Tables 45 and 46 (continued) 137 



REDUCTION OF THE MERCURY COLUMN TO STANDARD TEMPERATURE 



Millimeter or millibar barometers. — For millimeter or millibar barometers (Fortin 

 type) the formula for reducing observed readings to the standard temperature, 0° C, 

 becomes 



C = - 5<ff^ (3) 



1 + mt 



in which C and B are expressed in the same units and t is in centigrade degrees. 



For temperatures above 0° centigrade the correction is negative, and hence is to be 

 subtracted from the observed readings. 



For temperatures below 0° centigrade the correction is positive, and from 0° C. down 

 to — 20° C. the numerical values thereof, for ordinary barometric work, do not materially 

 differ from the values for the corresponding temperatures above 0° C. Thus the correction 

 for — 9° C. is numerically the same as for + 9° C. and is taken from the table. In 

 physical work of extreme precision, the numerical values given for positive temperatures 

 may be used for temperatures below 0° C. by applying to them the following corrections : 



Corrections to be applied to the tabular values of Table 46 in order to use them when the 

 temperature of the attached thermometer is below 0" centigrade 



English barometers. — For English barometers (Fortin type) the formula for reducing 

 observed readings to a standard temperature becomes 



c== B m(t - 32°) -l(t- 62°) (4) 



where 



1 + m{t — 32°) 



C = correction for temperature in inches, 

 B = observed height of the barometer in inches, 

 t = temperature of attached thermometer in degrees Fahrenheit, 



m = 0.0001818 X I = 0.000101, 



/ = 0.0000184 X | = 0.0000102. 



The combined reduction of the mercury to the freezing point and of the scale to 

 62° Fahrenheit brings the point of no correction to approximately 28.5° Fahrenheit. 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. 



{Continued) 



SMITHSONIAN METEOROLOGICAL TABLES 



