218 
THE PHYSICAL GEOGRAPHY OF THE SEA. 
please — upon these trades to turn about and restore the equilib- 
rium which the deserts destroy. There being no, or few such re- 
gions in the rear of the southeast trades, they obey the first im- 
pulse, push and press 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 I 
— 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 i 
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 would 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 
