BAROMETRICAL MEASUREMENTS. 
207 
numbers, which are probably somewhat nearer the truth 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 difficulty 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, are undoubtedly 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- 
