384 THE PHYSICAL GEOGRAPHY OF THE SEA. 



and September, the months of extreme heat and extreme cold to 

 the inhabitants of the " great deep." Corresponding isotherms 

 for any other month will fall between these, taken by pairs. 

 Thus the isotherm of 70° for July will fall nearly midway be- 

 tween the same isotherms (70°) for March and September. 



723. A careful study of this plate, and the contemplation of 

 Plate IV. the benign influence of the sea upon the climates 

 which we enjoy, suggest many beautiful thoughts; for by such 

 study we get a glimpse into the arrangements and the details of 

 that exquisite machinery in the ocean which enables it to perform 

 all its offices, and to answer with fidelity its marvelous adapta- 

 tions. How, let us inquire, does the isothermal of 80°, for instance, 

 get from its position in March to its position in September? Is 

 it wafted along by currents, that is, by water which, after having 

 been heated near the equator to 80*^, then flows to the north with 

 this temperature? Or is it carried there simply by the rays of 

 the sun, as the snow-line is carried up the mountain in summer ? 

 We have reason to believe that it is carried from one parallel to 

 another by each, of these agents acting together, but mostly 

 through the instrumentality of currents, for currents are the chief 

 agents for distributing heat to the various parts of the ocean. 

 The sun with, his rays would, were it not for currents, raise the 

 water in the torrid zone to blood heat; but before that can be 

 done, they run off with it toward the poles, softening, and miti- 

 gating, and tempering climates by the way. The provision for 

 this is as beautiful as it is benign ; for, to answer a physical adap- 

 tation, it is provided by a law of nature that when the tempera- 

 ture of water is raised, it shall expand ; as it expands, it must be- 

 come lighter, and just in proportion as its specific gravity is al- 

 tered, just in that proportion is equilibrium in the sea destroyed. 

 Arrived at this condition, it is ordained that this hot water shall 

 obey another law of nature, which requires it to run away, and 

 hasten to restore that equilibrium. Were these isothermal lines 

 moved only by the rays of the sun, they would slide up and down 

 the ocean like so many parallels of latitude — at least there would 

 be no break in them, like that which we see in the isotherm of 

 80° for September. It appears from this line that there is a part 

 of the ocean near the equator, and about midway the Atlantic, 

 which, with its waters, never does attain the temperature of 80° 



