298 THE PHYSICAL GEOGRAPHY OF THE SEA. 



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 there- 

 fore 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 mo- 

 tion, and in the season of the slow motion principally by a grad- 

 ual process of calorific absorption on the one hand, and by a grad- 

 ual process of cooling on the other. 



858. We have precisely such phenomena exhibited by the wa- 

 ters 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 over- 

 lapping 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 than those of the sea, may, therefore,- though colder, be 

 lighter than the warmer waters of the ocean. And thus we have 

 repeated here, though on a smaller scale, the phenomenon as to tlie 

 flow of cold waters from the north, which force the surface iso- 

 therm of 60° from latitude 56° to 40° during three or four months. 



859. Changes in the color or depth of the water, and the shape 

 of the bottom, etc., would also cause changes in the temperature 

 of certain parts of the ocean, by increasing or diminishing the ca- 

 pacities of such parts to absorb or radiate heat ; and this, to some 

 extent, would cause a bending, or produce irregular curves in the 

 isothermal lines. 



860. After a careful study of this plate, and the Thermal Charts 

 of the Atlantic Ocean, from which the materials for the former were 

 derived, I am led to infer that the mean temperature of the atmos- 

 phere between the parallels of 56° and 40° north, for instance, and 

 over that part of the ocean in which we have been considering the 

 fluctuations of the isothermal line of 60°, is at least 60° of Fah- 

 renheit, and upward, from January to August, and that the heat 

 which the waters of the ocean derive from this source — atmos- 

 pherical contact and radiation — is one of the causes which move 

 the isotherm of 60° from its January to its September parallel. 



