372 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. 



isotherms. Passing, therefore, from a higher to a lower tempera- 

 ture 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 interesting and important 

 fact 



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

 The effects of night latitudos at soa, WO should have more clouds; and 

 temper^it^STo{lta therefore it requires a longer time for the sun, 

 '«^'ater. 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 5(j^ 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 

 which 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 the sea, we must take cogni- 

 zance of its actinometry also, for here 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 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. Now, returning towards the south : we may, on the other 

 A belt of uniform hand, infer that the mean atmospherical tempera- 

 temperature at sea. ^^j.^ f^j. ^j^g 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 fact has been pretty clearly established by the dis- 

 The western half of covories to which the wind and current charts have 

 hati the'eastern. *^ led, that ilic westom half of the Atlantic Ocean is 



