C 421 ] 
nicety. I have therefore contrived one of a different 
fort, which meafures the expanfion with certainty, 
to much lefs than the 1 oooth part of the whole. It 
confifts of a bulb of glafs fomething in the fhape of 
a pear, and fufficient to hold about half a pound of 
quickfilver. It is open at one end, and at the other 
is a tube hermetically clofed at top. By the help of 
a nice pair of fcales, I found what proportion of 
weight a column of mercury, of a certain length, 
contained in the tube, bore to that, which filled the 
whole veffel. By thefe means I was enabled to mark 
divifions upon the tube, anfwering to a 1 oooth part 
of the whole capacity, which being of about one 
tenth of an inch each, may, by eflimation, be cafily 
fubdivided into fmaller parts. This gage, during the 
exhaufting of the receiver, is fufpended therein by a 
flip-wire. When the pump is worked as much as 
fhall be thought neceffary, the gage is pufhed down, 
till the open end is immerged in a ciflern of quick- 
filver placed underneath : The air being then let in, 
the quickfilver will be driven into the gage * ; till 
the air remaining in it becomes of the fame denfity 
with the external ; and as the air always takes the 
higheft place, the tube being uppermoft, the expan- 
fion will be determined by the number of divifions 
occupied by the air at the top. 
The degree, to which I have been able to rarefy 
the air in experiment, has generally been about 1000 
. times. 
* The bulb of the gage may be emptied of its quickfilver, with- 
out taking that out of the tube j and the tube being held horizon- 
tal, the column of mercury in it will have power to contract or 
expand the air at the top. 
