SEA ROUTES, CALM BELTS, AND VARIABLE WINDS. 319 



calm belt is (§ TjIT) there is the cloud-ring, with its constant pre- 

 cipitation. Therefore, if there be any indications that the 

 southern edge of the great desert is gradually approaching the 

 equator, it would favour the supposition that the southern 

 hemisphere is growing warmer ; but if the indications be that the 

 southern edge of the desert is receding from the equator, then 

 the fact would favour the supposition that the southern hemi- 

 sphere is growing still cooler. Nor are these the only latchets 

 which a study of this calm belt and of the winds enables us to 

 lift. 



647. Temperature of the trade-winds and calm belts. — Theory sug- 

 gests, and observation, as far as it goes, seems to confirm the sugges- 

 tion, that the N.E. andS.E. trade-winds enter the equatorial calm 

 belt at the same temperature. I have followed 100 vessels with 

 their thermometer across the equatorial calm belt of the Atlantic, 

 and another 100 across it in the Pacific. Assuming its mean 

 position to be as these observations indicate it to be — viz., between 

 the parallels 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 temperature of the air Avas noted also in each trade at the 

 distance of 5^ from its edge of the calm belt. Thus the tem- 

 perature of the N.E. trades, 5° from the north edge of the calm 

 belt, or in 14^ N., 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. Fi'om 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 5° 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 difi^erences 

 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 tem- 

 perature of the N.E. in O'' N., the latter being 81°, the former 

 80''.2. 



648. TiiG thermal equator. — The foregoing observations show 



