C 95 ] 
POSTSCRIPT. 
I cannot help exprefting my furprize, that fo clear 
and intelligible an account, of Mr. smeaton’s 
air-pump, fhould have been before the public fo 
long, as ever ftnce the publication of the forty- 
feventh volume of the Philofophical Tranfac- 
tions, and yet that none of our philofophical in- 
ftrument makers fhould attempt the conftrudtion. 
The fuperiority of this pump, to any that are made 
upon the common plan, is, indeed, prodigious. 
Pew of them will rarefy more than ioo times, 
and, in a general way, not more than 60 or 70 
times; whereas this inftrument muff be in a poor 
Hate indeed, if it do not rarefy 200 or 300 times ; 
and when it is in good order, it will go as far as 
1000 times, and fometimes even much farther 
than that ; befides, this inftrument is worked with 
much more eafe, than a common air-pump, and 
either exhaufts or condenfes at pleafure. In fhort, 
to a perfon engaged in philofophical purfuits, this 
inftiument is an invaluable acquifition. I (hall 
have occafion to recite fome experiments, which 
I could not have made, and which, indeed, I fhould 
hardly have dared to attempt, if I had not been 
poffeffed of fuch an air-pump as this. It is much 
to be wiftied, that l'ome perfon of fpirit in the 
trade would attempt the conftrudtion of an in- 
ftrument, which would do great credit to himfelf, 
as well as be of eminent fervice to philofophy* 
IX. Farther 
