210 



MB. T. HOPKINS ON THE INFLDENCE OF SUN-HEATED LAND 



atmosphere."* This calm is however itself a consequence of 

 the absence of all condensation of vapour, 



A wind is occasionally felt on the south-west coast of Italy 

 which is said to come from Africa — the following account of 

 it is from Milner. "It is (he says) a hot south-east wind, 

 prevailing in the Mediterranean, in Italy and in Sicily, but 

 felt most violently in the country around Naples and at 

 Palermo. It sometimes commences faintly about the summer 

 solstice, but blows occasionally with great force in the month 

 of July. There can be little doubt (he continues) that this 

 hot south-east wind sweeps across the Mediterranean from 

 Africa." But if it does, it evidently comes as a lower wind 

 sweeping the surface of the sea, from which it no doubt 

 takes up the moisture for which it is remarkable : as proof, 

 therefore, that the Desert heats air and causes it to ascend, 

 as heated air is assumed to rise within the tropics, it entirely 

 fails. 



If air from the Mediterranean Sea or the Atlantic Ocean 

 flowed freely towards the interior of this African desert, and 

 there became heated and ascended to a considerable height, 

 it would by that ascent expand and cool; and having taken 

 up vapour from the surface of the sea, as it must have done, 

 would produce first cloud and in due time rain. It is because 

 there is not a sufiicieut swelling and rising over the desert 

 that the country is so dry. There is no land sufficiently 

 elevated to force up the air to such a height as shall bring 

 on successive condensation and create ascending currents, 

 strong enough to cause air to flow as a wind to the desert. 

 Air rushes up the Valley of the Nile to the mountains of 

 Abyssinia, and from the Gulf of Guinea to other mountains, 

 because the vapour that it carries is condensed against their 

 sides ; but air does not flow from the adjoining seas into 

 the Sahara, because there is no suflicient condensation of 

 vapour there. If the Abyssinian mountains extended across 



AtpeeU of Nature, vol. i., p. 117. 



