DETERMINATION OF HEIGHT 



121 



distance from the bottom of a scale. The scale is placed 

 beside a slit near the top of a metallic tube which is firmly 

 fastened to the cistern 

 and surrounds and pro- 

 tects the glass tube. 



When it is desired to 

 read the barometer, the 

 sliding bottom of the 

 cistern is raised or low- 

 ered 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 



Fig. 60. 



order to determine the height with great precision there 

 is generally attached to the metallic tube a sliding vernier 



which moves in the slit. 

 The aneroid barome- 

 ter consists in general 

 of a corrugated metallic 

 box from which the air 

 has been partially - ex- 

 hausted. 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 multiplied and indicated by a 

 pointer arranged to move over a dial. The dial has a 

 scale upon it and thus the air pressure is registered. 



BAROGRAPH. 



This is arranged so as to record the 

 air pressure automatically for a week 

 at a time. 



