328 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. 



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

 strength of the should be stronsfer than the summer. In our sum- 



trade-wiiids varies 



with the seasons, mer, the air which the north-east trade-wiads put in 

 motion has its temperature raised and brought more nearly up to 

 that of the air in the cahn belt. At the same time, the temperature 

 of the air which the south-east trade-winds put in motion is pro- 

 portionably lowered. Thus they increase in strength, wliile the 

 north-east diminish ; the consequence is, they push their place 

 of meeting mth the north-east trades far over on tliis side of the 

 equator, and for two or thi'ee 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-A^^inds are diminished so as to admit of the edge of the calm 

 belt being pressed dovm only as far as 5° 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, 

 aftei' the period of maximum heat in one case and of minimum in 

 the other. 



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

 Sailing through them coLQcident with that of the minimum in this. These 

 in fall and winter, doductious aro also Confirmed by observations; for 

 &uch is the difference as to strength and regularity of the north-east 

 trade-winds in September and February, that the average passage 

 thi'ough them from New York to the line is 26.4 days in the vdnter 

 against 38.8 in the fall month. 



640. Thus it appears that the equatorial calm belt is made to 

 A thermal adjust- shift its placo with the seasons, not by reason of 

 °^^"^' the greater intensity of the solar ray in the lati- 

 tude where the calm belt may be at that season, but by reason of 

 the annual variations in the energy of each system of trades ; which 

 variations (§ 638) depend upon the changes in the temperature 

 and barometric weight of the air wliich each system puts in motion. 

 This calm zone, therefore, may be considered as a thermal adjust- 

 onent — the dynamical null-helt — between the trade-winds of the two 

 hemispheres. 



641. The observations on the barometer at sea (§ 858) shed 

 The barometer in the light ou this subject. Accordiug to the Dutch, that 

 cqiutoriai^^tms. instrument stands higher by 0.055 inch in the south- 



