^ Ot = 1.2° 



[HeadAOc for gh of 0.7 



I'avo 

 and divide by 10). 



Figure 1 3. 



gram by use of a protractor is carried out below 

 for the case where contours are simple and a is 

 always less than 80°. The steps are fully ex- 

 plained under each figure. The protractor shown 

 is the type I. 

 C. Special Procedure 



The special method is shown for the tw o special 

 cases. First, when the angle of approach is greater 

 than 80° to the contours and, second, where com- 

 plex hydrography is encountered. The steps are 

 again explained under each figure. 



IV. DISCUSSION OF THE METHOD 



The method has given results in close agreement 

 with those obtained by the wave front method, and 

 apparently more nearly correct. See figures 19 

 and 20. 



Its particular advantages are: 



1. Speed — orthogonals can be constructed in 

 about a fourth of the time required by the wave 

 front method. 



2. Independence of the instrument from the 

 chart scale and wave period. 



3. The elimination of an entire step in the con- 

 struction of refraction diagrams. 



The disadvantages are: 



1. A higher degree of training is required for 

 the use of the protractor than for the wave front 

 method. 



2. It is not possible to draw an orthogonal from 

 shore seaward as is the case of the final step in the 

 wave front method. This is not an actual com- 

 parative disadvantage, however, for in the wave 

 front method the wave fronts first must be 

 drawn to shore before the orthogonals can bo 

 drawn seaward. 



21 



