208 GEOLOGY OF THE SECOND DISTRICT. 



minations of high mountains have been comparatively vague. One cause for this difference, 

 consisls 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 

 tlu-ough mediums of changing relations, and elevated tracts comparatively unknown. Re- 

 fraction, too, depends not only upon the affections of the air, but upon 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, when, 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, exliibits the results : 



Geometrical measurements made on land. 



By P. Feuillee, made in 1724, 2213 toises. 



The same result, modified by Bouguer, do 2062 " 



By Heberden and Cross, five operations, do 1752, 2408 " 



By Hernandez, do 1742, 2658 " 



By Borda and Pingre, do 1771, 1742 " 



By Borda, do 177G, 1905 " 



Geometrical measurements made under sail. 



By Mannevillette, do 1749, 2000 " 



By Borda and Pingre, ^ do 1771, 1701 " 



By Churacca, do 1788, 2193 " 



By Johnston, 2023 " 



Barometrical measurements, calculated after the formula of Laplace. 



By Feuillee and Verguin, do 1724, 2025 " 



By Borda, _ do 1776, 1976 



By Lamanon, do 1785, 1902 " 



By Cordier, do 1803, 1920 



