polar coordinates, the location of points of equal phase . Our pur- 

 pose is to calculate the wave heights, that is to calculate q . 



It is convenient to define the geometric shadow as the zone 

 situated behind the jetty corresponding to the geometric shadow of 

 optics. 



I. GRAPHS 



Figures 1, 2, and 3 show three series of curves corresponding 

 to the following wave incidences? 



1. direction of propagation normal to the jetty, i.e., 

 normal incidence 3 



2. direction of propagation makes an angle of 4.5° with 

 the jettyj 



3. direction of propagation makes an angle of 135° with 

 the jetty. 



For each angle of incidence we have drawn curves showing the 

 distribution of wave height on the radius vectors drawn from the end 

 of the jetty and making a certain angle with the direction of the 

 jetty. The curves are drawn to r = 5 L, beyond that one may utilize 

 the asymptotic formulae. 



It is necessary to distinguish several zones; 



a.) within the geometric shadow, (i.e., 9<9 ) the height 

 decreases in a uniform fashion as one moves along the radius vector. 

 One may interpolate without difficulty between the different curves, 



b.) outside the geometric shadow, (i.e., 9 < 9<7f), the 

 height presents a series of maxima or minima, as functions of r/L 

 and 9. These maxima and minima may be very sharp and very close for 

 certain values of the angle 9$ for other values they are not so 

 evident or are quite separated. As an example of the first case we 

 may cite the curve corresponding to a value of 9 = 56° for an incidence 

 of 4-5 5 as an example of the second case we may cite the curve correspond- 

 ing to 9 = 67° for an incidence angle of U5° • For these reasons the 

 drawing of the curve is very laborious and presents great difficulty, 

 and for the same reason it is impossible to interpolate between curves. 

 It may be states however, that in the neighborhood of the origin there 

 exists a minimum which may be very sharp. When r increases indefinite- 

 ly the ratio of the amplitude to the amplitude in the open sea 

 approaches unity. 



c.) on the geometric shadow the amplitude decreases 

 regularly to a value 0.5 5 



17 



