XVI INTRODUCTION. 



thermometer will indicate the temperature of the emergent mercurial column 

 to an accuracy sufficient for many purposes. Under conditions ordinarily 

 met with in practice it is desirable to place the bulb of the auxiliary ther- 

 mometer at some point below the middle of the emergent column. 



It is to be noted that the correction sought is directly proportional to the 

 value of a, and that this may vary for glass stems of different composition 

 from 0.00015 to 0.000165 for Centigrade temperatures. For thermometers 

 ordinarily used in meteorological work, however, 0.000155 appears to be a 

 good average value for Centigrade temperatures (0.000086 for ■ Fahrenheit 

 temperatures), and the correction formulae, therefore, are, 



T — t — 0.000086 n (/' — /) Fahrenheit temperatures. 

 T = t — 0.000155 n (/' — /) Centigrade temperatures. 



In the above, T = Corrected temperature. 

 / = Observed temperature. 



t' = Mean temperature of the glass stem and emergent mer- 

 cury column. 

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



When /' is < . [ than / the numerical correction is to be < ... ' > 



( lower } ( added. ) 



Table 7 gives corrections computed to o-oi for Fahrenheit thermometers 

 from the equation C = — 0.000086 n (/' — /). The side argument, n, is given 

 for io° intervals from io° to 130 ; the top argument, t' — t, for io° intervals 

 from io° to ioo°. 



Table 8 gives corrections computed to 0.01 for Centigrade thermometers 

 from the equation C = — 0.000155 n (/' — t). The side argument, n, is 

 given for io° intervals from io° to ioo°; the top argument, /' — /, for io° 

 intervals from io° to 8o°. 



Example: 



The observed temperature of a black-bulb thermometer is 120.4 ^m 

 the temperature of the glass stem is 55°2 F., and the length of mer- 

 cury in the emergent stem is 130 F. To find the corrected tempera- 

 ture. With n = 130 F. and t' — t = — 65 F., as arguments, Table 

 7 gives the correction o?7 F., which by the above rule is to be added 

 to the observed temperature. The corrected temperature is therefore 

 121. 1 F. 



CONVERSIONS INVOLVING LINEAR MEASURES. 



The fundamental unit of length is the meter, the length of which is equal 

 to the distance between the defining lines on the international prototype 

 meter at the International Bureau of Weights and Measures (near Paris) 

 when this standard is at the temperature of melting ice (o° C). The relation 



