§ G49, G50. SEA ROUTES, CALM BELTS, ETC. S49 



latent heat is much more effective than is the direct heat of the 

 sun in rarefying the air ; consequentfy 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 actinometer ^cj couducts US. Parallel for parallel (§ 446), the 

 in the trade-^nd3. gQuthem hemisphere is cooler than the northern ; 

 that is, the mean temperature for the parallel of 40° south, for ex- 

 ample, is below the mean temperature for the parallel of 40° north, 

 and so of corresponding parallels between 40° and the equator. 

 It appears, moreover, that the mean temperature of the northeast 

 trade-winds as they cross the parallel of 9° north, and the mean 

 temperature of the southeast trade-winds as they cross the equa- 

 tor, is about the same (§ 647). The difference of temperature, then, 

 between the southeast trades as they cross the parallel of 9° south, 

 and as they "cross the equator, expresses the difference in the ther- 

 mal forces which give difference of energy to the dynamical pow- 

 er 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. This actinometer measures for us the heat which the south- 

 iieat daily received by east trade- wluds recclvc betwccn the moment of 

 v^inds. " crossing thc 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 southeast up to the temperature which the 

 northeast trades have as they cross the parallel of 9° north. To 

 complete this measurement of heat, we should know how long it 

 takes the southeast trade-winds to get from the parallel of 9° south 

 to the equator. According to the estimate, it takes about a day ; 

 but, knowing the exact time, we should have in the band of winds 

 an actinometer which would disclose .to us the average quantity of 

 heat daily impressed by the sun upon the atmosphere at sea be- 

 tween the equator and 9° south. I say it takes about a day, and 

 SD infer from these data, viz. : The mean annual direction of the 

 southeast trade-winds between 10° south and the line is south 40° 

 e';ist.^ We suppose their average velocity to be (§ 843) about 25 

 miles an hour. At this rate it would take them 29h. 22m. SOs. to 

 reach the equator. During this time they receive more heat than 



* Maiiry's Nautical Monogrnj)h, No. 1. 



