202 DISCUSSION OF THE DESICCATION QUESTION. 



what centrally situated in the midst of a vast area of land, and therefore 

 must partake of the conditions imposed by the necessities of such a position ; 

 these, as has already been explained, include deprivation of rain-fall, to an 

 extent proportioned to the size of the continental area, and tlie manner in 

 Avhich the necessary moisture is cut off on its edges by interposing mountain 

 ranges. The Mediterranean is not large enough to do away with the char- 

 acter of a single mass of land which the three continents — Europe, Asia, and 

 Africa — would really have in perfection, were it not for the interposition of 

 that inland sea. The most that it can do is, to mitigate in some degree, 

 in the immediately adjacent region, what would otherwise be an entirely 

 insupportable dryness. 



Besides this, another unfavorable condition for the countries adjacent 

 to the Mediterranean, and especially for those to the east and south of it, 

 results from their position with reference to the great mass of intensely 

 heated land of Equatorial Africa, and the proximity of the trade-wind belt 

 on the south. The effect of this is, that the winds sweeping over the region 

 in question are mainly northerly, and are not moisture-bearing, because 

 they are moving toward warmer, and not toward colder, regions. 



The reversal of the trade-wind by the mass of Central Asia, producing — 

 as has been described — copious rains, in ordinary years, in India, is only 

 very slightly effective in Persia or Arabia, and hardly at all in Northern 

 Afiica. The Monsoon system of India and the region flirther east depends 

 on the proximity of the ocean, which cannot be heated wp by the summer 



Sahara : ve have thpiefore no reason to expect to find in that desert any constant current of air coming from 

 Central Asia. The northeast winds which, during the winter, blow in the Sahaia, have their origin chiefly in 

 Africa itself. In the summer Asia is still less the cause of tin? dryness of the Sahara, for at that season the baro- 

 metric pressure in the interior of that continent is less than it is in Africa, and the wind blows toward Asia from 

 eastern Europe : Asia, in fact, forms in summer a gathering-place [Sammelplatz] for the winds. Since in summer 

 the atmospheric pressure over the Sahara is small, especially over the southern part of the Desert, the air draws 

 thither from the Mediterranean Sea, from a ]iortion of the Atlantic Ocean ; and since the pressure in general in 

 the eastern part of the Sahara is comparatively low, north winds arise (tliat is to say, westerly or northwesterly 

 winds, wdiieh by the effect of the earth's rotation are converted into northerly winds). TIh^ constancy of the 

 summer north winds is proved by the observations of all travellers, not only in the Sahara but in Egyiit. Since 

 these winds come from the Mediterranean Sea, which at that season is much colder than the Desert, they of course 

 cannot bring rain. Even along the coasts of Algiers, in Sicily, Malta, etc., it hardly ever rains during the sum- 

 mer on account of the prevalence of north winds ; and the same result is still more reasonably to be expected in 

 the Sahara, since there the air is still warmer and drier. Northern Arabia, as well as a part of Mesopotamia and 

 Syria, is also extremely dry. The constancy of the north winds during the sunmier causes the scarcity of rain at 

 that season ; during the winter southerly winds do blow occasionally, and do bring some rain, but on tlie level 

 areas only in very small quantity, since the air is too dry, and obstacles which would cause the air to rise and be- 

 come suddenly cooled arc not present. In the mountains and in their vicinity the precipitation at this season is 

 more copious." 



