inflowing air were scarcely perceptible, the difference of specific gravity between the inflowing wind and the uprising air would be scarcely perceptible, and the movement of the inflowing wind 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 inrushing wind proportionably great. Because the southern half of the torrid zone is the cooler, the diflerence in temperature between the air of the calm belt and the air of the trade-winds is greater, parallel for parallel, in the south-east than in the north-east trade-winds; consequently, the south-east trade-winds 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) with facts.
638. Strength of the trade-winds varies with the seasons.—It follows from these premises that the winter trade-winds should be stronger than the summer. In our summer, the air which the north-east trade-winds put in motion has its temperature raised and brought more nearly up to that of the air in the calm belt. At the same time, the temperature of the air which the south-east trade-winds put in motion is proportionably lowered. Thus they increase in strength, while the north-east diminish; the consequence is, they push their place of meeting with the north-east trades far over on this side of the equator, and for two or three months of the year maintain the polar edge of the calm belt as high up as the parallel of 15° N. But with the change of seasons these influences are all transposed and brought into play on opposite sides—only in the southern summer the strength of the south-east and the temperature of the north-east trade-winds are diminished so as to admit of the edge of the calm belt being pressed down only as far as 5° instead of 15° S. The causes which produce this alternation of trade-wind strength are cumulative; consequently, the north-east trade-winds should be weakest in August or September, strongest in February or March, after the period of maximum heat in one case and of minimum in the other.
639. Sailing through them in fall and winter.—In the other hemisphere, the period of strongest trades is coincident with that of the minimum in this. These deductions are also confirmed by observations; for such is the difference as to strength and regularity of
