PNEUMATICS. 71 



width for a barometer. If the tube be perfectly clean, 

 pure mercury has always a convex surface ; if soiled 

 or stained, the mercury will become flat or concave.* 



Natural Condition of the Barometer. 



The fall of the barometer is connected, not with 

 the existence of clouds or fall of rain, but with their 

 formation, for it may fall while clouds are forming, 

 and rise when this process has ceased although rain 

 fall. 



The state in which the barometer continues longest 

 without change, may be considered its natural condi- 

 tion, and it is for much longer periods high than low. 

 An approach to its highest elevation therefore may 

 be considered its natural condition. 



Mensuration of Heights. 



At the surface of the earth the mean density or 

 pressure is considered equal to the support of a 

 column of mercury - 30 inch. high. 



2000 - - ditto - - 27-86 



3000 - - ditto - - 26-85 



4000 - - ditto - - 25-87 



5000 - - ditto - - 24-93 



1 mile - ditto - - 24-67 



2 - - ditto - - 20-29 



3 - - ditto - - 16-68 



4 - - ditto - - 13-72 



5 - - ditto - ; ' - 11-28 

 10 - - ditto - - 4-24 

 15 - - ditto - - 1-60 

 20 - - ditto - - 0-95 



* Mr. Daniel says, the common instruments are mere play 

 things, scarcely two agreeing within a quarter of an inch, whereas 

 the questions of meteorology, now of interest, require the mea- 

 surement of jfoth part of an inch of the mercurial column. 



