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XXIX. On the Determination of Heights by the Boiling Point of Water. By 

 JAMES D. FORBES, Esq., F.R.S., Sec.R.S.Ed., Inst. Reg. Sc. Paris. Corresp., 

 and Professor of Natural Philosophy in the+University of Edinburgh. 



(Read 6th March 1843.) 



IT was observed by FAHRENHEIT, that the boiling point of water depends on 

 the height of the barometer, the pressure of the air hindering the conversion of 

 water into steam by a resistance which must be overcome by an increase of heat. 

 DELUC* and DE SAUSSUREJ contrived apparatuses for making the observation in 

 the open air, and at great heights, and appear to have contemplated the substi- 

 tution of the thermometer for the barometer upon occasion. They, as well as 

 Dr HoRSLEY,t Sir GEORGE SCHUCKBUEGH, and Mr CAVENDISH, || seem to have 

 regarded the question as one which concerned the fixity of the point used in gra- 

 duating thermometers, and its requisite corrections, rather than as applicable to 

 barometric purposes generally. Several of them have given empirical tables for 

 correcting the boiling point within the limits of the usual barometric variations, 

 but one only, M. DELUC, has given a formula for connecting the indications of the 

 barometer with the boiling point of water throughout the range which the baro- 

 meter has been observed to vary on the earth's surface. This is the only formula 

 immediately deduced from direct observations of the boiling point ; and having 

 been verified by DE SAUSSURE at a height greater than the limits for which it was 

 constructed, and having elsewhere been declared by him to be so accurate as to 

 supersede farther experiment on the subject, it might have been expected to be 

 generally adopted, or at least known. We find, however, that though it has been 

 occasionally copied into the formal articles of Encyclopaedias, as a correction in 

 graduating thermometers, observers who have used the boiling point for the 

 determination of heights, have always preferred the ordinary tables which give 

 the elasticity of steam in terms of its temperature, determined from experiments 

 of quite a different kind from the boiling of water. 



Dr DALTON, indeed, has given a table from observation under the air-pump 

 of the boiling point,^[ and that table shews a manifest deviation from the elasti- 

 cities and temperatures of vapour determined by himself, and now generally 

 accepted as the most accurate below 212. In boiling, the temperature requires 



* Modifications de 1' Atmosphere, torn. ii. t Voyages, 1275, 2011. 

 J Phil. Trans, vol. Ixiv. Ibid. vol. Ixix. || Ibid. vol. Ixvii. p. 816. 



*[ Meteorological Essays, 2d edit. p. 127. 

 VOL. XV. PART III. 5 8 



