lent. Now, in each case, the prevailing winds blow (§ 657) from the high to the low barometer (Plate I.).
660. The barometric ridges.—The fact of two barometric ridges encircling the earth, as the high barometer of the tropical calm belts do, and as they may be called (Plate I.), suggests a place of low 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. They make a depression in the atmosphere.—Let us contemplate for a moment this accumulation of air in the tropical belt about the earth in each hemisphere. Because it is an accumulation of atmospheric 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 precipitation 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 flow^s 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 winds to blow, but it so flows by reason of difference as to level.
662. The upper surface of the atmosphere.—The tropical calm belts (§ 278) are places where the mean amount of precipitation is small. The air there is 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
