IN PBODUCING ASCENDINO ATMOSPHliRIC CUEEENTS. 



205 



these countries and warms them. A moderate flow of air 

 from the north appears to pass over Algiers and Tripoli to the 

 desert in an irregular and fluctuating way ; but it passes over 

 the whole desert, leaving it nearly destitute of water, and 

 probably in some manner, not open to observation at present, 

 feeds the north-east trade wind of the Atlantic. But the 

 only continuous winds that have a decided character in the 

 whole of this extensive area, are the north wind that blows 

 up the Valley of the Nile, and the Harraattan, which issues 

 from the desert and terminates in the Atlantic. 



That wind which blows up the Nile becomes strong in the 

 month of June, a time when the desert is much heated by the 

 sun, and it is known to continue blowing up the river as far 

 as Abyssinia. Speaking of it, a compiler says — " The mists 

 which are exhaled from the Mediterranean by heat, and which 

 frequently obscure the Egyptian sky, are carried by the north 

 winds over the flat country, without interruption, until they 

 are arrested by the mountains of central Africa."* But this 

 part of Africa is, as compared with the desert, a cold country, 

 being much elevated ; the wind therefore blows along a border 

 of the heated desert to feed a distant part. As stated by 

 Lizars, it may be distinctly traced as coming from the Medi- 

 terranean and the countries beyond it, and, in blowing up the 

 Valley of the Nile, it passes near the imaginary vacuum over 

 the desert, and proceeds to a part where we find, as we did 

 near the Himalaya Mountains, that at the time rain is falling 

 very freely, as is proved by the subsequent rise of the Nile. 

 Now if the hot Sahara heated the air that rested upon it, and 

 made it swell and ascend to the higher regions, allowing cooler 

 air to flow in below to supply its place, in the way that has 

 been asserted, this current of air which now goes to Abyssinia 

 would most undoubtedly be drawn towards the interior of the 

 desert, to feed the imaginary ascending current. But as it 

 is not so drawn, it is to be presumed that no such current 

 exists over the desert. 



* Lizare, p. 262. 



