SEA ROUTES, CALM BELTS, AND VARIABLE WINDS. 341 



barometer on the polar side as naturally as the ascent of a hill on 

 one side suggests to the traveller a descent on the other ; and, had 

 not actual observations revealed the fact, theory should have taught 

 us (§ 654) the existence of a low barometer towards the polar 

 regions as well as towards the equatorial. 



661. Let us contemplate for a moment this accumulation of air 

 They make a depres- in the tropical belt about the earth in each hemi- 



sion in the atmo- ■, \^ • i • i i • o i 



sphere. spliero. i^ecauso it IS an accumulation oi atmo- 



spheric air about the calms ; — because the barometer stands higher 

 under the calm belt of Capricorn, for mstance, than it does on any 

 other parallel between that calm belt and the pole on one side, or 

 the equator on the other, it is not to be inferred that therefore 

 there is a piling — a ridging up — of the atmosphere there. On the- 

 contrary, were the upper surface of our atmosphere visible, and 

 could we take a view of it from above, we should discover rather- 

 a valley than a ridge over this belt of greatest pressure ; and over- 

 the belt of least pressure, as the equatorial calm belt, we should 

 discover (§ 520), not a valley, but a ridge, and for these reasons : 

 In the belts of low barometer, that is, in both the equatorial and 

 polar calms, the air is expanded, made light, and caused to ascend, 

 chiefly by the latent heat that is liberated by the heavy precipita- 

 tion which takes place there. This causes the air which ascends 

 there to rise up and swell out far above the mean level of the great 

 aerial ocean. This intumescence at the equatorial calm belt has 

 been estimated to be several miles above the general level of the 

 atmosphere. This calm belt air, therefore, as it boils up and flows 

 off through the upper regions, north and south, to the tropical calm 

 belts, does not so flow by reason of any difference of barometric 

 pressure, like that which causes the surface ^dnds to blow, but it so 

 flows by reason of difference as to level. 



662. The tropical calm belts (§ 278) are places where the mean 

 The upper surface of amouut of procipitation is small. The air there is 

 the atmosphere. comparatively dry air. So far from being expanded 

 by heat, or swelled out by vapour, this air is contracted by cold, for 

 the chief source of its supply is through the upper regions, from 

 the equatorial side, where the cross section between any two given 

 meridians is the larger ; and this upper current, while on its way 

 from the equator, is continually parting with the heat which it 

 received at and near the surface, and which caused it to rise under 

 the equatorial cloud-ring. In this process it is gradually con- 

 tracted, thus causing the upper surface of the air to be a sort of 



