GEOGRAPHICAL BOTANY. 379 



In the rainless zone of the north of Africa, in conse- 

 quence of the duration of the polar currents, the great 

 diurnal differences of temperature allow of the formation 

 of dew to a slight extent ; it occurs very copiously in the 

 lower valley of the Nile, is formed in Upper Egypt, and 

 appears also to fertilize the oases. Russeger, however, 

 did not meet with any dew in the Desert of Nubia, but 

 in that of Lybia it is common (ii, p. 253). The oases 

 lying to the west of Egypt, according to Russeger, gene- 

 rally obtain their soil-water from the Nile, which flows 

 laterally to them over beds of clay (p. 271). Thus they 

 form a valley which is filled with springs, excavated 

 below the level of the Nile, and parallel to this river. 

 The other oases of the Sahara appear to be produced 

 merely by the formation of dews. Borgu, Darfur, and 

 Kordorfan, however, in this sense are not oases, but 

 savannahs, situated within the rainy climate (p. 283). 



The tropical rains extend in most years to, at the 

 most, 18 N. lat. (i, p. 224), i. e. two degrees north of 

 Chartum, the point at which the two arms of the Nile 

 become confluent. The heavy rains fall there in the 

 summer, and correspond to the south winds which blow 

 at this time, and which prevail below 15 N. lat. from 

 April to September, and alternate every six months with 

 the north winds. The northern border of this monsoon- 

 zone, which in the south of the Desert or Soudan pro- 

 duces savannahs, is not accurately determined. A short 

 rainy season may occasionally occur beyond 18 N. lat., 

 when the south winds blow as far as this part. How- 

 ever, the dry chamsin of the Desert, which blows from 

 the same direction, and which Russeger regards as a 

 local and electrical phenomenon, must not be confounded 

 with these general south winds which bring rain. Even 

 between 16 and 18 N. lat. the rainy period is irregular, 

 and in many seasons abbreviated : at Chartum it lasts 

 five months. Russeger assumes the following mean 

 values as marking the north border of the tropical rainy 

 zone throughout Africa : 21 N. lat. at the Red Sea, 



