11 S STEPPES AND DESERTS. 



so much the more striking. Melloni, (Memoria suh" abas- 

 samento di temperatura durante le notti placide e serene, 

 1847, p. 55), ascribes this cold, produced doubtless by the 

 radiation from the ground, less to the great purity and 

 serenity of the sky, (irrigiamento calorifico per la grande 

 serenita di cielo nell' immensa e deserta pianura del? Africa 

 centrale), than to the profound calm, the nightly absence 

 of all movement in the atmosphere. (Consult also, re- 

 specting African meteorology, Aime in the Exploration de 

 FAlgerie, Physique generate, T. ii., 1846, p. 147.) 



The southern declivity of the Atlas of Morocco sends to 

 the Sahara, in lat. 32, a river, the Quad-Dra (Wady-Dra), 

 which for the greater part of the year is nearly dry, and 

 which Renou (Explor. de FAlg. Hist, et Geogr., T. viii. 

 p. 65-78) considers to be a sixth longer than the Rhine. It 

 flows at first from north to south, until, in lat. 29 N. and 

 long. 5 W., it turns almost at right angles to its former 

 course, runs to the west, and, after passing through 

 the great fresh water Lake of Debaid, enters the sea at 

 Cape Nun, in lat. 28 46' N. and long. 11 08' W. This 

 region, which was so celebrated formerly in the history of 

 the Portuguese discoveries of the 15th century, and was 

 afterwards wrapped in profound geographical obscurity, is 

 now called on the coast "the country of the Sheikh Bei- 

 rouk," (a chief independent of the Emperor of Morocco.) 

 It was explored in the months of July and August 1840, 

 by Captain Count Bouet-Yillaumez of the French Navy, 

 by order of his government. Prom the official Reports 

 and Surveys which have been communicated to me in 

 manuscript, it appears evident that the mouth of the 



