102 niYSICAL GEOGRArnY OF THE SEA, AND ITS METEOROLOGY. 



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. TJie counter trades-^ihey 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 hemi- 

 sphere 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 difterence how- 

 ever : 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. Tliey turn with the hands of a icatch 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, con- 

 sequently, 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 con- 

 sequently there they revolve about it icith 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 repre- 

 sent, by their flight, the mean direction of the wind, and by 



