MEASUREMENT OF TEMPERATURE. 
175 
The temperature of the mercury is repeatedly taken in the course of the observations 
by the aid of a delicate thermometer immersed in the mercury of the open limb. It 
almost always differs slightly from that of the air. 
The correction for temperature is only applied to the nearest tenth-millimetre, 
partly because the readings are only accurate to this order, and partly because, since 
the temperature is varying and is not the same throughout, the correction itself is 
liable to a slight uncertainty. To avoid inaccuracy, the calculations were usually 
carried to the next figure, to which, however, but little weight can be attached. 
The Barometer. 
Readings of the atmospheric pressure were taken with the standard Fortin baro¬ 
meter. Each reading is subject to the following corrections :— 
1 . The most important is the correction for temperature. The scale is of brass, 
graduated in English inches, and is correct at a temperature of 62° Fahr. The scale 
errors are negligible. 
The density of the mercury is normal at Oh 
Thus for a height of 76 - 0 cm. the correction is —'024 cm. at 0° Cent., since the brass 
scale is incorrect; and at 62° Fahr., wll'en the brass scale is correct, the mercury 
correction is —■'228 cm., and the change in the correction is ‘0123 cm. per degree 
Centigrade. 
The correction is uncertain, because it does not follow that the thermometer 
indicates the mean temperature of the mercury and scale. 
2 . The correction for capillary depression and vapour tension of mercury amounts 
altogether to about +'005 cm. at 15° Cent. 
3. The corrections for gravitation and height above sea-level need not be applied, 
since absolute measures of pressure are not required (except for determining the 
temperature of saturated steam at atmospheric pressure). 
4. The level of the cistern of the barometer is at a height of 4 metres above the 
manometer. A constant correction of + 04 cm. is applied to allow for this. 
5. Sources of Error .—The atmospheric pressure varies incessantly and capriciously, 
so that the reading of the barometer cannot be relied on beyond the nearest tenth- 
millimetre * (Regnault), even if all the mechanical adjustments could be perfectly 
effected and the temperature correction were certain. The barometer is, therefore, 
read several times in the course of each series of observations, and the corrections are 
applied to each reading to the nearest tenth-millimetre. 
6 . The constant errors of the attached thermometer are sufficiently small to be 
negligible. 
In reading the barometer, all precautions were taken to avoid inaccuracy in the 
mechanical adjustments, the probable error from this source being only about '001 in. 
* ‘ Paris, Memoires de TTnstitut,’ tom. 21, p. 69. 
