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The Temperature Correction of Barometers. By R. T. Omond.* 



In correcting the reading of a barometer to the freezing-point, it is customary to ascer- 

 tain the temperature of the instrument by observing a thermometer attached to the 

 brass frame, but it is doubtful how far this attached thermometer indicates the 

 temperature of all parts of the barometer under the conditions in which most observa- 

 tions are made. In a Fortin barometer the level of the mercury in the cistern is 

 adjusted to a fixed point at each observation, and we have only to consider the 

 expansion with heat of the mercury and of the brass scale. As the expansion 

 of mercury is about ten times that of brass, the former is the chief factor in the 

 problem, and a simple piece of apparatus was constructed to show how far the 

 attached thermometer differed from the temperature of the mercury. A brass tube, 

 such as forms the casing of an ordinary standard barometer, was taken, and inside this 

 a glass tube of the same diameter and thickness as is generally used was placed. The 

 glass tube was closed at the lower end, and filled with mercury to near the top, in 

 which a cork was inserted ; passing through this cork was a thermometer whose bulb 

 dipped into the mercury, and whose scale could be read through the glass tube and the 

 slots in the upper part of the brass tube, where the scale is in a finished barometer. 

 This thermometer, therefore, gave directly the temperature of the upper part of the 

 mercury column ; and as mercury is a good conductor of heat, this was assumed to be 

 the temperature of the whole column for the purposes of the experiment. Another 

 thermometer was attached outside the brass scale in the usual manner, and the experi- 

 ments consisted in comparing the readings of the two thermometers under varying 

 temperature conditions. 



For sudden changes of temperature a very simple law was found to govern the 

 relationship of the two thermometers. When the instrument is taken from a warm 

 room to a cold, or vice versa, the outer thermometer alters most rapidly ; it takes about 

 an hour for the two to come into agreement again for differences between the rooms 

 ranging from 7° to 20° ; the maximum difference between the two thermometers occurs 

 about ten minutes after the change is made, and this difference amounts to 40 per cent, 

 of the whole change of temperature. For example : on 23rd December 1899, at 5 p.m. 

 the two thermometers read 44°"1 and 44 o, ; the instrument was then moved into a 

 warmer room ; at 5.11 p.m. the readings were 52° - 2 and 48°*0 — a difference of 4 0, 2, the 

 outside attached thermometer being the higher, and this was the greatest difference 

 observed. The difference between them decreased till at 5.47 p.m. they read 54°*9 and 

 54°"7, showing a total change of 10°"8, — (44°'l to 54 0, 9). Similarly, for a change of 



* See Journal of Scottish Meteorological Society, vol. xi. p. 298. 



