frequencies ojm ^i^d^^;^ to internal waves of the first mode occurs, producing 

 internal waves of amplitudes of several meters. These internal waves have the 

 same characteristics as the swell modulation. Their period coincides with that 

 of the modulation, and the wavelength is the same. Additionally, they travel in 

 the same direction as the modulation. 



The agreement between internal wave periods and the periods of the swell 

 modulations was tested. Data from the wave-height sensors for the time from 4 

 October 1966, 1900 to 5 October 1966, 0145, were used to determine the periods 

 of the modulations. The internal wave periods were taken from the analog 

 records for the time 3 October, 1500 to 5 October, 0300. Histograms of both 



10^ 



SECONDS 

 18 16 14 12 10 



^ 10 



I I I I I — \ r 



"1 r 



J L 



C/SEC 



Figure 3. Swell spedra of wove height sensors ' and 2 of positions near the NEL 

 Tower, and phase difference between both records (4 October 7 966, 1230-1330; 

 sampling rate, 1 second); the arrow indicates the 95-percent confidence limit. 



25 



