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WIND CURRENTS AND WIND WAVES 



between the two, and it is seen that this roughly corresponds to the 

 observed pattern. The discrepancies are accounted for partly by the 

 fact that the record was obtained only about 300 m from the beach, 

 where the depth to the bottom was about 6 m, for which reason the waves 

 were somewhat deformed, and partly by the fact that waves of shorter 

 periods apparently were present. Such relatively clear-cut cases are 

 rare because, for the most part, waves of so many different period lengths 

 are present that the resulting pattern of interfering waves is extremely 

 complicated. The lower curve in the figure reproduces a record which 



Fig. 36. Upper curve: Record of waves at the end of the Scripps Institution pier, 

 showing interference. Middle curve: Computed pattern of wave interference. Lower 

 curve: Example of the ordinary type of records of waves at the Scripps Institution 

 pier, showing very complicated conditions. 



has been selected at random and which shows isolated high waves occur- 

 ring at apparently irregular intervals. 



These considerations help to explain the occurrence of a sequence of 

 high waves followed by a sequence of low ones, but the}^ do not explain 

 the irregular pattern of ''waves" that is called "cross sea." Progress 

 toward explaining the typical cross sea, however, has recently been made 

 by H. Jeffreys. Jeffreys calls the waves that have been discussed so 

 far long-crested waves, because it has been assumed that the crest of the 

 wave is very long compared to the wave length. Mathematically 

 speaking, it is assumed that the wave crests are of infinite length. Jeff- 

 reys had introduced the so-called short-crested waves, which can be repre- 

 sented by the formula 



^ = a cos {kx — (Tt) cos K'y, 



