300 THE PHYSICAL GEOGRAPHY OF THE SEA. 



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, etc., to disturb the laws of atmospherical circu- 

 lation as they apply to the ocean. 



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

 latitudes, we should have more clouds ; and therefore it w^ould re- 

 quire a longer 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 these cool surface currents to float it down. After this 

 southward motion of the isotherm of 60° has been checked in De- 

 cember by the cold, and after the sources of the current which 

 brought it down have been bound in fetters of ice, it pauses in the 

 long nights of the northern winter, and scarcely commences its re- 

 turn till the sun recrosses the equator, and increases its power as 

 well in intensity as in duration. 



864. Thus, in studying the physical geography of the sea, we 

 have the effects of night and day, of clouds and sunshine, upon its 

 currents and its climates, beautifully developed. These effects are 

 modified by the operations of certain powerful agents which re- 

 side upon the land ; nevertheless, feeble though those of the for- 

 mer class may be, a close study of this plate will indicate that they 

 surely exist. 



865. Now, returning toward the south : we may, on the other 

 hand, infer that the mean atmospherical temperature for the par- 

 allels between which the isotherm of 80° fluctuates is below 80°, 

 at least for the nine months of its slow motion. This vibratory 

 motion suggests the idea that there is, probably, somewhere be- 

 tween the isotherm of 80° in August and the isotherm of 60° in 

 January, a line or belt of invariable or nearly invariable temper- 

 ature, which extends on the surface of the ocean from one side of 



