214
Art of Travel.
amount of the current, in different years, might materially affect the place and the character of the deposition of these clay strata.
In searching the beds of partly dried-up watercourses, the fact must never be forgotten, that it is especially in little tributaries at the point where they fall into the main one, that most water is to be found ; and the most insignificant of these should never be overlooked. I presume that the bar, which always accumulates in front of tributaries, and is formed of numerous layers of alluvial deposit, parallel to the bed of the great stream, is very likely to have one, at least, of its layers of an impervious character. If so, the bar would shut in the wet sand of the tributary, like a wall, and prevent it from draining itself dry.
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
