326 NOETH-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 vs^aters 

 are derived from the Nile, and they even pretend to have observed a certain 

 increase in their volume during the period of the great inimdations. But this 

 would be very surprising considering the great extent of sands which the under- 

 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 

 underground 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 knnats and 

 khariz 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-Belâ-mâ, 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." Although 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 



