384 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. 



nearly midway between the same isotlierms (70'') for March and 

 September. 



723. Plate IV. — A careful study of this plate, and the con- 

 templation of ^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 marvellous 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 instnimentality of 

 currents, for currents are the chief agents for distributing heat to 

 the various parts of the ocean. The sun with its 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 

 towards the poles, softening, and mitigating, and tempering 

 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 

 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 awa}^ 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° in September. More- 

 over, this isotherm of 80^ will pass in the North Atlantic, from 

 its extreme southern to its extreme northern declination — nearly 

 two thousand miles — in about three months. Thus it travels at 

 the rate of about twenty-two miles a day. Surely, without the 



