THE EARTH. 25<J 



amaze ourselves at the weight of air we sustain. 

 It has been computed, and found, that our ordi- 

 nary load of air amounts to within a little of forty 

 thousand pounds : This is wonderful ! but won- 

 dering 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 some- 

 times more compressed, and sometimes more elas- 

 tic 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 

 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 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 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 



