4 PHYSICAL NATURE OF WIND WAVES 



given instant, throughout the whole of the depth zone that is affected 

 by the wave. Thus, at the instant of passage of the top of the crest, 

 all the water particles that lie below it are moving horizontally for- 

 ward. During the passage of the back of the crest, all of the water 

 particles below it are moving first obliquely forward and downward, 

 next perpendicularly downward, then obliquely downward and back- 



Figvke 2. — Directions of orbital movement of water particles in different parts 

 of wind waves that are advancing in the direction of the long arrow. (Adapted 

 from Berget.) 



Figure 3. — Photograph showing movements of water particles in a wave in water 

 so shallow that their orbits are elliptical. The exposure was for half a wave 

 period. (After Alborn.) 



ward. At the instant of passage of the bottom of the trough, all of 

 the water particles below 7 it are moving horizontally backward. 

 During the passage of the front of the next crest, all the water particles 

 below it are moving first obliquely upward-backward, next vertically 

 upward, then obliquely upward-forward, to move horizontally for- 



