an improved Air-Pump, &c. 449 
exbnufted fpace within the pump is fmall, but it becomes 
quite inconfiderable when the receiver is upon the plate, its 
quantity bearing a very fmall proportion to the exhaufted 
fpace. 
It is of great advantage in this pump that very little oil can 
be lodged in it, becaufe then the elaftic fluid yielded by this 
fluid is in fo fmall a quantity as not to a fled the experiments. 
As for the oil, which by the working of the pump is accumu- 
lated into the oil veflel, it cannot interfere with the exhauftion 
of the pump, flnee it does not communicate with the cavity 
of the receiver. 
The exhaufting power of this pump was next examined 
with the pear-gage, made and placed under a receiver after the 
uiual manner ; and by this it appeared that the pump exhaufted 
fo far as to remain within the receiver lefs than the thoufandth 
part of the air it contained before the exhauftion *. In that 
cafe, the quickfilver in a fhort barometer-gage came to the fame 
level in both tubes, which proves that, by this laft mentioned 
gage in that ftate, the air is ftiewn to be rarefied at leaft one 
thoufand times. 
Laftly, a long tube, or what is called a long barometer- 
gage, was adapted to the pump by means of a bent brafs tube. 
This glafs tube went down along the fide of the wooden frame, 
and its lower end was immerfed in fome quicklilver kept in a 
proper ciftern. 
O11 working the pump, when all the three gages were an- 
nexed to it, there appeared, that the quickfilver of the fhort 
gage came to the fame level in the infide as well as outfide tube, 
* Whenever the pear-gage was ufed, care was taken to keep the infide of the 
receiver, of the pear-gage, and of the pump, as free from moiflure as it was 
pofiible. 
N n n 2 
that 
