SEA BOUTES, CALM BELTS, AND VARIABLE WINDS. 327 



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

 one " trade " be stronger than the other, the stronger will preyail 

 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 

 mil recede before the stronger wind, mitil the momentmn of the 

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

 reduced as exactly to be counterbalanced by the weaker wind. 

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

 from some cause, one or the other of the meeting mnds 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 wmds keep the calm belt in a trembling state, 

 mo\ing now to the north, now to the south, and always shilling 

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

 sphere. 



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

 The calm belts occupy noi^thern, and, parallel for parallel, the south-east 

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

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

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

 makes room for the inflowing ciUTent 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 ^vill be 

 the stronger. This is ob^sdous, for it is evident that if the difference 

 of temperature of the ascendiug column and the inflowing ak were 

 scarcely perceptible, the difference of specific gravity between the 

 iuflowiag wuid and the uprisiag air would be scarcely perceptible, 

 and the movement of the inflo^^dng wiud would be very gentle ; but 

 if' the difference of temperature were very great, the difference of 

 specific gravity would be very great, and the violence of the inrush- 

 iug wind proportionably great. Because the southern half of the 

 torrid zone is the cooler, the difference in temperature bet^veen the 

 air of the calm belt and the air of the trade-'^inds is greater, 

 parallel for parallel, iu the south-east than in the north-east trade- 

 winds ; consequently, the south-east trade-wiads should be — as 

 observations show them to be — stronger than the north-east ; — and 

 consequently, also, their meeting should take place, not upon the 

 equator, but upon that side of it where the weaker winds prevail, 

 and this is also in accordance (§ 343) mth facts. ^ 



