340 
[Assembly 
As I have not learned the distance that Mr.- Johnson actually used> 
it is proper to remark, that if we should assume the distances 40 and 
45, instead of 35 and 40, our conclusions would not differ so much from 
those above, as to vitiate the argument. Using these latter numbers, 
which are probably somewhat nearer the truth than the former, an er- 
ror of 5 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^h'ave 
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 ob- 
servers. 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 evidently 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 ob- 
servations at the same station should always be taken, which will ena- 
ble the observer to guard against error, either by rejecting all, or se- 
lecting 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 
