a 
aoe 
~ 
[-: qox 4] 
and then turning off at right angles to communicate with the 
perforation. Through this paflage the mercury overflows until 
it has defcended to its ftandard level, and through this it is 
forced back again from the bag into the bafon when it is necef- 
fary to fill it. For conveniently effeGting this, the lower box is 
open at the bottom m wide enough to admit the thumb, and 
¢has in it a loofe, piece # with a cavity anfwering the convexity 
of the bottom of the upper box, which is prefled by the thumb 
againft the bag, and forces (with little difficulty) up into the 
bafon the mercury, which finds no paflage but through the 
\perforation., A {mall air-hole o is in the top of the upper 
box, which;is opened or fhut by a {mall ivory peg or ftop- 
cock. 
In order to render the barometer portable, firft clofe the air- 
hole to prevent the mercury from running out through it, then 
invert the barometer, prefs the bag with the loofe piece {fo as to 
fill completely the cavity of the bafon (if any air is fuppofed to 
remain in the bottom of the bafon), now the top, unfcrew a 
{mall fteel pin « which pafles from the outfide of the upper box 
into the bottom of the bafon, and force the mercury out of the 
bag until it appears in this hole, the air is then perfectly ex- 
cluded and the cavity full of mercury, fhut the ftop-cock by 
turning it by the fquare g with its key one-quarter or one-half 
round, and {crew again into its place the fteel fcrew. It will 
now be found very portable, particularly if carried in an in- 
verted pofition. 
