Page:Physical Geography of the Sea and its Meteorology.djvu/73

 Rh plan of oceanic circulation and in the system of unequal heating and cooling, been brought together.

129. Waters of the ocean kept in motion by thermo-dynamical means.—The waters of the Gulf Stream form by no means the only body of warm water that the thermo-dynamical forces of the ocean keep in motion. Nearly all that portion of the Atlantic which lies between the Gulf Stream and the island of Bermuda has its surface covered with water which a tropical sun and tropical winds have played upon—with water, the specific gravity of which has been altered by their action, and which is now drifting to more northern climes in the endless search after lost equilibrium. This water, moreover, as well as that of the Gulf Stream, cools unequally. It would be surprising if it did not: for by being spread out over such a large area, and then drifting for so great a distance, and through such a diversity of climates, it is not probable that all parts of it should have been exposed to like vicissitudes by the way, or even to the same thermal conditions: therefore all of the water over such a surface cannot be heated alike; radiation here, sunshine there; clouds and rain one day, and storms the next; the unequal depths; the breaking up of the fountains below, and the bringing their cooler or their warmer waters to the surface by the violence of the waves, may all be expected, and are well calculated, to produce unequal heating in the torrid and unequal cooling in the temperate zone; the natural result of which would be streaks and patches of water differing in temperature. Hence it would be surprising if, in crossing this drift and stream (Plate VI.) with the water-thermometer, the observer should find the water all of one temperature. By the time it has reached the parallel of Bermuda or "the Capes" of the Chesapeake, some of this water may have been ten days, some ten weeks, and some perhaps longer on its way from the "caldron" at the south. It has consequently had ample time to arrange itself into those differently-tempered streaks and layers (§ 127) which are so familiar to navigators, and which have been mistaken for "forks of the Gulf Stream."

130. Fig. A, Plate VI.—Curves showing some of these variations of temperature have been projected by the Coast Survey on a chart of engraved squares (Fig. A, Plate VI.). These curves show how these waters have sometimes arranged themselves off the Capes of Virginia into a series of thermal elevations and depressions.