126 



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



Fig. 612. 



column of mercury which 

 will balance the weight of 

 atmosphere ; so that any 

 change in the pressure of 

 the atmosphere, will have 

 an equal effect on the mer- 

 cury at the closed end and 

 at the open end ; and thus, 

 through whatever space the 

 mercury may rise in the 

 closed end, it will be de- 

 pressed to the same extent 

 in the open end ; the tubes 

 at these parts being of pre- 

 cisely the same diameter, 

 while between them the 

 tube is contracted. Upon 

 the surface of the mercury 

 in the open end floats a glass 

 weight, suspended by a silk 

 thread over and around a 

 brass pulley, to the other 

 end of which is attached 

 another weight, not quite as 

 heavy, acting as a counter- 

 poise to the former. The axis 

 of the pulley passes through 

 the frame and centre of the 

 dial plate, and carries a 

 blued steel hand, which re- 

 volves as the pulley turns 

 round. The weight on the 

 surf-.ice of the mercury being 

 nearly supported by the 

 counterpoise, rises or falls 

 freely as the surface of the 

 mercury on which it floats 

 is elevated or depressed by 

 the weight of the air. If 

 the circumference of the 

 wheel be one inch, then one 

 entire revolution of the 

 wheel will correspond to an 



