By Sea-shore.--Fresh water is frequently to be found under the very
sands of the sea-shore, whither it has oozed underground from the upper
country, and where it overlies the denser salt water; or else abuts
against it, if the compactness of the sand resists free percolation. In
very many places along the skirt of the great African desert, fresh water
is to be found by digging two or three feet.
Fountains.--Fountains in arid lands are as godsends. They are far more
numerous and abundant in limestone districts than in any others, owing to
the frequent fissures of those rocks: therefore, whenever limestone crops
out in the midst of sand deserts, a careful search should be made for
water. In granite, and other primary rocks, many, but small springs, are
usually seen.
The theory of ordinary fountains is simple enough, and affords help in
discovering them. In a few words, it is as follows:--All the water that
runs from them has originally Been supplied by rain, dew, or fog-damp,
falling on the face of the land and sinking into it. But the subsoil and
rocks below, are far from being of a uniform character: they are full of
layers of every imaginable degree of sponginess. Strata of clay wholly
impenetrable by water, often divide beds of gravel that imbibe it freely.
There are also cracks that make continuous channels and dislocations that
cause them to end abruptly; and there are rents, filled with various
materials, that may either give a free passage or entirely bar the
underground course of water.
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