35 J: THE PHYSICAL GEOGRAPHY OF THE SEA. 



US (§ 654:) the existence of a low barometer toward the polar re- 

 gions as well as toward the equatorial. 



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

 Theymakeadepre.;- in the tropical bclt about the carth in each hemi' 



sion in the atmos- _^ , . ■> • n 



piiere. Sphere, iiecause it is ati accumulation of atmos- 



pheric air about the calms — because the barometer stands higher 

 under the calm belt of Capricorn, for instance, 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 trop- 

 ical calm belts, does not so flow by reason of any difference of 

 barometric pressure, like that which causes the surface winds 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 aixLOunt of prccipltation is small. The air there is 

 the atmosphere. Comparatively dry air. So far from being expand- 

 ed by heat, or swelled out by vapor, 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 un- 

 der 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 

 double inclined plane, descending from the equator and from the 

 poles to the place of the tropical calm belts. 



