92 TIEWS OF NATURE. 



The riches of the desert in rock-salt, and its employment 

 for purposes of building, have been known since the time of 

 Herodotus. The salt zone of the Sahara (zone salifere du 

 desert) is the most southern of the three zones which pass 

 through Northern Africa from south-west to north-east, and 

 is believed to be connected with the beds of rock-salt in Sicily 

 and Palestine described by Friedrich Hoffman, and by Ro- 

 binson*. 



The trade in salt with Sudan, and the possibility of culti- 

 vating the date-tree in the many Oasis-like depressions, 

 caused probably by earth-slips in the beds of tertiary chalk or 

 Keuper-gypsum, have equally contributed to animate the 

 desert, at various parts, by human intercourse. The high 

 temperature of the air, which renders the day's march so 

 oppressive across the Sahara, makes the coolness of the night 

 (of which Denham and Sir Alexander Burnes frequently com- 

 plained in the African and Asiatic deserts) so much the more 

 remarkable. Mellonif ascribes this coolness (which is proba- 

 bly produced by the radiation of heat from the ground), not 

 to the great purity of the heavens (irraggiamento calorifico per 

 la grande serenita di cielo nelT immensa e deserta pianura 

 dell' Africa centrale), but to the extreme calm, and the ab- 

 sence of all movement in the air throughout the whole nighty. 



The river Quad-Dra (Wadi Dra), which is almost dry the 

 greater part of the year, and which, according to Renou§, is one- 

 sixth longer than the Rhine, flows into the Sahara in 82° north 

 latitude, from the southern declivity of the Atlas of Morocco. 

 It runs at first from north to south, until in 29° north lat., 

 and 5° 8' west long., it deflects at right angles to the west, and 

 traversing the great fresh-water lake of JDebaid, flows into 

 the sea at Cape Nun, in lat. 28° 46', and long. 1 1° 8'. This re- 

 gion, which was first rendered celebrated by the Portuguese 

 discoveries of the fifteenth century, and whose geography has 

 subsequently been shrouded in the deepest obscurity, is now 

 known on the coast as the country of the Scheik of Beirouk 



* Fournel, Sur les Gisemens de Muriate de Sonde en Algerie, pp. 

 28-41 ; and Karsten, Ueber das Vorkommen des Kochsalzes auf der 

 Oberjlache der Erde, 1846, s. 497, 648, 741. 



+ Memoria sulV abbassamento di temperatura durante le notti 

 placide e serene, 1847, p. 55. 



X Consult, also, on African Meteorology, Aime, in the Explor. de 

 V Algerie, Phys. Gener. t. ii., 1846, p. 147. 



§ Explor. de VAlg., Hist, et Gcogr. t. viii. pp. 65 —78. 



