meafuring Heights with the Barometer. 691 
All the experiments were made when the barometer 
was at, or near, 30 inches. When the bulb was down- 
wards, the height of the barometer at the time of obfer- 
vation, augmented, and when upwards, diminifhed by 
the number of inches of quickfilver in the tube of the 
manometer, expreffed the denfity of the confined air. 
Pounded ice and water were ufed to fix a freezing 
point on the tube ; and by means of fait and ice, the air 
was farther condenfed, generally four, and fometimes 
five or fix degrees below zero. The thermometer and 
manometer were then placed in the tin yeflel, among 
water which was brought into violent ebullition ; where 
having remained a fufficient time, and motion being 
given to the manometer, a boiling point was marked 
thereon. After this the fire was removed, and the gra- 
dual defcents of the piece of quickfilver, correfponding 
to every 20 degrees of change of temperature in the ther- 
mometer, were fucceffively marked on a deal rod ap- 
plied to the manometer. It is to be obferved, that both 
inftruments, while in the water, were in circum fiances 
perfectly fimilar; that is to fay, the ball and bulb were 
at the bottom of the veffel. 
In order to be certain that no air had efcaped by the 
fide of the quickfilver during the operation, the mano- 
meter was frequently placed a fecond time in melting 
Vol. LXVII. 4 U 
ice. 
