Page:Physical Geography of the Sea and its Meteorology.djvu/187

Rh for each of the four quarters in every band was ascertained. Considering difference of temperature between these various bands to be one of the chief causes of movement in the atmosphere,—that the extremes on one hand are near the equator, and on the other about the poles;—considering that the tendency of every wind (§ 234) is to blow along the arc of a great circle, and that consequently every wind that was observed in any one of these bands must have moved in a path crossing these bands more or less obliquely, and that therefore the general movements in the atmosphere might be classed accordingly, as winds either with northing or with southing in them;—we have so classed them; and we have so classed them that we might study to more advantage the general movements of the great atmospherical machinery. See Plate XV.

353. The medial hands.—Thus, when, after so classing them, we come to examine those movements in the band between 5° and 10° south, and to contrast them with the movements in the band between 55° and 60° south, for example, we find the general movements to be exactly in opposite directions. Observations show that during the year the winds in the former blow towards the equator 283, and from it 73 days;, and in the latter they blow toward the pole for 224, and from it 132 days. These facts show that there must be a place of rarefaction—of low barometer, an indraught towards the poles as well as the equator;—and that consequently, also, there must be a medial line or band somewhere between the parallels of 10° and 55° south, on one side of which the prevailing direction of the wind is towards the equator, on the other towards the pole. So, in the northern hemisphere, the same series of observations point this medial band out to us. They show that one is near the calm belt of Capricorn, the other near the calm belt of Cancer, and that they both probably lie between the parallels of 35° and 40°, where the winds north and south are equal, as per table, page 162.

The wind curves (Plate XV. and the table) afford a very striking view of these medial bands, as the parallels in either hemisphere between which the winds with northing and the winds with southing are on the yearly average exactly equal. In the northern hemisphere the debatable ground appears by the table to extend pretty nearly from 25° to 50° N. By the plate the two winds first become equal between 25° and 30°; the two curves then recede and approach very closely again, but without