386 THE PHYSICAL GEOGRAPHY OF THE SEA. 



usual limits of the Gulf Stream. The outer edge of this cold wa- 

 ter, 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, 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 phenomenon as to the flow 

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

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

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

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

 tain parts of the ocean, by increasing or diminishing the capaci- 

 ties of such parts to absorb or radiate heat ; and this, to some ex- 

 tent, would cause a bending, 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 the former were derived, I am led to infer that the mean tem- 

 perature 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, from January to 

 August, 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 southwardly 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, con- 

 sequently, as a general rule, at sea the mean dew-point due the 

 isotherm of 60° is higher than the mean dew-point along the isO' 

 therm 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 should then have the atmosphere which 

 crosses the isotherm of 60°, with a mean dew-point of 55°, gradu- 

 ally precipitating its vapors 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 



