Before proceeding with, a discussion, of how the two separate 

 models were approached, it mi^ht be well to mention that the 

 general conclusion reached by most investigators is that the 

 greatest single factor in determining the nature o!^ the thermal 

 structure at any one time is the internal wave of tidal period. 

 Consequently most of the investigations have centered around 

 waves of semidiurnal and diurnal periods, 



2. The two-layer system 



This problem resolves itself mainly into a consider- 

 ation of the depth and the amplitudes of the oscillations of the 

 thermocline, because of the fact that the maximum amplitude of 

 the wave occurs at the interface betvreen two layers of differ- 

 ent densities. It also can be shown, from the boundary conditions 

 and the fact that density Is constant through each layer, that the 

 amplitude must decrease linearly from the interface and approach 

 zero at both tiie free surface and the bottom. 



Although the problem of the internal wave was first 

 attacked by Stokes (Lamb, 1932), who derived a good m.any of 

 the relationships, the subject did not excite much practical 

 interest until the work of Ekman (I90i|). Ekman used the concept 

 of the internal wave to explain the peculiar phenomenon of 

 "dead water" experienced by slow moving sailinr: vessels and 

 steamships of that period. He shov^ed that v/hen "dead water" 

 vjas experienced, there existed a layer of light, fresh wai-er 

 overlying a heavier, more saline water belov/. The energy 

 which the ship usually used to overcome the resistance ox" the 

 vrater itself went instead to thfe generation and maintenance of 



33 



