356 THE PHYSICAL GEOGRAPHY OF THE SEA. 



666. It appears, moreover, that the polar rarefaction is greater 

 Polar rarefaction, than the equatorial, for the mean height of the aus- 

 tral barometer is very much below that of the equatorial, and, 

 consequently, its influence in creating an indraught is felt at a 

 gTcater distance — even at the distance of 50° of latitude from the 

 south pole, while the influence of the equatorial depression is felt 

 only at the distance of 80° in the southern, and of 25° in the 

 northern hemisphere. The difference as to degree of rarefaction 

 is even greater than this statement implies, for the influx into the 

 equatorial calm belt is assisted also by temperature in this, that 

 the trade-winds blow from cooler to warmer latitudes. The re- 

 verse is the case with the counter-trades ; therefore, while differ- 

 ence of thermal dilatation assists the equatorial, it opposes the po- 

 lar influx. 



667. Thus we perceive that the tropical calm belts are simply 

 The tropical calm ^^^ adjustmcut bctwecn the polar and equatorial 

 ^lar anTequaforkii calms ; that the tropical calm belts assume their po- 

 '^^^™^- sition and change their latitude in obedience to the 

 energy with which the influence of the heated and expanding col- 

 umns of air, as they ascend in the polar and equatorial calms, is 

 impressed upon them. 



668. This explanation of the calm places and of the movements 

 The meteorological ^^ ^hc low austral baromctcr shows, comparatively 

 power of latent heat, gpgr^i^ing, how much the latcut hcat of vapor, and 

 how little the direct heat of the sun has to do in causing the air 

 to rise up and flow off* from these calm places, and, consequently, 

 bow little the direct action of the solar ray has to do either with 

 the trades or the counter-trades. It regulates and controls them ; 

 it can scarcely be "said to create them. 



669. The fact of a low barometer off Cape Horn was pointed 

 Tiie low b-arometer ^^^^^ ^^ ^^^E ^S^ ^^ 1834. It was cousidcrcd an 

 off Cape Horn. auomaly peculiar to the regions of Cape Horn. It 

 is now ascertained by the comparison of 6455 observations on the 

 polar side of 40° south, and about 90,000 in all other latitudes, 

 that the depression is not peculiar to the Cape Horn regions, but 

 that it is general and alike in all parts of the austral seas, as the 

 following tables, compiled from the log-books of the Observatory 

 by Lieutenants Warley and Young, show : 



* American Journal of Science, vol. xxvi., p. 54 (1834). 



