[142 ] 
FF Are two femicircular parts of a tube joined together, to keep 
the barometer perpendicular. 
G The orifice of the tube of the barometer. 
HH A circular piece of light wood, cemented to the tube 
of the barometer, of a fize fufficient to make the barometer 
float at its proper height. 
IIII The refervoir for the mercury. 
KK Awooden cover, which fcrews into the refervoir to ad- 
mit the circular piece of wood HH, 
. 
LIZ The tube of the refervoir, which is cemented to the 
cover K K. 
MM _ The cylinder of the barometer. 
MG _  Thetube of the barometer. 
yx Is a mark on the tube of the barometer, which fhews the 
depth at which the barometer floats when the mercury ftands 
at 293. 
Tue paper 4.444 moves horizontally and preffes againft the pen- 
cil D. The pencil would trace an horizontal Jine upon the paper if 
the mercury remained ftationary ; but fuppofe the mercury defcends 
in 
