SEA WATER 239 



math of a heavy storm or the herald of wind to come. 

 Waves are of great importance in helping to keep the surface 

 waters of the sea mixed, while in the tidal zone they have 

 left their mark in the majiy adaptations shown by littoral 

 animals for protection against the pounding surf. 



Waves are generally the result of the action of wind 

 on the sea surface, from the faintest ripple caused by the 

 light airs of summer to the tumultuous mountains of water 

 raised by the full force of a winter's gale (Plate 83). The 

 size of the wave depends upon the strength of the wind 

 and upon the distance through which the wind can act. 

 The largest waves therefore occur in the oceans where 

 there has been sufficient distance over which the wind 

 has blown. Mr. Vaughan Cornish in his book on Ocean 

 Waves states that a fetch of some 600 to 900 miles is re- 

 quired to produce the largest weaves. The height of a 

 wave is the vertical distance between the summit of the 

 crest and the deepest part of the trough. Figures for the 

 height of waves in relation to the speed of the wind are given 

 by Cornish as follows : With wind of 31 miles per hour the 

 height of the wave is 21ft., at 50 m.p.h. it is 35ft., at 63.5 

 m.p.h. 45 ft., and above 75 m.p.h. about 70ft. ; these figures 

 are under conditions when the wind has had a long fetch. 

 In a wind of 30 m.p.h. the speed of the wave is about 

 25 m.p.h., at 50 m.p.h. it is 40 m.p.h., and at 68 m.p.h. 

 it is 54. 



The wave in the open ocean is of the type known as an 

 oscillatory wave. That is, while an undulation passes 

 through the water, after the wave has passed, the water 

 particles are still where they were and have not received 

 any movement in a horizontal direction Actually the 

 movement of the water particles is a circular one, backward 

 and upward on the lower half of the wave front, then forward 

 and upward to the summit of the crest, forward and down- 



