366 Professor Fournet's Besearches on the 



tends the chain of the Mokattam, which may be considered 

 as the line of the disjunction of Asiatic monsoons and the 

 trade-winds ; upon it, in consequence of its summits, and of 

 the phenomenon of the monsoons, there are as violent and 

 frequent winter rains as in Palestine, for they cause the moun- 

 tain torrents to swell to overflowing ; but this result is not to 

 be regarded as contradicting the general law, for close to 

 Qoceyr (lat. 26° 7' N.) the sky is constantly serene, and no 

 house possesses a cistern, although the nearest spring is dis- 

 tant a day's march. On the opposite side from the Red Sea, 

 Thebes (lat. 25° 43' N.) receives rain only during a small num- 

 ber of days in the year ; it then ceases in Upper Egypt, but 

 recommences in Dongolah in 20"" N., where it becomes estival. 

 There, also, the arid soil and the naked rocks cease, as well as 

 the cloudless sky of Upper Egypt and Lower Nubia; but 

 nevertheless, the pluvial intermission is still distinctly dis- 

 played in Dongolah ; according to some, Gerry, in lat. 16° 15' 

 N., and according to others, lat. 17° N., is to be regarded as 

 the true limit of the regular intertropical rains of that portion 

 of the continent. 



Various oases, or depressions of the surface (the oasis of 

 Siwah is 100 feet below the level of the sea), are scattered 

 here and there in the midst of the sand, and owe their 

 fertility to springs. Among these there is one which should 

 fix our attention in a particular manner, because it forms a 

 sort of diaphragm, so to speak, dividing the Sahara into two 

 equal parts, and serving as the great highway for the caravans 

 from Fezzan to the centre of Africa. At the epochs of the year 

 when the temperature is lowered, that is to say, from October 

 to February, the caravans of the north, of the east, and of the 

 west, meet at Mourzouk ; they take fifty-seven days' march to 

 reach Birney (the capital of Bornou, to the west of Lake 

 Tchad, in lat. 16° N.), following a chain of rocks more or less 

 abrupt, that enclose a sort of long valley (Wady Kawas), in 

 which there are numerous stations, and consequently likewise 

 that watery subsoil, without which the journey would be im- 

 possible. This line is only interrupted here and there by 

 saline tracts, and by the sandy desert of Timtuma ; and it is, 

 therefore, not astonishing that it served as the foundation of 



