the south." 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 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 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 meeting 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, from 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 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 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 difference of temperature of the ascending column and the
