197 



LECT. 

 VI. 



And hence it is easy to prove by calculation, that a 

 cubic inch of such air as we breathe, would be so much 

 rarefied at the altitude of 500 miles, that it would fill a 

 sphere equal in diameter to the orbit of Saturn. 



The weight or pressure of the air is exactly deter- 

 mined by the following experiment. 



Take a glass tube about three feet long, and open at The Tori- 

 one end ; fill it with quicksilver, and, putting your fin- 

 ger upon the open end, turn that end downward, 

 and immerse it into a small vessel of quicksilver, with- 

 out letting in any air: then take away your finger; 

 and the quicksilver will remain suspended in the tube 

 29i inches above its surface in the vessel ; sometimes 

 more, and at other times less, as the weight of the air 

 is varied by winds and other causes. That the quick- 

 silver is kept up in the tube by the pressure of the 



