SEA ROUTES, CALM BELTS, AND VARL\BLE WINDS. 341 



inflowing air were scarcely perceptible, the diirerence of specific 

 gravity between 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 diflerence of temperature were 

 ver}'- great, the diff'erence of specific gravity would be very great, 

 ;ind the violence of the inrushing wind proportionably great. 

 Because the southern half of the torrid zone is the cooler, the 

 diflerence in temperature between the air of the calm belt and 

 the air of the trade -winds is greater, parallel for parallel, in the 

 south-east than in the north-east trade-winds ; consequently, the 

 south-east trade-winds should be — as observations show them to 

 be — stronger than the north-east ; — and consequently, also, their 

 meeting should take place, not upon the equator, but upon that 

 side of it where the Aveaker winds prevail, and this is also in 

 accordance (§ 343) with facts. 



638. Strength of the trade-ivinds varies with the seasons. — It follows 

 from these premises that the winter trade-wdnds should be 

 stronger than the summer. In our summer, the air which the 

 north-east trade-winds put in motion has its temperature raised 

 and brought more nearly up to that of the air in the calm belt. 

 At the same time, the temperature of the air which the south- 

 east trade-winds put in motion is proportionably low^ered. Thus 

 they increase in strength, while the north-east diminish ; the 

 consequence is, they push their place of meeting with the north- 

 east trades far over on this side of the equator, and for two or 

 three months of the year maintain the polar 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 summer the strength of 

 the south-east and the temperature of the north-east trade-winds 

 are diminished so as to admit of the edge of the calm belt being- 

 pressed down only as far as o"" instead of 15° S. The causes which 

 produce this alternation of trade-wind strength are cumulative ; 

 consequently, the north-east 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 minimum in the 

 other. 



639. Sailing through them in fall and winter. — In the other hemi- 

 sphere, the period of strongest trades is coincident with that of the 

 minimum in this. These deductions are also confirmed by observa- 

 tions ; for such is the diff'erence as to strength and regularity of 



