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CHAPTER II. 



Winds. 



"He spoke, and, at his call, a mighty Wind, 

 Not like the fitful blast, with fury blind, 

 But deep, majestic, in its destined course, 

 Sprung with unerring, unrelenting force, 

 From the bright East. Tides duly ebbed and flowed; 

 Stars rose and set; and new hoiizons glowed; 

 Yet still it blew." Rogers. 



Ix the preceding chapter we have alluded to the weight and 

 decreasing density of the air. In the present we shall consider 

 it in its relations to heat and moisture, in which connection it per- 

 forms a most important part in the economy of nature. Air, like 

 all material bodies, expands when heated, and thus its specific 

 gravity being lessened, it rises. Every one is familiar with this 

 fact. The little whirligigs placed, on stoves, are turned by the 

 ascent of the heated air, and the draft of chimneys and pipes de- 

 pends upon the same principle. We might infer that if air ex- 

 pands upon being heated, it will part with this heat on being con- 

 densed. This is the fact, upon forcing a close fitting piston down 

 a tight tube, suddenly compressing the air before it, a heat is 

 evolved, sufficient to inflame good tinder. We well remember 

 in our younger days performing this feat, using a leaden tube for 

 the pipe, and a bit of wood with a circular and greased "leather 

 nailed to one end, for a piston, placing the "punk" in the folds 

 of the leather. 



The ascent of heated air from the earth is one of those silent 

 and often .unobserved agencies employed to produce a general 

 equilibrium in the temperature and moisture of the earth. The 

 air itself being transparent, is scarcely heated by the.passage of 

 the suns rays, but the stratum nearest the earth absorbing a por- 

 tion of the heat, rises and gives place to a colder portion, which 

 in its turn is displaced by another, and thus the excessive heat 

 which, in some portions of our globe, would parch up the earth 

 and destroy all life, is rapidly carried off, and a more genial tern- 



