Personal Narrative Of Travels To The Equinoctial Regions Of America During The Years 1799-1804 - Volume 1 - By Alexander Von Humboldt And Aime Bonpland.
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It Appears That
The Transparency Of The Air Is Prodigiously Increased, As We Have
Already Observed, When A Certain Quantity Of Water Is Uniformly
Diffused Through The Atmosphere.
Independent of these observations,
it is not astonishing, 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, my 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 with perpetual snows. The Piton, 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 part of the total elevation. The
flanks of the volcano are covered either with blocks of black and
scorified lava, or with a luxuriant vegetation, the masses of which
reflect the less light, as the leaves of the 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, according to the
expression of Bouger, are seen at considerable distances only in a
NEGATIVE MANNER, because they intercept the light which is
transmitted to us from the extreme limits of the 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 by the particles of air placed between
the mountains and the eye of the observer. As we withdraw from the
isle of Teneriffe, the Piton or Sugar-loaf is seen for a
considerable space of time in a POSITIVE MANNER, because it
reflects a whitish light, and clearly detaches itself from the sky.
But as this cone is only 80 toises high, by 40 in breadth at its
summit, it has recently been a question whether, from the
diminutiveness of its mass, it can be visible at distances which
exceed 40 leagues; and whether it be not probable, that navigators
distinguish the peaks as a small cloud above the horizon, only when
the base of the Piton begins to be visible on it. If we admit, that
the mean breadth of the Sugar-loaf is 100 toises, we find that the
little cone, at 40 leagues distance, still subtends, in the
horizontal direction, an angle of more than three minutes. This
angle is considerable enough to render an object visible; and if
the height of the Piton greatly exceeded its base, the angle in the
horizontal direction might be still smaller, and the object still
continue to make an impression on our visual organs; for
micrometrical observations have proved that the limit of vision is
but a minute only, when the dimensions of the objects are the same
in every direction. We distinguish at a distance, by the eye only,
trunks of trees insulated in a vast plain, though the subtended
angle be under twenty-five seconds.
As the visibility of an object detaching itself in a brown colour,
depends on the quantities of light which the eye meets on two
lines, one of which ends at the mountain, and the other extends to
the surface of the aerial ocean, it follows that the farther we
remove from the object, the smaller the difference becomes between
the light of the surrounding atmosphere, and that of the strata of
air before the mountain. For this reason, when less elevated
summits begin to appear above the horizon, they present themselves
at first under a darker hue than those we discern at very great
distances. In the same manner, the visibility of mountains seen
only in a negative manner, does not depend solely on the state of
the lower regions of the air, to which our meteorological
observations are limited, but also on the transparency and physical
constitution of the air in the most elevated parts; for the image
detaches itself better in proportion as the aerial light, which
comes from the limits of the atmosphere, has been originally more
intense, or has undergone less loss in its passage. This
consideration explains to a certain point, why, under a perfectly
serene sky, the state of the thermometer and the hygrometer being
precisely the same in the air nearest the earth, the peak is
sometimes visible, and at other times invisible, to navigators at
equal distances. It is even probable, that the chance of perceiving
this volcano would not be greater, if the ashy cone, at the summit
of which is the mouth of the crater, were equal, as in Vesuvius, to
a quarter of the total height. These ashes, being pumice-stone
crumbled into dust, do not reflect as much light as the snow of the
Andes; and they cause the mountain, seen from afar, to detach
itself not in a bright, but in a dark hue. The ashes also
contribute, if we may use the expression, to equalize the portions
of aerial light, the variable difference of which renders the
object more or less distinctly visible. Calcareous mountains,
devoid of vegetable earth, summits covered with granitic sand, the
high savannahs of the Cordilleras,* (* Los Pajonales, from paja,
straw. This is the name given to the region of the gramina, which
encircles the zone of the perpetual snows.) which are of a golden
yellow, are undoubtedly distinguished at small distances better
than objects which are seen in a negative manner; but the theory
indicates a certain limit, beyond which these last detach
themselves more distinctly from the azure vault of the sky.
The colossal summits of Quito and Peru, towering above the limit of
the perpetual snows, concentre all the peculiarities which must
render them visible at very small angles. The circular summit of
the peak of Teneriffe is only a hundred toises in diameter.
According to the measures I made at Riobamba, in 1803, the dome of
the Chimborazo, 153 toises below its summit, consequently in a
point which is 1300 toises higher than the peak, is still 673
toises (1312 metres) in breadth.
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