290 NORTH-EAST AFRICA. Climate. Nubia is divided into two climatic zones, whose limits, changing yearly, are defined by the struggle between the northern and southern winds. In summer, when the solar rays fall vertically on the soil between the equator and the northern tropic, the southern winds follow the sun into the northern hemisphere, carrying the rain-clouds with them ; but they scarcely ever reach the seventeenth degree of latitude. The last periodical rains fall in the valley towards the junction of the Atbara, the last tributary watercourse of the Nile. In this spacious region the force of the southern winds is neutralised by that of the aerial currents blowing from the north, the result of this struggle being the constant shifting of the parting-line. Travellers across the Bayuda territory in May or June have to struggle against the winds, being at one time forced from their course by the south wind at another by that of the north, their way thus lying between two conflicting tempests. However, the winds often blow alternately, that of the north prevailing during the day and being replaced at night-time by that of the south. To the south of this zone fall periodical rains, the more copious and longer in duration the nearer the equator is approached. To the north, the soil is not watered by the summer rains, being visited only by a few showers, which sometimes even cease for several years together. When the northern winds predominate in the inter- mediary zone and drive the opposing currents south of the usual limit, the drought becomes general, bringing to the Nubians distant from the Nile famine, involun- tary exile, and brigandage. The region of the border mountains, in the vicinity of the Red Sea, is more favoured than the countries of the interior. The abundance of the vapours in this littoral zone promotes the advance of the rain-bearing clouds towards the north. Instead of stopping at the seventeenth degree of latitude, they extend as far as the twenty-first degree of latitude, and beyond this limit occasional showers are frequent ; but brought down by the northern winds they fall in winter, whilst the opposed southern winds cause the moisture to be again precipitated in summer. The result of this relative abundance of the rains on the coast region is a great contrast between the Nubian districts near the Red Sea and those of the interior. To the east the Arab nomads find abundance of wells, fountains, and pasturages for their cattle ; but to the west nothing is visible but rocks and sands, the eye of the traveller seldom lighting upon a grove of palms or mimosas, or any trailing shrubs growing on the banks of the wadies. In some years the drought is so complete that no shepherd dare venture into the desert. Nubia thus offers some well-defined natural divisions. The southern part of the country, comprising nearly the whole peninsula of Bayuda, is a district of steppes ; the coast of the Red Sea presents an analogous appearance, whilst all the rest is desert and atmur, with the exception of the verdant and populous Nile Valley lying between these two dreary wastes. In many places this valley is reduced to a strip a few yards in width ; it even disappears altogether at tho
