angular harmonics a n , b n . For example, pressure fluctuations at one point give a , the 

 total energy; the shipborne wave recorder (see below) gives a , a x and b x \ a current 

 meter and pressure recorder give a Q , a x , b r , a 2 and b 2 . From a Q , a x and b t one could 

 determine, at least in principle, three parameters of a wave pattern, such as energy, 

 mean direction and angular spread (Longuet-Higgins, 1954). In practice these values are 

 subject to instrumental and statistical uncertainties, and it will be difficult to distinguish 

 between a narrow beam and a unidirectional beam. 



The problem can be put in another way. A unidirectional wavetrain of unit 

 energy has all its harmonics equal, so that if it comes from direction 6 = 0, an instru- 



Figure 3. The directional sensitivity pattern of a device measuring the zero order and first two 

 angular harmonics of the wave spectrum. The equation of the curve is r = 1 + cos + cos 20. 



51 



