INTRODUCTION TO PNEUMATICS. 



SECTION I. On the Mechanical Properties of Air. 



WE shall now examine the second class of fluids, distinguished by the 

 name of aeriform, or elastic fluids, the principal of which is the air we 

 breathe, which surrounds the earth, and is called the atmosphere. There 

 are a great variety of elastic fluids, but they differ only in their chemical, 

 not in their mechanical properties ; and it is the latter we are to examine. 

 There is no attraction of cohesion between the particles of elastic fluids, 

 so that the expansive power of heat has no adversary to contend with but 

 gravity; any increase of temperature, therefore, expands elastic fluids 

 prodigiously, and a diminution proportionally condenses them. The most 

 essential point in which air differs from other fluids is by its spring or 

 elasticity : that is to say, its power of increasing or diminishing in bulk, 

 according as it is less or more compressed a power of which liquids are 

 almost wholly deprived. 



The atmosphere is thought to extend to about the distance of 45 miles 

 from the earth ; and its gravity is such, that a man of middling stature is 

 computed, when the air is heaviest, to sustain the weight of about 14 tons. 

 Such a weight would crush him to atoms, were it not that air is also con- 

 tained within our bodies, the spring or elasticity of which counterbalances 

 the weight of the external air, and renders us insensible of its pressure. 

 Besides this, the equality of pressure on every part of the body enables 

 us more easily to support it : when thus diffused, we can bear even a 

 much greater weight, without any considerable inconvenience. In bathing 

 we support the weight and pressure of the water, in addition to that of 

 the atmosphere ; but this pressure being equally distributed over the body, 

 we are scarcely sensible of it : whilst if the shoulders, the head, or any 

 particular part of the frame were loaded with the additional weight of a 

 hundred pounds, we should feel severe fatigue. On the other hand, if 

 the air within a man met with no external pressure to restrain its elasticity, 

 it would distend his body, and at length bursting the parts which confine 

 it, put a period to his existence. The weight of the atmosphere, there- 

 fore, so far from being an evil, is essential to our existence. When a 

 person is cupped, the swelling of the part under the cup is produced by 

 taking away the pressure of the atmosphere; in consequence of which, 

 the internal air distends the part. The air-pump affords us the means 

 of making a great variety of interesting experiments on the weight and 

 pressure of the air. We have already seen, that in a vacuum produced 

 within the air-pump, substances of various weights fall to the bottom in 

 the same time. 



We shall now point out some experiments which illustrate both the weight 

 and elasticity of air. If a piece of bladder be tied over a glass receiver, 

 open both at the top and bottom, when the air is taken away from the 

 under surface, so that there is no longer any re-action to counterbalance 

 the pressure of the atmosphere, the bladder is pressed inwards in propor- 

 tion as the receiver is exhausted ; and before a complete vacuum or void 

 is formed within the receiver, the bladder, unable to sustain the violence 

 of the pressure, bursts with a loud report. 



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