Table 133. 



REDUCTION OF BAROMETRIC HEIGHT TO STANDARD TEMPERATURE.' 



* The height of the barometer is affected by the relative thermal expansion of the mercury and the glass, 

 in the case of instruments graduated on the glass tube, and by tlie relative expansion of the mercury and tlie 

 metallic inclosing case, usually of brass, in the case of instruments graduated on the brass case. This relative 

 expansion is practically proportional to the first power of the temjierature. The above tables of values of 

 the coefficient of relative expansion will be found to give corrections almost identical with those given in the 

 International Meteorological Tables. The numbers tabulated under a are the values of a in the equation 

 //( = //f' — a (f — t) where Ht is the height at the stand.nrd temperature, /A' the observed height at the tem- 

 perature /', and aJ the correction for temperature. The standard temperature is o ' C. for the metric system, and 

 28^.5 F. for the English system. The English barometer is correct for the temperature of melting ice at a 

 temperature of approximately 28^.5 F., because of the fact that the brass scale is graduated so as to be standard 

 at 62 ' F., while mercury has the standard density at 32° F. 



ExAMPLF. — A barometer having a brass scale gave //=:765 mm. at 25° C. ; required, the corresponding 

 reading at 0° C. Here the value of o is the mean of .1235 and . 1251, or .1243 ; .'. a (/' — t) =^ .1243 ^ 25 ^ 3"- 

 Hence //o= 765 — 3.11 =: 761.89. 



N. B. — Although o is here given to three and sometimes to four significant figures, it is seldom worth while 

 to use more than the nearest two-figure number. In fact, all barometers have not the same values for o, and 

 when great accuracy is wanted the proper coefficients have to be determined by experiment. 



Smithsonian Tables. 



120 



