_— a 
ae 
ee eS ee 
THERMOMETERS. lv 
the evening when the sun shone, the front was removed and suspended at an equal 
height, on the west or east wall of the Observatory, being kept apart from it by pro- 
jecting pins. Observations at different times shewed that, all other things being 
equal, the temperature was the same in all the positions ; but when the sun shone 
it might be one or two degrees less to the east or west than to the north. 
86. It sometimes happens when the air is very humid during frost, especially 
when the temperature is falling, that the dry bulb thermometer reads less than the 
wet bulb; when such is the case the readings of the wet bulb have not been printed 
in this volume, and in the summations for the abstracts the readings of the thermo- 
meters are considered as the same.* 
87. The maximum and minimum register thermometers, on RUTHERFORD’S con- 
struction, were made by Aprz and Son. Before January 24, 1843, they were placed 
4 feet from the ground, near the east window, facing the north, and were protected 
from the sun’s rays in the morning and evening by projecting spars of wood, After 
January 24 they were placed on the same board with the dry and wet bulb thermo- 
meters, 5 feet from the ground. 
88. The following table contains the corrections of the thermometers to a 
standard thermometer by NEwMaNn. The comparisons were made in melting ice or 
snow for the freezing point, and in water at different temperatures. This table also 
contains the correction for the bifilar and balance thermometers, which have not 
been applied (see No. 33). 
TABLE 23.—Corrections of Thermometers to the Standard by NEWMAN. 
Bifilar (Ross),| Balance. 
° 
—0-1 
—0-1 
—0-1 
—0:3 
—0:5 
— 0-5 
— 0-4 
—0-3 
— 0-3 
—0-2 
—0.1 
* The cause of this apparent anomaly, it is conceived, is this, that the moisture deposited on the silk 
_ cover of the wet bulb is frozen as it is deposited, until it becomes a thickish coat of silk and ice; the mer- 
_ cury in the bulb will thus, following the falling temperature of the air, contract slowly, and will be less 
affected by any evaporation proceeding on the outer surface of the coat; on the dry bulb, however, the 
_ frozen moisture is but a thin film, as the bulb is generally dried between the observations, it will thus 
_ he easily affected by any evaporation, and become, in fact, a wet bulb thermometer; it might be ad- 
i visable, therefore, instead of rejecting to substitute the readings of the wet bulb for the dry, and the 
‘ readings of the dry bulb for the wet, 
MAG. AND MET. oBs. 1843. P 
