BAROMETRICAL TABLES. XV 



When /' is I ^-7^33^' I than /, the numerical correction is to be | '^^^d^'^^'^- } 



Example : 



The observed temperature of a black bulb thermometer is 1 2o°4 F. , the 

 temperature of the glass stem is 55°2 F. and the length of mercury 

 in the stem is 130° F. To find the corrected temperature. 



With fi = 130° F. and — /' / = [— ] 65° F, as arguments, the table gives 

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

 observed temperature. The corrected temperature is therefore i2i.°i Z'. 



BAROMETRICAI. TABLES. 



REDUCTION TO A STANDARD TEMPERATURE OF OBSERVATIONS MADE 

 WITH BAROMETERS HAVING BRASS SCALES. 



The indicated height of the mercurial column in a barometer varies not 

 only with changes of atmospheric pressure, but also with variations of the 

 temperature of the mercury and of the scale. It is evident therefore that 

 if the height of the barometric column is to be a true relative measure of 

 atmospheric pressure, the observed readings must be reduced to the values 

 they would have if the mercury and scale were maintained at a constant 

 standard temperature. 



This reduction is known as the reduction for temperature, and combines 

 both the correction for the expansion of the mercury and that for the expan- 

 sion of the scale, on the assumption that the attached thermometer gives the 

 temperature both of the mercury and of the scale. 



The freezing point is universally adopted as the standard temperature 

 of the mercury, to which all readings are to be reduced. The temperature 

 to which the scale is reduced is che normal or standard temperature of the 

 adopted standard of length. For English scales, which depend upon the 

 English yard, this is 62° Fahrenheit. For metric scales, which depend 

 upon the metre, it is 0° Centigrade. 



As thus reduced, observations made with English and metric barometers 

 become perfectly comparable when converted by the ordinary tables of linear 

 conversion, viz. : millimetres to inches and inches to millimetres (see Tables 

 64, 65), for these conversions refer to the metre at 0° Centigrade and the 

 English yard at 62° Fahrenheit. 



The general formula for reducing barometric readings to a standard tem- 

 perature is 



i+m(t-T) 



