When using Plates II to V it should be borne in nind that the 

 curves are constructed on the assumption that a constant wind sud- 

 denly starts to blow over an undisturbed water surface. If the 

 wind velocity changes gradually, an average velocity has to be 

 introduced according to rules which are discussed when dealing 

 with the practical applications. Also, allowances laust be iiiade 

 for- waves that are 'oresent when the wind starts blowing. 



Some other characteristics of the grovdng waves are shovra in 

 Figure 9- In the upper curve the wave steepness as expressed by 

 the ratio H/L is plotted against the fetch for a wind of 30 knots. 

 The curve shov/s the steady state and the horizontal lines show 

 the stage of development after 10, 20, and 30 hours. Before a 

 steady state has been reached, that is, v;hen the duration is 

 shorter than the minimum duration, the steepness decreases with 

 time, and when a steady state has been established it decreases 

 v/ith fetch. 



In the lower curve of Figure 9 the wave age as expressed by 

 the ratio, v/ave velocity to wind velocity, C/U, is plotted against 

 fetch. The wave age increases with duration before the minimum 

 value is reached and with fetch after the establishment of a steady 

 state. 



If the corresponding values of H/L and C/U are plotted in a 

 graph with wave steepness, H/L, and wave age, C/U, as coordinates 

 they fall exactly on the curve in Figure 7, which represents the 

 empirical data. Actually, this curve has been used for determin- 

 ing the constants needed for carrying out all computations. By 



24 



