GENERAL BIOLOGY 



709 



we see that the surface temperatures of the North Atlantic 

 change very considerably from February to August. In 

 February the isotherm of 15" C. follows approximately the 40th 

 degree of latitude, while in August it reaches the north-western 

 corner of Iceland, north of the 50th degree. The isotherm of 

 10' C. has in February a course approximating to that of the 

 15 isotherm in August, when the isotherm of 10° runs far 

 north in the Norwegian Sea, where the seasonal difference is 



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Yic. 509.— Variation of Temperature according to Depth during different 

 Seasons, off the Norwegian West Coast. 



Still more pronounced. Fig. 509 shows the vertical distribution 

 of temperature during approximately fifteen months, as observed 

 by me in the 'nineties of last century while making repeated 

 investigations in one locality off the west coast of Norway. 

 We perceive that during the summer months warm temperatures 

 occur in the upper 50 metres, temperatures which during 

 winter we can find in the Atlantic only south of the 40th degree 

 of latitude (see Fig. 159, p. 227). During autumn high 

 temperatures (8° C.) pass down through the water column, so 

 that towards the close of the year the warmest water is found 

 at 250 metres. At the same time the surface -layers cool 



