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Blount, in the manner faid to be moft approved of by the 

 Royal Society. In this inftrument the box has a leathern bag 

 or moveable bottom, which being fcrewed up raifes the mercury 

 till it fills both the box and the tube ; then the hole made 

 for admitting the floating gage is Hopped and the inftrument 

 becomes portable. Thefe contrivances for keeping the box 

 and tube full of mercury feem to have been thought neceffary 

 from a miftaken notion, that if air was included in the box 

 its elafticity would (when the infirument was fuddenly inclined) 

 force the mercury againft the top of the tube fo violently as to 

 break it, which has often happened in an open barometer ; but 

 this is not the cafe, for I have feen your clofe barometer 

 fuddenly inclined, and the included air did not make the 

 mercury flrike the top of the tube with any violence. I am 

 therefore of opinion that your barometer, if the box was made 

 to be occafionally opened or fhut, would have the moft fimple 

 and convenient form, and would be lefs liable than any 

 other to be put out of order, or to require readjuftment or 

 repairs, as I am told Dodtor Ufsher's barometer, now in the 

 obfervatory, frequently does. The true altitude of the mercury, 

 in a barometer, is the diftance between the furface of the 

 mercury in the tube and in the box ; when therefore the furface 

 in the box is fo large that it will not rife or fall fenfibly, as the 

 mercury falls or rifes in the tube, the common fcale, if rightly 

 adjufted at firft to the height of the mercury, will continue to 

 point out its true height afterwards. This is the cafe in fixed 

 barometers, which have ufually very large velTels to hold the 



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