XVI INTRODUCTION. 



in which C = Correction for temperature. 



B = Observed height of the barometric cokimn. 



i = Temperature of the attached thermometer. 

 T= Standard temperature of the mercury. 

 m. = Coefficient of expansion of mercury. 



/ = Coefficient of Hnear expansion of brass. 



— 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 1° C.) = 10-9(181792-1- 0.175/ + 0.035116/2). 



As a sufficiently accurate approximation, the intermediate value 



m — 0.00018 18 



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 



/ = 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 10. Redziction of the barometer to standard temperattire — English 

 measures. 



For the English barometer the formula for reducing observing readings 

 to a standard temperature becomes 



C= - y^K^-32°)-/(/'-62°) 

 1 -\- m {t — 32°) 



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



/ = Temperature of attached thermometer in degrees Fahrenheit. 



m = 0.0001818 X I = o.oooioi 



/ = 0.00001S4 X ^ = 0.0000102 



The combined reduction of the mercury to the freezing point and of the 

 scale to 62° Fahrenheit brings the point of no correction to approximately 



