SicT. XV. THE TRADE-WINDS. 117 



and southerly impetus, which, combining with their de- 

 ficiency of rotatory velocity, will make them appear to 

 blow from the north-east on one side of the equator and 

 from the south-east on the other, which is the general 

 direction of the trade-winds. But they are modified 

 both hi intensity and direction by the seasons, by the 

 neighborhood of continents, and by the nature of the 

 soil, so that the phenomena are not the same in both 

 hemispheres. These winds, however, are not felt at all 

 under the line, because the easterly tendency of the 

 two great polar currents is gradually diminished as they 

 approach the equator by the friction of the earth, which 

 slowly imparts a portion of its rotatory velocity to them 

 as they pass along, and when they meet in the equator 

 they destroy one another's impetus. The equator does 

 not exactly coincide with the line which separates the 

 trad^-winds north and south of it. That line of separa- 

 tion depends upon the total difference of heat in the two 

 hemispheres, arising from the distribution of land and 

 water, and other causes. 



The polar currents from defect of rotatory velocity 

 tend, by their friction near the equator, to r diminish the 

 velocity of the earth's rotation ; while, on the contrary, 

 the equatorial or upper currents carry their excess of 

 rotatory velocity north and south. And as they occa- 

 sionally come to the surface in their passage to the poles, 

 they act on the earth by their friction as a strong south- 

 west wind in the northern hemisphere, and as a north- 

 west wind in the southern. In this manner the equili- 

 brium of rotation is maintained. Sir John Herschel 

 ascribes to this cause the western and south-western 

 gales so prevalent in our latitudes, and also the west 

 winds which are so constant in the North Atlantic. 



There are many proofs of the existence of the coun- 

 ter-currents above the trade-winds. On the Peak of 

 Teneriffe the prevailing winds are from the west. The 

 ashes of the volcano of St. Vincent's, in the year 1812, 

 were carried to windward as far as Barbadoes by the 

 upper current. The captain of a Bristol ship declared 

 that on that occasion dust from St. Vincent's fell to the 

 depth of five inches on the deck at the distance of 500 

 miles to the eastward. Light clouds have frequently 



