Birds.—­Some species of birds (as water-fowl, parrots, and the diamond bird) or animals (as baboons) afford surer promise; but the converging flight of birds, or the converging fresh tracks of animals, is the most satisfactory sign of all.  It is about nightfall that desert birds usually drink, and hence it often happens that the exhausted traveller, abandoning all hope as the shades of evening close in, has his attention arrested by flights of birds, that give him new life and tell him where to go.

Tracks.—­In tropical countries that have rainy and dry Seasons, it must be recollected that old paths of men or wild animals only mislead; they go to dry ponds that were full at the time they were trodden, but have since been abandoned on becoming exhausted.

Other Signs.—­Well-water may be sought where the earth is still moist, though arid all around, or, failing that, where birds and wild animals have lately been scratching, or where gnats hover in swarms.

To find the Spring—­From the number of birds, tracks, and other signs, travellers are often pretty sure that they are near water, but cannot find the spring itself.  In this case the party should at once be spread out as skirmishers, and the dogs cheered on.

To probe for Well-water.—­It is unusual, when no damp earth can be seen, but where the place appears likely to yield well-water, to force an iron ramrod deep into the soil; and, if it bring up any grains that are moist, to dig.

Pools of Water.—­For many days after there has been rain, water is sure to be found among mountains, however desert may be their appearance; for not only does more wet fall upon them, but the drainage is more perfect; long after the ravines and stream-beds are quite dry, puddles and cupfuls of water will be found here and there, along their courses, in holes and chinks and under great stones, which together form a sufficiency.  A sponge tied to the end of a stick will do good service in lapping these up.

The sandy Beds of Watercourses in arid countries frequently contain pools of stagnant water; but the places where these pools are to be found are not necessarily those where they have been found in preceding years.  The conditions necessary for the existence of a pool are not alone those of the rocky substratum of the river-bed, but more especially, the stratifications of mud and clay left after each flooding.  For instance, an extensive bed of sand, enclosed between two layers of clay, would remain moist, and supply well-water during the dry season; but a trivial variation in the force and 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.