300 PHYSICAL GEOGllAPilY OF THE SEA, AND ITS METEOROLOGY. 



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551. The Bed Sea and its vajjours. — It bliould be borne in mind 

 that, by this hypothesis, the south-east trade-winds, after they 

 rise up at the equator (Plate I.), have to overleap the north-east 

 trade-winds. Consequently, they do not touch the earth until 

 near tlie tropic of Cancer (see the bearded arrows, Tlate VIL), 

 more frequently to the north than to the south of it ; but for a 

 part of every year, the place where these vaulting south-east 

 trades first strike the earth, after leaving the other hemisphere, 

 is very near this tropic. On the equatorial side of it, be it 

 remembered, the north-east trade-winds blow ; on the polar side, 

 what were the south-east trades, and what are now the prevailing 

 south-westerly winds of our hemisphere, prevail. Now examine 

 Plate VII., and it will be seen that the upper half of the Eed Sea 

 is north of the tropic of Cancer ; the lower half is to the south of 

 it ; that the latter is within the north-east trade-wind region ; 

 the former, in the region where the south-west passage winds are 

 the prevailing winds. The Eiver Tigris is probably evaporated 

 from the upper half of this sea by these winds ; while the north- 

 east trade-winds take up from the lower half those vapours which 

 feed the Nile with rain, and which the clouds deliver to the cold 

 demands of the Mountains of the Moon. Thus there are two 

 " wind-roads " crossing this sea : to the windward of it, each 

 road runs through a rainless region ; to the leeward there is, in 

 each case, a river rained down. The Persian Gulf lies, for the 

 most part, in the track of the south-west winds ; to the windward 

 of the Persian Gulf is a desert ; to the leeward, the River Indus. 

 This is the route b}' which theory would require the vapour from 

 the Eed Sea and Persian Gulf to be conveyed, and this is the 

 direction in which we find indications that it is conveyed. For 

 to leeward do we find, in each case, a river, telling to us, by 

 signs not to be mistaken, that it receives more water from the 

 clouds than it gives back to the wands. 



552. Certain seas and deserts considered as counterpoises in the 

 terrestrial machinery. — Is it not a curious circumstance, that the 

 winds wdiich travel the road suggested from the southern hemi- 

 sphere should, when they touch the earth on the polar side of 

 the ti'opic of Cancer, be so thirst}', more thirsty, much more, than 

 those which travel on either side of their path, and which are 

 supposed to have come from southern seas, not from southern 

 lands? The Mediterranean has to give those winds three times 

 as much vapour as it receives from them (§ 547) ; the Eed Sea 



