THE EARTH. 293 



into a trade- wind, that continually blows from the 

 poles, from the north on one side, or the south 

 on the other, both directing towards the equator. 

 This also proceeds from a similar cause with the 

 former ; for the air being more rarefied in those 

 places over which the sun more directly darts its 

 rays, the currents will come both from the north 

 and the south, to fill up the intermediate vacuity. 



These two motions, namely, the general one 

 from east to west, and the more particular one 

 from both the poles, will account for all the phe- 

 nomena of trade- winds ; which, if the whole sur- 

 face of the globe were sea, would undoubtedly be 

 constant, and for ever continue to blow in one 

 direction. But there are a thousand circum- 

 stances to break these air currents into smaller 

 ones ; to drive them back against their general 

 course ; to raise or depress them ; to condense 

 them into storms, or to whirl them in eddies. 

 In consequence of this, regard must be often had 

 to the nature of the soil, the position of the high 

 mountains, the course of the rivers, and even to 

 the luxuriance of vegetation. 



If a country, lying directly under the sun, be 

 very flat and sandy, and if the land be low and 

 extensive, the heats occasioned by the reflection 

 of the sun-beams, produce a very great rarefac- 

 tion of the air. The deserts of Africa, which are 

 conformable to this description, are scarcely ever 

 fanned by a breath of wind by day ; but the 

 burning sun is continually seen blazing in intoler- 

 able splendour above them. For this reason, all 

 along the coasts of Guinea, the wind is always 



