TNEUMATICS. 135 



stands in the barometer at the time. Then it is 

 plain, that the air in the shorter leg will be com- 

 pressed with a force twice as great as at first, when 

 it possessed the whole space C D; for then it was 

 compressed only with the weight of the atmo- 

 sphere, but now it is compressed by that weight 

 and the additional equal weight of a column of 

 mercury. The surface of the mercury will now 

 be at E ; and it will be found, upon measuring it, 

 that the space D E, into which the air is now com- 

 pressed, is just half the former C D. If another 

 column of mercury were added, equal to the for- 

 mer, it would be reduced into one-third of the space 

 it formerly occupied. 



Hence the density of the air is proportional to the 

 force that compresses it. 



As all the parts of the atmosphere gravitate, or 

 press upon each other, it is easy to conceive, that 

 the air next the surface of the earth is more com- 

 pressed and denser than what it is at some height 

 above it. Thus, if wool were thrown into a deep 

 pit until it reached the top, the wool at the bottom, 

 having all the weight of what was above it, would 

 be squeezed into a less compass; the layer or stra- 

 tum above it would not be pressed quite so much; 

 the one above that, still less; and so on, till the 

 upper one, having no weight over it, would be in 

 its natural state. This is the case with the air, or 

 atmosphere, that surrounds our earth, and accom- 

 panies it in its motion round the sun. On the tops 

 of lofty buildings, but still more on those of moun- 

 tains, the air is found to be considerably less dense 

 than at the level of the sea. 



The height of the atmosphere has never yet been 

 exactly ascertained; indeed, on account of its great 

 elasticity, it may extend to an immense distance, 



K 4) 



