Page:Physical Geography of the Sea and its Meteorology.djvu/411

aid of currents, the rays of the sun could not drive it along that list. In this fact we have another link in the chain of proof (Chap. XXII.), going to show that the sea receives more heat than it radiates off again. Being now left to the gradual process of cooling by evaporation, atmospherical contact, and radiation, this isotherm occupies the other eight or nine months of the year in slowly returning south to the parallel whence it commenced to flow northward. As it does not cool as rapidly as it was heated, the disturbance of equilibrium by alteration of specific gravity is not so sudden, nor the current which is inquired to restore it so rapid. Hence the slow rate of movement at which this line travels on its march south. Between the meridians of 25° and 30° west, the isotherm of 60° in September ascends as high as the parallel of 56° N. In October it reaches the parallel of 50° north. In November it is found beneath the parallels of 45° and 47°, and by December it has nearly reached its extreme southern descent between these meridians, which it accomplishes in January standing then near the parallel of 40°. It is all the rest of the year in returning northward to the parallel whence it commenced its flow to the south in September. Now it will be observed that this is the season—from September to December—immediately succeeding that in which the heat of the sun has been playing with greatest activity upon the polar ice. Its melted waters, which are thus put in motion in June, July, and August, would probably occupy the fall months in reaching the parallels indicated. These waters, though cold, and rising gradually in temperature as they flow south, are probably fresher, and if so, probably lighter than the sea water; and therefore it may well be that both the warmer and cooler systems of these isothermal lines are made to vibrate up and down the ocean principally by a gentle surface current in the season of quick motion, and in the season of the slow motion principally by a gradual process of calorific absorption on the one hand, and by a gradual process of cooling on the other. We have precisely such phenomena exhibited by the waters of the Chesapeake Bay as they spread themselves over the sea in winter. At this season of the year, the charts show that water of very low temperature is found projecting out and overlapping the usual limits of the Gulf Stream. The outer edge of this cold water, though jagged, is circular in its shape, having its centre near the mouth of the bay. The waters of the bay, being fresher