454 ON THE BOILING-POINT THERMOMETER. Appendix J. 



but often as I have tried common river- water for comparison, I never 

 found that it made any difference in the temperature of the boiling- 

 point. Even the mineral- spring water at Yeumtong, and the 

 detritus-charged glacial streams, gave no difference, and I am hence 

 satisfied that no objection can be urged against river waters of 

 ordinary purity. 



On several occasions I found anomalous rises and falls in the 

 column of mercury, for which I could not account, except theoreti- 

 cally, by assuming breaks in the column, which I failed to detect 

 on lifting the instrument out of the water ; at other times, I 

 observed that the column remained for several minutes stationary, 

 below the true temperature of the boiling water, and then suddenly 

 rose to it. These are no doubt instrumental defects, which I only 

 mention as being sources of error against which the observer must 

 be on the watch : they can only be guarded against by the use of two 

 instruments. 



"With regard to the formula employed for deducing the altitude 

 from a boiling-point observation, the same corrections are to a great 

 extent necessary as with barometric observations : if no account is 

 taken of the probable state of atmospheric pressure at the level of 

 the sea at or near the place of observation, for the hour of the day 

 and month of the year, or for the latitude, it is obvious that errors 

 of 600 to 1000 feet may be accumulated. I have elsewhere stated 

 that the pressure at Calcutta varies nearly one inch (1000 feet), 

 between July and January ; that the daily tide amounts to one-tenth 

 of an inch ( = 100 feet) ; that the multiplier for temperature is too 

 great in the hot season and too small in the cold ; and I have expe- 

 rimentally proved that more accuracy is to be obtained in measuring 

 heights in Sikkim, by assuming the observed Calcutta pressure and 

 temperature to accord with that of the level of the sea in the latitude 

 of Sikkim, than by employing a theoretical pressure and temperature 

 for the lower station. 



In the following observations, the tables I used were those 

 printed by Lieutenant- Colonel Boileau for the East India Company's 

 Magnetic Observatory at Simla, which are based upon Eegnault's 

 Table of the 'Elastic Eorce of Vapour.' The mean height of 

 the barometrical column is assumed (from Bessel's formula) to 



