196 THE ATMOSPHERE. 



them before the moving solid ; and as no force would in that case be imparted 

 to the air, so no force would be lost by the solid ; in other words, the solic 

 would suffer no resistance to its motion. 



But every one's experience proves this not to be the case. Open an um- 

 brella and attempt to carry it along swiftly with its concave side presented for- 

 ward it wi'l immediately be felt to be opposed by a very considerable re- 

 sistance, and to require a great force to draw it along. Yet this force is noth- 

 ing more than what is necessary to push the air before the umbrella. 



On the deck of a steamboat propelled with any considerable speed, we fee 

 on the calmest day a breeze directed from the stem to the stern. This arises 

 from the sensation produced by our body displacing the air as we are carriei 

 through it. 



It is the inertia of the atmosphere which gives effect to the wings of birds 

 Were it possible for a bird to live without respiration, and in a space void oi 

 air, it would no longer have the power of flight. The plumage of the wings 

 being spread, beating with a broad surface on the atmosphere beneath them 

 is resisted by the inertia of the atmosphere, so that the air forms a fulcrum, as 

 it were, on which the bird rises by the leverage of its wings. 



As a body at rest manifests its inertia by the resistance which it offers when 

 put in motion, so a body in motion exhibits the same quality by the force wit] 

 which it strikes a body at rest. We have seen examples of the resistance 

 which the atmosphere at rest offers to a body in motion ; but the force with 

 which the atmosphere in motion acts upon a body at rest is exhibited by ex- 

 amples far more numerous and striking. Wind is nothing more than moving 

 air ; and its force, like that of every other body, depends on the quantity moved, 

 and the speed of the motion. Every example, therefore, of the effects of the 

 power of wind, is an example of the inertia of atmospheric air. In a wind- 

 mill, the moving force of all the heavy parts of the machinery is derived from 

 the moving force of the wind acting upon the sails, and the resistance of the 

 work to which the mill is applied is oveicome by the same power. A ship 

 is propelled through the deep, and the deep itself is agitated and raised in 

 waves by the inertia of the atmosphere in motion. As the velocity increases, 

 the force becomes more irresistible, and we find buildings totter, trees torn 

 from the roots, and even the solid earth itself yield before the force of the hur- 

 ricane. 



IMPENETRABILITY OF AIR. 



Since air may be seen and felt since it has color and weight and since 

 it opposes resistance when -acted upon, and strikes with a force proportionate 

 to the speed of its motion we can scarcely hesitate to admit that it has quali- 

 ties which entitle it to be classed among material substances ; but one other 

 quality still remains to be noticed, which perhaps decides its title to materiality 

 more unanswerably than any of the others. Air is impenetrable ; it enjoys 

 that peculiar property of matter by which it refuses admission to any other 

 body to fhe space it occupies, until it quit that space. This property air pos- 

 sesses as positively as adamant. The difficulty which is commonly felt in 

 conceiving the impel etrability of substances of this nature arises partly from 

 confounding the quality of impenetrability with that of hardness, and partly 

 from not attending to the fact that, when a body moves through the air, it ( 

 drives the air before it in the same manner as a vessel moving through the 

 water propels the fluid. 



Let a bladder be filled with air, and tied at the mouth : we shall then be abie ? 

 to feel the air it contains as distinctly as if the bladder were filled with a solid \ 



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