884 NORTH-EAST AFRICA. were experienced both by Cailliaud in the Siwah oasis, and by Rohlfs in that of Dakhel farther west. In the Arabian desert the sudden rains on one occasion swept away the village of Desara, near Atfieh which was afterwards rebuilt on a site farther removed from the bed of the wady. On the other hand, there has been at times a total absence of rain. Not a drop fell for the space of six years in the district between Kosseir and Keneh; all vestiges of herbage disappeared from the valleys, and of trees the acacia alone resisted the effects of this protracted drought. Nevertheless the cisterns, which were fed by rain water along the old highway between Coptos and Berenice, are sufficient proof that this district does not lie within the absolutely rainless zone. In certain places natural cisterns or basins are met, formed by the subsidence of the numraulitic rocks, and here the water is collected on an impermeable bed of siliceous formations. These so-called mgheta, which differ greatly from the surface springs, usually known by the name of el-din, nearly always contain excellent water, the existence of which the surrounding tribes endeavour carefully to conceal from Europeans. But however slight is the winter rainfall, it nevertheless suffices, even without the aid of irrigation, to impart to the vegetation an appearance of freshness and vitality, which again disappears during the summer months. In this respect the Egyptian winter season presents the most striking contrast to that of temperate Europe. In the delta, however, the rainfall represents a part only of the actual discharge. Here the night dews are tolerably abundant, especially during the prevalence of the marine breezes, when they are heavy enough to regularly moisten the roofs and balconies of the houses in Alexandria. But the amount of dew diminishes in direct proportion to the distance from the Mediterranean, and in the Nubian desert, there is a slight discharge only in the vicinity of the river. In the heart of the Egyptian solitudes, where the bare rocks and white sands cause the heat of the sun to radiate into space during the night, it often happens that the dew freezes towards the morning. At its rise the sun, which will in a short time raise the temperature of the ground to over 100° F., begins by melting the slight layer of hoar-frost covering the desert. Even on the arable lands the plants are occasionally frozen, and Mr. Maspero picked up an icicle on the route between Edf u and Esneh. The extremes of heat and cold, although less considerable than in Nubia, are nevertheless very great in Upper Egypt. They increase gradually, proceeding from the north southwards, ranging in this direction between the isothermal lines of 20° and 25°. Climatic Changes during the Historic Period. Egypt is one of those regions whose climate must have undergone the greatest changes within the historic period. To judge from the bas-reliefs decor^iting the walls of the Saqqarah necropolis, probably the oldest in the world, the habits of the people at that time were not those of a nation everywhere hemmed in by the wilderness. They had no knowledge of the camel, a domestic animal without
