TOL. XVI.] VHILOSOPHICAL TRANSACTIONS. 327 



heat, and consequently more ponderous, must have a motion towards those 

 parts, w here it is more rarefied and less ponderous, to bring it to an equilibrium ; 

 and secondly, the presence of the sun continually shifting to the westward, that 

 part towards which the air tends, by reason of the rarefaction made by his 

 greatest meridian heat, is with him carried westward, and consequently the 

 tendency of the whole body of the lower air is that way. Thus a general 

 easterly wind is formed, which being impressed on all the air of a vast ocean, 

 the parts impel one another, and so keep moving till the next return of the sun, 

 by which so much of the motion as was lost is again restored, and thus the east- 

 erly wind is made perpetual. 



From the same principle it follows, that this easterly wind should, on the 

 north side of the equator, be to the northward of the east, and in south lati- 

 tudes to the southward of it ; for near the line the air is much more rarefied 

 than at a greater distance from it; because the sun is twice a year verticle, and 

 at no time distant above 23° and a half; at which distance, the heat, being as 

 the sign of the angle of incidence, is but little short of that of the perpendi- 

 cular ray. Whereas under the tropics, though the sun continue long vertical, 

 yet he is as long 47° off; which is a kind of winter, when the air is so cooled^ 

 that the summer heat cannot warm it to the same degree with that under the 

 equator. Therefore the air to the northward and southward, being less rarefied 

 than that in the middle, it follows, that from both sides it ought to tend to- 

 wards the equator : this motion, compounded with the former easterly wind, 

 answers all the phaenomena of the general trade winds, which, if the whole sur- 

 face of the globe were sea, would undoubtedly blow all round the world, as they 

 are found to do in the Atlantic and Ethiopic oceans. 



But as such great continents interpose and break the continuity of the 

 oceans, regard must be had to the nature of the soil, and the position of the 

 high mountains, which I suppose the two principal causes of the several varia^ 

 tions of the winds, from the former general rule : for if a country lying near 

 the sun prove to be flat, sandy, low land, such as the desarts of Lybia are re- 

 ported to be, the heat occasioned by the reflection of the sun beams, and its 

 retention in the sand, is incredible to those that have not felt it; by which the 

 air being exceedingly rarefied, it is necessary that the cooler and more dense 

 air should run thither to restore the equilibrium. This I take to be the cause, 

 why near the coast of Guinea the wind always sets in upon the land, blowing 

 westerly instead of easterly, there being sufficient reason to believe that the in- 

 land parts of Africa are prodigiously hot, since its northern borders were so in- 

 temj)erate, as to give the ancients cause to conclude that all beyond the tropic 

 was uninhabitable by excess of heat. From the same cause it happens, that 



