THE CLIMATES OF THE SEA. 387 



lower temperature over the ocean, the total amount of vapour 

 deposited by any given volume of atmosphere, as it is blown 

 from the vicinity of the tropical towards that of the polar regions, 

 is greater than that which is taken up again. This is an in- 

 teresting and important fact. 



724. Tlie effects of night and day upon the temperature of sea water. 

 — Having, therefore, more precipitation in. high than in low 

 latitudes at sea, we should have more clouds ; and therefore it 

 requires 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° down 

 to the parallel of 40°, than it did for those cool surface currents 

 to float it down. After this southwardly motion of the isotherm 

 of 60° has been checked in December by the cold, and after the 

 sources of the current whicb have brought it down have been 

 bound in fetters of ice, it pauses in the long nights'of the northern 

 winter, and scarcely commences its return till the sun recrosses 

 the equator, with increased powers both as to intensity and 

 duration. Thus, in studying the physical geography of thue sea, 

 we must take cognizance of its actinometry also, for here we 

 have the eifects 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 

 reside upon the land ; nevertheless, feeble though those of the 

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

 they surely exist. 



725. A belt of uniform temperature at sea, — l^ow, returning 

 towards the south : we may, on the other hand, infer that the 

 mean atmospherical temperature for the parallels 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 between' the isotherm of 

 80° in August and the isotherm of 60° in January, a line or belt 

 of invariable or nearly invariable temperature, which extends on 

 the surface of the ocean from one side of the Atlantic to the 

 other. This belt or band may have its cycles also, but they are 

 probably of a long and uncertain period. 



726. The western half of the Atlantic warmer tlum the eastern. — The 

 fact has been pretty clearly established by the discoveries to 

 which the wind and current charts have led, that the western 

 half of the Atlantic Ocean is heated up, not by the Gulf Stream 



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