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FIGURE I. 



A B reprefents a fedion of the barometer longitudinally, 

 when put together and ready for ufe. F the ivory cylinder. 

 C D the fcale, with a vernier that Aides fo as to cover the 

 operture when the inftrument is put by. E the attached ther- 

 mometer in its cafe, G G the brafs caps that fecures the ends. 



FIGURE II. 



A B reprefents a fedion of the ivory cylinder with its cork 



C, and its tube T S the furface of the mercury. M the mafs 

 of mercury, E E the flioulders that keep the cork C in its 

 place ; and F F its bottom that fcrews on tight. 



To ufe the inftrument, you have nothing to do, but taking 

 it lightly between the finger and thumb of the right hand near 



D, fig. I. gently turn up the point A, and looking through 

 oppofite to the light (for the outer cafe is to be cut to give this 

 advantage, and alfo for the purpofe of a fcale and vernier 

 divifion on each fide) you will obferve the mercury to fink 

 gradually and gently to its due ftation ; and with your left hand 

 following the fubfiding furface of the mercury with the bottoms 

 of the vernier Aide (which arc to be made exadly fquare 

 and of the fame length) determine their contad with the top of 

 the quickfilver when fettled, and finally read off" the obfervation, 



N 2 on 



