§ 222-234. THE ATMOSPHERE. 35 



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

 by sloughing it off and forcing it up from the lower (§ 228), ac- 

 cording as the wind is approaching the pole or the equator. 



232. Indeed, the point may be well made whether the horizon- 

 verticai and horizon- tal circulatioii of the air be not dependent upon 



tal movements in the ^ ^ . . , . ^ . 



air consequents of, auQ a conscQuent 01 its vcrtical circulation — so near- 



and dependent upon t-itt t. • .,. ,, 



each other. Jy aiiied are the two motions m their relations as 



cause and effect. Upward and downward movements in fluids 

 are consequent upon each other, and they involve lateral move- 

 ments. The sea, with its vapor, is the great engine w^hich gives 

 upward motion in the air (§ 227). As soon as aqueous vapor is 

 formed it rises ; the air resists its ascent ; but it is lighter than 

 the air, therefore 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 vapor left 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 vapor is con- 

 densed, and its latent heat liberated and set free in the upper air, 

 we often have the most terrific storms. 



233. Now suppose the surface from which this vapor rises, or 

 Cold belts. 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 somew^here, for example, in the tem- 

 perate 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), going from 

 larger to smaller parallels, and giving rise to ascending columns; 

 while that from the northern side, moving in the opposite direc- 

 tion, is, like the trade-winds (§ 223), bringing down air from 

 above. 



234. By the motion of the clouds upper currents of wind arc 

 The upper currents disccmed in the skv. Thcv are arranged in layers 



—their numbers and ^ "^ " pi 



offices. or strata one above the other. The clouds 01 each 



stratum are carried by its winds in a direction and with a velocity 

 peculiar to their stratum. How many of these superimposed cur- 

 rents of wind there may be between the top and bottom of the 

 atmosphere w^e know not. As high up as the cloud-region sev- 

 eral are often seen at the same time. They are pinions and 



