THE EARTH. 



89 



Thus we see that the air at the surface of 

 the earth is just as heavy as thirty-two feet 

 of water, or twenty-nine inches and a half of 

 quicksilver ; and it is easily found, by com- 

 putation, that to raise water thirty-two feet 

 will require a weight of fifteen pounds upon 

 every square inch. Now, if we are fond of 

 computations, we have only to calculate 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, and found, that our or- 

 dinary 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 

 sometimes more elastic or springy, which 

 produces the same effects as an increase of 

 its weight. The air which at one time raises 

 water thirty-two feet in the tube, and quick- 

 silver 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 compu- 

 tation, 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 pres- 

 sure 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 con- 

 tinually 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 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, expe- 

 riment has hitherto found its attempts in- 

 definite. In every situation, it retains its 



elasticity ; and the more closely we compress 

 it, the more strongly does it resist the pres- 

 sure. 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 irresistible ; and a certain French 

 philosopher" supposed, that air thus confined, 

 and expanding, was sufficient for the explo- 

 sion of a world. 



Many instruments have been formed to 

 measure and determine these different pro- 

 perties of the air; and which serve several 

 useful purposes. The barometer serves to 

 measure its weight; to tell us when it is hea- 

 vier, 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 with- 

 out, and no air being in the tube at top, 

 the quicksilver will continue in the tube, be- 

 ing pressed up, as was said, by the air, on the 

 surface of the bason below. The height at 

 which it is known to stand in the tube, is 

 usually about twenty-nine inches, when the 

 air is heavy; but not above twenty-six, when 

 the air is very light. Thus, by this instru- 

 ment we can, with some exactness, determine 

 the weight of the air; and, of consequence, 

 tell before-hand the changes of the weather. 

 Before fine dry weather, the air is charged 

 with a variety of vapours, which float in it 

 unseen, and render it extremely heavy, so 

 that it presses up the quicksilver ; or, in other 

 words, the barometer rises. In moist, rainy 

 weather, the vapours are washed down, or 

 there is not heat sufficient for them to rise, 

 so that the air is then sensibly lighter, and 

 presses up the quicksilver with less force ; 

 or, in other words, the barometer is seen to 

 fall. Our constitutions seem also to corres^ 

 pond with the changes of the weather-glass ; 

 they are braced, strong, and vigorous, with 



a Monsieur Amontous. 



