to lower the water to the line B _b_. If so lowered that
it can no longer overflow at A or at S, and the surface from A to O
is drained, so far as the springs are concerned, though our section
can only represent one spring, and one summit-overflow, it is
manifest that, however long the horizontal line of junction between
the gravel and clay may be, however numerous the weak places
(springs) in the overlap, or dam, and the summit-overflows, they
will all be stopped, provided they lie at a higher level than the
line B _b_. If Elkington had driven his drain forward from B to
_n_, he would, at least, equally have attained his object; but the
bore-hole was less expensive. He escapes the deepest and most
costly portion of his drain. At _x_, he might have bored to the
centre of the earth without ever realizing the water in this
gravel. His whole success, therefore, depended upon his sagacity in
hitting the point Z. Another simple and very common case, first
successfully treated by Elkington, is illustrated by our second
drawing.

[Illustration: Fig. 2.]

"Between gravel hills lies a dish-shaped bed of clay, the gravel
being continuous under the dish. Spring