Page:Popular Science Monthly Volume 26.djvu/552

536 fissures or crevasses in which the waters of the Baltic and the Mediterranean, incessantly swelled by the rivers and by the currents of the Sund and of the Strait of Gibraltar, constantly lost themselves. During the last three years, the question of the evaporation of seawater has been much discussed between the partisans of the Saharian sea and their adversaries. The great point was to ascertain whether the proposed sea would not in the end become an enormous marsh. The sub-commission of the French Academy of Sciences was of the opinion that, other things being equal, salt water would evaporate less rapidly than fresh. Experiments by M. Dieulefait, on the other hand, indicated that a nearly equal loss would occur in the case of salt water and of fresh.

Fresh water freezes at 32° Fahr., but a liquid charged with salt congeals at lower temperatures; the rule is about the same as for the maximum of density, except that water slightly salt undergoes its contraction before being converted into ice, while normal sea-water acquires its minimum volume only in a state of surfusion, or when maintained artificially in a fluid state in capillary tubes. In this condition, a number of substances, water among them, are susceptible of being cooled considerably below their point of congelation and still remaining liquid. In the Baltic and White Seas, the waters of which for some depth are but weakly charged with salt, ice forms on the surface when the surrounding temperature has become low enough, while immediately below are strata more dense and relatively warmer. But suppose that below a certain depth of a brackish and warm liquid there is a cold salt current; the latter would produce such a refrigeration in the mixed intermediate strata that a mass of ice would be formed in the interior of the sea at the expense of the less saline zone. The block when formed would rise to the surface by virtue of its specific levity. This is what happens at the mouths of the great Siberian rivers. The Lena, in particular, pours out an enormous mass of warm water which overwhelms the salt waves from the polar regions. Even in the most favorable seasons, the navigator sails in the midst of floating ice-cakes that constitute a constant source of danger, while the thermometer dipped in the sea will indicate a temperature above the freezing-point. The depth of the warm stratum varies with the year, the place, and the prevailing winds; and hence we account for some explorers declaring impracticable tracks which others have easily sailed over. The northeast passage along the Siberian coast can never become a regular commercial route, unless, by repeated soundings accompanied by attentive studies, we can finally discover regular and periodical laws for the phenomena under consideration.

The Swedish physicist, Edlund, having inquired of the Scandinavian fishermen, was assured that they had sometimes, though rarely, seen the sea near the fiords of their country "vomit fragments of ice." The following is a textual reproduction of the story of one of the