762 cox [chap. 22 



variety of directions. The half angle of the beam of directions cannot, however, 

 be larger than 0.4 radians. The coherence observed between other pairs of 

 stations drops off with frequency more rapidly than such a beam of directions 

 would permit. Changes of phase velocity of a few centimeters per second by 

 various causes are fully capable of causing the additional decrease of coherence. 



At frequencies below 0,05 c/min, neither the directional properties nor 

 changes in phase velocity of internal waves can account for the observed 

 coherences. By elimination, one is forced to conclude that irregular motions, 

 perhaps associated with turbulence, are responsible. Part of the irregular 

 motion can be contributed by instrumental noise in the thermocline followers, 

 but the contribution of this noise appears to be one hundred times below the 

 observed spectral density at low frequencies, while the observed coherence 

 indicates that the non-wavelike fluctuations have a spectral intensity one 

 third as large as that of the internal waves. 



The demonstration that fluctuations of isotherms are non-coherent within a 

 comparatively small number of wavelengths, and that the coherence even at a 

 small fraction of a wavelength is well below unity, emphasizes the importance 

 of making an unambiguous distinction between internal waves and other, less 

 coherent motions. The origin and nature of the latter are obscure. 



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