13(. A HISTORY OF 



one nvre plausible ; which seems to be the reigning system of the 

 day. 



To conceive his opinion clearly, let us for a moment suppose tha 

 whole surface of the earth to be an ocean, and the air encompassing 

 it on every side, without motion. Now it is evident, that that part ot 

 the air which lies directly under the beams of the sun, will be rarefied ; 

 and if the sun remained for ever in the same place, there would be a 

 great vacuity in the air, if I may so express it, beneath the place where 

 the sun stood. The sun moving forward, from east to west, this va- 

 cuity will follow too, and still be made under it. But while it goes 

 on to make new vacuities, the air will rush in to fill up those the sun 

 has already made ; in other words, as it is still travelling forward, the 

 air will continually be rushing in behind, and pursue its motions from 

 east to west. In this manner the air is put into motion by day ; and 

 by night the parts continue to impel each other, till the next return of 

 the sun, that gives a new force to the circulation. 



In this manner is explained the constant east wind that is found 

 blowing round the globe, near the equator. But it is also known, that 

 as we recede from the equator on either side, we come 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 

 phaenomena of trade-winds ; which, if the whole surface of the globe 

 were sea, would undoubtedly be constant, and for ever continue to 

 blow in one direction. But there are a thousand circumstances 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, re- 

 gard 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 re- 

 flection of the sun-beams produces a very great rarefaction 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 intolerable splendour above them. For 

 this reason, all along the coasts of Guinea, the wind is always perceiv- 

 ed blowing In upon land, in order to fill up the vacuity caused by the 

 sun's operation. In those shores, therefore, the wind blows in a con- 

 rary direction to that of its general current ; and is constantly found 

 setting in from the west. 



Krom the same cause it happens, that those constant calms, attend- 

 ed with deluges of rain, are found in the same part of the ocean. For 

 this tract being placed in the middle, between the westerly winds 

 blowing on the coast of Guinea, and the easterly trade-winds thai 



