IO2 Chemical and Physical Notes 



Ttie Necessity of the Knowledge of Calorimetric Factors in 

 Connection with the Use of the Barometer. It is an interesting 

 historical fact that Fahrenheit, to whom we owe the thermo- 

 meter as a physical instrument of precision, got the idea of 

 filling his thermometer with mercury by observing and being 

 troubled by the irregularities of the barometer due to change 

 of temperature. It is also probably not altogether accidental 

 that the length of one degree of his thermometric scale corre- 

 sponds to one ten-thousandth of the volume of the mass of 

 mercury used in the thermometer 1 . If mercury expands by 

 10 ft -0 for every Fahrenheit's degree, then an error of i F. in 

 the estimation of the temperature of the barometer introduces 

 an error in the barometric pressure of T Q^Q of the whole height 

 of the column of mercury. If that height is 30 inches the 

 error is three-thousandths of an inch. If the height is 760 mm. 

 the error is 0*076 mm. It is obvious, therefore, that unless we 

 can be perfectly certain that the temperature of the barometer 

 is within ^ F. of what we take it to be, there is no sense in 

 reading the barometer to units in the third decimal place of 

 the inch, and still less so to units in the second decimal place 

 of the millimetre. But, it will be said, we have the " attached 

 thermometer," and it can easily be read to ^ F. Completing 

 the suggested syllogism, we have : Now the temperature of 

 the attached thermometer is always identical with that of the 

 mercury of the barometer to which it is attached ; therefore, 

 we know the temperature of the mercury of our barometer to 

 y 1 ^ F. ; and by consequence we are justified in reading our 

 barometer to 3 in the fourth decimal place of the inch, and to 

 7'6 in the third decimal place of the millimetre. If the 

 premises are granted, the conclusion is necessary. But the 

 second premise is true only in certain conditions which do 

 not usually obtain. If the temperature of the room in which 

 the barometer is hung is invariable, the premises may be 

 granted and the conclusion is valid. If the temperature is 

 not invariable, the premises cannot be granted, and the con- 

 clusion is false. Sufficient attention is not paid to this source 



1 This fact gives Fahrenheit's thermometer a genuine title to the name centi- 

 grade which Celsius' scale lacks. 



