116 BENJ. PIKE'S, JR., DESCRIPTIVE CATALOGUE. 



Fig. 601. Fig . 603. 



Fig. G02. 



A 



Price of Fig. 601, $1.50. 

 " 602, $1.25. 

 " 603, $1.75. 



THE BAROMETER. 



The barometer is an instrument for measuring the weight 

 or pressure of the atmosphere, and sometimes called a 

 weather glass. It consists of a cylindrical glass tube, 

 whose diameter is about half of an inch, and its length 

 thirty -four inches, filled with prepared mercury ; one end 

 of the tube is hermetically sealed, and to the open end a 

 reservoir of hard wood is cemented, having a soft flexible 

 leather bag for its bottom, the whole containing the mercury, 

 but not quite full ; the atmosphere pressing on the surface 

 of the leather, and causing it to yield to the pressure of the 

 external air, sustains the mercury in the tube to the same 

 height as if exposed to the atmosphere. 



The tube and reservoir are fixed to a frame of wood, and 

 suspended in a vertical position, having a scale graduated 

 from twenty-seven to thirty one-inches, which is the distance 

 from the surface of the mercury in the reservoir ; the inches 

 are divided into tenths, and in the best instruments there is 

 a vernier moving in a groove in the plate, and subdividing 



