fame Experiments made with an Air-pump . 619 
of the quickfilver in the ciftern, by which means the dif- 
ference of the two furfaces could be feen to a great nicety. 
This kind of gage is called the fliort barometer-gage. 
The other tube, which was cut off to thirty-three 
inches, being perfectly full of the boiled quickfilver, was 
alfo carefully inverted into a glafs ciftern containing 
boiled quickfilver, to fuch a depth that from the furface 
of the quickfilver in the ciftern to the top of the tube was 
twenty-nine inches; this had likewife a piece of ivory, 
with divifions on it, put over the tube, fo as to float on 
the furface of the quickfilver in the ciftern, in the fame 
manner as the other. 
This long tube and the fhort barometer-gage being- 
put both of them at the fame time under the receiver, 
which was carefully cemented to the plate of the pump ; 
the pump was then worked for ten minutes, and the fur- 
face of the quickfilver in both the tubes came down very 
nearly to within one-twentieth or five hundredth parts of 
an inch of the furface of the quickfilver in their refpec- 
tive citterns. 
The air was then let in, and the receiver being taken 
from the pump, the long tube was raifed up fo far in the 
ciftern as to let the quickfilver come down from the top 
of the tube, fo that it now became a common barometer, 
4 1/ 2 and 
