._,.. TABLE 99. 117 



REDUCTION OF BAROMETRIC HEICHT 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 inclosing case, usually of brass, in the case of instruments graduated 



numbers tabulated under a are the values of a in the equation Ht = Hf a (t f /) where Iff is the 

 height at the standard temperature, Hf the observed height at the temperature f, and a (/' t) the 

 correction for temperature. The standard temperature is o C. for the metric system and 28.s 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 cor- 

 responding reading at o C. Here the value of a is the mean of .1235 and .1251, or. 1243; . . a (t't) 

 .1243 X 25 = 3.11. Hence HO = 765 3.11 = 761.89. 



N. B. 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 deter- 

 mined by experiment. 



SMITHSONIAN TABLES. 



