XIV INTRODUCTION. 



Example, Table 8 : 



Observed temperature at an elevation of 500 metres, i2.°5 C. 



Reduction to sea level for an assumed decrease in tempera- 

 ture of 1° C for ever}^ 200 metres, + 2°s 



Temperature reduced to sea level, i5°o C. 



CORRECTION FOR THE TEMPERATURE OF THE MERCURY IN THE THER- 

 MOMETER STEM. 



Table 9. Fahrenheit thermometers ; Centigrade thermoineters. 



W^hen the temperature of the thermometer stem is materialh- differ- 

 ent from that of the bulb, a correction needs to be applied to the observed 

 reading in order to correct it for the difference in the length of the mer- 

 cun,^ column caused by this difference in its temperature. This correction 

 frequentl}' becomes necessary in physical experiments where the bulb only 

 is immersed in a bath whose temperature is to be determined, and in 

 meteorological obser\'ations it ma}' become appreciable in wet-bulb, dew 

 point, and solar radiation thermometers, when the temperature of the bulb 

 is considerablj' above or below the air temperature. 



If /' be the average temperature of the mercury column, t the observed 

 reading of the thermometer, n the length of mercury in the stem in scale 

 degrees, and a the apparent expansion of mercurj^ in glass for i", the 

 correction is given by the expression 



— an {f — t) 



in which, for Centigrade temperatures, a = 0.000154 or 0.000155. 



The average temperature of the mercury column can not be directly 

 observed and is diflBcult to determine, for it diflfers from the temperature 

 of the glass stem by an amount depending on the conduction of heat 

 between the bulb and the mercury column. Practicalh- however it is 

 possible to use the actually observed temperature of the glass stem as the 

 value of /' by making a small compensating change in the value of a, 

 and this appears to be the simplest method that has been proposed. Mr. 

 T. E. Thorpe (^Journal of the Chemical Society, vol. 37, 1880, p. 160) has 

 determined by a series of experiments that the proper thermometric cor- 

 rections will be obtained by this method if 0.000143 be used as a coefficient 

 (for Centigrade temperatures) instead of the value of a given above, and 

 this value has been adopted in the present tables. 



The correction formulae are, then, 



T =■ t — 0.0000795 ^^ (^ ~ O Temperature Fahrenheit. 

 T= t— 0.000143 '^ i.i' ~ O Temperature Centigrade, 

 in which T = Corrected temperature. 

 t = Observed temperature. 

 t' = Mean temperature of the glass stem. 

 n = Length of mercury in the stem in scale degrees. 



