514 — Mr. PRince’s Account of 
length, the! diameters being the fame in both, becaufe the capá- 
_city of the barrel being doubled, thé vacancy bears fo mach tefe 
proportion to-it than to one of fix inches. And ifthe arcan | 
be rarefied in proportion to the difference between the vacancy 
sand the capacity of the barrel, by leffening ‘this proportion, 
which, after having made the work to fit as well as poffible, is 
! to be done by enlarging the capacity of the barrel, the enor of 
: the pamp muft be increafed. 
This, Sir, is reafoning from snide but thefe circumftances, 
-Ithink, ought to be confidered in the conftruétion of an air- 
pump ; and experiment only muft determine how far i an atten- 
tion-to them may be ufeful. 
_ The rarefaction which a pump will produce, by enpe, 
may. come very far fhort of what it ought to do by the. theory 
~of its conftru&ion. If the common pump will, in experiment, 
rarefy the air only one hundred times, when in its beft ftate, and 
Mr. Smeaton’s, by conftru&ion, in duplicate proportion to this, 
at ought to-go to ten thoufand ; every thing being fuppofed per- : 
fet : but in its beft ftate, Mr. Smeaton’s pump will only rarefy 
the air about one thoufand times; fo that the nine-tenths 
which it falls fhort of what it ought to do by theory, is to be: 
attributed either to the imperfection of the machine alone; or to: 
the nature of the air, in not permitting the rarefaction to go fur- 
ther than one thoufand times, or both thefe caufes together. 
The way to prove how far this is owing to the air itfelf, is by 
making a machine, which, in theory, will carry the rarefaction: 
further. A pump conftructed without the valves, as mine is, 
‘ought to. rarefy the air in duplicate proportion of what Mr. 
 Smeaton’s fhould do by theory, and in quadruplicate proportion 
of the common pump, which would be one hundred million, 
allowing 
