THE ATMOSPHERE. 89 



in arcs of great circles, and not in small ones* — creates a vertical 

 circulation either by dragging down air from tlie 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 j^oint may be well 

 made whether the horizontal circulation of the air be not de- 

 pendent 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 conse- 

 quent 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 tempe- 

 rate zone of the northern hemisphere. The wind that comes rush- 

 ing 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 hori- 

 zontal, 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, tlien 

 draw great circles to a point in the middle of -"'t, 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). 



