502 KEPORT— 1891. 



7. On the Barometric Determination of Altitudes. 



By E. DoBsoN, M.Inst. C.E. 



The object of this paper is to explain a method of obtaining 

 altitudes from barometric observations to a close degree of 

 accuracy by reference to a couple of bench-marks of which the 

 height above the sea has been determined by the spirit-level, 

 without the necessity of making any correction for tempera- 

 ture, wind, or moisture. 



The tables to be used have been calculated by the formula, 



X = 60235 X — 

 b' 



(in which 



X = altitude in feet above the sea of the upper station ; 

 30 == height of the mercurial column in inches at the sea-level ; 



h' — barometer reading in inches at the upper station ; 

 the mean temperature of the atmosphere being assumed as 

 32° Fahr.), 



and then corrected for temperature to show the heights cor- 

 responding to a temperature of 32° Fahr. at the sea-level, but 

 decreasing at the rate of 1° for every 300ft. of elevation. 



So long as the deviations of the synchronous barometer 

 readings from the normal readings in the tables decrease in au 

 ascending scale at the rate of g\j for every inch of barometric 

 pressm-e, the altitudes will be correctly given by the following 

 equation : — 



n_ (B-A) X [c-a) . 

 (h-a) ^ ' 



in which 



a — tabular height corresp. to barom. reading at lower B.M. 



h = „ „ „ upper „ 



c = „ „ „ intermediate 



station. 



A = height above sea-level of the lower B.M. 



B = „ „ upper „ 



C = „ „ intermediate station. 



If the differences from the normal barometer readings vary 

 in any other ratio than the above, a correction must be 

 applied, the mode of calculating which is shown in the 

 paper, which also contains tables for the altitudes correspond- 

 ing to every hundredth of an inch of normal barometer pres- 

 sure from 30in. to 26in., with a temperature of 32° Fahr. at 

 the sea-level. 



