300 Introduction 



movements in the sea and allow a deeper insight into the basic causes of the phe- 

 nomena observed. This is not entirely sufficient, however, because water masses of 

 the ocean show quite large deviations from ideal homogeneity and their density de- 

 pends strongly on the temperature and salinity. It is therefore necessary to take into 

 account the stratification and compressibility of the water masses. Classical hydro- 

 dynamics is thus replaced by physical hydrodynamics, for which at the present time 

 only for some of the more important parts a precise theoretical basis has been de- 

 veloped. (Bjerknes and co-workers, 1932 or Godske, Bergeron, Bjerknes and 



BUNDGAARD, 1957). 



