326 PHYSICAL GEOGBAPHY OF THE SEA, AND ITS METEOROLOGY. • 



633. That the south-east trade-wmds should, as observations 

 S^tradV-'^"re'h ^^ ^^^^ ^^^'® showii, be stioiiger than the north- 

 stronger. '^ east trade-winds, is due in part also to the well- 

 established fact that the southern (§ 446) is cooler than the northern 

 hemisphere. The isothermal lines of Dove show that the air of the 

 south-east is also cooler than the ak of the north-east trade-winds. 

 Being cooler, the air from the cool side would, for palpable causes, 

 rush with greater velocity into the equatorial calm belt than should 

 the lighter air from the warmer or northern side. The fact that the 

 air in the lower latitudes of the southern hemisphere is the cooler 

 will assist to explain many other contrasts presented by the meteoro- 

 logical conditions on opposite sides of the equator. Plate XIII. 

 shows that we have more calms and more fogs, more rains and more 

 gales, \vith more thunder, on om- side than on the other, and that 

 the atmosphere preserves its condition of unstable equihbrium 

 with much more uniformity, being subject to changes less fre- 

 quent and violent on the south side of the equator than on the 

 north side. 



634. The highest summer temperature in the w^orld is to be 

 Theiriiniformity of fouiid in the oxtra-tropical countries of the north. 

 temperature. rj^j^^ greatest extrcmos of temperatm^e are also to be 

 found among the valleys of the extra-tropical north. In the extra- 

 tropical south there is but httle land, few vaUeys, and much water ; 

 consequently the temperatm-e is more uniform, changes are less 

 sudden, and the consequent commotions in the air less violent. 



635. Following uj) these facts with their suggestions, we dis- 



S^^e"ua"oriaTca'm ^^^^^^ ^^^ ^^J ^^ Hiany phenomena which before 

 belt. ' " were locked up in "the chambers of the south." 

 The belt of equatorial cahns which separates the two systems of 

 trade-Avinds is, as we know (§ 295), variable as to its position. It 

 is also variable in breadth. Sometimes it covers a space of several 

 degrees of latitude, sometimes not more than one. Its southern 

 edge, in spring, sometimes goes dovm to 5° S. ; its northern edge, 

 in autumn, often mounts up to the parallel of 15° N. The key to 

 these phenomena has been foimd ; with it in hand, let us proceed to 

 unlock, first remarking that the mean position of the equatorial cahn 

 belt in the Atlantic is between the equator and 9° N., and that as it 

 is there, so I assume it to be in other oceans. 



636. This calm belt is produced by the meeting of the two 

 . Never at rest, trade-wdnds, and it occupies strictly a medial position 



between them. It is in the barometric valley, between the two 



