Page:Physical Geography of the Sea and its Meteorology.djvu/254

228 the south, which, by virtue of its saltness (see Fig. 2), is heavier than the cool and upper current which runs out of the polar basin, and which is known as an ice-bearing current. It is the same which is felt by mariners as far down as the Grand Banks of Newfoundland, and recognized by philosophers off the coast of Florida. This upper current, though colder than its fellow below, is lighter, because it is not so salt. Figure 2 reveals to us a portion of sea between the parallels of 34° and 40° north, exactly in such a physical category as that in which this theory presents the Arctic Ocean. Here, along our own shores, the thermal curve loses 12° of heat; and what does the specific gravity curve gain in the same interval? Instead of increasing up to 1.027, according to the thermal law, it decreases to 1.023 for the want of salt to sustain it. Now recollect that the great American chain of fresh-water lakes never freezes over. Why? Because of their depth and their vertical circulation. The depths below are continually sending water above 32° to the surface, which, before it can be cooled down to the freezing-point, sinks again. Now compare the shallow soundings in these lakes with the great depths of the Arctic Ocean; compute the vast extent of the hydrographic basin which holds this polar sea; gauge the rivers that discharge themselves into it; measure the rain, and hail, and snow that the clouds pour down upon it; and then contrast its area, and the fresh-water drainage into it, with the like of Long Island Sound, Delaware Bay, and the Chesapeake; consider also the volume of diluted sea water between our shore-line and the Gulf Stream; strike the balance, and then see if the arctic supply of fresh water be not enough to reduce its salts as much as our own fresh-water streams are diluting the brine of the sea under our own eyes. The very Gulf Stream water, which the observing vessel left as she crossed 34° and entered into those light littoral waters, was bound northward. Suppose it to have flowed on as a surface current until it, with its salts, was reduced to the temperature of 40°. Its specific gravity at that temperature would have been 1.030, or specifically 30 per cent, heavier than the sea water of our own coasts. Could two such currents of water meet anywhere at sea, except as upper and under currents? If water that freezes at 32°, that grows light and remains on the surface as you cool it below 39°, is prevented from freezing in our great fresh-water lakes by vertical circulation, how much more would both vertical and horizontal circulation prevent congelation in the open polar