meafuring Heights with the Barometer. 667 
top of the veffel, which, in the firft fet of the third clafs 
of experiments, was little more than 29 inches high. 
The top of the tube palling through the aperture in the 
lid, one and a half or two inches of the vacuum gene- 
rally rofe above the veffel. That the expanlion of the 
column might be meafured as nearly as poflible in that 
part of the tube fronting the center of the eyes, more or 
lefs quicklilver, according to the It ate of the atmofphere, 
was occalionally put into the cittern, to raife or deprefs 
the furface of the column to the proper height. A thin 
brafs ring, whofe lower parts were made to fpring, em- 
braced with fufficient force the upper part of the tube, 
permitting it at the fame time to be moved freely with 
the hand. It carried along with it a nonius index, pro- 
jecting as far as the center of the rod, and confequently 
applying itfelf to the divilions of the fcale, which was 
kept in its proper polition by palling through a flit fitted 
for it in an arm attached to the lid. The divilions on the 
nonius being the fame with thofe of the barometer for- 
merly defcribed, the height of the quickfilver could 
always be read off to — th part of an inch. 
The quickfilver having been carefully boiled, as on 
former occafions, in the tube ; and that being filled com- 
pletely, and held with its open end upwards; the tin 
veffel was inverted over it, and lowered gradually, till the 
4 R 2 ground 
