IN PRODUCING ASCENDING ATMOSPHERIC CUKRENTS. 



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the west, that is, from the Gulf of Guinea towards the main 

 land of Africa. But it passes over the equatorial parts 

 towards a mountainous region where heavy rains are falling, 

 and not towards the heated desert. Thus, among all the 

 winds that are found to blow in or about this desert country 

 of northern Africa, the surface of which is in the summer 

 greatly heated, not one blows in a direction and with a 

 constancy that accords with the assertion that heated air 

 ascends within it ; whilst two winds evidently blow from it — 

 one, the Harmattan, on the western side, and the other on 

 the eastern side, carrying sand towards the Nile. 



The imperfect accounts which we have of the interior of 

 this great desert of stones and sand, with a few oases, state 

 that although the ground is greatly heated by the sun during 

 the day, yet at night its temperature sinks to a low degree, as 

 compared with that of the day ; the mass of the atmosphere 

 is therefore probably not greatly heated. From many facts, 

 it appears that the solar heat which is accumulated near the 

 ground in the day, is rapidly dissipated by radiation through 

 a clear sky at night ; and the mean temperature of the air at 

 a moderate height, if taken at each of the twenty-four hours, 

 would probably be found not high. In the parts near to 

 the Mediterranean where scarcely any rain falls, heavy dew 

 moistens the earth ; from which moisture the deceptive mirage 

 so often described is probably produced. But further in the 

 interior no dew is formed even by the great cold of the nights; 

 the air must therefore be very dry, and the sky is said to be 

 generally without clouds. Humboldt says that " the high 

 temperature of the air of the desert which makes the day's 

 march so oppressive, renders the cold of the nights (of which 

 Denham complained so often in the African, and Sir A. Burnes 

 in the Asiatic desert) so much more striking. Melloni ascribes 

 this cold, produced doubtless by radiation from the ground, 

 less to the great purity and serenity of the sky than to the 

 profound calm, the nightly absence of all movement in the 



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