336 PHYSICAL GEOGRAPHY OF THE SEA, AXD ITS METEOROLOGY. 



It appears, moreover, that tlie temperature of the S.E. trade- wind 

 is in 2'S. below the temperature of the N.E. in 9^N., the latter 

 behig 81°, the former 80^2. 



6i8. The foregoing observations show that after these winds 

 The thermal equator, enter the calm belt, the air they bring into it con- 

 tinues to rise, and this also is what might well be anticipated, for 

 the sun continues to pour down upon it. But while the tempera- 

 ture of the surface is kept down by the ram-drops from above, the 

 temperature of the air in the whole belt is raised botli 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 dnect heat of the 

 sun in rarefying the air ; consequently we here unmask the influ- 

 ences which place the thermal equator in the northern hemisphere. 



649. Nor is this the only chamber into which this calm belt 

 A natural act ino- kov couducts US. Parallel for parallel (^ 446), the 



meter in the trade- -^ . , , . ^ . -. ^-, ,i ■ / 



winds. southern hemisphere is cooler than the northern j 



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 wliich give difference of energy to the dynamical power of the 

 trade- winds. Not only so : it expresses the difference of tempera- 

 ture 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 dehcate and beautiful kind. 



650. This actinometer measures for us the heat ^diich the south- 

 Heat daily received east trade-winds receive between the moment of 

 trade-'A^hids."'^^^ crossing the parallel of 9° south and theu^ 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-Avinds 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, kno^ving the exact 



