AitendixJ. ON THE BOILING-POINT THERMOMETER 453 



ON THE MEASUREMENT OF ALTITUDES BY THE BOILING-POINT 



THERMOMETER. 



The use of the boiling-point thermometer for the determination 

 of elevations in mountainous countries appearing to me to be much 

 underrated, I have collected the observations which I was enabled 

 to take, and compared their results with barometrical ones. 



I had always three boiling-point thermometers in use, and for 

 several months five ; the instruments were constructed by Newman, 

 Dollond, Troughton, and Simms, and Jones, and though all in one 

 sense good instruments, differed much from one another, and from 

 the truth. Mr. Welsh has had the kindness to compare the three 

 best instruments with the standards at the Kew Observatory 

 at various temperatures between 180° and the boiling-point ; from 

 which comparison it appears, that an error of 1-| may be found at 

 some parts of the scale of instruments most confidently vouched for 

 by admirable makers. Dollond' s thermometer, which Dr. Thomson 

 had used throughout his extensive west Tibetan journeys, deviated 

 but little from the truth at all ordinary temperatures. All were so far 

 good, that the errors, which were almost entirely attributable to care- 

 lessness in the adjustments, were constant, or increased at a constant 

 ratio throughout all parts of the scale ; so that the results of the diffe- 

 rent instruments have, after correction, proved strictly comparable. 



The kettle used was a copper one, supplied by Newman, with free 

 escape for the steam ; it answered perfectly for all but very high 

 elevations indeed, where, from the water boiling at very low tempe- 

 ratures, the metal of the kettle, and consequently of the thermo- 

 meter, often got heated above the temperature of the boiling water. 



I found that no confidence could be placed in observations taken 

 at great elevations, by plunging the thermometer in open vessels of 

 boiling water, however large or deep, the abstraction of heat from the 

 surface being so rapid, that the water, though boiling below, and hence 

 bubbling above, is not uniformly of the same temperature throughout. 



In the Himalaya I invariably used distilled, or snow or rain-water ; 



