340 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. 



the south." The belt of equatorial calms which separates ilie 

 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 several degrees of latitude, sometimes not more than 

 one. Its southern 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 9° N., and that as it is there, so I assume it to 

 be in other oceans. 



636. Never at rest. — This calm belt is produced by the meeting- 

 of the two trade-winds, and it occupies strictly a medial position 

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

 barometric ridges (§ 667), from which the trade-winds flow. If 

 one " trade " be stronger than the other, the stronger will pre- 

 vail 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 

 meeting will recede before the stronger wind, until the momen- 

 tum 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, imtil, from some cause, one or the other of the meeting 

 winds gains strength or loses force ; then the stronger will press 

 upon the Aveaker, 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 atmosphere. 



637. The calm hells occupy medial positions. — The southern half 

 of the torrid zone is cooler than the northern, and, parallel for 

 parallel, the south-east trade-winds are consequently cooler than 

 the north-east. They both blow into this calm belt, where the- 

 air, expanding, ascends, flows off above, produces a low barometer, 

 and so makes room for the inflowing current below. Now if th& 

 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 Avhich flows in on the other, the wind from the he-avy 

 side will be the stronger. This is obvious, for it is evident that 

 if the difference of temperature of the ascending column and the 



