SEA ROUTES, CALM BELTS, AND VARIABLE WINDS. 335 



Tnimaii life, afford unerring indications of a change of climates, if 

 any such change were going on ? If the temperature of the S.E. 

 trade-winds, or the barometric jDressui'e upon the N.E. (§ 641), 

 were to be diminished, the S.E. trades would force tliis calm belt 

 still farther to the north, and we might have a regular rainy sea- 

 son in what is now the great desert of Sahara ; for where this calm 

 belt is (§ 517) there is the cloud-ring, with its constant precipita- 

 tion. Therefore, if there be any indications that the southern edge 

 of the great desert is gradually apf)roachmg the equator, it would 

 favour the supposition that the southern hemisphere is growling 

 warmer ; but if the mdications be that the southern edge of the desert 

 is receding from the equator, then the fact would favour the sup- 

 position that the southern hemisphere is groT\dng still cooler. Nor 

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

 the wuids enables us to lift. 



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

 tod^windraod *^'^ ^^ confirm the suggestion, that the N.E. and S.E. 

 calm belts.'' tradc-Avinds 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 m 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 temperatm^e 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 tempera- 

 ture of the air was noted also m each trade at the distance of 5° 

 from its edge of the calm belt. Thus the temperature of the N.E. 

 trades, 5° from the north edge of the calm belt, or in 14° N., is 

 78 .2; at a hke 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 tvdce as much as the temperatm-e 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 tvdce 

 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 traversmg given differences of latitude. Thus the 

 position of the cahn belt, the barometer, the thermometer, and the 

 rate of sailing, all mdicate the S.E. trade-wmds to be the strono-er. 



