THE ATMOSPHERE. . 85' 



utmospiiere being required of the winds in their circuits, is it 

 possible for the human mind to conceive of the appointment of 

 "cii'cuits" for them (§ 216) which are so admirably designed and 

 exquisitely adapted to the pm'pose as are those which this view 

 suggests ? 



231. As a physical necessity, the vertical circulation of the air 

 Its verucai move- sccms to bc uo Icss important than its horizontal 

 <iuced. movements, which we call wind. One begets the 

 other. The wind, when it blows across parallels of latitude — as 

 it always must, except at the equator, for it blows in arcs of great 

 cii'cles, and not in small ones * — creates a vertical circulation either 

 by dragging down air from the upper regions (§ 224), or by slough- 

 ing it off and forcing it up fi'om the lower (§ 228), according as the 

 wind is approaching the pole or the equator. 



232. Indeed the point may be well made whether the horizontal 

 SnS4mH^t'S?S;circlllation of 'the air be not dependent upon and a 

 air consequents of, consequeut of its Vertical circulation; — so nearly 



And dependent upon n • T a! J. f ' lX. • i ±- *^ 



each other. alliecL are the two motions m then- relations as cause 



and effect. Upward and downward movements in fluids are conse- 

 quent upon each other, and they involve lateral movements. The 

 sea, mth 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 pro- 

 duces ascending columns in the atmos2}here. 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 p(rce'ved. 

 But when the ascending vapour is condensed, and its latent heat 

 liberated and set fi'ee in the upper air, we often have the most 

 terrific storms. 



* The tendency of all bodies, wlien pnt in motion on the surface of the eartli, 

 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 tJie motion is hori- 

 zontal, is an arc of a great circle. If we imagine a partial vacuum to be formed at 

 the north pole, wo can readily enougli perceive that the wind for 5^, 10°, 20° of 

 polar distance, all around, would tend to rush nortli, and strive to get there along 

 the meridians— arcs of great circles. This would be the case whether the earth 

 be supposed to be Avith 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 rush- 

 ing into the vacuum, seek to reach it by these great circles. Furce may turu it 

 aside, but such is the tendency (§ 120j. 



