XXXli INTRODUCTION. 



in which C = Correction for temperature. 



B = Observed height of the barometric column. 

 / = Temperature of the attached thermometer. 

 T = Standard temperature of the mercury. 

 m = Coefficient of expansion of mercury. 

 / = Coefficient of linear expansion of brass. 

 6 = Standard temperature of the scale. 

 The accepted determination of the coefficient of expansion of mercury 

 is that given by Broch's reduction of Regnault's experiments, viz: 



m (for i° C.) = io" 9 (181792 + 0.175/ + 0.035116/ 2 ). 



As a sufficiently accurate approximation, the intermediate value 



m = 0.0001818 



has been adopted uniformly for all temperatures in conformity with the 

 usage of the International Meteorological Tables. 



Various specimens of brass scales made of alloys of different com- 

 position show differences in their coefficients of expansion amounting to 

 eight and sometimes ten per cent, of the total amount. The Smithsonian 

 Tables prepared by Prof. Guyot were computed with the average value 

 / (for i° C.) = 0.0000188; for the sake of uniformity with the International 

 Meteorological Tables, the value 



I = 0.0000184 



has been used in the present volume. For any individual scale, either value 

 may easily be in error by four per cent. 



A small portion of the tables has been independently computed, but the 

 larger part of the values have been copied from the International Meteoro- 

 logical Tables, one inaccuracy having been found and corrected. 



Table 46. Reduction of the barometer to standard temperature — English 

 measures. 



For the English barometer the formula for reducing observed readings 

 to a standard temperature becomes 



m (t - 32 ) -l(t- 62°) 

 L D i+m(t- 32 ) 



in which B = Observed height of the barometer in English inches. 



t = Temperature of attached thermometer in degrees Fahrenheit. 



m - 0.0001818 X - = o.oooioi 



y 



/ = 0.0000184 x - = 0.0000102 

 9 



