THE ATMOSPHERE AND ITS CURRENTS. 



435 



marshes, which only inferior orders of animals can occupy. The antagonist power is the 

 subterranean upheaving agency a rare visitant often at rest for ages and then 

 counteracting in the twinkling of an eye the effect of the rains, rills, torrents, rivers, 

 atmosphere, and seas, that have been preying upon the soil. 



CHAPTER XIII. 



THE ATMOSPHERE AND ITS CURRENTS. 



T has been mentioned as a part of the planetary 

 constitution of our globe, that a gaseous envelope . 

 environs its mass the atmosphere which re- 

 quires the attention of the astronomer, on ac- 

 count of its influence in displacing the celestial 

 bodies, and contributing to their visibility by re- 

 fracting and reflecting the rays of light. This 

 elastic fluid is the scene of interesting phenomena, 

 and performs important functions in the economy 

 of nature. Besides being essential to the life of man, and the 

 animal races, whose existence would terminate in a few 

 minutes without the respiration of it; the exhalation of 

 moisture from the surface of the earth is mainly owing to 

 the common air we breathe, which receives and sustains the 

 vapours formed into clouds, distributes them over different 

 regions by its incessant motions, and tempers by its currents 

 those extremes of heat and cold to which various localities 

 are subject. It is in these last-named offices that the 



atmosphere demands the notice of the physical geographer. The consideration of its 

 actual constitution does not belong to his province, but a general view of the fluid 

 may be appropriate before we proceed to those agitations and changes which are in 

 constant action, and upon which the welfare of organised beings so materially depends. 



The atmosphei'e is, then, an integral portion of the earth, a body of air revolving with 

 the solid mass upon its axis, the higher strata, of course, increasing in velocity with the 

 distance from the axis of revolution. From hence a conclusion may be drawn respecting 

 its height, for an absolute limit is put to its elevation by this feature of its physical 

 condition. There is a point where the centrifugal force, or the tendency to fly off from 

 the centre, will counterbalance the centripetal, or the gravitation towards the centre, and 

 beyond that point the latter will be vanquished. It is obvious that no portion of the 

 atmosphere can extend beyond the point where the two influences balance, or are in 

 equilibrium, and the projectile force becomes greater than that of gravitation, or its pro- 

 jection into space would follow. At the distance of 6'6 radii from the centre of the earth, 

 or at an elevation of 22,200 miles, about the eleventh part the distance of the moon, this 

 point is fixed, beyond which it is impossible for the atmosphere in the smallest quantity 

 to extend. This consideration is only of importance to show that physical laws rigidly 

 restrict it within finite bounds, for any portion of air at that distance must have a tenuity 

 which is utterly inconceivable. The indications of the height of the atmosphere drawn 

 from its weight, as shown by the barometer, reduce its elevation within a vastly circum- 



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