Step 5. — Measuring Aai with protractor pinned at Pi, 

 the line P2O2' is the new direction of the orthogonal. 



"V\ hen a drafting machine is used, Aa is turned on the 

 protractor head. Aa is always turned in a direction so 

 that a is decreased except in the rare case when the wave 

 is progressing from shallow to deeper water. In this case 

 Aa is turned so that a is increased. 



Step 6. — At P2, 02 is measured in respect to a line P2P2' 

 which is drawn in by ej'e. P2P2' has a direction midway 

 between the directions of the contours dsda' and didi at the 

 points where they intersect the line PiOi. A 02 is turned 

 off at P2 and P3 is established in the same manner as P2. 

 This is continued to the beach. In the example Aa at 

 P2 was greater than 13°. Thus P2' and P2'' were estab- 

 lished using the line P2P2' as an intermediate contour with 



J — values of 10.5 on each side. This step is rarely neces- 



sary in practice. The true orthogonal corresponds with 

 the line Pi . . . P5 only at the, intersections with con- 

 tours. ' For all ordinary purposes, however, the line 

 Pi . . . Ps is an orthogonal. 



Fisure 17. 



26 



