§ G38, 039. SEA ROUTES, CALM BELTS, ETC. 311 



ence of temperature of the ascending column and the inflowing 

 air were scarcely perceptible, the difference of specific gravity be- 

 tween the inflowing wind and the uprising air would be scarcely 

 perceptible, and the movement of. the inflowing wind would be 

 very gentle ; but if the difference of temperature were very great, 

 the difference of specific gravity would be very great, and the vi- 

 olence of the inrushing wind proportionably great. Because the 

 southern half of the torrid zone is the cooler, the difference in tem- 

 perature between the air of the calm belt and the air of the trade- 

 winds is greater, parallel for parallel, in the southeast than in the. 

 northeast trade-winds ; consequently, the southeast trade-winds 

 should be — as observations show them to be — stronger than 

 the northeast, and consequently, also, their meeting should take 

 place, not upon the equator, but upon that side of it where the 

 weaker winds prevail, and this is also in accordance (§ 343) with 

 facts. 



638. It follows from these premises that the winter trade-winds 

 strength of the trade- should bc strougcr than thc summer. In our sum- 



vdnds varies Avith the t • i • i i i t • i 



seasoas. Hicr, the air which the northeast trade-winds put m 



motion has its temperature raised and brought more nearly up to 

 that of the air in the calm belt. At the same time, the tempera- 

 ture of the air which the southeast trade-winds put in motion is 

 proportionably lowered. Thus they increase . in strength, while 

 the northeast diminish ; the consequence is, they push their place 

 of meeting with the northeast trades far over on this side of the 

 equator, and for two or three months of the year maintain the po- 

 lar edge of the calm belt as high up as the parallel of 15° N. But 

 with the change of seasons these influences are all transposed and 

 brought into play on opposite sides — only in the southern sum- 

 mer the strength of the southeast and the temperature of the north- 

 east trade-winds are diminished so as to admit of the edge of thc 

 calm belt being pressed down only as far as 5° instead of 15° S. 

 The causes which produce this alternation of trade- wind strength' 

 are cumulative ; consequently, the northeast trade-winds should 

 be weakest in August or September, strongest in February or 

 March, after the period of maximum heat in one case and of min- 

 imum in the other. 



639. In the other hemisphere, the period of strongest trades is 

 coincident with that of the minimum in this. These deductions are 



