meafuring Heights with the Barometer. 709 
with which it is loaded for, in the firft cafe, it bears 
the proportion of 484 to 434; and in the laft, it is (from 
the 
(f) This difference between the elaftic force of common air, and that which 
is artificially rendered more or lefs denfe, by the addition or fubtra&ion of 
weight, particularly the latter, is truly remarkable, and contradi&s the expe- 
rience of boyle, marriotte, &c. It could not arife from the adhefion of 
the quickfilver to the tube, though in the denfe experiments a column of 72 
inches was once made ufe of; becaufe the conftant motion given to the mano- 
meter before the fpaces were marked, muft either have prevented any irregu- 
larity whatever, or made the apparent expanfion fometimes too great, and at 
others too little. But the rare experiments ferve to put this matter out of 
doubt; for if the adhefion of the quickfilver to the tube had tended to leffen 
the apparent expanfion of the air, beneath the truth in one cafe, it muft have 
had a direft contrary effe£t in the other, and augmented it above the truth, , 
which it evidently doth not. 
Thefe experiments on the expanfion of air lefs denfe than the atmofpliere, 
were extremely difficult and troublefome; and it was not till after feveral 
fruitlefs attempts that, with the afliftance of Dr. lind, an apparatus was pre- 
pared for making them with fufficient accuracy. The vefiel employed for this 
purpofe was made of the brafs tube of a large telefcope, near four inches in 
diameter; it was divided into four pieces, which, when fcrewed together, made 
a pot of fix feet in height. This was mounted on a platform laid over the 
area rails, for the reception of the manometer, which was placed therein, with 
the bulb uppermoft, the lower extremity of the tube palling through’ a focket at 
the bottom of the vefiel, and then through a collar compofed of many thick - 
neffis of flannel. By means of a brafs plate and three long ferews, the collar was 
made to embrace the tube fo clofely, as to fuffer very little water to pafs: fuch as 
did iflue, oozed off along the fides of a paper funnel, bound round the end of 
the tube, without entering into the bore. In this pofition, it required fome degree 
of force to pufti the manometer up, or draw it down, till the top of the quick- 
filver confining the air, juft appeared without the collar, fo as to admit the 
fpaces to be meafured, from a fixed point marked on the tube. The vefiel being 
filled with boiling water, was kept to that temperature by means of lamps fui~ 
