Chapter III 



Observations and Measurements 

 of Ocean Waves 



1. Testing of the Theory by Laboratory Experiments 



The first attempts to check experimentally the theoretical results of the wave 

 theory date back as far as Leonardo da Vinci (1452—1519), who always 

 endeavoured to support his views by experiments. The first systematical 

 experiments in wave tanks were made by the Weber brothers (1825). However, 

 the wave tanks used by them were not large enough and too narrow to avoid 

 disturbances at the ends. The results were also influenced by friction at the 

 side walls. Later on, Scott Russell (1845, p. 311) undertook measurements 

 under more favourable circumstances; he was principally concerned with 

 a kind of waves different from the wind waves of the oceans (waves of trans- 

 lation) (see p. 116). Later on the theories by Stokes, Airy and others have 

 been tested in hydraulic dynamic laboratories which were better equipped. 

 But the depths in wave tanks are generally small in proportion to the generated 

 wave lengths, and the term ''surface waves" can only be used with certain 

 reservations. At the bottom of the tank the water particles are still in motion, 

 and the friction at the bottom is liable to retard the propagation of the waves. 

 Recent experiments with long and relatively deep tanks are significant and 

 important, as they permit a survey of the principles on which the whole wave 

 theory is built. 



We shall discuss the experimental tests made during 1938 and 1939 by 

 Mitchim in the hydraulic laboratory of the University of California (1940). 

 The tank had the shape of a canal with a length of slightly over 18 m, a depth 

 of 92 cm and a width of 30 cm. Through glass walls one could observe and 

 photograph waves travelling through the canal. He made 28 series of ob- 

 servations on waves with a constant wave length varying between approx- 

 imately I and lim. Such waves can still be considered as surface waves 

 in proportion to the depth h because hjX did not become smaller than 05; 

 according to Table 1 and Fig. 9, however, this is the lower limit for such 

 waves. We will show in diagrams the main results of the extensive observations. 

 Figure 17 shows for each series the relation between the velocity of pro- 

 pagation and the wave length and the solid line corresponds to the theoretical 



