325 



selves over the equator — i. e., across a great circle — into the region 

 of equatorial calms, while the north-east trade-winds discharge 

 lihemselves into the same region over a parallel of latitude, and con- 

 sequently over a small chicle. 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 south- 

 east trade-winds keep in motion more air than the north-east 

 do, by a quantity at least proportioned to the difference between 

 the cu'cumference of the earth at the equator and at the parallel 

 of latitude of 9° north. For if we suppose that those two per- 

 petual 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 akeady been shown (§ 343), 

 flow across the equator m a given time than would flow fi*om 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 north-east trade-winds is much greater than the quan- 

 tity 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 part which is 

 within the region of the south-east trade-winds. And as the north- 

 east trade-winds blow under the influence of a greater extent of 

 land surface than the south-east trades do, the former are more 

 obstructed in their course than the latter by the forests, the moun- 

 tain ranges, unequally heated surfaces, and other such like obstacles. 

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

 The land in the tum thcso wiuds is rendered still more probable 

 northern hemisphere. fj.Qjj^ ^j^jg cuTcumstance I All the gi'cat dcscrts are in 

 the northern hemisphere, and the land surface is also much greater 

 on our side of the equator. The action of the sun upon these un- 

 equally absorbing and radiating surfaces in and behmd, or to the 

 northward of the north-east trades, tends to check these winds, and 

 to draw in large volumes of the atmosphere, that otherwise would be 

 moved by them, to supply the partial vacuum made by the heat of 

 the sun, as it pours down its rays upon the vast plains of burning 

 sands and unequally heated land surfaces in our overheated hemi- 

 sphere. The north-west winds, of the southern are also, it may be 

 inferred stronger, than the south-west winds of the northern hemi- 

 sphere. 



