SECT, xv.] THE TRADE-WINDS. 137 



spheres. 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 an- 

 other's impetus. The equator does not exactly coincide 

 with the line which separates the trade-winds north and 

 south of it. That line of separation 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 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 occasionally 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 hemi- 

 sphere, and as a north-west wind in the southern. In this 

 manner the equilibrium 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 counter- 

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

 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 been seen moving rapidly from west to east 

 at a very great height above the trade-winds, which were 

 sweeping along the surface of the ocean in a contrary direc- 

 tion. Rains, clouds, and nearly all the other atmospheric 

 phenomena, occur below the height of 18,000 feet, and gene- 



