Equinoctial Regions of America, vol 2 | Page 9

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