Page:Physical Geography of the Sea and its Meteorology.djvu/185

Rh equator (Plate I.); on the north side of it, the prevailing winds come from it also, but they go towards the north-east. They are the well-known westerly winds which prevail along the route from this country to England in the ratio of two to one. But why should we suppose a crossing to take place here? We suppose so from these facts: because throughout Europe,—the land upon which these westerly winds blow,—precipitation is in excess of evaporation, and because at sea they are going from a warmer to a colder climate; and therefore it may be inferred that Nature exacts from them what we know she exacts from the air under similar circumstances, but on a smaller scale, before our eyes, viz., more precipitation than evaporation. In other words, they probably leave in the Atlantic as much vapour as they take up from the Atlantic. Then where, it may be asked, does the vapour which these winds carry along, for the replenishing of the whole extra-tropical regions of the north, come from? They did not get it as they came along in the upper regions, as a counter-current to the north-east trades, unless they evaporated the trade-wind clouds, and so robbed those winds of their vapour. They certainly did not get it from the surface of the sea in the calm belt of Cancer, for they did not tarry long enough there to become saturated with moisture. Thus circumstances again pointed to the south-east trade-wind regions as the place of supply. This question has been fully discussed in Chapter V., where it has been shown they did not get it from the Atlantic. Moreover, these researches afforded grounds for the supposition that the air of which the north-east trade-winds are composed, and which comes out of the same zone of calms as do these south-westerly winds, so far from being saturated with vapour at its exodus, is dry; for near their polar edge, the north-east trade-winds are, for the most part, dry winds.

350. Wet and dry air of the calm belts.—Facts seem to confirm this, and the calm belts of Cancer and Capricorn both throw a flood of light upon the subject. These are two bands of light airs, calms, and baffling winds, which extend entirely around the earth. The air flows out north and south from these belts. That which comes out on the equatorial side goes to feed the trades, and makes a dry wind; that which flows out on the polar side goes to feed the counter-trades (§ 349), and is a rain wind. How is it that we can have from the same trough or receiver, as these calm belts may be called, an efflux of dry air