SEA ROUTES, CALM BELTS, AND VARIABLE WINDS. 34B 



666. It appears, moreover, that the polar refraction 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, con- 

 sequently, its influence in creating an indraught is felt at a greater 

 distance (Plate XY.) — even at the distance 50^ of latitude from the 

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

 only at the distance of 30° in the southern, and of 25^ in the 

 northern hemisphere. The difierence as to degree of rarefaction is 

 even greater than this statement imphes, for the influx into the 

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

 trade- vinds blow from cooler to warmer latitudes. The reverse is 

 the case with the counter-trades ; therefore, while difference of ther- 

 mal dilatation assists the equatorial, it opposes the polar influx. 



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

 The tropical calm adjustment between the polar and equatorial cahns ; 

 polSTd equ^oriai ^^^^^ ^^e tropical cahn_ belts assume theii^ position and 

 calms. change their latitude in obedience to the energy with 

 which the influence of the heated and the expanding columns 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 of the low austral barometer shows, comparatively 

 power of latent heat, speaking, how much the latent heat of vapour, and 

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

 rise up and flow ofi" from these calm places, and consequently, how 

 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 ofi" Cape Horn was pointed out* 

 The low barometer ^^ ^^ug as 1834. It was Considered an anomaly 

 off Cape Horn. pecuhar to the regions of Cape Horn. It is now 

 ascertamed by the comparison of 6455 observations on the polar side 

 of 40'' south, and about 90,000 in all other latitudes, that the de- 

 pression 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 Lieu- 

 tenants Warley and Young, show : — (See Table, page 344.) 



671. These are the observed heights; for the want of data, no 

 Barometer at the correctious have bccu apphcd to them ; and for the 

 ^^^^^' want of numbers sufficient to give correct means, they 



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



