380 GEOGRAPHICAL BOTANY. 



18 at the Nile, 16 north of Tschad (according to 

 Denham), and 20 in Senegambia (ii, p. 546). He forms 

 a law on the great diurnal differences of temperature be- 

 tween the night and day, even within the rainy zone, 

 which, if generally confirmed, would constitute a charac- 

 teristic peculiarity of tropical Africa. 



The whole of Nubia, as far south as 18 N. lat., ex- 

 cept the valley of the Nile and the coast, consists, like 

 Egypt, of rocky and sandy deserts. The heights here 

 extend scarcely 1000' above the plain, on the coast only 

 ascending to 4000', and in Dschebel Olba, according to 

 Wellsted, to 8000'. The coast of the Red Sea is not 

 free from rain ; but on the Nubian side, the summer 

 rains produced by the south-west monsoon extend almost 

 as far as the latitude at which the tropical winter rains 

 (as in Lower Egypt) commence. Suakim is situated on 

 the northern border of the full rainy season (19 N. lat.); 

 here, however, it occurs six weeks later (middle of July) 

 than below 17, and the summer rains are proportionately 

 retarded and abbreviated as far as 21 N. lat., from which 

 latitude northwards the winter rains commence. Al- 

 though the upper portion of the sea north of Suakim is 

 set in motion throughout the entire year by north winds, 

 yet the African coast of the Arabian Sea is never free 

 throughout the entire year from humid currents of air. 

 This explains how it is that the entire coast line of Nubia 

 is furnished with willows and other trees, whilst the inner 

 country does not contain even oases. During the journey 

 through the Desert, from Korosco to El Muchaireff, 

 which occupied fifty hours^ and is usually made to avoid 

 the great bend of the Nile, Russeger only once met with 

 brackish water, and that was in the middle of the 

 journey. 



The Nile leaves the zone of tropical rain at the 

 influx of the Atbara, and again comes in contact with it 

 by its bend at Dongola for a short distance. South of 

 the mouth of the Atbara, savannahs begin to alternate 

 with tropical forests, and this is the case throughout 



