XX.] THE MOVEMENTS OF THE EARTH. 345 



Such a case as this which has just been discussed is by 

 no means imaginary. As a matter of fact, a stream of heated 

 moist air constantly rises, by its relative lightness, from the 

 neighbourhood of the equator, where the heat is greatest 

 and the evaporation most rapid. To supply the place of 

 the air which is thus raised, colder and denser air rushes 

 in from the north and the south of the equatorial belt. Yet 

 this inflowing air does not take the shape of a due north 

 wind, in the one hemisphere, and of a due south wind, in 

 the other hemisphere. The air comes from places where the 

 velocity of rotation is less ; and, therefore, it lags behind the 

 earth in its rapid rotation from west to east. Hence, the 

 wind reaches the equatorial zone from the north-east on 

 the north side, and from the south-east on the south side, 

 of the equator. In this way it comes about, that winds, 

 more or less constant in direction, blow across those parts 

 of the Atlantic and Pacific oceans which lie for some 

 distance on the two sides of the equator ; the direction 

 being from N.E. in the northern tropics, and from S.E. in 

 the southern tropics. Such steady winds were of so much 

 importance to navigation, before the days of steam-ships, 

 that much of the world's commerce depended upon them, 

 and they were therefore called trade winds. 



It has just been said that the trade-winds blow in a 

 direction " more or less constant." This qualification is 

 needed, because the character of the wind is greatly modified 

 by local circumstances, such as the distribution of land and 

 water, and the altitude of neighbouring land. The trade- 

 winds are not equally well marked in the two great oceans ; 

 nor are they equally strong at all seasons. 



It may be inquired, what becomes of the air which 



named after the point towards which they set. Hence a N.E. wind 

 blows from the N.E. ; but a N.E. current flows to the N.E 



