a new-conflirutded Air- Pump. $13 
piftons every time the pump is ufed ;. the piftons being always 
kept moiftened with oil in the cifterns, where they ftand when 
-the pump is not in ufe; and in part, to the power which the 
 piftons have over thefe valves, by condenfing theair againft them, 
In the common pump, and in Mr. Smeaton’s, the valves, at the 
. bottom of the barrels, can only be opened by the fpring of the 
„air acting againft them: but in this pump the valves are forced 
open, by raifing the piftons, and muft, —— Lom much 
longer to the power applied in this way. 
..,,I mentioned. above, that the piftons in this pump did not 
move the whole length of the barrels ; but were interrupted by 
the plate, a little more than half way from the bottom, for còn- 
venience: but on this conftruction, they may be made to move 
through the. whole. length, as. in Mr. Smeaton’s pump ; arid 
then it will exhauft a.receiver in half the time that his will, if 
the .capacity of cach barrel inthe two pumps be equal. And 
perhaps the air may.be further rarefied by a pump on this con- 
Atruction without,the, valves, whofe barrels are of greater length 
than the barrels nof. my pump. For ince the: pifton may be 
„made to. fit as well;to the-top of one barrel as another, if the 
length of. the barrel, through which the pifton moves, be'twelve 
inches inftead of fix, the vacancy, which is unavoidably Teft be- 
tween the, top-plate and the :pifton, when the latter is drawn up 
tothe former, will beatiadefs proportion to the capacity of the 
whale barrel. Suppofe, then, :the valve.en the ‘top-plate will 
rife only tili the air be expanded onc hundred times in a barrel 
of. fix. inches. length; becaufe this is the proportion which the 
“yaeancy. bears to.the. capacity of the whole barrel, (the refiftance 
of the yalve not being taken into theaccount) it will rife till the 
sir is, expanded, two. hundred times.in a barrel of twelve inchés 
Rrr — length, 
