SECT. 5] WIND WAVES 695 



antennae used by radio astronomers. Because wave detectors are usually costly 

 to instal and maintain, it is not usually practicable to employ more than a few, 

 but as many as six detectors are being used in this manner by the Woods Hole 

 Oceanographic Institution for the study of wind waves (Farmer and Ketchum, 

 1960). By a judicious choice of unequal spacings it has been possible to increase 

 the angular resolving power of the array (Barber, 1959). 



Two fixed detectors have been used to examine the direction of approach of 

 swell to the New Zealand coast (Barber and Doyle, 1956) and three detectors 

 are currently being used for a similar purpose by the Scripps Institution, 

 California (Munk, 1959). Such simple arrays seem very primitive but can give 

 useful information about wave directions. With so few detectors it is best to 

 express the results as correlations (coherence and phase angle) between all the 

 various pairs of signals for each frequency that is examined. Wave directions 

 can be deduced from these coefficients. Indeed it has been realized that only 

 two detectors are necessary in principle (O'Brien, 1954; Barber, 1954), pro- 

 vided that they can be used in succession at a variety of different spacings so 

 as to build up a picture of the space-time correlogram. A Fourier transforma- 

 tion of the correlogram gives the directional power spectrum (see, for example. 

 Cox and Munk, 1954). But it is not usually practicable to move wave detectors 

 to new positions. 



As a further alternative it may be remarked that the pitch, roll and heave of 

 a simple floating buoy can give information about wave directions (Longuet- 

 Higgins, Cartwright and Smith, in press) about as much as could be got from 

 three fixed detectors. An array of buoys would give fuller information. The 

 general conclusion, however, is that a practical and rapid means of finding the 

 directional wave spectrum in the open sea has not yet been developed. 



References 



Numerous references are to papers in Proceedings of Conferences. To avoid repetition 

 of the details, these will be abbreviated in the references. The full descriptions are as 

 follows (the date is the date of publication, not of the conference) : 



Proceedings of the First Conference on Coastal Engineering, 1951, published by the 

 Council on Wave Research, the Engineering Foundation, University of California, 

 Berkeley, California. 

 Abbreviation: 1951, Proc. 1st Conf. on Coastal Eng. 



Similarly for subsequent conferences in this series. 



Proceedings of the First Conference on Coastal Engineering Instruments, 1956, pub- 

 lished by the Council on Wave Research, as above. 

 Abbreviation: 1956, Coastal Eng. Instruments. 



Proceedings of the First Conference on Ships and Waves, 1954, published by the Council 

 on Wave Research and the Society of Naval Architects and Marine Engineers, 1955. 

 Abbreviation: 1955, Ships and Waves. 



Proceedings of the Symposium on the Behaviour of Ships in a Seaway, 1959, published 

 by H. Veenman and Zonen, N.V., Wageningen. 

 Abbreviation: 1959, Behaviour of Ships in a Seaway. 



