296 THE PHYSICAL GEOGRAPHY OF THE SEA. 



are the store-houses in which the sui'plus heat of summer is stored 

 away against the severity of winter, and its waters continue to 

 grow warmer for a month after the weather on shore has begun 

 to get cooL This brings the highest temperature to the sea in 

 September, the lowest in March. Plate IV. is intended to show 

 the extremes of heat and cold to which the waters — not the ice — 

 of the sea are annually subjected, and therefore the isotherms of 

 40°, 50°, 60°, 70°, and 80° have been drawn for March and Sep- 

 tember, the months of extreme heat and extreme cold to the in- 

 habitants 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 between the same 

 isotherms (70°) for March and September. 



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

 benign influences 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 adaptations. 



854. How, let us inquire, does the isotherm 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 mitigating, and temper- 

 ing climates by the way. The provision for this is as beautiful 

 as it is benign ; for, to answer a physical adaptation, it is provided 

 by a law of nature that when the temperature of water is raised, 

 it shall expand ; as it expands, it must become lighter, and just in 

 proportion as its specific gravity is altered, just in that proportion 



