236 PROFESSOR J. D. FORBES ON THE MEASUREMENT OF HEIGHTS 



circumstances * The formula of one degree of lowering of the boiling point for 

 550 feet of elevation, in an atmosphere at 32°, I stated to represent my observa- 

 tions quite sufficiently, and better than D Alton's Table of the Elasticity of Va- 

 pour, which was the one then commonly in use. I refer to my former paper 

 for a description of the boiling apparatus, consisting of a thin copper pan heated 

 by a " Russian furnace," having a powerful jet of inflamed alcoholic vapour. The 

 thermometer (contrary, I believe, to the usual practice) had its bulb in the water, 

 not in the steam. 



In 1844 M. Regnault published in the Annales de Chimie a table of the elas- 

 ticities of vapour _at moderate temperatures, and a comparison with some boiling 

 water experiments in Switzerland and the Pyrenees. He also contrived (I am 

 not sure at what date) a small apparatus for the use of travellers, somewhat 

 resembling Archdeacon Wollaston's. 



In 1845 he published a second paper on the same subject in the same Journal, 

 in which he quotes my observations, which he rejects as not conforming to his 

 law of Elasticities of Steam, and attributes their discrepancy to faults in the boil- 

 ing apparatus, and to errors of graduation of the thermometer. 



The slightest comparison of M. Regnault's paper with mine, shows, how- 

 ever, that the discrepancies complained of do not argue anything against the ac- 

 curacy either of his Table of Elasticities, or of my mode of observing ; but they 

 disappear almost entirely when the correction for the index error of the thermo- 

 meter I used is applied to the temperatures observed. This index error (0°62 in 

 excess) is given in my paper (page 414), and, of course, should have been applied 

 when it was intended to compare absolute temperatures with absolute pressures, 

 but had not been used by me when my object was merely to ascertain the rela- 

 tive variation of those quantities, as in page 412. When the index correction is 

 applied, the deviations of my observations from M. Regnault's Table fall, as will 

 be immediately seen, considerably within those of M. Marie quoted by him, of 

 which he says that they " s'accordent avec la formule aussi bien qu'on peut le 

 desirer."f 



To the observations of 1842, made in the Alps, and published in my former 

 paper, I have now added a fresh series, made in 1846, with the same apparatus 

 and thermometer, which confirms them in a remarkable manner. These series 

 are denoted by I. and II. in the following Table. They were projected in the 

 manner described in my former paper, the ordinates being the temperatures, the 

 abscissse the logarithms of the corresponding observed barometric pressures, which 

 numbers are proportional to the heights in an atmosphere of uniform tempera- 

 ture. Through the points a straight line may be drawn, with a very close approxi- 

 mation (See Plate III., fig. 1). The deviations of the observations from this 



* Transactions of the Royal Society of Edinburgh, vol. xv., p. 411. 

 t Annales de Chimie, 3^e Serie, vol. xi., p. 332. 



