ON THE SURFACE TEMPERATURE OF THE NORTH 

 SEA AND OF THE NORTH ATLANTIC. 



By D'ARCY WENTWORTH THOMPSON. 



It is generally recognised that we cannot hope to understand the 

 ways of life of Fishes and other inmates of the sea, unless we first 

 acquire a sound knowledge of the sea itself and all its physical con- 

 ditions. With these physical conditions of the sea, the life and 

 habits of the fish are continually bound up. The temperature, the 

 salmity, the contained gases of the sea must be so ordered and 

 regulated as to be in keeping with the fishes' vital needs ; as they 

 alter, the fish will be affected for better or for worse ; as they vary 

 from place to place, one species or race of fish will tend to make way 

 for another. Abnormal conditions may lead to scarcity, or some- 

 times to unusual abundance of fish ; and such changes as come year 

 by year with the revolving seasons must be in great part the cause of 

 the fish's periodic movements and migrations. 



With the exception of the Tides, whose immediate practical 

 importance has led to their being studied with especial care, we 

 know more about the Temperature of the sea, and its variations 

 from place to place and from season to season, than we do about 

 any of the other physical phenomena with which the science of 

 hydrography deals. But even here our knowledge is still far from 

 adequate and far from accurate. In the first place, though we know 

 a great many important and fundamental facts about the temperature 

 of the deeper waters, our knowledge of the seasonal and other fluctua- 

 tions of these deep-sea temperatures is very limited indeed ; and, in 

 the- second place, though we know a good deal more about the dis- 

 tribution and the periodic variations of temperature at the surface of 

 the sea, yet, in the narrow seas, such as the North Sea and all our 

 coastal waters, even the surface temperature presents us with 

 problems of very great complexity, and as yet we can by no means 

 claim a complete understanding of all its various phenomena. 



In such a phenomenon as Temperature, whose interest lies 

 chiefly if not wholly in its fluctuations, our knowledge grows slowly, 

 and by successive well-marked stages. We learn, to begin with, the 

 average temperature of the sea in some particular localities. As 

 we study it in a larger and larger number of places we come to see 

 the gradual changes, and the underlying laws, which connect one 

 locality with another ; till at length (though our points of observation 

 may not be very numerous) we are enabled to map out the mean 

 temperature over a large area of sea, or a long stretch of coast-line. 

 All physical science rests upon this principle of Continuity, this 

 tendency of phenomena to vary in an orderly way, from place to 



M&GLtd Wt 5177/392 2-16 750 G. 2 



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