THE CLIMATES OF THE SEA. 371 



mouth of the bay. The waters of the bay, being fresher than 

 those of the sea, are therefore, though colder, yet lighter (§ 426) 

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

 here, though on a smaller scale, the phenomena as to the flow 

 of cold waters from the north, which force the surface isotherm 

 of 60^ from latitude 56"^ to the parallel of 40° during three or 

 four months. Changes in the colour or depth of the water, and 

 the shape of the bottom, etc., are also calculated to cause changes 

 in the temperature of certain parts of the ocean, by increasing or 

 diminishing the capacities of such parts to absorb or radiate heat ; 

 and this, to some extent, assists to bend or produce irregular 

 curves in the isothermal lines. After a careful study of this 

 plate, and the Thermal Charts of the Atlantic Ocean, from which 

 the materials for it are derived, I am led to infer that from 

 January to August the mean temperature of the atmosphere 

 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 

 Fahrenheit, and upward, and that the heat which the waters of 

 the ocean derive from this source — atmospherical contact and 

 radiation — is one of the causes which move the isotherm of 60^ 

 from its January to its September parallel. 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 south- 

 wardly motion to the isotherm of 60°. We know the mean dew- 

 point must always be below the mean temperature 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 each isotherm 

 be 5° lower than the mean temperature, we sliould then have the 

 atmosphere which crosses the isotherm of 60°, with a mean dew- 

 point of 55°, gradually precipitating its vapours until it reaches 

 the isotherm of 50^, with a mean dew-point of 45° ; by which 

 difference of dew-point the total amount of precipitation over the 

 entire zone between the isotherms of 60° and 50° has exceeded 

 the total amount of evaporation from the same surface. The 

 prevailing direction of the winds to the north of the fortieth 

 parallel of north latitude is from the southward and westward 

 (Plate YIII.); in other words, it is from the higher to the lower 



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