CHAPTER VII
THE HEIGHT OF DENALI, WITH A DISCUSSION OF THE READINGS ON THE SUMMIT AND DURING THE ASCENT
The determination of the heights of mountains by triangulation is, of course, the method that in general commends itself to the topographer, though it may be questioned whether the very general use of aneroids for barometric determinations has not thrown this latter means of measuring altitudes into undeserved discredit when the mercurial barometer is used instead of its convenient but unreliable substitute.
The altitude given on the present maps for Denali is the mean of determinations made by triangulation by three different men: Muldrow on the Sushitna [7] side in 1898, Raeburn on the Kuskokwim side in 1902, and Porter, from the Yentna country in 1906. In addition, a determination was made by the Coast and Geodetic Survey in 1910, from points near Cook’s Inlet. “The work of the Coast Survey,” writes Mr. Alfred Brooks, “is more refined than the rough triangulation done by our men; at the same time they were much further away.” “It is a curious coincidence,” he adds, “that the determination made by the Coast Survey was the mean which we had assumed from our three determinations” (twenty thousand three hundred feet).
Theodolites and Barometers
There are, however, two sources of error in the determination of the height of this mountain by triangulation—a general one and a particular one. The general one lies in the difficulty of ascertaining the proper correction to be applied for the refraction of the atmosphere, and the higher the mountain the greater the liability to this error; for not much is positively known about the angle of refraction of the upper regions of the air. The officers of the Trigonometrical Survey of India have published their opinion that the heights of the great peaks of the Himalayas will have to be revised on this account. The report of the Coast Survey’s determination of the height of Denali claims a “co-efficient of refraction nearer the truth” than the figure used on a previous occasion; but a very slight difference in this factor will make a considerable difference in the result.
The particular source of error in the case of this mountain lies in the circumstance that its summit is flat, and there is no culminating point upon which the cross-hairs of the surveying instrument may intersect.
The barometric determination of heights is, of course, not without similar troubles of its own. The tables of altitudes corresponding to pressures do not agree, Airy’s table giving relatively greater altitudes for very low pressures than the Smithsonian. All such tables as originally calculated are based upon the hypothesis of a temperature and humidity which decrease regularly with the altitude, and this is not always the case; nor is the “static equilibrium of the atmosphere” which Laplace assumed always maintained; that is to say an equal difference of pressure does not always correspond to an equal difference of altitude. There is, in point of fact, no absolute way to determine altitude save by running an actual line of levels; all other methods are approximations at best. But there had never been a barometric determination of the height of this mountain made, and it was resolved to attempt it on this expedition.