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 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.
As examples of ordinary cases of fountains, we will take those represented in the following figures. Fig. I is a mountain. Fig. 2 is a model, made to explain more clearly the conditions represented in fig. I. It will be observed that there is a ravine, R, in front; a line of fault, L, M. N, on its left side, Supposed to be filled with water-tight rock; and a valley, V (fig. 1), on the extreme right. The upper part of the mountain is supposed to be much more porous than its base, and the plane which divides the porous from the non-porous rock, to cut the surface of the mountain along the line, A, N, M, B, C, D, E, F. The highest point of the plane is F, and the lowest point A. The effect of rain upon the model fig. 2 would be, to wet its upper half: water would ooze out along the whole of the lines A, N, and M, B, C, D, E, F; and there would be a small fountain at A, and a large one at M. But in the actual mountain, fig. 1, we should not expect to find the same regularity as in the model. The rind of the earth, with its vegetation and weather-impacted surface, forms a comparatively impermeable envelope to the mountain, not likely to be broken through, except at a few places. But ravines, such as r, would be probably denuded of their rind, and there we should find a line of minute fountains at the base of the porous rock. If there be no actual fountains, there would at least be some vegetation that indicated dripping water: thus the appearance is well known and often described, of a ravine utterly bare of verdure above, but clothed with vegetation below a sharply defined line, whence the moisture proceeds that irrigates all beneath. We should also be almost certain of finding a spring breaking forth near m or even near a. But in the valley V we should only see a few signs of former moisture, along e, f; such as bunches of vegetation upon the arid cliff, or an efflorescence of salts. Whenever a traveller remarks these signs, he should observe the inclination of the strata, by which he would learn the position of m, where the probability of finding water is the greatest. In a very arid country, the anatomy of the land is so manifest, from the absence of mould, that geological indications are peculiarly easy to follow.
Wells.--Digging Wells.--In default of spades, water is to be dug for with a sharp-pointed stick. Take it in both hands, and, holding it upright like a dagger, stab and dig it in the ground, as in fig. 1; then clear out the loose earth with the hand, as in fig. 2. Continue thus working with the stick and hand alternately, and a hole as deep as the arm is easily made. In digging a large hole or well, the earth Must be loosened in precisely the same manner, handed up to the surface and carried off by means of a bucket or bag, in default of a shovel and wheelbarrow.
After digging deeply, the sand will often be found just moist, no water actually lying in the well; but do not, therefore, be disheartened; wait a while, and the water will collect. After it has once begun to ooze through the sides of the well, it will continue to do so much more freely. Therefore, on arriving at night, with thirsty cattle, at a well of doubtful character, deepen it at once, by torch-light, that the water may have time to collect; then the cattle may be watered in the early morning, and sent to feed before the sun is hot.