338 rnYsioAL geogbapht of the sea, and its meteobology. 



more or less heated, there is a call — a pulling back, if you 

 please — upon these trades to turn about and restore the equili- 

 brium which the deserts destroy. There being few or no such 

 regions in the rear of the south-east trades, the south-east trade- 

 wind force prevails, and carries them over to the northern 

 hemisphere. 



631. Dhirnal rotation. — We see by the plate that the two 

 opiDOsing currents of wind called " the trades," are so une*qually 

 balanced that the one recedes before the other, and that the cur- 

 rent from the southern hemisphere is larger in volume ; i. e., it 

 moves a greater zone or belt of air. The south-east trade-winds 

 discharge themselves over the equator — i. e., across a great circle 

 — into the region of equatorial calms, while the north-east trade- 

 winds discharge themselves into the same region over a parallel 

 of latitude, and consequdntly over a small circle. If, thejefore, 

 we take what obtains in the Atlantic as the type of what obtains 

 entirely around the earth, as it reg^ards the trade-winds, we shall 

 see that the south-east trade-winds keep in motion more air than 

 the north-east do, by a quantity at least proportioned to the dif- 

 ference between the circumference of the earth at the equator 

 and at the parallel of latitude of 9° north. For if we suppose that 

 those two perpetual currents of air extend the same distance 

 upward from the surface of the earth, and move with the same 

 velocity, a greater volume from the south should, as has already 

 been shown (§ 343), flow across the equator in a given time than 

 would flow from the north over the parallel of 9° in the same 

 time ; the ratio between the two quantities wovld be as radius to 

 the secant of 9°. Besides this, the quantity of land lying within 

 and to the north of the region of the north-east trade- winds is 

 much greater than the quantity within and to the south of the 

 region of the south-east trade- winds. In consequence of this, the 

 mean level of the earth's surface within the region of the north- 

 east trade-winds is, it may reasonably be supposed, somewhat 

 above the mean level of that paii: which is within the region of the 

 south-east trade-winds. And as the north-east trade-winds blov/ 

 under the influence of a greater extent of land surface than the 

 south-east tradog do, the former are more obstructed in their course 

 than the latter by the forests, the mountain ranges, unequally 

 heated surfaces, and other such like obstacles. 



632. The land in the northern hemisphere. — That the land of the 

 northern hemisphere does assist to turn these winds is rendered 



