98 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. 



law (§ 260), a rarefaction in tlie upper regions, and an ascent of 

 air about tlie poles similar to that about tbe equator, with this dif- 

 ference however : the place of ascent over the equator is a Hne, or 

 band, or belt ; about the poles it is a disc. The air rushing in 

 from all sides gives rise to a vdnd, 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 spu^als from the south- 

 west. 



265. If we draw a circle about this pole on a common terres- 

 Tiiey turn ^^^^^^^^ trial globo, aud iutcrscct it by spirals to represent 

 about the south pole, the dii'ection of the wind, we shall see that the wind 

 r north'!'"' '^'^' enters aU 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 ivith 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 pre- 

 vailing direction of the wind at the surface of the earth on the 

 polar side of these calms. 



266. The arrows that are drawn about the axis of this diagram 

 The arrows in the aro intended to represent, by their flight, the mean 

 wfnds. direction of the wind, and by their length and their 

 feathers the mean annual dm-ation from each quadrant. Only the 

 arrows nearest to the axis in each belt of 5° of latitude are cLra^vsTi 

 with such nicety. The largest arrow indicates that the wind in 

 that belt blows annually, on the average, for ten months as the 

 arrow flies. The arrow from the next most prevalent quarter is 

 half-feathered, provided the average annual duration of the wind 

 represented is not less than four months. The unfeathered aiTOws 

 represent winds having an average dm-ation of less than three months. 

 The arrows are on the decimal scale ; the longest arrow — which is that 

 representing the south-east trade-winds between 5° and 10° S., where 

 their average dm^ation is ten months — being half an inch. Winds that 

 blow five months are represented by an arrow half this length, and so 

 on. The haK-bearded arrows are on a scale of two for one. It appears, 

 at first, as a singular coincidence that the wind should whirl in these 



