130 THE ATMOSPHERE AND ITS SERVICE TO MAN 



ANEROID BAROMETER 



until the top of the mercury in the cistern just touches the 

 bottom of the ivory peg. The height of the top of the 



mercury column is then 

 read from the scale. In 

 order to determine the 

 height with great preci- 

 sion there is generally 

 attached to the metallic 

 tube a sliding vernier 

 which moves in a slit. 



The aneroid barometer 

 consists in general of a 

 corrugated metallic box 

 from which the air has 

 been partially exhausted. 

 Within the box is a stiff spring so that the pressure of the 

 air will not cause it to collapse. Attached to the box are 

 levers by which any 

 change in the volume of 

 the box will be multi- 

 plied and indicated by 

 a pointer arranged to 

 move over a dial with a 

 scale upon it. 



Instruments called 

 barographs are con- 

 structed in which a long 

 lever provided with a 

 pen point is attached to 

 the aneroid and made to 



record on a cylinder revolved by clockwork. Thus a con- 

 tinual record is made of barometric readings. 



BAROGRAPH 



This is arranged so as to record the air 

 pressure automatically for a week at a 

 time. 



