that after these winds enter the calm belt, the air they bring into It continues to rise, and this also is what might well be anticipated, for the sun continues to pour down upon it. But while the temperature of the surface is kept down by the rain-drops from above, the temperature of the air in the whole belt is raised both by the direct heat of the sun and the latent heat which is set free by the constant (§ 515) and oftentimes heavy precipitation there. This latent heat is much more effective than is the direct heat of the sun in rarefying the air; consequently we here unmask the influences which place the thermal equator in the northern hemisphere.
649. A natural actinometer in the trade-winds.—Nor is this the only chamber into which this calm belt key conducts us. Parallel for parallel (§ 446), the southern hemisphere is cooler than the northern; that is, the mean temperature for the parallel of 40° south, for example, is below the mean temperature for the parallel of 40° north, and so of all corresponding parallels between 40° and the equator. It appears, moreover, that the mean temperature of the north-east trade-winds as they cross the parallel of 9° north, and the mean temperature of the south-east trade-winds as they cross the equator, is about the same (§ 647), The difference of temperature, then, between the south-east trades as they cross the parallel of 9° south, and as they cross the equator, expresses the difference in the thermal forces which give difference of energy to the dynamical power of the trade-winds. Not only so: it expresses the difference of temperature between the two corresponding parallels of 9° north and 9° south, and discovers to us a natural actinometer on a grand scale, and of the most delicate and beautiful kind.
650. Heat daily received by the south-east trade-winds.—This actinometer measures for us the heat which the south-east trade-winds receive between the moment of crossing the parallel of 9° south and their arrival at the equator, for the heat thus received is just sufficient (§ 644) to bring so much of the south-east up to the temperature which the north-east trades have as they cross the parallel of 9° north. To complete this measurement of heat we should know how long the south-east trade-winds are on their march from the parallel of 9° south to the equator. According to the estimate, it takes them about a day to accomplish this distance; but, knowing the exact time, we should have in the band of winds an actinometer which would disclose to us the average
