XVlll 



INTRODUCTION. 



Corrections to be applied to the tabtdar values of Table 1 1 iji order to use them 

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



Example : 



Observed height of barometer, 763. ly'""'-; 



attached thermometer, — 12° C. 

 Numerical value of the reduction for + 12° C 

 Correction for temperature below 0° C. 



Reduction for — 12° C 

 Observed height of barometer 



Barometer corrected for temperature 



Temperature of the 

 = 1.50 



= + O.OI 



= + 1.50 

 = 763-17 



= 764.68 



REDUCTION OF THE BAROMETER TO STANDARD GRAVITY 



AT LATITUDE 45° 



The atmospheric pressure is mea.sured by the weight of the mer- 

 curial column of the barometer, but by common usage the pressures are 

 expressed in terms of the height of the barometric column instead of by 

 its weight. The observed height however is not a true measure of the 

 pressure, becau.se it changes with the temperature of the mercury and 

 with the variations in the value of gravity. Therefore to obtain a height 

 that .shall be a true relativ-e measure of the atmospheric pressure, the ob- 

 served height of the mercurial column must be reduced to that which 

 would be measured at a standard temperature and under a uniform stand- 

 ard value of gravity. 



