1 he subject of submarine waves, i. e. of waves in the boundary between two layers of 

 water of different density, is one that has of late attracted much attention from the part 

 of hydrographic writers. It has been pointed out, especially by Scandinavian authors, 

 that changes in the salinity and temperature distribution of the sea may be successfully 

 treated as the consequences of immense boimdary-waves, following the laws of propagation 

 found by Stokes. ' For example, in their great work on the Hydrography of the Norwe- 

 gian Sea, Helland-Hansen & Nansen 2 mention the formation of such waves as a possible 

 cause (among others) of the »puzzling variations or irregularities, found in the horizontal 

 distribution of the temperature, salinity, and density in the upper strata of the sea ». These 

 authors also hint at the possibility of boundary -waves being created under tidal influence, 3 

 though their observations are too few and too far apart to allow definite conclusions in 

 that respect. 



The first to have definitely proclaimed the existence of boundary tides is O. Petters- 

 son, who inferred this from researches in the »Store Belt » in 1907. 4 The salt water of the 

 Kattegat, penetrating into the Baltic across the bottom-ridge of the Store Belt, is separated 

 from the generally outwards flowing fresher water of the Baltic by a thin stratum of in- 

 termediate salinity. Pettersson found that, synchronously with the tidal pulsations 

 of the sharply def ined system of surface- and bottom-current, the isohalines of that inter- 

 mediate stratum underwent elevations and depressions of an amplitude far exceeding that 

 of the ordinary tide as observed in the surface. This phenomenon he ascribed to boundary - 

 waves caused in some way by the tides and possessing the tidal period. He attachés 

 to these waves an interest reaching beyond that of mere curiosity by attributing to 

 them a dominating influence upon the circulation of the Baltic, thereby replacing the 



1 On the theory of oscillatory waves, Math. and Phys. Papers, vol. I. 



8 The Norwegian Sea, Report on Norwegian Fishery- and Marine Investigations. Vol. II, Part. I. 



3 Cf. loc. cit. p. 105: »We consider it probable for instance that the tidal waves passing into a basin across 

 a suboceanic ridge like that between Scotland and Greenland may give regular impulses such as these [to the for- 

 mation of boundary- waves]. It is even possible that the tidal waves may t hus to some extent be transformed into 

 boundary-waves which will advance with much reduced velocities. » 



4 Strömstudier vid Östersjöns portar, Svenska Hydrografisk-Biologiska Kommissionens skrifter III. (Resumé 

 in German. ) 



