. ** improved Air-Pump, kc. u ~ 
the under part of the barrel remains without an y air; confe- 
quently the valve at the bottom of the barrel, having no pref- 
lure on one fide, will be pi, Hied up by the air in the receiver, 
which expanding comes through the tube RS, and part of it 
pafles into the barrel. Then the pifton on being let down, the 
air pafles through the valve of the pifton, to the upper part of 
the barrel, and when afterwards the pifton is drawn up, this air is 
forced through the valve at E into the tube Dr, from thence 
into the veflel q s, through the channel/;-/,. and, laftly, it will 
be expelled into the atmofphere through the aperture k. 
As fome fmall quantity of oil is always neceflary to be put 
into the pump, this oil, by the affion of the pifton, is brought, 
together with the air, towards the tube Dr, and. would come 
out of the hole k it the veflel qs had not been placed to re- 
ceive it ; and it is for this reafon, that the lower part of the 
piece pn is fhaped as fhewn in the figure, and that the tube Dr 
is made to proceed almoft as far as the top of the veflel q s ; for 
it the oil was permitted to come out of the aperture k it 
would be fcattered about the inftrument and the operator, by 
the violence of the air coming out of l\ 
As this pump exhaufts exaftly in the fame manner as other 
pumps do, the lever which opens the valve at the bottom of 
the barrel is not to be moved, except when fuch a degree of 
exhauftion is requned as cannot be made bv the inftrument 
itfelf, viz., when worked in the ordinary way. In faff, it will 
be feen by the gage, that when the mercury cannot fall any 
lower by the ufual way of working the pump, it will be in- 
ftantly deprefled by opening the valve at the bottom of the 
barrel, which evidently fhews the great advantage of the im- 
provement. In general, the lever may be begun to be prefl'ed, 
or, which is the fame thing, the valve to be opened, when the 
