CHAPTER XI.
THE SAHARA.
HE term Sahara (Sah'rā) meaning a vast plain, waste, or wilderness, appears to have no very precise value. It is a geographical expression applied by various writers to an aggregate of regions to which very different superficial areas are assigned. In its general acceptation it comprises the almost waterless and very sparsely inhabited zone which separates the Barka plateaux and the Mauritanian uplands from the countries watered by the Senegal, the Niger, the affluents of Lake Tsad, and the headstreams of the White Nile. But where are we to draw the line of separation between the "Greater Africa" and the region which has been called the "Lesser Africa"? According to some writers, its northern limit skirting the foot of the Atlas merges eastwards in the shores of the Syrtes, thus embracing the whole of Tripolitana and coinciding at one point with the Mediterranean seaboard. But account is more usually taken of the political frontiers traced to the south of the Barbary States, and many tracts which in their physical aspect and climate present features common to both zones, are thus excluded from the Sahara and comprised in the Mediterranean basin.
Extent — Population.
The natural limits of the Sahara are indicated both by the nature of the soil and the shifting phenomena of its climate. Wherever regular rains cease to fall, the desert begins. But no fixed barriers can be assigned to the movement of the moisture-bearing clouds. In their relation to the arid zone they advance to a greater or less distance, gaining or losing ground according to the cycles of years or centuries. Where no precise boundaries are laid down by mountain ranges such as the Atlas, or by river valleys such as the Nile, the transition from the Sahara to the surrounding regions is effected through intervening zones of varying breadth. Nor have all the frontier lands yet been accurately explored, so that its outlines can only be approximately indicated on the maps.
In its widest extent the Sahara covers an area almost as large as Europe, itself. From east to west, that is, from the banks of the Nile to the Atlantic seaboard, it stretches for a distance of 3,000 miles, with a mean breadth of perhaps 900 or
