[ I04 ] 



liable to in the ufe of the floating guage, or any other contrivance 

 fubftituted in its room, that requires to be adjufted at every 

 feparate obfervation. 



REMARK III. 



On the Dimen/wns of the Injlrument. 



As fimplicity and portablenefs are the great advantages of 

 this inftrument, I would recommend the keeping the width 

 of the aperture of the tube as much under as polfibie, as well 

 to prevent accidental breaking from the weight of a thick column 

 of mercury, as alfo to leave the more room for the defcending 

 mercury in the ciftern. If the diameter of the tube is a full 

 tenth of an inch, and that of the ciftern i, 2, there will be 

 ample room to meafure a height of 3000 feet, which is enough 

 for any thing intended to be accomplifhed by this inftrument. 

 The ftiell of the glafs lliould be of ftrong and tough metal, and 

 the fealing well annealed, as there is nothing but the gradual 

 admiflion of the air through the cork to check the force of the 

 mercury againft the top of the barometer on inverting it. It 

 is alfo plain that the length of the tube ftiould be as little more 

 than that of the greateft ufual height of the mercury at the level 

 of the fea as poflible, becaufe when the barometer is ufed all 

 the fuperfluous mercury that runs down from above the height 

 will contract the neceffary fpace in the ciftern. 



REMARK 



