Mr. cavallo’s Defer iption of 
gnge (hews that the exhauftion of the air is to about ioo times* 
viz. that the quantity of the air remaining within the receiver* 
Is about the one-hundredth part of the air that was contained 
in it before the exhauftion was commenced. Care muff be 
taken to open the valve only whilft the pifton is drawing up, 
and to remove the foot from the lever the moment that the 
pifton is impelled downwards, otherwife the work is ufelefs. 
In the fituation in which tlife pump is (hewn by fig. i. pi. III. 
it is evident, that as the adtion of the pump determines the air to 
move from the tube RS into the barrel and from the barrel up 
the tube Dr, &c. ; it is evident, I fay, that if a receiver is placed 
upon the plate jj\ the air will be exhaufted from it. But if the 
ftop-cocks Z and h are fo turned as that the fide hole of the 
cock >6, as well as that of the cock Z, is turned towards X, 
then, by the action of the pump, the air, inftead of being 
exhaufted, is condenfed into the receiver properly placed upon 
the plate ff ; for now the air coming from the atmofphere 
through the aperture X goes down the cavity of the tube RS, 
enters the barrel, and from the barrel is impelled upwards 
through Dr, through pn, through a , b, d , and laftly into the 
receiver, which in that cafe muft be prefled down, as is ufually 
done in condenfing engines. This fltuation of the ftop-cocks, 
viz. when the inftrument is to be ufed as a condenfer, is (hewn 
in fig. 2 . of plate III. Two letters, E and C, marked upon 
the fquare top of each cock, diredt the operator how to turn 
them, whether for exhau fling or for condenfing. 
The conical holes X and k, fig. i. plate III. are made to re- 
ceive occafionally the extremities of two ftop-cocks. Thofe 
ftop-cocks are fitted to the holes H and k by grinding, and a 
bladder is adapted to each of them. The ufe of this contri- 
vance is to introduce into the glafs receiver fome particular fort 
of 
