The Sahara has been defined as the region which receives only as an exception any rainfall, whether of Mediterranean origin, or from the tropical regions of West Africa. It is only relatively a desert in the strict sense of the word; no part of it is absolutely without rain, and even in the districts which are reputed the most dry the traveller may meet with violent storms. The generally arid nature of the soil is due to the fact that water circulates not on the surface, but underground. Where it comes to light either by natural or artificial means, a focus of intense cultivation, an oasis, is produced. The Algerian Sahara is only a portion of the great desert of Northern Africa. Yet it is ten times the size of Algeria itself. It consists roughly of two great depressions separated by an isthmus of calcareous hills. Each of these basins contains a great expanse of dunes, and the two chief groups of oases occur in their lower levels. A generation ago it was commonly believed that the Sahara was the bed of a sea which had disappeared at no very distant date; and projects were formed of admitting the Mediterranean by means of a canal. But more precise knowledge has shown that its sterility is due to other causes; that like the rest of the continent it has its ancient conformation of mountain and plain; that it has distinctive flora and fauna long established; and that the portion which lies below the level of the Mediterranean is of very small extent. Between eighty and ninety per cent of the surface is of rock, slightly undulating and broken occasionally by perpendicular ravines and large crevasses. Here, as a rule, no water can be found, and the only vegetation is an occasional thorny shrub. With the regions of the sand dunes it is different. Their sterility is by no means absolute. They have a vigorous vegetation of their own, which will support camels, and even sheep at a favourable season. They absorb eagerly the rainfall which runs off the rocky plateaux, and acting as a sponge retain it for a long period. Their comparative barrenness is due only to the dryness of the climate; wherever they can be irrigated they become fertile. Of the underground rivers the best known is the Oued Rir, which is met with about fifty miles to the south of Biskra, and extends as far as Temacin, fourteen miles south-west of Touggourt. Its course is marked by a number of oases, some of which have been created, and others much improved by the Artesian wells of the French engineers. The first experiment of this sort was tried as early as 1856 at the oasis of Tamerna. After twenty-two days of work, in the presence of a crowd of incredulous and scarcely friendly natives the bore produced a veritable river of a thousand gallons a minute. “In the desert a fountain is springing.”