348 THE PHYSICAL GEOGRAPHY OF THE SEA. 



are these the only latchets which a study of this calm belt and of 

 the winds enables us to lift. 



647. Theory suggests, and observation, as far as it goes, seems 

 Temperature of the to confirm the suggcstion, that the N.E. and S. E, 



trade-winds and calm - . - " . ^ i i i 



belts. traae-wmds enter the equatorial calm belt at the 



same temperature. I have followed 100 vessels with their ther- 

 mometer across the equatorial calm belt of the Atlantic, and an- 

 other 100 across it in the Pacific. Assuming its mean position to 

 be as these observations indicate it to be — viz., between the paral- 

 lels 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 temper- 

 ature 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 K.E. 

 trades, 5° from the north edge of the calm belt, or in 14° I^., 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 6° 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 foregoing observations show that after these winds 

 The thermal equator, enter thc Calm belt, thc 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 rain-drops from above, the 

 temperature of the air in the whole belt is raised both by the di- 

 rect heat of the sun and the latent heat which is set free by the 

 constant (§ 514) and oftentimes heavy precipitation there. This 



