136 



WIND CURRENTS AND WIND WAVES 



have been compiled by different authors. Table 15 contains one of the 

 compilations made by Kriimmel, from which it is evident that the 

 observed values are in fair agreement with the theoretical expectation. 



Table 15 



The longest wave periods observed at sea rarely exceed the value 

 of 13.5 sec that Cornish reports from the Bay of Biscay. It has been 

 established, however, that the swell reaching the shore may have much 

 longer periods and correspondingly longer wave lengths and greater 

 velocities of progress. The longest period that Cornish has observed is 

 about 22.5 sec, corresponding to a wave length in deep water of about 

 850 m and a velocity of progress of 35 m/sec. This difference between 

 the waves of the open ocean and the swells that reach the coast will be 

 dealt with later. 



The theory of waves leads not only to a relation between the wave 

 length and the velocity of progress, but also to results concerning the 

 profile of the wave. These results are based on the concepts of classical 

 hydrodynamics and have been derived from the hydrodynamic equations, 

 omitting friction but taking the boundary conditions into consideration. 

 In an ideal fluid the free surface of the waves, according to Stokes, will 

 very nearly take the shape of a trochoid — that is, a curve that is formed 

 by the motion of a point on a disk when this disk rolls along a level sur- 

 face. If the amplitude is small, the trochoid approaches in shape a sine 

 curve, but at great amplitude the crests become narrower and the troughs 

 longer. 



