THE CLIMATES OF THE OCEAN. 243 



resents the mean or average limits of these constant flows — polar 

 and equatorial ; whereas, with almost every wind that blows, and 

 at every change of season, the line of meeting between their w^a- 

 ters is shifted. In the next place, this line of meeting is drawn 

 with a free hand on the plate, as if to represent an average ; 

 whereas there is reason to believe that this line in nature is vari- 

 able and unstable as to position, and as to shape rough and jag- 

 ged, and oftentimes deeply articulated. In the sea, the line of 

 meeting between waters of different temperatures and density is 

 not unlike the sutures of the skull-bone on a grand scale — very 

 rough and jagged ; but on the plate it is a line drawn with a free 

 hand, for the purpose of showing the general direction and po- 

 sition of the channels in the sea, through which its great polar and 

 equatorial circulation is carried on. 



525. Now, continuing for a moment our examination of Plate 

 IV., w^e are struck with the fact that most of the thermal lines there 

 drawn run from the western side of the Atlantic tow^ard the east- 

 ern, in a northeastwardly direction, and that, as they approach the 

 shores of this ocean on the east, they again turn down for lower 

 latitudes and w^armer climates. This feature in them indicates, 

 more surely than any direct observations upon the currents can 

 do, the presence, along the African shores in the North Atlantic, 

 of a large volume of cooler waters. These are the waters which, 

 having been first heated up in the caldron (§ 509) of St. Roque, in 

 the Caribbean Sea, and Gulf of Mexico, have been made to run 

 to the north, charged with heat and electricity to temper and reg- 

 ulate climates there. Having performed their offices, they have 

 cooled down ; but, obedient still to the " Mighty Voice" which the 

 winds and the waves obey, they now return by this channel along 

 the African shore to be again replenished with warmth, and to 

 keep up the system of beneficent and wholesome circulation de- 

 signed for the ocean. 



