66 Experimental Philosophy. [Lecture 6. 



to fill which the water is pressed up, as before de- 

 scribed. It would rise, if there were no impedi- 

 ment, to the height of thirty-two feet, because, 

 as I explained in my last lecture, a column of 

 the atmosphere is at a medium equal in weight 

 to a column of water of that height. 



The weight of the air, or rather of the atmo- 

 sphere, is, however, exactly determined by the 

 following experiment. 



Take a glass tube about three feet long, open 

 at one end ; fill it with quicksilver, putting the 

 finger upon the open end, turn that end down- 

 ward, and immerse it into a small vessel of quick- 

 silver, without admitting any air : then take away 

 the finger, and the quicksilver will remain sus- 

 pended in the tube twenty-nine inches and a half 

 above its surface in the vessel ; sometimes more, 

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

 is varied by winds, vapours, and other causes. 

 That the quicksilver is kept up in the tube by 

 the pressure of the atmosphere upon that in the 

 bason, is evident ; for, if the bason and tube are 

 put under a glass, and the air is then taken out 

 of the glass, all the quicksilver in the tube 

 will fall down into the bason ; and if the air is ad- 

 mitted again, the quicksilver will rise to the same 

 height as before. The air's pressure therefore on 

 the surface of the earth, is equal to the weight of 

 twenty -nine inches and a half depth of quicksilver 

 all over the earth's surface, at a mean rate. 



A square column of quicksilver, twenty-nine 



