Mr. J. Burgess on the Measurement of Altitudes. 29 



under similar conditions. Schonbein explains all these results 

 by the combination of nitrogen with the elements of water, 

 producing at the same time ammonia and nitrous acid. As he 

 has well remarked, this reaction serves to explain the absorption 

 of nitrogen by vegetation, and, through the oxidation of nitrites, 

 the formation of nitrates in nature. By these elegant experi- 

 ments, he has confirmed in a remarkable manner my theory of 

 nitrificatiou, and of the double nature of free nitrogen. It is 

 however evident that, since the publication of my note of March 

 1861 referred to above, we cannot say with Schonbein that the 

 generation of nitrite of ammonia from nitrogen and water is " a 

 most wonderful and wholly unexpected thing." (Letter from Sch 6n- 

 bein to Faraday, Philosophical Magazine, June 1862, p. 467). 

 I cannot, however, admit with these gentlemen that the results of 

 Schonbein are due to evaporation, except in so far as the co- 

 operation of water, and a slightly elevated temperature, are 

 necessary conditions of the reaction. 



IV. On the Measurement of Altitudes by means of the Temperature 

 at which Water Boils. By J. Burgess, Esq. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 



SINCE Archdeacon Wollaston first employed the temperature 

 of ebullition as a means of measuring altitudes, the subject 

 has been several times discussed. The late Mr. James Prinsep 

 calculated a table of heights corresponding to boiling-points for 

 every degree of temperature from 176° to 214° P., which was 

 employed by Colonel Sykes*, and has been several times 

 reprintedf; and, more recently, Professor J. D. Forbes, in 1843 

 and again in 1854, contributed two papers on the subject to the 

 f Transactions of the Royal Society of Edinburgh/ By projecting 

 graphically the altitudes derived from barometrical measure- 

 ments made by him among the Alps, but uncorrected for tem- 

 perature, in terms of the observed boiling-points, Professor 

 Forbes found that they lay almost exactly in a straight line. 

 Hence he inferred that " the temperature of the boiling-point varies 

 in a simple arithmetical ratio with the height "%. 



2. But the general expression for difference of level in terms 



* Phil. Mag. (1835) vol. vii. p. 31 1. 



t Daviesand Peck's 'Math. Diet.,' art. "Levelling;" Smith and Thuil- 

 lier's ' Manual of Surveying for India,' p. 435 ; Journal of the Royal Geo- 

 graphical Society, vol. viii. 



X Transactions of the Royal Society of Edinburgh, vol. xv. 



