EXPLANATION OP OCEAN TEMPERATURE CHART No, 38. 



The air and surface isotherms of 40°, 45°, and 50°, Fahrenheit, at the light-house stations o*f the eastern coast of 

 the United States, during the years 1881 and 1883. 



This chart has been prepared to permit of a comparison of the surface with the air isotherms at the several light- 

 houses and light-ships now under consideration. It has been noticed by previous observers that, in certain locali- 

 ties, the rise and fall in the snrfaoe temperatures maintain a nearly constant relation to the rise and fall in the air 

 temperatures at the same place. For example, the surface temperatures of 40°, 45°, and 50° may follow the air tem- 

 peratures of the same value at more or less regular interyals, and the length of these intervals may be sufficiently 

 uniform to permit of a prediction of the surface temperature several days in advance, with approximate accuracy. 

 Such predictions would be of great practical value in determining the time when schools of those fishes that regulate 

 their migrations by the surface temperature of the water might be expected at certain fishing grounds. It is very 

 probable, for example, that the spring migrations of mackerel and menhaden are regulated mainly, if not entirely, by 

 changes of surface temperature. 



The writer has made many plottings of the air and surface isotherms conjointly, without discovering a constant 

 ratio at any of the stations. The two sets of isotherms shown on the accompanying chart are presented as illustra- 

 tions of the variations that occur. 



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