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fed vacuum could be made in the rec' and gage-vefTel, It 

 would fink more or lefs bentath the level of the furrounding 

 mere', as the tube is narrower or wider, even fo as to dif- 

 appear, if the tube were very narrow ; fo that it mufl not be 

 lefs than rV^hs of an inch in diameter ; but it would be better 

 if it were tV^^s. If it were obferved how much the mere'', 

 in a tube open at both ends, would fink beneath the .level of 

 that in a ciftern, in the open air, it might be pretty nearly efti- 

 mated how great is the rarefadlion in this gage veflel and con- 

 fequently in the rec'. 



This reduced barometer is the ordinary gage I ufed, by 

 which a perfon accuftomed to obferve it may know what is 

 nearly the rarefadlion when it does not exceed two or three 

 thoufand times ; for which purpofe there fhould be marks 

 made with a file on the outfide of the tube, dividing it into 

 inches and decimal parts : if the common whole barom' gage, 

 and this reduced one, were applied to the pump together, the 

 altitude of the mere'' in the latter would be equal to the dif- 

 ference between that of the former and a ftandard barom'. 

 When the mere", having funk to the bottom of this fliort tube, 

 rifes to the top again on admiflion of air, all the invifible air 

 bubbles which were in it, will afcend and form one air bubble 

 at the top, which will be the lefs, and the gage the more exadl, 

 as the tube had been more perfedlly filled, and this will alfo 

 fhew how far the ordinary Torricellian vacuum is from being 



' fo 



