APPENDIX. UPON BOILING-POINT OBSERVATIONS. 419 



about equal to 900 feet of altitude. The value of a degree of the Fahrenheit 

 scale is therefore equal to about 531 feet. 



At the height of the summit of Mont Blanc the value of one degree of 

 the Fahrenheit scale is about equal to 0-375 of an inch on a barometer, and 

 at this elevation the value of one inch on a barometer is about equal to 1600 

 feet. In this case, the value of a degree of the Fahrenheit scale is therefore 

 about 600 feet. 



From comparison of the means of a number of observations of the boil- 

 ing-point of water, made by myself in Ecuador, against the mercurial barome- 

 ter, when the latter was standing at 16*522 to 17 '427 inches, the value 

 of a degree of a Fahrenheit scale, at that pressure, appears to be 613 

 feet. 



From the previous paragraphs it is seen that the value of each degree of 

 a thermometric scale is greater the higher we ascend. It is consequently 

 necessary, in order to obtain as good proportional results at a high level as at 

 the level of the sea, that personal errors, and errors arising from method or 

 instrumental defects must be lesseiied. This, in practice, will not, I think, be 

 found possible. The higher we ascend the greater are the difficulties of 

 observation. 



Should it, however, be found possible to obtain absolutely perfect instru- 

 ments, and to observe without introducing personal errors, it remains to be 

 seen whether the observations will accord with the ' corresponding barometric 

 pressure.' ^ 



Few observations for comparison have hitherto been made of the boiling- 

 point of water against the mercurial barometer at great heights, and I am not 

 acquainted with any which have been made at greater elevations than those 

 by Dr. (now Sir Joseph) Hooker in 1848-50, by the brothers Schlagintweit 

 in 1855-56, and by M. Wisse in Ecuador in 1844-49. Dr. Hooker quotes ^ 

 observations for comparison at seventeen stations above 13,000 feet, his 

 loftiest being the Donkia Pass, 18,466 feet. The brothers Schlagintweit 

 record^ observations at five stations above 13,000 feet, their most elevated 

 point of observation being their camp on Ibi Gamin, 19,323 feet.* M. 

 Wisse ^ observed at only two stations above 13,000 feet. My own series 



1 The remarks which accompcany the section in the SmUhsonian Tables (Section iv. 

 p. 96') upon the thermometrical measurement of heights well deserve attention. " It 

 may be seen that the heights determined by the means of the temperature of boiling- 

 water are less reliable than those deduced from barometrical observations. Both derive 

 the difference of altitude from the difference of atmospheric pressure. But the tempera- 

 ture of boiling -water gives only indirectly the atmospheric pressure, which is given 

 directly by the barometer. This method is thus liable to all the chances of error which 

 may affect the measurements by means of the barometer, besides adding to them new ones 

 peculiar to itself, the principal of which, not to speak of the differences exhibited in the 

 various tables of the force of vapour, is the difficulty of ascertaining with the necessary 

 accuracy the true temperature of boiling-water." 



"^ In Himalayan Journals, vol. ii. p. 458 ; 8vo, London, 18.54. 



=* In their Results of a Scientific Mission to India and High Asia, vol. ii, p. 28 ; 4to, 

 London and Leipzig, 1862. 



* At p. 336 of the same volume, the height of this camp is said to be 19,094 feet. 

 I am not aware which determination is the more correct one. 



5 The observations are given by Regnault in Annates de Chimie et de Physique, vol. 

 28, p. 123, 1850. 



