104 



Generation, Growth and Propagation of Waves 



48J n.m. per hour. The wind-sea might have had periods of 8, maximum 

 9 sec i.e. velocities of 24 to 27 n.m. per hour in the storm region, and the 

 velocity of propagation of the swell can be assumed to have been accelerated 

 with increasing age. Thorade wanting a more precise answer to this question 

 has submitted the values of Table 14 to an analysis by the method of the 

 least squares and found that the observations are in somewhat better 



Table 14. Tracking a heavy North Atlantic swell from 25th February 



to 1st March 1! 



agreement with the hypothesis of accelerated progression of the Poisson- 

 Cauchy theory than the hypothesis of uniform velocity. Unfortunately, no 

 other similar cases are available for a check on the theory, although the 

 observations for this could probably be found in the ship journals. The 

 questions, how swell propagates in the ocean, how their wavelength and 

 period increase in progressing belonged until recently to the unsolved problems 

 of ocean waves. Thanks to the new theory of Sverdrup and Munk (1947), 

 we now know more about the development of the swell, even though the 

 observations along this line still leave very much to be desired. 

 (b) Theory for the Decay of Waves and Application of Same to the Swell 

 In order to survey the values characteristic of the swell, it will be suitable 

 to realize the conditions as given by Fig. 46 a. At the point P a swell is 

 observed whose characteristic wave values are H D and T D , while t D is the 

 time of occurrence. This swell component comes from the point A on the 

 windward side of the area of wave generation with a fetch F and a width B. 

 The lateral spreading is given by the angle 6, and at the time t = the wave 

 values in A are H F and T F . The problem of the swell consists in computing 

 the transformation of the waves outside their area of generation. 



