[+i7] 
proportion of the remainder would be taken away, 
was it not afteCted by the impediments I have men- 
tioned : fo that, when the fpring of the air in the 
receiver becomes fo weak, as not to be able to over- 
come the cohefion of the bladder to the plate, occa- 
fioned by the fluid between them, the weight of the 
bladder, and the refiftance that it makes by being 
flretch’d, the rarefaction cannot be carried farther, 
tho’ the pump fhould ftill continue to be worked. 
It is evident, that the larger the * hole is, over 
which the bladder is laid, a proportionably greater 
force is exerted upon it by the included air, in order 
to lift it up ; but the aperture of the hole cannot 
be made very large, becaufe the preflure of the in- 
cumbent air would either burfl: the valve, or fo far 
force it down into the cavity, as to prevent its lying 
flat and clofe upon the plate, which is abfolutely 
neceflary. 
To avoid thefe inconveniences as much as pofiible, 
inftead of one hole, I have made ufe of feven, all of 
equal fize and flhape ; one being in the centre, and 
the other fix round it : fo that the valve is fupported 
at proper diflances, by a kind of grating, made by 
the folid parts between thefe holes : And to render 
the points of contaCl, between the bladder and grating, 
as few as poflible, the holes are made hexagonal, and 
the partitions filed almofl: to an edge. As the whole 
preflure of the atmofphere can never be exerted upon 
* If we examine the force, that air rarefied 140 times can exert 
in a common valve through a hole of one tenth of an inch diameter, 
we fhall find it not to exceed 6 grains at a medium. 
G gg 
