EXPLANATION OP OCEAN TEMPERATURE CHART No. 38. 



The air and surface isotherms of 40°, 45", and 50°, Fahrenheit, at the light-house Btations of the eastern coast of 

 the United States, during the years laSl and 1883. 



This chart has heen prepared to permit of a compsirison 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 surface 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 intervals, and the length of these intervals may bo sntBciently 

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

 Such predictions would he 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 he 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 eorface temperature. 



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

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

 tions of the variations that occur. 



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