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 dilFerence 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 VIIL) ; 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 VIIL 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° dilFerence, 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 
