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itfelf fo much the more, as the valve of the pifton 
opens more eafily, when prefled by the rarefied air 
above it, than when prefled by the whole weight of 
the atmofphere. Hence, as the pifton may be made 
to fit as nearly to the top of the cylinder, as it can 
to the bottom, the air may be rarefied as much above 
the pifton, as it could before have been in the receiver. 
It follows therefore, that the air may now be rarefied 
in the receiver, in duplicate proportion of what it 
could be upon the common principle ; every thing 
elfe being fuppofed perfect. 
Another advantage of this conftru&ion is, that tho’ 
the pump is compofed of a Angle barrel*, yet the pref- 
fure of the outward air being taken off by the upper 
plate, the pifton is worked with more eafe § than the 
common pumps with two barrels : And not only 
fo, but when a confiderable degree of rarefaction is 
defired, it will do it quicker ; for the terms of the 
feries exprelling the quantity of air taken away at 
each ftroke do not diminifh fo faft, as the feries an- 
fwering to the common one. 
I have found the gages, that have been hitherto 
made ufe of, for meafuring the expanfion of the air, 
very unfit to determine in an experiment of fo much 
nicety. 
* It is obvious that thcfe improvements will equally obtain, whe- 
ther the pump is conftrufted with a Tingle or a double barrel. 
§ Becaufe, tho’ the preflure of a column of air, equal to the di- 
ameter of the pifton-rod, ftill prefles upon it, yet, as there is only 
the friction of one pifton, and that not loaded with the weight of 
the atmofphere; the fridtion of the leather againft the fide of the 
barrel, and that of the rack and wheel, is much lefs : fo that, 
notwithftanding the addition of friction in the collar of leathers, 
that of the whole will be lefs. 
3 
