[ 4i9 ] 
And tho r the contrivance I made ufe of feemed to me 
lefs liable to objection than any thing I was acquainted 
with,, that had been defigned for that purpofe ; yet I 
did not find it to anfwer the end better than what I 
have already deferibed ; and therefore laid it afide, as 
it render’d the machinery much more complex, and 
troublefome to execute. 
But fuppofing all thofe difficulties to be abfolutely 
overcome, the other defedt, that I mentioned in the 
common conftrudtion, would hinder the rarefaction 
from being carried on beyond a certain degree. For,, 
as the pifton cannot be made to fit fo clofe to the 
bottom of the barrel, as totally to exclude all the air; 
as the pifton rifes, this air will expand itfelf ; but ftill 
preffing upon the valve, according to its denfity,. hin- 
ders the air within the receiver from coming out : 
Hence, were this vacancy to equal the 1 50th part of 
the capacity of the whole barrel, no air could ever 
pafs out of the receiver, when expanded 150 times, 
tho’ the pifton was conftantly drawn to the top ; 
becaufe the air in the receiver would be in aqui- 
librio with that in the barrel, when in its moft ex- 
panded ftate. This I have endeavoured to overcome, 
by fhutting up the top of the barrel with a plate, 
having in the middle a collar of leathers, through 
which the cylindrical rod works, that carries the 
pifton. By this means, the external air is prevented 
from preffing upon the pifton ; but that the air, that 
pafles thro’ the valve of the pifton from below, may 
be difeharged cut of the barrel, there is alfo a valve 
applied to the plate at the top, that opens upwards. 
The confequence of this conftrudtion is, that when 
the pifton is put down to the bottom of the cylinder, 
the air in the lodgment under the pifton will evacuate 
Ggg2 itfelf 
