Sais ae 
re Cpa — 
Sy ear a ee ~ : —— 
—— a NS I ee ae See a 
oe Pn TIKES FT 
ind cn GEMS ET Etta geaPIa oe mee 
36 DISTANCE AT WHICH MOUNTAINS — 
ought to be visible. at the distance of 148 miles, 
supposing the eye at the level of the ocean, and the 
refraction equal to 0-079 of the distance. Navigators 
who frequent these latitudes find that the Peaks of 
Teneriffe and the Azores are sometimes observed at 
very great distances, while at other times they can- 
not be seen when the interval is considerably less, al- 
though the sky is clear. Such circumstances are of 
importance to navigators, who, in returning to Eu- 
rope, impatiently wait for a sight of these mountains 
to rectify their longitude. ‘The constitution of the 
atmosphere has a great influence on the visibility of 
distant objects, the transparency of the air being 
much increased when a certain quantity of water is 
uniformly diffused through it. 
It is not surprising that the Peak of Teneriffe 
should be less frequently visible at a great distance 
than the tops of the Andes, not being like them in- 
vested with perpetual snow. The Sugar-loaf which 
constitutes the summit of the former, no doubt re- 
flects a great degree of light, on account of the white 
colour of the pumice with which it is covered ; but 
its height does not form a twentieth part of the total 
elevation, and the sides of the volcano are coated 
with blocks of dark-coloured lava, or with luxuriant 
vegetation, the masses of which reflect little light, 
the leaves of the trees being separated by shadows 
of greater extent than the illuminated parts. 
Hence the Peak of Teneriffe is to be referred to 
the class of mountains which are seen at great dis- 
tances only in what Bouguer calls a negative man- 
ner, or because they intercept the light transmitted 
from the extreme limits of the atmosphere ; and we 
perceive their existence only by means of the dif- 
