XXXIV 



INTRODUCTION. 



Corrections to be applied to the tabular values of Table 47 in order to use them 

 when the temperature of the attached thermometer is below o° Centigrade. ■ 



Example: 



Observed height of barometer, 763.i7 mm- : Temperature of the attached 



thermometer, — 12° C. 

 Numerical value of the reduction for + 12 C. 

 Correction for temperature below o° C. 

 Reduction for — 12 C. 

 Observed height of barometer 

 Barometer corrected for temperature = 764.68 



REDUCTION OF THE MERCURIAL BAROMETER TO STANDARD GRAVITY. 

 Tables 48,49, 50. 



The mercurial barometer does not directly measure the atmospheric 

 pressure. The latter is proportional to the weight of the mercurial column, 

 and also to its height after certain corrections have been applied. Since 

 the height of the barometric column is easily measured, by common con- 

 sent the pressures are expressed in terms of this corrected height. 



The observed height of the barometer changes with the temperature 

 of the mercury as already shown, and also with the variations in the value 

 of gravity, as well as with the pressure. Therefore, to obtain a height that 

 shall be a true relative measure of the atmospheric pressure, the observed 



