436 Mr. cavallo’s Defcription of 
•then the air may by it be rarefied about 600 times, and not 
farther *. 
The principal caufe which prevents this pump to exhaufl 
farther than that limit is the weakened elafticity of the air 
remaining within the receiver, which, decreafing in proportion 
as the quantity of the air within the receiver is diminifhcd, 
becomes at lad; incapable of lifting up the valve, which opens 
the communication between the receiver and the barrel ; confe- 
quently no more air Can in that cafe pafs from the former to 
the latter. 
To remove this principal imperfection of the bed; air-pumps 
had been attempted by feveral ingenious perfons ; but, as far 
as I know, was never obtained before the happy contrivance of 
the air-pump, which I am going to deferibe in the following 
pages. 
Being in want of a good air-pump, and imagining that the 
opening and fhutting the communication between the barrel 
and the receiver might, inftead of the valve, be performed bv 
means of a hop -cock, particularly conftruCted upon an idea of 
mine; I communicated my plan, about the latter end of 
the lad; year, to Mr. Jacob barnard haas, an ingenious 
workman in the philofophical-indrument way, who, in part- 
nerfhip with Mr. john henry kurter, had lately effablilhed 
a manufactory of philofophical indruments. Mr. haas re- 
marked, that according to my plan the friClion of the parts of 
* The degree of rarefaftion lhewed by what is called the pear-gage, when any 
Vapour of water is within the receiver, is not to be confldered as the degree of 
Varefa&ion of the elaflic fluid in the receiver, but only of the air; for though the 
air may be exhaufted, yet the vapour of water will fupply its place ; we fliall, 
therefore, only take notice of the exhauftion when no vapour or moiflure is within 
the receiver. See nairne’s Experiments, Phil. Trail f. vol, LXVII. 
I 
the 
