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 Datron’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. Reenautr 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. RecnavLt’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. Reanautt’s Table fall, as will 
be immediately seen, considerably within those of M. Marre quoted by him, of 
which he says that they “s’accordent avec la formule aussi bien qu’on peut le 
désirer.”’+ 
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 IL. in the following Table. They were projected in the 
manner described in my former paper, the ordinates being the temperatures, the 
abscissee 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 IL., 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™¢ Serie, vol. xi., p. 332. 
Es Se 
