2&2 



m^TullV UF 



that to raise water tliirty-two feet, will require a weight of fii. 

 teen pounds upon every square inch. Now, if we are I'ond of 

 computations, we have only to calculate how many square iiicliep 

 are ui the surface of an ordinary human body, and allowing every 

 inch to sustain fifteen pounds, we may amaze ourselves at the 

 weight of air we sustain. It has been computed, and found, 

 that our ordinary load of air amounts to within a little of forty 

 thousand pounds -. this is wonderful ! but wondering is not the 

 way to grow wise. 



Notwithstanding this be our ordinaiy load, and our usual sup- 

 ply, there are, at different times, very great variations. The aii 

 is not, like water, equally heavy at all seasons ; but sometimes 

 is lighter, and sometimes more heavy. It is sometimes more 

 compressed, and sometimes more elastic or springy, which pro- 

 duces the same effects as an increase of its weight. The air, 

 which at one time raises water thirty-two feet in the tube, and 

 quicksilver twenty-nine inches, will not at another raise the one 

 to thirty feet, or the other to twenty- six inches. This makes, 

 therefore, a verj' great difference in the weight we sustain ; and 

 we are actually known, by computation, to carry at one time 

 four thousand pounds of air more than at another. 



The reason of this surprising difference in the weight of air, 

 is either owing to its pressure from above, or to an increase of 

 vapour floating in it. Its increased pressure is the consequence 

 of its spring or elasticity, which cold and heat sensibly affect, 

 and are continually changing. 



This elasticity of the air is one of its most amazing proper- 

 tics ; and to which it should seem nothing can set bounds. A 

 body of air that may be contained in a nut shell, may easily, 

 with heat, be dilated into a sphere of unknown dimensions. 

 On the contrary, the air contained in a house, may be compressed 

 into a cavity not larger than the eye of a needle. In short, no 

 bounds can be set to its confinement or expansion ; at least, ex- 

 periment has hitherto found its attempts indefinite. In every 

 situation, it retains its elasticity ; and the more closely we com- 

 press it, the more strongly does it resist the pressure. If to the 

 increasing the elasticity on one side by compression, we increase 

 it on the other side by heat, the force of both soon becomes ir- 

 resistible ; and a certain French philosopher '^ supposed that aix 

 1 MuiJsieur AmonUms. 



