826 NORTH-EAST AFRICA. and Farafreh cannot have their origin in the districts themselves, for rain is here the rarest of phenomena. The natives are thoroughly convinced that these waters are derived from the Nile, and they even pretend to have observed 'a certain increase in their volume during the period of the great inundations. But this would be very surprising considering the great extent of sands which the imder- ground currents would have to filter through. Yet the explorers Cailliaud and Russegger accepted the theory of the natives that the oases derive their supplies from the Nile. But Dakhel being at a much higher level than the main stream under the same latitude, the source of its springs must in any case be sought in the upper reaches of the Nile. They probably come from the southern regions lying within the zone of the tropical rains. But, however this be, the high temperature attained by the current during its imdergroimd passage shows that it must flow at a depth of several hundred yards below the surface of the ground. All the springs have a mean temperature of from 98° to 100° F., and they are utilised as well for the cure of certain maladies as for irrigation purposes. Since the year 1850 their volume has been consider- ably increased in the Farafreh oasis, thanks to the intelligence of a native, who after travelling with the French engineer Lefebvre, returned to his home, where he sank a number of wells and carried out a regular system of irrigation. Care was also taken to construct underground galleries analogous to the kanats and kJiariz of Persia, Afghanistan, and other parts of the Iranian plateau, in order to prevent excessive evaporation. So far the new wells do not appear to have at all diminished the abundance of the old sources, so that the underground supply seems to be practically inexhaustible. In the oasis of Beris, south of Khargeh, two hundred wells have been choked with the sands. But there still remain twenty-five whose thermal water ranging from 77° to 86° F. is highly ferruginous, and is found only at a depth of 200 feet from the surface. According to the ancient writers some of the wells in the great oasis had in former times been sunk to a depth of over 650 feet. The walls of the shafts are supported by beams of acacia wood affording access to the bottom. But the boring of new wells and the work of clearing the old pits of their accumulated sands are not unattended by danger. After the last layer of sand is pierced, wherever the flow is abundant, as in Dakhel and still more in Khargeh, the water tends to spread out in malarious swamps. The Natron Lakes. ' North of the Bahr-Bela-m&, and parallel with the series of depressions collec- tively known by this name, a valley of more regular form running south-east and north-west is occupied in its lowest cavities by seven shallow morasses. These are the so-called " Natron Lakes." Althotfgh separated from the nearest bend of the Nile by a shingly desert over 24 miles broad, the El-Natrun Valley most probably receives its supply of moisture from the river. During the three months following the autumnal equinox the water, "of a dark blood-red colour," due
