50 Observations and Measurements of Ocean Waves 



pictures. These pictures do not give any information as to the motion of the 

 waves. We might consider perhaps the instantaneous pictures of the sea surface 

 between two wave crests or between two wave troughs as a picture giving 

 the change of the surface during a wave period. Kohlschuetter has interpreted 

 the pictures of the sea surface at a definite interval as a wave motion and 

 to derive from this motion the orbits of average waves. A complete wave 

 length was divided in twelve equal parts and around each division he placed 

 an orbit according to the trochoid theory, but instead of taking the corres- 

 ponding point of the trochoid, he took the point of the real wave profile 

 lying vertically beneath the trochoid point. Although these twelve points 

 are side by side in space they are considered to succeed each other in time. 

 These twelve points were then considered as an orbit of the observed wave 

 profile. This is only permissible if the wave profile remains constant during 

 its travel, which might be acceptable for one wave length if the movement 

 is harmonic. Figure 30 represents one of the six cases on which Kohlschuetter 



Fig. 30. wave profile according to stereophotogrammetric pictures taken on the 



"Planet"; , trochoid of the same height; , real orbit (wave height not exagerated 



in height) (Kohlschiitter). 



worked. The wave profile shows again the characteristic deviation from the 

 form of a trochoid. On the left, the poi-nts give the derived orbit. The troughs 

 are flatter and the crests are steeper, and, therefore, the orbit is not circular 

 but oval, with the pointed end on top. These experiences of Kohlschuetter 

 have not been repeated recently, probably because the fundamentals are not 

 considered too accurate. 



A good way to broaden our knowledge in the mechanics of ocean waves 

 would be to take a stereophotogrammetric moving picture. This would be 

 essential for the research on orbits. 



5. Apparent and Real Characteristics of Waves, the Complexity of Wind- 

 Generated Waves 



In former timeS, the numerical investigation of the wind-generated sea 

 consisted in determining some average value of wave period, wave length 

 or propagation velocity, and wave height. Such average values apply only 

 to ideal wave trains, that is, to an infinite succession of congruent waves 

 of permanent form. Therefore, it was not to be expected to find definite 

 relations between these quantities on the one hand and to the wind force 

 on the other. The wind does not generate a single wave, and especially not 

 a simple harmonic wave. As soon as the first formation of waves appears 



