138 WINDS OF THE NORTIIEKN HEMISPHERE. 



18. On the whole, do not the results in Series C authorize us to lay down the 

 following, as a general description of the winds of the northern hemisphere? 1st. 

 That from high northern latitudes the Avinds proceed in a southerly direction, but 

 veer toward the west, as they approach a limit ranging from about latitude 56° on 

 the western continent to about latitude 08° on the eastern, where they become 

 irregular and disappear. The area of the zone occupied by these winds is about 

 11,800,000 square miles. 2d. That farther south there is a belt of westerly winds, 

 less than 2000 miles in breadth, entirely encircling the earth ; the westerly direc- 

 tion being clearly defined in the middle of the belt, but gradually disappearing as 

 we approach the limits on either side. The area of this zone is estimated to be 

 about 25,870,000 square miles. 3d. That south of the zone last named, the mean 

 direction of the wind is easterly. This area is estimated to contain 60,760,000 

 square miles.' 



Theoretical Considerations. 



In looking for the causes of winds, there are two which are obvious; 1st, the 

 diurnal revolution of the earth upon its axis, and 2d, the unequal distribution of 

 heat over different parts of its surface ; and we apprehend that these two, taken 

 conjointly, are sufficient to account for all the leading observed phenomena. Dr. 

 Halley, in a paper read before the Royal Society in 1686, undertook to explain the 

 phenomena of the trade-winds, by taking into account only the latter cause ; or at 

 least introducing the former only so far as it affects the temperature of places near 

 the equator at different hours of the day. His view (as explained by Professor 

 Mitchell in his article already referred to) was, that the rarefaction of the air over 

 the spot where the sun is vertical, and the continual motion of this spot westward 

 by the diurnal motion of the earth, generated a series of vortices, moving westward 

 below and eastward above, and that the lower parts constituted the trade-winds. 



According to his views, the motion would be as 

 in the accompanying figure, in which A M N B 

 represents a section of the atmosphere resting 

 on the equator A M, as seen from the north 

 side, and the different arrows show the direc- 

 tion in which the air is supposed to move. 

 Others, on the contrary, have maintained 

 that the mere rotation of the earth on its axis, combined with its annual revolution 

 round the sun, is sufficient to account for the leading phenomena of winds, without 

 any aid from heat. If at one and the same time the entire atmosphere were 

 reduced to a perfect calm all over the surface of the earth, and if the temperature 

 were everywhere the same, they have supposed that mere cosmical influences, such 

 as we have named, would, in some unexplained way, create just such currents as 



' Profussor Dove, of Berlin, maintains tbat there are but two systems, viz. our 1st and 3(1. See his 

 Letter to Col. Sabine, published in the Report of the British Association for 1845. 



