THE EARTH. 123 



late how many square inches are in 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, ana 

 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 ordinary load, and our usual supply, 

 there are, at different times, very great variations. The air is not. 

 like water, equally heavy at all seasons ; but sometimes is lighter, and 

 sometimes more heavy. It is sometimes more compressed, and some- 

 times more elastic or springy, which produces the same effects as at 

 increase of its weight. The air, which at one time raises water thirty- 

 \wo 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 very great difference in the weight we sus- 

 tain ; and we are actually known, by compulation, 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 properties ; and 

 to which it should seem nothing can set bounds. A body of air that 

 may be contained in a nutshell, 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 ex- 

 pansion ; at least, experiment has hitherto found its attempts indefi- 

 nite. In every situation, it retains its elasticity ; and the more closely 

 we compress 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 irresisti- 

 ble ; and a certain French philosopher* supposed, that air thus con- 

 fined and expanding, was sufficient for the explosion of a world. 



Many instruments have been formed to measure and determine these 

 different properties of the air ; and which serve several useful pur- 

 poses. The barometer serves to measure its weight ; to tell us when 

 t is heavier, and when lighter. It is composed of a glass tube or pipe, 

 of about thirty inches in length, closed up at one end ; this tube is 

 then filled with quicksilver ; this done, the maker clapping his finger 

 upon the open end, inverts the tube, and plunges the open end, finger 

 and all, into a bason of quicksilver, and then takes his finger away ; 

 now the quicksilver in the tube will, by its own weight, endeavour to 

 descend into that in the bason ; but the external air, pressing on the 

 surface of the quicksilver in the bason without, and no air being in 

 the tube at top, the quicksilver will continue in the tube, being press- 

 ed up, as was said, by the air, on the surface of the bason below. Th 

 height at which it is known to stand in the tube, is usually about twea- 



* Mons'^ur Atnontons. 



