60 



IJJOKTS IIYDHOGRAPIIIC-BIOLOGICAL STUDIES 



The complete cycle of changes is shown in the following tables 

 compiled from Hjort's data : 



In the " typical winter period " (February and March) the tempera- 

 ture of the sea rises as one descends from the surface through the 

 successive layers of increasing salinity. We have just seen that the 

 minimum temperature is at the surface in the cold fresh water from 

 the Baltic, while the maximum temperature is at the bottom in 

 the salt Atlantic water. The interesting thing to notice is that at 

 this season of the year increase of temperature and increase of salinity 

 go together ; and that, within certain limits, the temperature of the 

 water during this period may be used as an index of salinity. Thus, 



as a rule : 



6° to 7° C. indicates Atlantic water. 



5° ,. North Sea water. 



4° to 3° 

 2° to 1° 



Bank water. 



the Baltic current. 



The establishment of this relation is valuable, since it enables the 

 temperature data of previous years to be translated — so far as the 

 winter season is concerned — into terms of salinity, and thus permits 

 to a considerable extent the study of the winter characters of the 

 water off the Norwegian coast for many years back. Hjort's results 

 under this head will be found below in the section on "Fisheries." 



3. Origin and Fate of Bank Water. 



The facts that bank water lies between Baltic water and North Sea 

 water, that its characters are for the most part intermediate between 

 those of these two layers, and that bank water is most abundant in 

 autumn and winter, when the Baltic current is usually at its lowest, 

 render it probable that bank water is produced by a mixture of the 

 summer water of the Baltic current with the subjacent water of the 

 North Sea. 



This view, at any rate, is taken by Hjort (pp. 22, 27), although 



