370 PHYSICAL GEOGEAPHY OF THE SEA, AND ITS METEOROLOGY. 



Thus it travels at the rate of about twenty-two miles a day. 



Surely, without the aid of currents, the rays of the sun could not 



drive it along that fast. 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, 



a,nd radiation, this isotherm occupies the other eight or nine 



months of the year in slowdy 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 required 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 prpcess of calorific absorption on the one 



hand, and by a gradual process of cooling on the o^her. 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 



