XXXVni INTRODUCTION. 



Example : 



Observed heights of the mercury in the manometer tubes (in.), +6.258 



and -4-375- 

 Difference in height of the two cohimns 10.633 



Attached thermometer, 72 °4 F. 



Correction for temperature — .042 



Manometer reading corrected for temperature 10.591 



For temperatures above 28? 5 Fahrenheit, the correction is subtractive, 

 and for temperatures below 28^5 Fahrenheit, the correction is additive, as 

 indicated by the signs ( + ) and ( — ) inserted throughout the table. 

 Table 47, Reduction of the mercurial column in U-shaped manometers with 

 brass scales to standard temperature. Metric measures. 



This table is an extension of Table 45 to the small differences in height 

 of the mercurial columns as determined with a U-shaped manometer. The 

 values have been obtained from the corrections given in that table by the same 

 process as those given in Table 46 were obtained from Table 44. 



Example : 



Observed heights of the mercury in the manometer tubes (mm.), + 12 1.5 



and —86.7. 

 Dift'erence in height of the two columns 208.2 



Attached thermometer, i8?4 C 

 Correction for temperature — 0.6 



Manometer reading corrected for temperature 207.6 



For temperatures above 0° C. the correction is negative, and hence is to 

 be subtracted from the observed readings. For negative temperatures see the 

 explanation of Table 45. 



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 consent 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 height 

 of the mercurial column must not only be reduced to what its height would 

 be if at a standard temperature, but also to what it would be at a standard 

 value of gravity. 



