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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This Banded Magnetic Iron-Ore Is
Found Also In The Sierra Nevada Of Merida.
Between the two peaks of
the Silla, angular fragments of cellular quartz are found, covered
with red oxide of iron.
They do not act on the needle. This oxide
is of a cinnabar-red colour.)
The temperature of the atmosphere varied on the summit of the Silla
from eleven to fourteen degrees, according as the weather was calm
or windy. Every one knows how difficult it is to verify, on the
summit of a mountain, the temperature, which is to serve for the
barometric calculation. The wind was east, which would seem to
prove that the trade-winds extend in this latitude much higher than
fifteen hundred toises. Von Buch had observed that, at the peak of
Teneriffe, near the northern limit of the trade-winds, there exists
generally at the elevation of one thousand nine hundred toises, a
contrary current from the west. The Academy of Sciences recommended
the men of science who accompanied the unfortunate La Perouse, to
employ small air-balloons for the purpose of ascertaining at sea
the extent of the trade-winds within the tropics. Such experiments
are very difficult. Small balloons do not in general reach the
height of the Silla; and the light clouds which are sometimes
perceived at an elevation of three or four thousand toises, for
instance, the fleecy clouds, called by the French moutons, remain
almost fixed, or have such a slow motion, that it is impossible to
judge of the direction of the wind.
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