calm belt is (§ 517) there is the cloud-ring, with its constant precipitation. Therefore, if there be any indications that the southern edge of the great desert is gradually approaching the equator, it would favour the supposition that the southern hemisphere is growing warmer; but if the indications be that the southern edge of the desert is receding from the equator, then the fact would favour the supposition that the southern hemisphere is growing still cooler. Nor are these the only latchets which a study of this calm belt and of the winds enables us to lift.
647. Temperature of the trade-winds and calm belts.—Theory suggests, and observation, as far as it goes, seems to confirm the suggestion, that the N.E. and S.E. trade-winds enter the equatorial calmbelt at the same temperature. I have followed 100 vessels with their thermometer across the equatorial calm belt of the Atlantic, and another 100 across it in the Pacific. Assuming its mean position to be as these observations indicate it to be—viz., between the parallels of 3° and 9° N.—the mean temperature is 81° at its northern, 81°.4 at its southern edge, and 82'^ in the middle of it. These 200 logs were taken at random, and for all months. The temperature of the air was noted also in each trade at the distance of 5° from its edge of the calm belt. Thus the temperature of the N.E. trades, 5° from the north edge of the calm belt, or in 14° N., is 78°.2; at a like distance in the S.E. trades from the equatorial edge, or in 2^ S., the mean temperature is 80.°2. From this it would seem that, in traversing this belt of 5°, the temperature of the N.E. is raised twice as much as the temperature of the S.E. trades ; which is another indication that the velocity of the S.E. is nearly or quite double the velocity of the N.E. trades (§ 642). For if it be supposed that it takes the N.E. trades twice as long to traverse 5° of latitude as it does the S.E., it is evident that the former would be exposed twice as long to the solar ray, and receive twice the amount of heat that is imparted to the S.E. trade-winds in traversing given differences of latitude. Thus the position of the calm belt, the barometer, the thermometer, and the rate of sailing, all indicate the S.E. trade- winds to be the stronger. It appears, moreover, that the temperature of the S.E. trade-wind is in 2° S. below the temperature of the N.E. in 9° N., the latter being 81°, the former 80°.2.
648. The thermal equator.—The foregoing observations show
