350 



PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOEOLOGY. 



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 antici- 

 pated, for the sun continues to jDour 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 precipita- 

 tion 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-icinds.—lsov 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 be- 

 tween 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 equa- 

 tor, expresses the difference in the thermal forces which give dif- 

 ference of energy to the dynamical power of the trade-winds. 

 Kot only so : it expresses the difference of temperature between 

 the two corresponding parallels of 9° north and 9° south, and dis- 

 covers to us a natural actinometer on a grand scale, and of the 

 most delicate and beautiful kind. 



650. Heat daily received hy the south-east trade-winds.- This acti- 

 nometer 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 dis- 

 tance; but, knowing the exact time, we should have in the band 

 of winds an actinometer which would disclose to us the average 



