BAROMETRICAL MEASUREMENTS. 207 



numbers, whicli arc proliably somewhat nearer the trutli than the former, an error of five 

 miles in distance would still induce an error in elevation of 718 feet. 



Refraction is another cause of deviation from exactness, of which, however, I shall merely 

 remark in this connection, that it may occasion an error of about 100 feet ; and that, too, 

 even after a correction has been applied according to the best authorities. 



A discussion of the comparative merits of barometrical and angular measurements of great 

 elevations, would prolong this communication to an unreasonable length. I shall pass over 

 it, therefore, at present, with one or two brief remarks. The barometer and theodolite have 

 their peculiar capabilities and defects ; and the exact measurement, by either, of a mountain 

 covered with clouds during the greatest portion of the year, and surrounded by an atmosphere 

 subject to incessant change, demands more perfect instruments and skill in their use, than is 

 generally apprehended. 



The chief difhculty that the barometer has to contend with, and one over which it has no 

 direct control, is a want of uniformity in the changes of atmospheric pressure in different 

 places at corresponding times. As correct observations have been multiplied, more harmony 

 in this respect has been detected than had formerly been supposed. Indeed, this is not the 

 only department in which nature has been held accountable for blunders due to clumsy 

 instruments and unskilful observers. This difficulty, without doubt, exists to such an extent 

 as to impair confidence in single sets of observations at least, with whatever care they may 

 have been made. The atmosphere, whether charged uniformly with vapor or not, must evi- 

 dently have a strong tendency to equilibrium ; and a derangement of it, within moderate 

 distances, must consequently be transient. For this reason, a course of consecutive obser- 

 vations at the same station should always be taken, which will enable the observer to guard 

 against error, either by rejecting all, or selecting those that, in this way, are shown to be 

 worthy of confidence. 



The corrections for the hygrometrical state of the atmosphere, arc imdoubtedly more or less 

 imperfect ; although that portion of the error which yet remains unprovided for, I apprehend, 

 is comparatively inconsiderable. Laplace measures these effects by the temperature of the 

 air, and observes that this hypothesis very nearly satisfies the observations that have hitherto 

 been made. The agreement of my results, where courses of observations were taken, inti- 

 mates with what degree of approximation I have corrected for the changes of the weather. 

 This agreement is particularly worthy of remark in relation to Long lake, where the obser- 

 vations were protracted in time, and the weather singularly variable. But notwithstanding 

 all this, I am free to admit, that these corrections are still less perfect than could be desired. 

 The theodolite is above the need of eulogium from any one ; but, like every other human 

 invention, it has its proper capabilities and defects. An indispensable condition to the accuracy 

 of angular measurements, is the exact determination of a base line ; a work which requires 

 that skill, variety of delicate instruments, time and means, which, in this country, are not 

 generally at the command of a single individual. Triangulations, embracing great extent, 

 have been executed with astonishing precision ; and the results of similar measurements, 

 properly conducted, are entitled to the utmost confidence : on the contrary, the angular deter- 



