CLIMATES OF THE OCEAN. 299 



861. 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 

 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 

 w^iich is taken up again. 



862. The area comprehended on Plate YIII. 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 isotherms of 10° difference, 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 tliere are 

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



