456 ON THE -BOILING-POINT THERMOMETER. Appendix J. 



observations amounts to 65°1 Fahrenheit, using the Calcutta and 

 upper station observations, and 65°3 on the assumption of a fall of 

 1° for every 330 feet. To show, however, how great an error may 

 accrue in individual cases from using the formula of 1° to 330, I 

 may mention that on one occasion, being at an elevation of 12,000 

 feet, with a temperature of the air of 70°, the error amounted to 

 upwards of 220 feet ; and as the same temperature may be recorded 

 at much greater elevations, it follows that in such cases the formula 

 should not be employed without modification. 



A multitude of smaller errors, arising from anomalies in the distri- 

 bution of temperature, will be apparent on consulting my observa- 

 tions on the temperature at various elevations in Sikkim ; practically 

 these are unavoidable. I have also calculated all my observations 

 according to Professor J. Forbes's formula of 1° difference of tem- 

 perature of boiling- water, being the equivalent of 550 feet at all 

 elevations. (See Ed. Phil. Trans., vol xv. p. 405.) The formula is 

 certainly not applicable to the Sikkim Himalaya ; on the contrary, 

 my observations show that the formula employed for Boileau's tables 

 gives at all ordinary elevations so very close an approach to accuracy 

 on the mean of many observations, that no material improvement in 

 its construction is to be anticipated. 



At elevations below 4000 feet, elevations calculated from the 

 boiling-point are not to be depended on ; and Dr. Thomson remarked 

 the same in north-west India : above 17,000 feet also the observations 

 are hazardous, except good shelter and a very steady fire is obtain- 

 able, owing to the heating of the metal above that of the water. At 

 all other elevations a mean error of 100 feet is on the average 

 what is to be expected in ordinary cases. For the elevation of 

 great mountain masses, and continuously elevated areas, I conceive 

 that the results are as good as barometrical ones ; for the general 

 purposes of botanical geography, the boiling-point thermometer 

 supersedes the barometer in point of practical utility, for under every 

 advantage, the transport of a glass tube full of mercury, nearly three 

 feet long, and cased in metal, is a great drawback to the unrestrained 

 motion of the traveller. 



In the Khasia mountains I found, from the mean of twelve stations 

 and twenty-three observations, the multiplier as derived from the 



