non~condu 5 llng Power of a perfect Vacuum. 2*7 
learned from him, that he has alfo afcertained the non-cond lifting 
power of a perfeft vacuum ; but what fteps he took for that pur- 
pofe I know not. Of his accuracy, however, I am fo well con- 
vinced, that hadlnever made an experiment myfelf,I 1 ho 11 Id, upon 
his teftimony alone, have been equally affured of the faft. To 
moft of the preceding experiments Dr. Price, Mr. I^ane, and 
fome others of my triends, have been eye-witneffes, and I be- 
lieve that they were as thoroughly latisfied as, myfelf with the 
refults of them. I mult beg leave toobferveto thofe who wifh 
to repeat them, that the firfi experiment requires fome nicety* 
and no inconfiderable degree of labour and patience. I have 
boiled many gages for feveral hours together without fuccefs, 
fo as to be fufficiently long to take hold of, when the mercury is boiling. The 
bulb (K) is defigned as a receptacle for the mercury, to prevent its boiling over, 
and the bent figure of the tube is adapted for its inverfion into the cittern ; for by 
breaking off the tube at (M) within for f of an inch of the angle, the open 
end of the gage may be held perpendicular to the horizon when it is dipped into 
the mercury in the cittern, without obliging us to bring our finger, or any other 
fubftance , into contaft with the mercury in the gage, which never fails to render the 
inttrument imperfeft. ‘It is neceflary to obferve, that if the tube be fourteen 
or fifteen inches long, I have never been able to boil it effeftually for the experfo 
ments mentioned in this paper in Jefs than three or four hours, although Mr* 
Brook feems to prefcribe a much ftiorter time for the purpofe ; nor will it even 
then fucceed, unlefs the greateft attention be paid that no bubbles of air lurk 
behind, which to my own mortification I have frequently found to have been the 
cafe ; but experience has at length taught me to guard pretty well againft this 
difappointment, particularly by taking care that the tube be completely dry before 
the mercury is put into it ; for if this caution be not observed, the inttrument can 
never be made perfeft. There is, however, one evil which I have not yet been able to 
remedy; and that Is, the introduction of air into the gage, owing to the unboiled 
mercury in the cittern ; for when the gage has been a few times exhaufted, the mer- 
cury which originally filled it becomes mixed with that into which it is inverted, and 
m confequence the vacuum is rendered ids and lefs perfeft, till at latt the inftru- 
ment is entirely fpoiled. I -have juft conftrufted a gage fo as to be able to boil the 
merciuj/ in the cittern, but haye net yet afcertained its fuccefs 0 
