330 A Table for reducing Barometrical Observations. [No. 4. 



enters as a factor, because it is that which expanded produces the 

 observed mercurial column B. 



In like manner the correction for the dilation of the brass scale 

 will be found to be of the form -f- B. (t — 62°) b, which is additive, 

 because the mercurial column measured by the expanded scale being 

 B, it would be more, or B + B. (t — 62°) b if the scale remained 

 unaltered by temperature. 



Combining the corrections for mercury and brass according to their 

 signs, there will arise the following equation. 



_ C = — (B — C) (t — 32°) m -f B (t — 62°) b 

 which solved in the usual manner will give 



C==B ( fc — • 32 °) m — (t — 62°) b 



1 + (t — 32°) m 

 which formula has accordingly been made use of in the computation 

 of the following Table. 



The arrangement and use of the Table will be best understood from 

 the following example. 



Suppose it is required to compute the correction for Barometer 

 29780 inches and Thermometer 83°. 3. 



The Tabular number for 29.8 Fahrenheit, .145 



Alteration for 0°.3 Fahrenheit deduced by the common 1 ftft 

 rule of proportion, J 



■r — 



Kequired correction, .146 



Observed height of the Barometer, 29*780 



Height reduced to 32° Fahrenheit, 29*634 



It will be remembered that the Tabular correction is always nega- 

 tive. 



