J. Crease 137 
face tide to the internal tide. Rattray [11] (1960) has considered this problem in 
some detail and given numerical examples. It is perhaps worth quoting Nansen 
(1909), who said soon after the first observations were made that tidal waves 
crossing from the North Atlantic into the polar basin across a ridge might be 
expected to generate internal waves. The presence of semidiurnal waves in open 
ocean at the Meteor station might now perhaps be accounted for by its location, 
the mid-Atlantic ridge. Further effects of submarine topography will be found 
in the production of lee waves. Frassetto [12] (1960), for example, has obtained 
records with a thermistor chain in the straits of Gibraltar which suggest the 
possible presence of lee waves. 
In conclusion, theory suggests that free internal waves will exist in a band 
of frequencies f <o<WNmax. It appears that the spectrum has the character of noise 
with discrete tidal frequencies and lee waves appearing at abrupt changes in 
depth, such as the continental shelves. Significantly, the outermost of Reid's 
stations referred to above, although showing no coherent picture, did contain as 
much mean square fluctuation as the innermost station with its regular oscilla - 
tions. This could be explained by the waves' breaking and distributing their 
energy over a broad band of frequencies. 
REFERENCES 
1. O.S. Lee, "Effect of an Internal Wave on Sound in the Ocean,” J. Acoust. Soc. Am., Vol. 33, 5 (1961). 
2. H. Lamb, Hydrodynamics, 6th ed. (Cambridge, 1932). 
3. J.E. Fjeldstadt, "Interne Wellen,” Geofys. Publikasjoner, Vol. 10, No. 6 (1933). 
4, C. Eckart, Hydrodynamics of Atmospheres and Oceans (Pergammon, 1960). 
5. P. Groen, "Contribution to the Theory of Internal Waves," Kon. Ned. Met. Inst., B, II, 11 (1948). 
6. B. Haurwitz, H. Stommel, and W.H. Munk, "On the Thermal Unrest in the Ocean,” Rossby Memorial 
Volume, 74-94 (1958). 
7. C. W. Ufford, "Internal Waves Measured at Three Stations,” TAGU, Vol. 28, 1 (1947). 
8. A. Defant, Physical Oceanography, Vol. II (Pergammon, 1961). 
9. "Power Spectrum Analysis of Internal Waves from Operation Standstill,” USHO, TR-26 (1955). 
10, J. L. Reid, "Observations of Internal Tides in October 1950," TAGU, Vol. 37, 3 (1956). 
1l. M. Rattray, "On Coastal Generation of Internal Tides,” Tellus, Vol. 12, 1 (1960). 
12. R. Frassetto, "A Preliminary Survey of the Thermal Microstructure in the Straits of Gibraltar," Deep- 
Sea Research, Vol. 7, 152 (1960). 
13. B. Helland-Hansen, and F. Nansen, "The Norwegian Sea,” Report on the Norwegian Fisheries and 
Marine Investigation, II, No. 2, Bergen (1909). 
DISCUSSION 
MR. D. E. WESTON commented upon the considerable fluctuations in level 
found inmost sound propagation experiments which could arise from multiple-path 
interference effects of various kinds, effects of reflection from a moving surface, 
and inhomogeneities in the water, of which internal waves are a special case. 
Because it is difficult to state the precise cause of the fluctuations, in any investi- 
gation of the effects of internal waves on acoustic fields care should be taken to 
arrange special experiments in the regions where high-amplitude internal waves 
are known to exist. Mr. Weston also sought information on the usefulness of bio- 
logical markers. He mentioned the existence of the sound scatterers which con- 
gregate at thermoclines, so permitting the latterto be recorded on echo sounders. 
There was also the phenomenon of "slicks", whichhave recently been shown to be 
associated with the periodic convergences over internal waves; these may possi- 
bly be rendered visible by biological effects. 
