90° and 135° with the objective of verifying Equation U- The test 

 basin, the same which was utilized by Mr. Dixon, was 25 meters 

 (82.5 feet) long by U meters (13.2 feet) wide, and 0.5 meter 

 (1.65 feet) deep. Since maritime ports are established always in 

 relatively shallow depths, we limited ourselves to the case of 

 diffraction in shallow water. Although very simple in appearance, 

 this condition necessitates the use of a basin with an absolutely 

 horizontal bottom, which was not easily obtained. It is known, 

 in fact, that in shallow water the wave refracts and reflects in 

 relation to the bottom that it encounters. The modifications in 

 propagation which result may cause, sometimes at large distances 

 from their original topographic cause, notable wave reinforcement 

 and interference resulting in augmentation or diminution of the 

 instantaneous height. We think however, that we have limited this 

 cause of error because the difference in elevation between any two 

 points of the bottom of the basin was, on the average, of the order 

 of a 0.5 mm. and never exceeded 1.5 mm. 



The apparatus used in the basin was the same as that used by 

 Mr. Dixon. The wave generator was a three -section paddle driven by 

 a 2 HP AC motor and gear box. Absorbent beaches of round gravel 

 were used to surpress reflection at the basin end and along the 

 wall of the basin sheltered by the jetty. A straight jetty with 

 vertical walls was built from one of the longitudinal walls of the 

 basin at a distance of 9 meters (29.7 feet) from the wave generator, 

 leaving an opening of 1.2 meters (3.96 feet) minimum width. To 

 avoid reflection of the incident wave on the jetty an absorbent 

 beach was built in front of it$ a flat guide of sheet metal 0.5 

 meters (1.65 feet) long, placed perpendicular to the extremity of 

 the jetty avoided any lateral reduction of the wave penetrating 

 into the opening. Wave heights were measured by means of a resistance 

 gauge 5 the variations of voltage at the electrodes of the sounding 

 gauge as a function of the depth of immersion of the points of the 

 electrodes were registered photographically through the medium of 

 a cathode oscillograph. 



The results obtained with a wave 76 cm. long in a depth of 

 water of 13 cm. are represented by points on Figure U° The curves 

 of this figure correspond to equation U for the angles of incidence 

 9 Q = 45°, 90°, and 135°. The portions of the curves that are 

 dotted correspond to the region where 



r ^ 1 



< 



L £ sin £ -f- 



i.e. where the approximation of Equation 2 is no longer valid. The 

 curves give values that are greater than true values in this region. 



23 



