* 
[ 422 j 
times, when the pump is put clean together ; But the 
moifture, that adheres to the inlide of the barrel, as 
well as other internal parts, upon letting in the air, 
is in the fame fucceeding trials worked together with 
the oil, which loon renders it fo clammy, as to ob- 
ftrudt the adtion of the pump upon a fluid fo fubtil 
as the air is, when fo much expanded ; but in this 
cafe it feldom fails to adt upon the air in the receiver, 
till it is expanded foo times : And this I have found 
it to do, after being frequently ufed for feveral months, 
without cleaning. I have alio generally found it to 
perform beft, the firft trial at each time of ufing j 
tho* nothing had been at it from the time preceding ; 
which, after a great many trials made with this 
view, I alfo attribute to the vapours of the air mix- 
ing with the oil. An experiment, where the air was 
expanded 1000 times, was tried about two years 
fince in your pretence ; at which were prefent alfo 
Dr. Knight and Mr. Canton ; and I lately did the 
fame thing with Mr. Watfon. The pump, which I 
intend myfelf the honour of fhewing the Society, is 
the fame, that I juft now mention’d, and the fecond 
that I made, with a view to improve upon this prin- 
ciple. 
The degree of rarefadtion, produced by the beft of 
the three pumps, that you procured the trial of, and 
which you efteemed good in their kind, and in com- 
plete order, never exceeded 140 times, when tried by 
the gage above delcribed. 
I have alfo endeavoured to render the pneumatic 
apparatus more Ample and commodious, by making 
this air-pump adt as a condenflng engine at pleafure, 
by fingly turning a cock. This not only enables us 
to 
