340 THE PHYSICAL GEOGRAPHY OF THE SEA. 



The belt of equatorial calms which separates the two systems of 

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

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

 eral degrees of latitude, sometimes not more than one. Its south- 

 ern edge, in spring, sometimes goes down to 5° S ; its northern 

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

 key to these phenomena has been found ; with it in hand, let us 

 proceed to unlock, first remarking that the mean position of the 

 equatorial calm belt in the Atlantic is between the equator and 

 0° E"., 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. tradc-wiuds, and it occupies strictly a medial posi- 

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

 two barometric ridges (§858, Fig.) from which the trade- winds 

 How. If one " trade" be stronger than the other, the stronger will 

 prevail so far as to force their place of meeting over and crowd it 

 back upon the weaker wind. It is evident that this place of meet- 

 ing will recede before the stronger wind, until the momentum of 

 the stronger wind is so diminished by resistance, and its strength 

 so reduced as exactly to be counterbalanced by the weaker wind. 

 Then this calm place will become stationary, and so remain, until, 

 for some cause, one or the other of the meeting winds gains 

 strength or loses force ; then the stronger will press upon the 

 weaker, and the calm belt will change place and adjust itself to 

 the new forces. The changes that are continually going on in the 

 strength of the winds keep the calm belt in a trembling state, 

 moving now to the north, now to the south, and always shifting 

 its breadth or its place under the restless conditions of our atmos- 

 phere. 



637. The southern half of the torrid zone is cooler than the 

 The calm belts occupy northcm, and, parallel for parallel, the southeast 

 medial positions. tradc-wiuds arc consequently cooler than the north- 

 east. They both flow into this calm belt, where the air, expand- 

 ing, ascends, flows off above, produces a low barometer, and so 

 makes room for the inflowing current below. Now if the trade- 

 wind air which flows in on one side of this calm belt be heavier, 

 whether from temperature or pressure, than the trade-wind air 

 which flows in on the other, the wind from the heavy side will be 

 the stronger. This is obvious, for it is evident that if the differ- 



