On Atmospheric Density and Pressure, 421 



sequcntly lie concluded that the mercury would rise^th the 

 height of the water : this he found exactly to answer. He 

 filled a glass tube about three feet long, closed at one end and 

 exhausted of its air, and inverted it into a bason of mercury ; 

 the mercury rose £9 inches, = T ^th the height of the column 

 of water ; the remaining part of the tube was of course a 

 vacuum (the Torricellian vacuum). 



This mercurial column 29 inches high, is therefore equal 

 to a column of the atmosphere extending from its summit 

 to the earth. 



The atmosphere presses equally on all bodies at the surface 

 of the earth, or equally distant from it. 



This pressure decreases in a direct ratio (generally) with, 

 its distance upwards from the earth's surface, (the atmo- 

 spheric column being shortened,) or rather from the level of 

 the sea, it being the only natural uniform level from whence 

 such indications are deducible. Hence it follows, that it 

 increases directly with its distance downwards from such 

 level — (the atmospheric column being lengthened) — so that 

 in pits the pressure must (comparatively speaking) be at its 

 maximum : of this, however, I believe we have no direct 

 proofs. 



Thus the atmosphere may be said to constitute a mass 

 consisting of strata lying on each other, each stratum pressing 

 on the one next it with not only its own individual weight, 

 but with that collected weight with which it is pressed from 

 above, so that the lower stratum has the whole superjacent 

 pressure as it were on its back, and with it presses the 

 earth. 



Hence (supposing the ratio to be direct), if heights in the 

 atmosphere be taken in arithmetical proportion, its rarity 

 (from want of pressure) will be in geometrical progression ; 

 thus, at 7 miles above the earth it is 1 times rarer, at 14 

 miles it is 16 times rarer, at 21 miles it is 64 times rarer, 

 and so on, the rarity increasing in proportion to the height 

 as 4M. 



This principle of the atmosphere decreasing in pressure 

 as we ascend, and this decrease being (generally) equable, 



Pd3 has 



