218 THE PHYSICAL GEOGRAPHY OF THE SEA. 



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

 um which the deserts destroy. There being no, or few such re- 

 gions in the rear of the southeast trades, the trade-wind force pre- 

 vails, and carries them over into the northern hemisphere. 



464. By resolving the forces which it is supposed are the prin- 

 cipal forces that put these winds in motion, viz., calorific action 

 of the sun and diurnal rotation of the earth, we are led to the con- 

 clusion that the latter is much the greater of the two in its effects 

 upon those of the northern hemisphere. But not to such an ex- 

 tent is it greater in its effects upon those of the southern. We see 

 by the plate that those two opposing currents of wind are so une- 

 qually balanced that the one recedes before the other, and that the 

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

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

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

 — into the region of equatorial calms, while the northeast trade- 

 winds discharge themselves into the same region over a parallel 

 of latitude, and consequently over a small circle. If, therefore, 

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

 entirely around the earth, as it regards the trade-winds, we shall 

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

 the northeast do, by a quantity at least proportioned to the differ- 

 ence 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 from 

 the surface of the earth, and move with the same velocity, a 

 greater volume from the south would flow across the equator in a 

 given time than w^ould flow from the north over the parallel of 9° 

 in the same time ; the ratio between the two quantities would 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 northeast trade- 

 winds is much greater than the quantity within and to the south 

 of the region of the southeast trade-winds. In consequence of 

 this, the mean level of the earth's surface within the region of the 

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

 above the mean level of that part which is within the region of the 

 southeast trade-winds. And as the northeast trade- winds blow 

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

 southeast trades do, the former are more obstructed in their course 



