236 THE PHYSICAL GEOGRAPHY OF THE SEA. 



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°, gradually 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 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 VIII.) ; in other words, it is from the higher to the lower 

 isotherms. Passing, therefore, from a higher to a lower tempera- 

 ture over the ocean, the total amount of vapor deposited by any 

 given volume of atmosphere, as it is blown from the vicinity of 

 the tropical toward that of the polar regions, is greater than that 

 which is taken up again. 



506. The area comprehended on Plate VIII. between the iso- 

 therms of 40° and 50° Fahrenheit is less than the area compre- 

 hended between the isotherms 50° and 60°, and this, again, less 

 than the area between this last and 70°, for the same reason that 

 the area between the parallels of latitude 50° and 60° is less than 

 the area between the parallels of latitude 40° and 50° ; therefore, 

 more rain to the square inch ought to fall upon the ocean between 

 the colder isotherriis of 10° diiference, than between the warmer 

 isotherms of the same difference. This is an interesting and an 

 important view, therefore let me make myself clear : the aqueous 

 isotherm of 50°, in its extreme northern reach, touches the paral- 

 lel of 60° north. Now between this and the equator there are 

 but three isotherms, 60°, 70°, and 80°, with the common differ- 

 ence of 10°. But between the isotherm of 40° and the pole, there 

 are at least five others, viz., 40°, 30°, 20°, 10°, 0°, with a com- 

 mon difference of 10°. Thus, to the north of the isotherm 50°, 

 the vapor which would saturate the atmosphere from zero, and 

 perhaps far below, to near 40°, is deposited, while to the south of 

 50° the vapor which would saturate it from the temperature of 

 50° up to that of 80° can only be deposited. At least, such would 

 be the case if there were no irregularities of heated plains, mount- 

 ain ranges, land, &c., to disturb the laws ^of atmospherical circu- 

 lation as they apply to the ocean. 



507. Having therefore, theoretically, at sea more rain in high 



