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XIV. — Further Experiments and Remarks on the Measurement of Heights by the 

 Boiling Point of Water. By James D. Forbes, D.C.L., F.R.S., Sec. R.S. Ed., &c, 

 Professor of Natural Philosophy in the University of Edinburgh. (With a 

 Plate.) 



(Read 4th December 1854.) 



In 1843 I presented a paper to the Royal Society of Edinburgh, giving an 

 account of experiments made on the boiling point of water in the Alps, un- 

 der various barometric pressures. My object was twofold : first, to describe an 

 apparatus which I considered more practically available than those previously in 

 use ; and, secondly, to give a simple, and, as I believed, new formula for comput- 

 ing heights from such observations. 



With reference to the second point, I became aware, some time after the pub- 

 lication of my paper, that Sir John Leslie had proposed to compute heights by 

 the thermometer, assuming the change of the boiling point to be exactly in pro- 

 portion to the height ascended. While cheerfully conceding to Sir John Leslie 

 priority on this point, I submit that he did not bring forward experiments to 

 justify its practical adoption. 



Of late years both the instrument and the formula have been objected to by 

 M. Regnault of Paris, and the latter by Dr Joseph Hooker, who finds that it 

 does not correctly represent his Indian observations. This has caused me to 

 examine the whole subject, and also Dr Hooker's observations on the boiling 

 point, with the particulars of which he has kindly furnished me, and I proceed 

 to lay the details before the Society. 



In 1843, when I wrote, the method of determining heights by boiling water 

 had fallen very much into abeyance, principally owing, as I believed, to the 

 inconvenient instruments employed, and partly to the uncertainty of the deduc- 

 tion of heights. As the thermometric method is principally valuable when baro- 

 meters cannot be safely transported, and must always be inferior in accuracy to 

 good barometric results, my intention was to do a service to physical geography, 

 by introducing a convenient and effective instrument, by means of which water 

 could be certainly made to boil even in untoward circumstances, and the tempe- 

 rature ascertained, not to the illusory nicety of two or three decimals of Fah- 

 [■ renheit's degree, as Archdeacon Wollaston attempted, but to within about 

 20th of a degree, corresponding to about 25 feet of elevation, which I stated as 

 the utmost degree of accuracy which I expected to attain, even in favourable 



vol. XXI. PART II. 3 R 



