This wave now plays a role similar to the wave a^ at t = 0, where- 

 as the wave a-, after about 21 seconds is to be considered as the 

 "middle wave" of this group. The former front wave b, has almost 

 disappeared after 21 seconds, while it seems as if this wave is re- 

 placed in the group by the former "middle wave" bg. The "group" 

 as a whole remains behind the waves. This well-known property of 

 wave groups is observed very often at sea and the wave profiles in 

 Figs. 8 and 9 show these variations by superposing the three char- 

 acteristic waves evaluated in Chapter 2, The displacement of the 

 highest (not individual) waves in Fig. 9 would lead to a "group 

 velocity" of about 8 m/sec, whereas the steepest characteristic wave, 

 the "sea" at a wind velocity of 19 nv/sec has a velocity of phase 

 propagation. of 16.4 m/sec (T, = 10,5 sec, according to formula (la), 

 ar^ = (g/2Tr)T3^). 



Theoretical "wave records" or "wave profiles" like those in 

 Figs. 8 and 9, are based on the assumption of three characteristic 

 waves, and are valid in the fully developed state of wave motion. 

 If the wave motion is not fully developed, it is first necessary to 

 calculate the dimensions of the characteristic waves in their dif- 

 ferent stages of generation at given wind velocities. In any case, 

 it Seems possible after these computations, to evaluate character- 

 istic wave profiles or "wave records" for practical purposes. These 

 profiles allow us to predict some striking features of composite 

 wave motion as required by practice, for example: 



(1) the periods of succeeding waves, the heights of succeeding 



waves and their steepness. The range of these elements 



and their average and maximum values* 



34 



