10 



cury in the tube be equal to ^ which, reduced to a 

 decimal, will be 



in. in. in. 



=0-025 for one inch., 0*013 for $ inch, 0-007 for in. 



in. 



Observed height. . . = 30-400 

 Neutral point =30-000 



400 



Difference above "I 

 neutral point / 

 Add for capacity + -010 



Correct height ... 30-410 



Observed height 29-500 



Neutral point 30-000 



Difference below neu- 



e below neu- 1 * ftn 

 tral point J 



Subtract for capacity '013 



Correct height 29-487 



The scale of the standard barometer used in fixed 

 observatories is made moveable, and terminates in an 

 ivory point, which is brought down to the surface of the 

 mercury : when this point and its reflection appear to 

 touch one another, the height indicated is correct. This 

 kind of barometer requires no adjustment or correction 

 for the cistern. 



The tubes of barometers vary in size : those of a large 

 diameter are preferable, as the motion of the fluid is 

 freer, and its friction against the sides of the tube is 

 nearly inappreciable ; tubes of small diameters require 

 correction for capillarity, or the depression of the mer- 

 cury caused by its adhesion to the sides of the tube. 



The range of the barometer, or the spaces passed 

 through by the mercury in its extreme depressions and 

 elevations, being limited to 3 inches, it is not usual to 

 graduate the scale from the lower end of the tube : the 

 divisions commence at 27 inches, and are continued to 

 31 inches. The graduations on Troughton's mountain 



