THE EARTH. I?31 



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 riversi, 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 heat occasioned 

 by the reflection of the sunbeams produces a very great rare- 

 faction of the air. The deserts of Africa, which are conform- 

 able 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 perceived blowing in 

 upon the land, in order to fill up the vacuity caused by the sun's 

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

 trary direction to that of its general current ; and is constantly 

 found setting in from the west. 



From the same cause it happens, that those constant calms, 

 attended 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 that move at some distance from shore, in a contrary 

 direction, the tendency of that part of the air that lies between 

 these two opposite currents is indifierent to either, and so rests 

 between both in torpid serenity ; and the weight of the incum- 

 bent atmosphere, being diminished by the continual contrary 

 winds blowing from hence, it is unable to keep the vapours sus- 

 pended that are copiously borne thither j so that they fall in con- 

 tinual rains. 



But it is not to be supposed, that any theory can account for 

 all the phenomena of even those winds that are known to be 

 most regular. Instead of a complete system of the trade-winds, 

 we must rather be content with an imperfect history. These,' 

 as was said, being the result of a combination of effects, assume 

 as great a variety as the causes producing them are various. 



Besides the great general wind above mentioned, in those parts 

 of the Atlantic that lie under the temperate zone, a north wind 

 prevails constantly during the months of October, November, 

 December, and January. These, therefore, are the most favour- 



1 JJuIioi;, vol. li. p. 230. 

 V 2 



