418 UPON BOILING-POINT OBSERVATIONS. appendix. 



sions are numerous on which, from one cause or another, it is impossible to 

 make observations of the boiling-point of water under such conditions as will 

 allow the observations to be of value. 



The experimentalist, when at considerable elevations, will soon learn to 

 distrust single readings or the use of single thermometers, through noticing how 

 seldom his observed boiling-points accord with the ' corresponding barometric 

 pressure,' or agree with each other ; and will consider it necessary to employ 

 at least several thermometers and to repeat his observations. I find, in 

 practice, that to do this out of doors, under favourable conditions, occupies as 

 great a length of time, or greater, than to take two careful observations 

 of the mercurial barometer, a quarter of an hour apart. ^ So far as con- 

 venience and rapidity of observation are concerned, the balance appears to be 

 in favour of the mercurial barometer when the conditions are favourable, and 

 more distinctly so when the conditions are adverse. 



Assuming that the thermometers which are employed are boiled in the 

 correct manner, ^ and with the utmost care, there yet remain several possible 

 causes of error, ^ and with thermometers of the kind usually employed, in 

 which a degree of Fahrenheit's scale seldom extends over more than the 

 eighth of an inch, I think an error of half a degree and upwards is probable 

 in a single observation of one thermometer.* To obtain a more extended 

 scale (to lessen the probability of error) the traveller must either carry an 

 embarrassing number of thermometers, or else employ instruments of unusual 

 and inconvenient length. 



Near the level of the sea, the value of one degree of the Fahrenheit scale 

 is about 0'590 of an inch on a barometer, and one inch of the barometer is 



> The amount of time actually consumed in making a series of observations of the 

 boiling-point is longer than is necessary for several observations of mercurial barometer ; 

 as in the former case undivided attention must be given to the operations, while in the 

 latter it is only necessary to inspect the instrument at intervals, other things being done 

 in the meanwhile. As it is seldom possible to remain more than an hour upon a very 

 elevated summit, every minute is of importance. 



2 They should not be boiled in open vessels, nor immersed in the water. This, how- 

 ever, is not unfi-equently done. See A note on an alleged ascent of CMmhorazo in 1856, 

 by Messrs. Betny and Brenchley, in the Alpine Journal, vol. x. pp. 226-31, 1881. Also 

 see Livingstone's Last Journals., vol. ii. p. 198, 1874. Livingstone enters in his Journal, 

 less than eleven months before his death, that " there is a full degree of difference between 

 boiling in an open pot and in Casella's apparatus." It may, I think, be taken as certain 

 that Dr. Livingstone would not have made this entry at such a time if he had been pre- 

 viously acquainted with the fact. 



3 Amongst others, the liability of mercurial thermometers to read too high by age. 

 See the Kew certificate at p. 397 (of the attached thermometer), and compare the error 

 therein stated with the error on return as stated in the Meteorological Office certificate. 



Also see S. W. Baker's Albert N'^Yanza., vol. ii. pp. 362-3, 8vo, 1866, for an instance 

 of a boiling-point thermometer acquiring an error of -t- 0*8 of a aegree Faht. in 4f years. 

 This thermometer was made by Casella, and was supplied by the Royal Geographical 

 Society. 



See also Sir S. W. Baker's Ismailia., vol. ii. p. 562, 8vo, 1874, for an example of a 

 boiling-point thermometer apparently changing its index-error from + 0*20 to — 0*10 

 Faht. 



< Much larger differences than half a degree Fahrenheit may be observed between 

 thermometers which are sent out by makers of the best repute, accompanied by 'verifi- 

 cations' stating only infinitesimal errors. 



