607 



TABLE 644.— CONSTANT a FOR 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 the relative expansion of the 

 mercury and the metallic enclosing 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 temperature. 

 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 Meteoroloeical Tables. The numbers tabulated under a 

 are the values of a in the equation Hi = Hi' — a {f — t) where Ht is the height at the standard tem- 

 perature, Hi' the observed height at the temperature t', and a (f — t) the correction for temperature. 

 The standard temperature is 0°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. 



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

 reading at 0°C. Here the value of a is the mean of .1235 and 1251, or .1243; .'. a (f - t) = .1243 X 

 25 = 3.11. Hence H — 765 - 3.11 = 761.89. 



Note.— Although a 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 a, and when great accuracy is wanted the proper coefficients have to be determined by 

 experiment. 



SMITHSONIAN PHYSICAL TABLES 



