551. The Red Sea and its vapours.—It should 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 the tropic of Cancer (see the bearded arrows, Plate VII.), 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 Red 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 River 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 by which theory would require the vapour from the Red 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 which travel the road suggested from the southern hemisphere should, when they touch the earth on the polar side of the tropic 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 Red Sea
