444 Mr. cavallo’s Defer ip t Ion cf 
To the fore- fide of the ball < 7 , on the ftrong piece of com- 
munication, is adapted by a crofs-piece and a ferew, the gage 
which (hews the exhauftion of the pump. This gage may be 
icon in fig. i. of plate II. It con fills of an outward glafs tube 
'Containing a little quickfilver ; and of an infide tube, which, 
like a barometer, is filled with, and inverted into, the quick- 
iilver of the large tube. The infide tube is fupported at top by 
.a Spring focke't. A finall ivory fcale, with divifions, encom- 
pafies the finall tube, and fwims upon the furface of the 
quickfilver in the large tube. By means of this fcale the ex- 
hauftion of the pump may be begun to be meafured, after that 
'the air is rarefied at leaft thirty times. 
To the other fide of the ball a , viz. oppofite to the gage, 
there is a ferew-nut, which, by means of a leather, fihuts a 
hole made in the laid ball, and ferves to open the communica- 
tion between the receiver and the atmofphere. This nut, be- 
fides a milled head, has a fquare filed on it, to which a key 
may be applied, in order to open the ferew more eafily, and by 
degrees without jerks, which can hardly be avoided when the 
nut is opened by one’s finger applied immediately to it. This 
nut is feen in plate I. 
1 fhall forbear from any more prolix description of other gages 
that may be adapted to this pump, as alfo of other parts iiecef- 
fary for performing experiments with it, thefe things being 
very well known at prefent, and containing nothing new. 
Now, as to the working of this pump, a bare infpedlion of 
fig. i. in plate III. will fihew, that by the adlion of the pifton, 
when moved up and down into the barrel, the air will be ex- 
baufted from the cavity of the tube RS, of the ball n, tube d, 
and of courfe from the glafs receiver that is put upon the plate 
ff; for when the pifton, after being let down, is drawn upwards, 
1 the 
