mn-comhcihig Power of a perfeci Vacuum. z^--] 



learned from him, that he has alfo afcertained the non-conc!u6ting 

 power of a perfedl 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 myfelfj fiiould,upoii 

 his teftimony alone, have been equally afllired of the facO:. To 

 mofl: of the preceding experiments Dr. Price, Mr. Lane, and 

 fome others of my friends, have been eye-witnelles, and I be- 

 lieve that they were as thoroughly fatisfied. as myfelf with the 

 refults of them. I muft beg leave to obferve to thofe who wiflT 

 to repeat them, that the firft experiment requires fome nicety, 

 and no inconliderable 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 ciftern ; for by 

 breaking off the tube at (M) within |- or | 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 ciftem, wiihuut obliging us to bring our finger, or any other 

 fubftance, into conta£l with the mercury in the gage,.which never fails to render the 

 inftrument imperfeft. It is necelTary to obferve,, that if the tube be fourteeii 

 or fifteen inches long, I have never been able to boil it effedually for the experi- 

 ments mentioned in this paper in lefs than three or four hours, although Mr. 

 Brook feems to prefcribe a m.uch fliorter time for the purpofe ; nor will it even 

 then fucceed, xinlefs 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 obferved, the inftrument can 

 never be made perfeft. There is, however, one evil which I have not yet been able to 

 remedy ; and that is, the introduftion of air into the gage, owing to the unboiled 

 mercury in the ciftern ; 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 

 ii^confequence the vacimm is rendered lefs and lefs perfect, till at laft the inftru- 

 ment is entirely fpoiled. I have juft conftrudled a gage fo as to be able to boil the, 

 mercury in the ciilern,. but have not yet afcertained its fuccefs,. 



and' 



