1852.] A Table for reducing Barometrical Observations. 329 



An account of the Table used for reducing Barometrical Observa- 

 tions to 32° Farenheity taken in the Surveyor General's Office, 

 Calcutta. By Babu Radhanath Sickdhar, chief Computer, Great 

 Trigonometrical Survey of India. Communicated by the Deputy 

 Surveyor General. 



The observed heights of a Barometer taken at different tempera- 

 tures, before they can be compared with each other, will require 

 reduction to one common temperature. The reduction consists of two 

 parts, one part being due to the dilation of the mercury, and the 

 other to that of the brass scale attached to the Barometer. Both 

 these corrections stand embodied in the following formula. 

 _ ( t — 32°) m — (t — 62°) b 

 C — B * 1 -f- (t — 32°) m 



C = Sum of the two corrections. 

 B = Observed height of the Barometer. 



f Observed temperature of the mercury, and of the 



I brass scale which are assumed to be equal, 

 m = .000100 Expansion of mercury for 1° of Faht. 

 b = .0000106 Expansion of brass for 1° of Faht. 

 32° = Standard temperature of mercury. 

 62° = Ditto ditto of brass. 

 The formula for C given above, is the same as that which Col. 

 Boileau makes use of in the computation of his Table XI. referring 

 his readers to p. 67 of Galbraith's Tables Edit. 1834, where he says 

 the formula will be found.* Col. Boileau has given no demonstration 

 of the process. Galbraith may have done so, but as the works of 

 the latter are not within my reach, I have been necessitated to satisfy 

 myself of the truth of the formula by the following investigation. 

 Now (B — C) is the observed Barometrical height reduced to 32°. 

 Taking this corrected height and multiplying it by the factor 

 (t — 32°) m, there will result the correction due to the expansion of 

 the mercury. This correction therefore is of the following form — 

 (B — C) (t — 32°) m, in which the corrected height of the Barometer 



* Vide p. IX. of Introduction to Boileau's Table 1849. 



