364 Professor Fournet's Besearches on the 



laws of continuity of nature, I have investigated the causes 

 of the anomaly, and have found that observation makes it 

 disappear, and shews us that the two preceding zones, far 

 from being in immediate contact, are, on the contrary, dis- 

 tinctly disjoined by bands of absolute dryness, at least in the 

 portion of the globe constituting the region of the trade-winds, 

 with which we shall, in the first place, occupy ourselves ; we 

 shall, afterwards, examine the phenomena connected with the 

 regions subject to the monsoons. 



Africa presents a very manifest demonstration of this cir- 

 cumstance. In the northern part of that continent, the band 

 of absolute dryness is represented by the Sahara, which we 

 may regard as prolonged, without interruption, from the 

 Atlantic to the Red Sea, and having a breadth which the 

 tropic divides into two nearly equal parts. In this manner, 

 by uniting under a collective name the Sahara properly so 

 called, the two deserts of Nubia and those of Egypt, the den- 

 ticulations of the band would reach to 15° of north latitude, 

 encompassing the ramifications of the central chains of the 

 continent ; while, on the opposite side, being bounded, for a 

 part of its course, by the masses of the Atlas, it would, 

 nevertheless, send off branches to the Mediterranean, as far 

 as 13° of north latitude, by the lower parts of Egypt, and the 

 salt plain of the Syrtis. Von Humboldt assigns to its sur- 

 face, not including Darfour and Dongolah, an extent of 

 194,000 square leagues — that is, more than double the extent 

 of the Mediterranean, which is 77,300 square leagues ; but 

 it must not be supposed that all this immense breadth be- 

 longs, properly speaking, to the Sahara- bela-md of the Arabs, 

 nor that it is in a state of absolute dryness, although the hot- 

 test climates are situated under the tropic of Cancer, and as 

 far as four or five degrees to the north. The true physiog- 

 nomy of the desert only presents itself progressively, by the 

 diminution of the vegetation, which, in proportion as the rains 

 become more rare, degenerates from the condition of forests 

 to that of brushwood, and is afterwards replaced by steppes, 

 and, finally, by the sands, pebbles, or naked rocks of the cen- 

 tral portions nearest the tropic. 



