28o DEPTHS OF THE OCEAN 



are here confronted with an inter-play of several different factors. 

 It is, by the way, worthy of notice that there is an interval of 

 twelve or thirteen hours between the two principal maxima of 

 temperature ; this agrees with the tide-period, and we know that 

 the velocity of the current varies with the tide. 



In previous investigations in the Norwegian Sea we have 

 several times encountered variations which are most naturally 

 explained by supposing that there are great undulatory move- 

 ments of the water-layers, and the investigations just described 

 strongly corroborate this supposition. The problem is one of 

 the greatest importance, and its solution will, in more ways than 

 one, lead to a fuller comprehension of the science of the sea, in 

 the first place with regard to the dynamics of the water-masses, 

 and in the second place with regard to certain biological 

 questions. The discontinuity-layer is often a boundary between 

 two different worlds of living organisms, and it is a point of 

 interest for the study of these to know if this boundary is 

 moving up and down, for this would probably imply that the 

 organisms themselves (possibly even shoals of fish) were also 

 being moved up and down. On the continental slope, just 

 below the edge, there live multitudes of marine animals, the 

 warm water having one characteristic fauna, and the deeper 

 cold water another. Now, if the fairly definite boundary 

 between the two water-masses swings up and down, one must 

 expect that there is a comparatively broad transitional region, 

 where the particularly hardy individuals of either of these 

 characteristic domains would live together. Where the change 

 of temperature is slow and regular the effect upon the organisms 

 would be of little importance ; not so, however, where there is 

 a marked discontinuity-layer, as for instance in the Norwegian 

 Sea. The proof that there are such oscillations would also be 

 of very great importance for our methods of studying the sea. 

 Let us look, for example, at Fig. 190, showing a section from 

 Shetland to the Faroe Islands, taken during the " Michael 

 Sars" Expedition on the loth and nth of August. The 

 positions of the stations are shown in Fig. 104, p. 122. 

 Isotherms are drawn at intervals of two degrees Centigrade ; 

 single hatching indicates salinities between 35.00 and 35.25 per 

 thousand, and cross-hatching salinities above 35.25 per thousand ; 

 in the deep layers the salinity was below 35 per thousand. 

 The lines both for temperature and salinity are strikingly wave- 

 like in the intermediate water-layers. The saltest water has 

 come from the Atlantic in the south, and the cold deep water 



