When a river-bed has been long followed by a traveller, and a frequent supply of water found along it, in pools or even in wells, say at every 5 or 10 miles—­then, should this river-bed appear to lose itself in a plain that is arid, there is no reason why the traveller should be disheartened; for, on travelling further, the water will be sure to be found again, those plains being always green and grassy where the water in such river-beds entirely disappears.

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.  Hence, when water has sunk into the earth, it does not by any means soak through it in an equable degree.  It is an easier matter for it to ooze many miles, along a layer of gravel, than to penetrate six inches into a layer of clay that may bound the gravel.  Therefore, whenever a porous earth or a fissured rock crops out to the light of day, there is, in ignorance of all other facts, some chance of a spring being discovered in the lowest part of the outcrop.  A favourable condition for the existence of a large and permanent fountain, is where a porous stratum spreads over a broad area at a high level, and is prolonged, by a gradually narrowing course, to an outlet at a lower one.  The broad upper part of the stratum catches plenty of water during the wet season, which sinks into the depths as into a reservoir, and oozes out in a regular stream at its lower outlet.  A fissured rock makes a still easier channel for the water.

[Fig 1 and Fig. 2].