sufficient to keep the air away from the poles and produce almost a vacuum there. In this state of things, the same air would continue to revolve about the poles were not some other agent, such as heat, brought in to expand and drive it away. Being expanded and puffed out above the general atmospherical level, but retaining its velocity—for the supposition is that it moves without friction—and returning through the upper regions, it would flow back as it went, viz., as a westerly wind, and arrive at its null belt in the direction of the meridian. But the wind has friction, and is resisted in every movement; the atmosphere partakes of the spheroidal form, which has been impressed upon the earth itself by its axial rotation. That form is to it the form of stability. The water at the pole is about 13 miles nearer to the centre of the earth than the water at the equator; but there is not on that account any tendency in the sea to flow back from the equator towards the poles; neither is there any tendency to motion one way or the other in the atmospherical ocean by reason of the oblateness of its surface. To produce the polar and equatorial movements of the air, there must be an agent both at the equator and the poles to prevent such stability by constantly disturbing equilibrium there, and that agent is heat; therefore, whatever be the degree of depression due the polar barometer in consequence of axial rotation, such depression could, of itself, produce neither trade nor counter-trade wind; it could no more produce currents in the air than in the sea, nor could axial rotation produce a high barometer at one pole, a low barometer at the other; consequently, the difference in the pressure of the atmosphere about the two poles, as shown by the diagram (Plate XVI.), cannot be ascribed to the influence of axial rotation. It is doubtless due to the excess in antarctic regions of aqueous vapour and its latent heat.
862. Psychrometry of polar winds.—The arctic circle lies chiefly on the land, the antarctic on the water. As the winds enter one, they are loaded with vapour; but on their way to the other they are desiccated (§826). Northern mountains and the hills wring from them water for the great rivers of Siberia and Arctic America. These winds, then, sweep comparatively dry air across the arctic circle; and when they arrive at the calm disc—the place of ascent there—the vapour which is condensed in the act of ascending does not liberate heat enough to produce a rarefaction sufficient to call forth a decided indraught from a greater
