called in to supply the indraught towards the north. Thus the north-east, trade-winds being checked, the south-east are called on to supply the largest portion of the air that is required to feed the ascending columns in the equatorial calm belt.
264. The counter trades-they approach the pole in spirals.—On the north side of the trade-wind belt in the northern, and on the south side in the southern hemisphere, the prevailing direction of the winds is not towards the equator, but exactly in the opposite direction. In the extra-tropical region of each hemisphere the prevailing winds blow from the equator towards the poles. These are the counter-trades (§ 204). The precipitation and congelation that go on about the poles produce in the amount of heat set free, according to Black's law (§ 260), a rarefaction in the upper regions, and an ascent of air about the poles similar to that about the equator, with this difference however: the place of ascent over the equator is a line, or band, or belt; about the poles it is a disc. The air rushing in from all sides gives rise to a wind, which, being operated upon by the forces of diurnal rotation as it flows north, for example, will approach the north pole by a series of spirals from the south-west.
265. They turn with the hands of a watch about the south pole, against them about the north.—If we draw a circle about this pole on a common terrestrial globe, and intersect it by spirals to represent the direction of the wind, we shall see that the wind enters all parts of this circle from the south-west, and that, consequently, there should be about each pole a disc or circular space of calms, in which the air ceases to move forward as wind, and ascends as in a calm; about the Arctic disc, therefore, there should be a whirl, in which the ascending column of air revolves from right to left, or against the hands of a watch. At the south pole the winds come from the north-west (§ 213), and consequently there they revolve about it with the hands of a watch. That this should be so will be obvious to any one who will look at the arrows on the polar sides of the calms of Cancer and Capricorn (Plate I., § 215). These arrows are intended to represent the prevailing direction of the wind at the surface of the earth on the polar side of these calms.
266. The arrows in the diagram of the winds.—The arrows that are drawn about the axis of this diagram are intended to represent, by their flight, the mean direction of the wind, and by
