THE CLIMATES OF THE OCEAN. 
235 
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 vi^inter. 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 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 
I 
