(b) when u-i is greater than 2 and U2 less than -2 the 

 amplitude has as its asymptotic value the modulus of the following 

 complex expressions 



1 cos-gL sine* — i ^rrr _ jJL 



A=l + 1 2 ^— e L 4 



rp cose — cosQ n 



In this zone it does not appear that the variation of the second 

 term of A can be greater than + 0„l6„ 



(c) when ui is smaller than 2 it is necessary to expect in 

 general a maximum of amplitude on the curve 



and a minimun on the curve 



x = 0,92 



VI 



9Z+Zy 



the axis Ox being the perpendicular of the polar angle 9 and the 

 axis Oy the perpendicular of the polar angle Q -{- JZL 



2 



We must now examine in what Treasure the preceding theory re- 

 presents the real phenomena,, 



In the article cited above Messrs Putnam and Arthur give the 

 results of experiments made at the University of California, They 

 were conducted in deep water , i<,e„ in a depth greater than L/2 and 

 they include angles of incidence of 4-5° , 90° and 135° j the height 

 of the diffracted wave was measured along parallels to the jetty and 

 compared to the corresponding theoretical curve,, These parallels were 

 situated at distances from the jetty varying from 3„38L to 17„85L, 

 with however, a predominance for the larger distance „ The agreement 

 was good between theory and experiment within the geometric shadow,, 



In a thesis submitted to the Faculty of Science of Grenoble, in 

 July 1949s and prepared in the Laboratoire Dauphinois d ! Hydraulique 

 Neyrpic, Mr„ Dixon studied the same problem for normal incidence 

 in a zone which had not been studied by Messrs,, Putnam and Arthur „ 

 The lines on which Dixon measured the height were situated at 

 distances from the jetty varying from 0„41L to 3«35L. Furthermore, 

 the study was conducted in shallow water, i,e» in a depth smaller 

 than one-half wave length (depth of 12 ci, for a wave length of 

 60 cm), The agreement with theory was good in the interior of the 

 geometric shadow and in the neighborhood of the jetty end„ 



Finally, in the course of studies on diffraction now under 

 way in the laboratoire Dauphinois d ' Hydraulique Neyrpic we have 

 made measurements along the jetty itself for incidences of 45°, 



21 



