230 HISTORY or 



not quite so absurd. He alleged that the earth went round 

 i'aster than its atmosphere at the equator ; so that its motion, 

 from west to east, gave the atmosphere an imaginary one from 

 east to west ; and thus an east wind was eternally seen to pre- 

 vail. Rejecting those arbitrary opinions, conceived without 

 force, and asserted without proof, Dr Halley has given one more 

 plausible ; which seems to be the reigning system of the day. 



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

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

 compassing it on every side, ^^ ithout motion. Now it is evi- 

 dent, that that part of the air which lies directly under the beams 

 of the sun, will be rarified ; 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 vacuity will follow 

 too, and still be made under it. But while it goes on to make 

 new vacuities, the air will rush in to till 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 mo- 

 tions from east to west. In this manner the air is put into mo- 

 tion 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 cir- 

 culation. 



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 di- 

 recting towards the equator. This also proceeds from a similar 

 cause with the former ; for the air being more rarified in those 

 places over which the sun more directly darts its rays, the cur- 

 /ents 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 ac- 

 count for all the phenomena of trade-winds j 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 niise 



