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 the relative expansion of the mercury an<} the 

 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 temperature. The above tables of values of 



lemperaiuie ui tpfjiuAiiii<ttciy ma .5 r., DCGBUVC ifi me idci 

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



EXAMPLE. A barometer having a brass scale gave t/ = 76$ mm. at 25 C. ; required, the corresponding 

 reading at o C. Here the value of a is the mean of .1235 ar >d .1251, or .1243; .'. a (t 1 t) = .1243 X 25 = 3.11. 

 Hence // = 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 a, and 

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



SMITHSONIAN TABLES. 



1 2O 



