SPITl VALLEY, &c. 



273 



The idea of other and still loftier ranges beyond those gelid scenes, 

 extending along the southern skirt of the Indus, is strengthened by the 

 information of the goatherds upon the spot ; but those observed from a 

 barometriclevel of 15.520, answering to eighteen thousand feet, where the 



regulate climate and vegetation, the sections of river courses and the planes of water communication 

 throughout a country admit of no other method. On comparing the circumstances which affect the 

 conditions of the respective operations, we shall see that the refractive power of the atmosphere 

 involves a source of error of infinitely greater extent and uncertainty than the variations in its gra- 

 vity which almost alone enter into Barometrical computations, and can be compensated by the 

 medium of a large range of simultaneous observations. Incases of small angles at great distances 

 the uncertainty of refraction must always prevail, and in the various degrees of temperature and 

 humidity of the medium through which a ray of light passes from an object in the Himalaya 

 to the eye of an observer upon the plain of India, if the angle is less than P, the undeter- 

 minable quantity might be sufficient to vitiate the whole calculation, if this is made with 

 reference to a fixed point, but without assuming the extreme limits of error which are liable 

 to result from the deflection of the visual ray in an atmosphere, varying in temperature within 

 the points of observations to 70 or tiO degrees, the uncertainty still remains as to the quan- 

 tity to be allowed for the intercepted arc, in cases where the three angles of the trian- 

 gle cannot be observed, which include all the grand points of the chain, and for which 

 allowance there is no precise measure, and a mean from the extremes only reduces the height of an 

 object within the limits of a very considerable space, in many cases exceeding a thousand feet. 

 With respect to Barometrical heights, much superfluous objection lias been made in regard to the 

 variations in the specific gravity of the mercury arising from natural impurities or adulteration, but 

 which are notwithstanding, limited to a mere imaijinary compass, from the impossibility of alloying 

 the metal to any appreciable extent, without rendering it useless for the purpose. These are how- 

 ever, determinable errors, which may be destroyed entirely. In the dry and brilliant regions which 

 have disclosed the scenes of gigantic grandeur alluded to, refraction becomes a computable element; 

 from stations elevated eighteen and nineteen thousand feet, the angles of the most distant objects 

 would be subject to little derangement from variation in the density of the atmosphere and 

 vertical bases which are generally within our reach, by their proximity to each other, would prevent 

 the accumulation of error by reducing the interval between the observation of the angles to an almost 

 cotemporary result ; another advantage occurs in Barometrical levels at very loftv stations in the 

 slight changes of atmospheric density, or at least the uniformity of the fluctuations. It is true that 

 it requires but half the extent of the oscillations in the mercurial column here, to produce the same 

 effect (error) as at the levi 1 ol'tho sea, hut tlii-; is eninlly npiireciahle at the highest as at the lowest 

 regions, nnd the discrepancy (whatever this may be in un altitude of four or five thousand feet, is not 

 liable to be augmented in that of eighteen or twenty thousand, a correction for the hygromctric 

 state of the air seems still a desideratum in liarometrical calculati. ns. Under all the circumstances 

 of the meaburemcDt when made with accuracy and the necessary coiii[)ens.itious, v\e may safely 



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