ATM0SPHE11IC PHEKOJIENA. 
41 
of attention because vessels returning to Europe, some- 
times wait impatiently for a sight of these mountains, to 
rectify their longitude; mid think themselves much farther 
off than they really are, when in fine weather these peaks 
are not perceptible at distances where the angles subtended 
must be very considerable. The constitution of the atmos- 
phere has a great influence on the visibility of distant 
objects. It may bo admitted, that in general the peak of 
Tenerifl'e is seldom seen at a great distance, in the warm and 
dry months of .Inly and August; and that, on the contrary, 
it "is seen at very extraordinary distances in the months of 
January and February, when’ the sky is slightly clouded, 
and immediately after a heavy rain, or a few hours befbiv 
it falls. It appears that the transparency of the air is prodi- 
giously increased, as we have already observed, when a certain 
quantity of water is uniformly diffused through the atmos- 
phere. Independent of these observations, it is not astonish- 
ing, that the peak of Teyde should be seldomer visible at a 
very remote distance, than the summits of the Andes, to 
which, during so long a. time, li.y observations were directed. 
This peak, inferior in height to those parts of the chain of 
Mount Atlas at the foot of which is the city of Morocco, is 
not, like those points, covered w ith perpetual snows. The 
Fit on, or Sugar-loaf, which terminates the peak, no doubt 
reflects a great quantity of light, owing to the whitish colour 
of the pumice-stone thrown up by the crater ; but the height 
of that little truncated cone does not form a twenty-second 
pax-t of the total elevation. The flacks of the volcano are 
covered cither with blocks of black and scorified lava, or 
with a luxuriant vegetation, the masses of which reflect the 
less light, as tlie leaves of tire trees are separated from each 
other by shadows of more considerable extent than that of 
the part enlightened. 
Hence it results that, setting aside the Piton, the peak 
of Teyde belongs to that class of mountains, which, accord- 
ing to the expression of 1 longer, are seen at considerable 
distances only in a negation manner, because they intercept 
the light which is transmitted to us from the extreme limits 
of tho atmosphere ; and we perceive their existence only on 
account of the difference of intensity subsisting between the 
aerial light which surrounds them, and that which is reflected 
