EASTING OF THE TRADE- WINDS, ETC. 169 



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, — precijoitation is 

 in excess of evaporation, and because at sea they are going from 

 a w^armer to a colder climate ; and therefore it may be inferred 

 that Nature exacts from them wliat we know she exacts from the 

 air under similar circumstances, but on a smaller scale, before 

 our ejes, 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 w^here, it may be asked, 

 does the vapour w^hich these winds carry along, for the re- 

 plenishing of the whoje 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 circum- 

 stances again pointed to the south-east trade-wind regions as the 

 place of supply. This question has been fully discussed in 

 Chapter Y., where it has been shown they did not get it from 

 the Atlantic. Moreover, these researches afforded gro-unds 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 

 wdth 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 

 "xlood 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 wdnd ; 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 



