Appendix J. ON THE BOILING-POINT THERMOMETER. 455 



be 29-924 at temp. 32°, in lat. 45°, which, differing only '002 from 

 the barometric height corresponding to 212° Fahrenheit, as deter- 

 mined experimentally by Begnault, -gives 29*921 as the pressure 

 corresponding to 212° at the level of the sea. 



The approximate height in feet corresponding to each degree of 

 the boiling-point, is derived from Oltmann's tables. The multipliers 

 for the mean temperature of the strata of atmosphere passed through, 

 are computed for every degree Fahrenheit, by the formula for 

 expansion usually employed, and given in Baily's Astronomical 

 Tables and Biot's Astronomie Physique. 



For practical purposes it may be assumed that the traveller, in 

 countries where boiling-point observations are most desired, has 

 never the advantage of a contemporaneous boiling-point observation 

 at a lower station. The approximate difference in height is hence, 

 in most cases, deduced from the assumption, that the boiling-point 

 temperature at the level of the sea, at the place of observation, is 

 212°, and that the corresponding temperature of the air at the 

 level of the sea is hotter by one degree for every 330 feet of difference 

 in elevation. As, however, the temperature of boiling water at the level 

 of the sea varies at Calcutta between July and January almost from 

 210 o, 7 to 212°6, 1 always took the Calcutta barometer observation at 

 the day and hour of my boiling-point observation, and corrected my 

 approximate height by as many feet as correspond to the difference 

 between the observed height of the barometer at Calcutta and 

 29921 ; this correction was almost invariably (always normally) 

 subtractive in the summer, often amounting to upwards of 400 feet : 

 it was additive in winter, and towards the equinoxes it was very 

 trifling. 



For practical purposes I found it sufficient to assume the Calcutta 

 temperature of the air at the day and hour of observation to be that 

 of the level of the sea at the place of observation, and to take out the 

 multiplier, from the mean of this and of the temperature at the upper 

 station. As, however, 330 feet is a near approach to what I have 

 shown (Appendix I.) to be the mean equivalent of 1° for all 

 elevations between 6000 and 18,000 feet; and as the majority of 

 my observations were taken between these elevations, it results that 

 the mean of all the multipliers employed in Sikkim for forty-four 



