287 



TABLE XX. 



THIS table has been calculated by using the same coefficients of dilatation as in the 

 preceding table x viz. : 



Brass, linear dilatation, from Laplace and Lavoisier for 100 C.= 0.0018782. 



Mercury, dilatation in volume, from Dulong and Petit for 100C.== 0.0180180. 



Dilatation of the mercurial column for 100 C. . . . = 0.0161398. 



Dilatation of the mercurial column for 1C. . . = 0.0001614. 



This table, calculated for the reduction of long series of meteorological observa- 

 tions, gives immediately the value of the correction for each tenth of a degree up to 

 35 C. above, and down to 35 C. below, the freezing point, and for mercurial columns 

 extending from 605 to 800 millimetres. 



Examples of Calculation. 



mm. 



Barometer, observed height, ..... 754.17 



Temperature of the attached thermometer, -f-17.8. 



For finding the correction, seek in the horizontal column, headed /barometer, at the 

 head of the pages, the corresponding height of the barometer; it will be found, p. 31, 

 barometer 755 mm - (from 752.50 to 757.50) ; next seek in the first vertical column, 

 containing the temperatures, 17, follow then horizontally this line as far as the col- 

 umn of 8 tenths, and you find there 2.17 millimetres, which is the correction, or the 

 quantity to be subtracted for reducing the observed height to zero. We have thus : 



mm. 



Observed height, 754.17 



Subtractive correction for +17.8 = '. . . 2.17 



Barometer at zero, 752.00 



If the temperature is below zero, the correction will be additive. 



mm. 



Observed height, 729.72 



Temperature of the attached thermometer, 8.4. 

 Additive correction, ...... -(-0.99 



Barometer at zero, 730.71 



