in arcs of great circles, and not in small ones[1]—creates a vertical circulation either by dragging down air from the upper regions (§ 224), or by sloughing it off and forcing it up from the lower (§ 228), according as the wind is approaching the pole or equator.
232. Vertical and horizontal movements in the air consequents of, and dependent upon each other.—Indeed the point may be well made whether the horizontal circulation of the air be not dependent upon and a consequent of its vertical circulation;—so nearly allied are the two motions in their relations as cause and effect. Upward and downward movements in fluids are consequent upon each other, and they involve lateral movements. The sea, with its vapour, is the great engine which gives upward motion in the air. As soon as aqueous vapour is formed it rises; the air resists its ascent; but it is lighter than the air, therefore (§ 177) it forces the resisting particles of air up along with it, and so produces ascending columns in the atmosphere. The juxta air comes in to occupy the space which that carried up by the vapour leaves behind it, and so there is a wind produced. The wind arising from this source alone is so slight generally, as scarcely to be perceived. But when the ascending vapour is condensed, and its latent heat liberated and set free in the upper air, we often have the most terrific storms.
233. Cold belts.—Now suppose the surface from which this vapour rises, or on which it is condensed, be sufficiently large to produce a rush of wind from afar; suppose it, moreover, to be an oblong lying east and west somewhere, for example, in the temperate zone of the northern hemisphere. The wind that comes rushing in from the south side will be in the category of the counter trades of the southern hemisphere (§ 228), viz.: going from larger to smaller parallels, and giving rise to ascending columns;
- ↑ The tendency of all bodies, when put in motion on the surface of the earth, is, "whether fluid, solid, or gaseous, to go from the point of departure to the point of destination by the shortest line possible; and this, when the motion is horizontal, is an arc of a great circle. If we imagine a partial vacuum to be formed at the north pole, we can readily enough perceive that the wind for 5°, 10° 20° of polar distance, all around, would tend to rush north, and strive to get there along the meridians—arcs of great circles. This would be the case whether the earth be supposed to be with or without diurnal rotation, or motion of any sort. Now suppose the place of refraction to be anywhere away from the poles, then draw great circles to a point in the middle of it, and the air all around would, in rushing into the vacuum, seek to reach it by these great circles. Force may turn it aside, but such is the tendency (§ 120).
