BAROMETRICAL TABLES. XXXI 



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 tem- 

 perature, and combines both the correction for the expansion of the mercury 

 and that for the expansion 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 arc to be reduced. The temperature 

 to which the scale is reduced is the 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 meter, it is o° Centigrade. As thus reduced, observations made with 

 English and metric barometers become perfectly comparable when con- 

 verted by the ordinary tables of linear conversion, viz: inches to milli- 

 meters and millimeters to inches (see Tables 9, 10), for these conversions 

 refer to the meter at o° Centigrade and the English yard at 62 Fahrenheit. 



Prof. C. F. Marvin in the Monthly Weather Review for July, 1898, has 

 pointed out the necessity of caution in conversion of metric and English 

 barometer readings: 



Example : 



Attached thermometer, 25.4 C. 

 Barometer reading, 762.15 mm. 



If the temperature is converted to Fahrenheit = 77.7 and the reading 

 to 30.006 in., the temperature correction according to table 47 would be 

 — 0.133 inch and the reduced reading 29.873. This would be erroneous. The 

 correct conversion is found by taking the correction corresponding to 25^4 C. 

 and 762 mm., i.e., — 3.15 mm., which gives a corrected reading of 759 mm., 

 and converted into inches gives 29.882 which is the correct result. 



Professor Marvin further remarks that circumstances sometimes arise 

 in which a Centigrade thermometer may be used to determine the tem- 

 perature of an English barometer, or a Fahrenheit attached thermometer 

 may be used with a metric scale. In all such cases the temperature must be 

 brought into the same system of units as the observed scale reading before 

 corrections can be applied, and the observed reading must then be cor- 

 rected for temperature before any conversion can be made. 



With aneroid barometers corrections for temperature and instrumental 

 error must be determined for each instrument. 



The general formula for reducing mercurial barometers with brass scales 

 to the standard temperature is 



m (t -T)-l(t-d) 

 C D 1 + m (t - T) ' 



