§ 724. THE CLIMATES OF THE SEA. 387 



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 temperature 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. The 

 area comprehended on Plate VIII. between the isotherms of 40° 

 and 50° Fahrenheit is less than the area comprehended between 

 the isotherms 50° and 60°, and this, again, less than the area be- 

 tween 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 iso- 

 therms 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 parallel of 60° 

 north. Now between this and the equator there are but three 

 isotherms, 60°, 70°, and 80°, with the common difference 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 common 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, mountain ranges, 

 land, etc., to disturb the laws of atmospherical circulation as they 

 apply to the ocean. 



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

 The effects of nijriit latitudcs, wc should havc morc clouds ; and there- 

 t^mpeSure °?f sea ^^^^ i* would Tcquirc a longcr time for the sun, 

 ****'"• with his feeble rays, to raise the temperature of the 



cold water which, from September to January, has brought the 

 isotherm of 60° from latitude 56° to 40°, than it did for those cool 

 surface currents to float it down. After this southward motion 

 of the isotherm of 60° has been checked in December bv the cold, 



