may exist at all depths, but higher frequencies are ex- 

 cluded from depths having low stability frequency. A 

 wider band of frequencies existing at depths of ^ 

 sustains a frequency channel for internal waves by a 

 selective process, as compared to a retaining process for 

 the sound channel. Each frequency transports energy at 

 its group velocity, and the frequency channel is therefore 

 a channel for horizontal flux of energy, which is the same 

 net effect of the sound channel. 



Little is known about the relative importance of the 

 various modes of internal waves in the ocean. On the other 

 hand, the nature of the stability frequency is such that fre- 

 quencies near the maximum of N must have their largest 

 displacements in the frequency channel. Waves of frequency 

 lower than N ■ have no such requirement, and it is not 

 known at what depths the various modes concentrate the 

 horizontal flux of energy. An estimate can be based upon 

 simple formulae. Starting with the energy equation for 

 internal waves and the approximation 



- 1 dp o 

 p = pe — _2- z, 



p dz ' 



o 



Kraus (1964) arrived at an expression for total potential 

 energy per unit area (horizontal) of the nth mode: 



dp 



E . - \g—-±a 2 H 

 pot p dz n 



where a is amplitude of the nth mode, H is total depth, g 



is the acceleration due to gravity, p is the zero-order 



density of seawater, and E is total potential energy per 



unit horizontal area. Energy per unit volume is estimated 



from 



dp 



E + - ig—-^a 2 

 pot p dz n 



H 



*Book in process 



