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



cient for a standard atmosphere at 32°. It is about 6 feet less for 1° than I formerly 

 deduced. If we include the whole of the observations (Edinburgh and the Alps), 

 the rate of ascent will not be sensibly altered, though the squares of the errors 

 will be somewhat increased. 



The coincidence of the formula with observation, shewn in column 6, is highly 

 satisfactory. In only three instances does it exceed a tenth of a degree of Fah- 

 renheit. 



The coincidence with M. Regnault's Table, shown in column 8, is not less 

 satisfactory (a very slight change in the index error* would neutralize the pre- 

 ponderance of positive errors). 



From the mode of experimentally obtaining his results, M. Regnault's Table 

 is in fact a table of boiling points (see Ann. de Chimie, 1844), which was not the 

 case with Dalton's Table, which differs from it sensibly : and I have no doubt 

 that M. Regnault's are, on the whole, the most accurate numbers we at present 

 possess.f 



Hence, 



1. Observations of height by the thermometer down to about 190°, or for 

 elevations not exceeding 12,000 feet, may, within the usual limits of error, be 

 reduced indifferently by M. Regnault's Table, or by my arithmetical proportion. 



2. The method of placing the thermometer in water instead of steam, and of 

 using a powerful alcoholic furnace, which may be removed to one side until the 

 escape of steam becomes uniform and moderate, \ appears to give remarkably 

 steady and consistent results. The graduation of the thermometer, as regards 

 the length of the degree (which was entrusted to Mr Adie, and on which M. 

 Regnault throws doubts), is sensibly correct. 



3. 543*2 feet per degree seems to express the observations better than 549 - 5, 

 formerly given. 



Let us now turn to Dr Hooker's observations. In his " Himalayan Journals," 

 vol. ii., p. 456, speaking of his numerous observations on the boiling point at a 



* The reason for this change in the index error, according to the two hypotheses, will be seen in 

 the concluding paragraph of this paper. 



f I still retain the doubt expressed in my former paper, as to whether the boiling point can be 

 taken correctly to represent the temperature of steam whose elasticity is that of the atmospheric 

 pressure at the time. This doubt is confirmed by the difference of M. Regnault's and Magnus's 

 Tables of Elasticity, as also by experiments of a different kind. I take this opportunity of adding, 

 that I have obtained true ebullition of water in an exhausted receiver at the low temperature of 46° ; 

 the syphon-gauge then stood at 0-25 inch, being -06 below the elasticity of vapour, at that temper- 

 ature, as given by M. Regnault. 



J The superabundance of heating power, and the mass of liquid in ebullition, I consider very 

 important to the good result. No other portable apparahis that I am aware of, gives so ready means 

 of adapting the force of the flame to the circumstances of the case. With a common spirit-lamp in 

 fixed position below a boiler, it is next to impossible to regulate the rate of boiling, especially in an 

 exposed situation. Mine is also the only instrument, so far as I know, which can be used in a gale 

 of wind. 



