As fresh and salt water do not attain the maximum of their density at the
same degree of temperature, and as the saltness of the sea lowers the
thermometrical degree corresponding to this point, we can understand how the
water drawn from breat depths of the sea during the voyages of the Kotzebue
and Dupetit-Thouars could have been found to have only the temperature of 37
degrees and 36.5 degrees. This icy temperatureof sea water, which is
likewise manifested at the depths of tropical seas, first led to a study of
the lower polar currents, which move from both poles toward the equator.
Without these submarine currents, the tropical seas at those depths could
only have a temperature equal to the local maximum of cold possessed by the
falling particles of water at the radiating and cooled surface of the
tropical sea. In the Mediterranean, the cause of the absence of such a
refrigeration of the lower strata is ingeniously explained by Arago, on the
assumption that the entrance of the deeper polar currents into the Straits
of Gibraltar, where the water at the surface flows in from the Atlantic
Ocean from west to east, is hindered by the submariine counter-currents
which move from east to west, from the Mediterranean into the Atlantic.
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