Chapter IV 



Generation, Growth and Propagation 



of Waves 



1. Observations of the Generation of Waves 



Observations on a large water surface show that, when the equilibrium 

 is disturbed, waves propagate from this point in all directions; the crests of 

 these waves are concentric circles. This is easy to understand, when we think 

 of the nature of wave motion. Pressure disturbances are transmitted from 

 the point of disturbance to the neighboring water particles and thus generate 

 a wave. It is more difficult to understand how a continuous force, like the 

 wind, blowing over the surface previously at rest, can put it into a rhythmic 

 oscillation. The effect of an air current on the water surface is not only the 

 development of a wind current, but also the generation of small horizontal 

 pressure differences which can cause wave formation. These pressure dif- 

 ferences are caused by the eddy motion of the wind, or its turbulence which 

 is the principal source of wave generation. 



We quote Scott Russell (1844, p. 317) and (Lamb, 1932, p. 630) the 

 description of the process of wave generation. "A wind velocity of less than 

 half a mile an hour (8| in. or 23 cm/sec) does not sensibly disturb the 

 smoothness of the reflecting surface. A gentle zephyr flitting along the surface 

 may destroy the perfection of the mirror for a moment, but when it disappears, 

 the surface is as smooth as before ; if the wind has a velocity of about a mile 

 an hour (45 cm/sec) the surface of the water becomes less capable of distinct 

 reflection, and it is to be noticed that the decrease of this reflecting power 

 is due to the presence of those minute corrugations of the superficial film 

 which form waves of the third order (capillary waves). At this first stage 

 of disturbance the phenomena on the surface cease almost simultaneously 

 with the intermission of the disturbing cause, so that a spot which is sheltered 

 from the direct action of the wind remains smooth. The waves of the third 

 order are incapable of travelling spontaneously to any considerable distance, 

 except when under the continued action of the original disturbing cause. 

 While it remains it gives that deep blackness to the water which the sailor 

 is accustomed to regard as the index of the presence of wind, and often as 

 the forerunner of more." 



