208 
GEOLOGY OF THE SECOND DISTRICT. 
minations of high mountains have been comparatively vague. One cause for this difference, 
consists in the great distances at which mountains are generally observed, and the consequent 
smallness of the angles of elevation. In this respect, a condition is almost necessarily vio¬ 
lated, which was scrupulously satisfied in the surveys just referred to. 
The chief source of error in mountain measurements, and one which distinguishes them from 
horizontal ones, is refraction. This difficulty, growing out of the condition of the air, and 
independent, therefore, of the instrument, is analogous to the one which the barometrical 
method is exposed to, with this difference in favor of the latter, that the atmospheric changes 
going on at both stations may be detected and compared. 
Refraction differs, in different countries and at different times, from one-fourteenth to one- 
eighth of the distance, reckoned in minutes. Such being the uncertainty as to the true path 
of light in low and familiar regions, it must be particularly difficult to follow it with precision 
through mediums of changing relations, and elevated tracts comparatively unknown. Re¬ 
fraction, too, depends not only upon the affections of the air, but iipon the relations of the 
line of sight with other objects. Every one who has used the spirit level, is aware of the 
errors that he is exposed to, Avhen, in clear weather, his line of collimation approaches logs 
and fences or the surface of the ground. 
The Peak of Teneriffe, from its great elevation, and the number of times that it has been 
measured according to both methods by distinguished observers, is a fair practical example 
illustrative of the foregoing remarks, and shows that barometrical measurements are not 
altogether unworthy of confidence, even when compared with angular ones by the same 
observers. The following table as published by Humboldt, exhibits the results : 
Geometrical measurements made on land. 
By P. Feuillee, .made in 
1724, 
2213 toises. 
The same result, modified by Bouguer,. 
do 
2062 
a 
By Heberden and Cross, five operations, _ 
do 
1752, 
2408 
i( 
By Hernandez,.... 
do 
1742, 
2658 
iC 
By Borda and Pingre,_____ 
do 
1771, 
1742 
u 
By Borda,. 
do 
1776, 
1905 
u 
Geometrical measurements made 
under 
sail. 
By Mannevillette,... 
do 
1749, 
2000 
By Borda and Pingre,.... 
do 
1771, 
1701 
(( 
By Churacca,___ 
do 
1788, 
2193 
(C 
By Johnston,.... 
2023 
(( 
Barometrical measurements, calculated after 
the formula of Laplace. 
By Feuillee and Verguin,. 
do 
1724, 
2025 
(C 
By Borda,.... 
do 
1776, 
1976 
(( 
By Lamanon,. 
do 
1785, 
1902 
6( 
By Cordier,....... 
do 
1803, 
1920 
(( 
