338 THE PHYSICAL GEOGRArUY OF THE SEA. 



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

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

 force prevails, and carries them over into the northern hemisphere. 



631. We see by the plate that the two opposing currents of 

 Diurnal rotation, wiud. Called "the tradcs," are so unequally bal- 

 anced that the one recedes before the other, and that, the current 

 from the southern hemisphere is larger in volume ; i. 6., 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 dis- 

 charge 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 difference 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 perpet- 

 ual 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 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 would be as radius to the secant of 9°. Besides 

 this, the quantity of land lying within and to the north of the re- 

 gion of the northeast trade- winds is much greater than the quan- 

 tity within and to the south of the region of the southeast trade- 

 winds. In consequence of this, the mean level of the earth's sur- 

 face within the region of the northeast trade-winds is, it may rea- 

 sonably 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 than the latter by the forests, the 

 mountain ranges, unequally heated surfaces, and other such like 

 inequalities, 



632. That the land of the northern hemisphere does assist to 

 Tiie land in the north- ^UHi thcsc wiuds is rendered still more probable 

 em hemisphere. from this circumstaucc : All the great deserts are 



