Personal Narrative Of Travels To The Equinoctial Regions Of America During The Years 1799-1804 - Volume 2 - By Alexander Von Humboldt And Aime Bonpland.
- Page 7 of 208 - First - Home
The
Deepest Places Are Between The Rocky Island Of Burro And The Point Of
Cana Fistula, And Opposite The High Mountains Of Mariara.
But in
general the southern part of the lake is deeper than the northern:
Nor
must we forget that, if all the shores be now low, the southern part
of the basin is the nearest to a chain of mountains with abrupt
declivities; and we know that even the sea is generally deepest where
the coast is elevated, rocky, or perpendicular.
The temperature of the lake at the surface during my abode in the
valleys of Aragua, in the month of February, was constantly from 23 to
23.7 degrees, consequently a little below the mean temperature of the
air. This may be from the effect of evaporation, which carries off
caloric from the air and the water; or because a great mass of water
does not follow with an equal rapidity the changes in the temperature
of the atmosphere, and the lake receives streams which rise from
several cold springs in the neighbouring mountains. I have to regret
that, notwithstanding its small depth, I could not determine the
temperature of the water at thirty or forty fathoms. I was not
provided with the thermometrical sounding apparatus which I had used
in the Alpine lakes of Salzburg, and in the Caribbean Sea. The
experiments of Saussure prove that, on both sides of the Alps, the
lakes which are from one hundred and ninety to two hundred and
seventy-four toises of absolute elevation* (* This is the difference
between the absolute elevations of the lakes of Geneva and Thun.)
have, in the middle of winter, at nine hundred, at six hundred, and
sometimes even at one hundred and fifty feet of depth, a uniform
temperature from 4.3 to 6 degrees: but these experiments have not yet
been repeated in lakes situated under the torrid zone. The strata of
cold water in Switzerland are of an enormous thickness. They have been
found so near the surface in the lakes of Geneva and Bienne, that the
decrement of heat in the water was one centesimal degree for ten or
fifteen feet; that is to say, eight times more rapid than in the
ocean, and forty-eight times more rapid than in the atmosphere. In the
temperate zone, where the heat of the atmosphere sinks to the freezing
point, and far lower, the bottom of a lake, even were it not
surrounded by glaciers and mountains covered with eternal snow, must
contain particles of water which, having during winter acquired at the
surface the maximum of their density, between 3.4 and 4.4 degrees,
have consequently fallen to the greatest depth. Other particles, the
temperature of which is +0.5 degrees, far from placing themselves
below the stratum at 4 degrees, can only find their hydrostatic
equilibrium above that stratum. They will descend lower only when
their temperature is augmented 3 or 4 degrees by the contact of strata
less cold. If water in cooling continued to condense uniformly to the
freezing point, there would be found, in very deep lakes and basins
having no communication with each other (whatever the latitude of the
place), a stratum of water, the temperature of which would be nearly
equal to the maximum of refrigeration above the freezing point, which
the lower regions of the ambient atmosphere annually attain. Hence it
is probable, that, in the plains of the torrid zone, or in the valleys
but little elevated, the mean heat of which is from 25.5 to 27
degrees, the temperature of the bottom of the lakes can never be below
21 or 22 degrees. If in the same zone the ocean contain at depths of
seven or eight hundred fathoms, water the temperature of which is at 7
degrees, that is to say, twelve or thirteen degrees colder than the
maximum of the heat* of the equinoctial atmosphere over the sea, I
think it must be considered as a direct proof of a submarine current,
carrying the waters of the pole towards the equator. (* It is almost
superfluous to observe that I am considering here only that part of
the atmosphere lying on the ocean between 10 degrees north and 10
degrees south latitude. Towards the northern limits of the torrid
zone, in latitude 23 degrees, whither the north winds bring with an
extreme rapidity the cold air of Canada, the thermometer falls at sea
as low as 16 degrees, and even lower.) We will not here solve the
delicate problem, as to the manner in which, within the tropics and in
the temperate zone, (for example, in the Caribbean Sea and in the
lakes of Switzerland,) these inferior strata of water, cooled to 4 or
7 degrees, act upon the temperature of the stony strata of the globe
which they cover; and how these same strata, the primitive temperature
of which is, within the tropics, 27 degrees, and at the lake of Geneva
10 degrees, react upon the half-frozen waters at the bottom of the
lakes, and of the equinoctial ocean. These questions are of the
highest importance, both with regard to the economy of animals that
live habitually at the bottom of fresh and salt waters, and to the
theory of the distribution of heat in lands surrounded by vast and
deep seas.
The lake of Valencia is full of islands, which embellish the scenery
by the picturesque form of their rocks, and the beauty of the
vegetation with which they are covered: an advantage which this
tropical lake possesses over those of the Alps. The islands are
fifteen in number, distributed in three groups;* without reckoning
Morro and Cabrera, which are already joined to the shore. (* The
position of these islands is as follows: northward, near the shore,
the Isla de Cura; on the south-east, Burro, Horno, Otama, Sorro,
Caiguira, Nuevos Penones, or the Aparecidos; on the north-west, Cabo
Blanco, or Isla de Aves, and Chamberg; on the south-west, Brucha and
Culebra.
Enter page number
PreviousNext
Page 7 of 208
Words from 6180 to 7188
of 211397