2IO n ESC RI PTI N of .a 



Fig. 4. ABCD is a glafs tube, bent in the manner repre- 

 fented, open at D, and hermetically fealed at A. From A to 

 B is 8 inches long, and about | of an inch cahber. From 

 B to C 317 inches long, and about ^ of an inch caliber. And 

 from C to D 44- inches long, and 7 inch caliber. 



The tube is filled with mercury, the length from B to E be- 

 ing 29T inches. When the tube is hung perpendicular, the 

 mercury will fall from B to E, leaving a vacuum in the upper 

 half of the tube from B to A. When the atmofphere becomes 

 heavier, the mercury falls in the tube DC, and when lighter it 

 rifes. The range of the fcale is about 3 inches, being equal to 

 that of a common barometer of the beft conftrudlion, which has 

 a bafon with a very broad furface. This inftrument moves in 

 a dire<5lion contrary to the common barometer, the one riling 

 while the other falls. 



Fig. 5. reprefents the tube DC, with the fcale placed above it, 

 of half the real dimenlions. F is a piece of ivory or glafs, of a co* 

 nical Ihape, of a proper weight, made to float on the furface of 

 the mercury, having a wire fixed to it reaching to G. From H to 

 H is a piece of fmall harpfichord-wire, or rather gold-wire, 

 flretched along the ivory or brafs plate on which the fcale is en- 

 graved. II are two indexes formed of the thinneft black oiled 

 iilk, pierced in fuch a manner by the fmall wire as to move up- 

 wards and downwards upon it with a very fmall force, not 

 more than two grain weight ; and thefe indexes, being not the 

 weight of half a grain, they do not defcend the wire by their 

 own weight, but remain where they are placed. 



The wire fixed to the float, (which we fhall call the float-wire), 

 has a knee bent at a right angle, and made to encompafs the 

 fmall wire between the two indexes, fo that, when the float rifes, 

 the upper index is carried up, and, when it falls, it leaves the 

 upper index, and puilies down the under index. 



In 



