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[Assembly 
of the air, but upon the relations of the hne 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 coUimation 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 distin- 
guished observers, is a fair practical example, illustrative of the fore- 
going remarks, and shows that barometrical measurements are not alto- 
gether unworthy of confidence, even when compared with angular ones 
by the same observers. The following table, as pubhshed by Hum- 
boldt, exhibits the results : 
Geometrical measurements made on land. 
By P. Feuillee, made in 1724, 2,213 toises. 
The same result, modified by Bouguer, . do 2,062 do 
By Heberden and Cross, five operations, do 1752, 2,408 do 
By Hernandez, do 1742, 2,658 do 
By Borda and Pingre, do 1771, 1,742 do 
ByBorda, „. do 1776, 1,905 do 
Geometrical measurements made under sail. 
By Mannevilette do 1749, 2,000 do 
By Borda and Pingre, do 1771, 1,701 do 
By Churacca, do 1788, 2,193 do 
By Johnston, 2,023 do 
Barometrical measurements^ calculated after the formula of La Place. 
By Feuellee and Verguin, do 1724, 2,025 do 
By Borda, do 1776, 1,976 do 
ByLamanon, do 1785, 1,902 do 
ByCordier, do 1803, 1,920 do 
In view of these measurements, Humboldt makes the following re- 
marks : 
" These measures, taken at different periods, vary from 1,700 to 2,- 
600 toises, and, what is remarkable enough, the results obtained by 
geometrical operations, differ more from each other, than those which 
were found by the barometer. It has, nevertheless, been extremely 
wrong to cite this want of harmony as a proof of the uncertainty of all 
measurements of mountains. Angles, the value of which is determin- 
