THE CLIMATES OF THE OCEAN. 335 



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

 ual process of cooling on the other. 



502. 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 pfrojecting 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 the 

 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. 



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

 of the bottom, &c., 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. 



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

 of the Atlantic Ocean, from which the materials for this plate 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. 



505. It is well to consider another of the causes which are at 

 work upon the currents in this part of the ocean, and which tend 

 to give the rapid southwardly motion to the isotherm of 60°. We 

 know the mean dew-point must always be below the mean tem- 

 perature of any given place, and that, consequently, as a general 

 rule, at sea the mean dew-point due the isotherm of 60° is higher 

 than the mean dew-point along the isotherm of 50°, and this, again, 

 higher than that of 40° — this than 30°, and so on. Now suppose, 

 merely for the sake of illustration, that the mean dew-point for 



