OSCILLATIONS OF THE "WINDS. 243 



after the 21st of September, crosses tlie equatorial line to tend towards 

 the tropic of Cancer, the centre of the trade-winds, and consequently 

 of the band of calms, moves at the same time towards the north; 

 on the contrary, when the sun returns to the tropic of Capricorn, the 

 most heated zone of air is gradually brought back to the south with 

 the whole circulatory system of the trade-winds. At the end of 

 March the northern limit of the equatorial calms of the Atlantic 

 is found, on an average, towards the 2nd degree of north latitude, 

 while at the end of September this same limit attains ordinarily the 

 13th or 14th degree.* As to the southern limit, it oscillates in the 

 same ocean from 1 to 4 degrees of north latitude. f In the equatorial 

 regions of the Pacific, the zone of calms is similarly displaced from 

 month to month, following the march of the sun, and its breadth 

 varies from 135 miles in the month of February to more than 840 

 miles in the month of August.iJ: In this respect the analogy is 

 almost complete in the two great oceans. In consequence of this 

 annual periodicity, the whole aerial system incessantly oscillates ac- 

 cording to the relative position of the sun, and it is for this reason 

 that, in the northern hemisphere, the north winds, being violently 

 attracted towards the south, are much stronger in winter. Besides 

 these, there probably exist monthly and semi-monthly oscillations 

 resulting from the declination of the moon.§ 



The central part of the zone of calms, which may be considered 

 as the meteorological equator of the world, does not correspond 

 with the equator properly so-called. On the earth, as in the 

 higher organisms, the principal seat of life is placed out of the geo- 

 metrical centre. The complete system of the winds inclines towards 

 the northern hemisphere, and it is to the north of the line that the 

 girdle of the equatorial calms is at all seasons developed. This pheno- 

 menon, which might seem at first sight strange, results from the 

 grouping of the greater part of the continental lands in the northern 

 hemisphere, and from the difference of temperature, which must be, at 

 least for one part of the world, the result of this unequal distribution 

 of solid and liquid. It is also in the northern hemisphere that we find 

 the desert of Sahara, the true geographical south of the earth, that im- 

 mense extent where wooded tracts are relatively few, and where the 

 reflection from the burning sands and rocks vaporize the clouds 



* D'Apr^s; — Dove, LaLoides Tempet.es (traduction Legras), p. 17. 



t Horsburgh, East India Directory, vol. i. p. 25. 



X Kerhallet, Considerations Generales sur V Ocean Pacijlque. 



§ Keller, Rennell. 



R 2 



