COMMON THINGS AIR. 



presses it downwards with a force of 15 Ibs., and the ELASTICITY of 

 the air confined within the tube presses it upwards with an equal 

 force of 15 Ibs. The piston is thus held in equilibrium, having 

 no tendency either to rise or fall in the tube. 



Indeed, the very fact that the piston inserted in the tube A B 

 has no tendency to descend into it, and to compress the air under 

 it, although it is urged downwards by the air above it with a force 

 of 15 Ibs., proves that the air below it must iirge it upwards by a 

 force exactly equal ; since, if it were urged upwards by any less 

 force, it would be pressed down by this excess of the force of the 

 external air, and if it were urged upwards by any greater force 

 than 15 Ibs., it would ascend with the excess of this upward force. 



Air, therefore, in its natural and usual state, has an elastic force 

 of 15 Ibs. per square inch, so that when it is shut up in any vessel 

 or other envelope, and cut off from all communication with the 

 external air, it will press on every square inch of the inner surface 

 of such envelope with a force of 15 Ibs. 



14. This elastic force increases in the same proportion as the 

 space in which the air is confined is diminished by compression, 

 and it decreases in the same proportion as the space into which it 

 is allowed to expand is increased. Thus, if we suppose that when 

 the air fills twelve inches of the tube A B, fig. 1, it has an elastic 

 force of 15 Ibs., it will have an elastic force of 30 Ibs. when it fills 

 six inches of the tube ; 45 Ibs. when it fills four inches ; 60 Ibs. 

 when it fills three inches, and so on. And in like manner when 

 it is allowed to expand so as to fill twenty-four inches, its elastic 

 force will be reduced to 7 Ibs. ; when it expands to thirty-six 

 inches, the elastic force will only be 5 Ibs. , and so on. 



15. The stratum of air which rests on the surface of the earth, 

 and in which the organised tribes that inhabit the earth live, 

 derives its pressure, elasticity, and density from the weight, of the 

 whole mass of the atmosphere which rests upon it. It must, there- 

 fore, be evident, that if we ascend to greater elevations, leaving 

 below us a certain stratum of the atmosphere, and having above 

 us a proportionally less quantity of air, the weight of the incum- 

 bent air being less, the pressure, elasticity, and density of the 

 stratum which surrounds us will be proportionally less. And we 

 find this actually to be the case. At great heights on mountain 

 chains, such as the Pyrenees or the Alps, the air is very sensibly 

 rarefied. It is lighter, and exercises a much less pressure. In 

 like manner, persons who ascend to great elevations in balloons find 

 much inconvenience from the thinness of the air. The fluids 

 confined within the body are much less resisted, certain organs 

 become dilated, and the effect of a cupping-glass is occasionally pro- 

 duced, attended with bleeding at the nose, and singing in the ears. 



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