120 A TEXTBOOK OF OCEANOGRAPHY 



When waves are short i.e., when the depth is great in 

 comparison with the length of the wave, as in the case of 

 ordinary waves in the open sea the motion is not sensible 

 except near the surface, where each particle moves uniformly 

 in a circle. 



In shallow water, where the length of the wave is great in 

 proportion to the depth of the water, each particle moves in 

 an ellipse. 



In the tide-wave travelling along a channel the water is 

 travelling forward with its greatest speed at the time of high- 

 water or at the top of the wave. When the water is at its mean 

 height its velocity is o that is, it is still water. When the 



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FIG. 20. MOTION OF A SURFACE PARTICLE ix A TIDAL WAVE 

 ix DEEP WATER. 



water is at its greatest depression it is running backwards 

 with its greatest velocity. 



Consider the motion of a surface particle in a tidal wave in 

 deep water. The periodicity is 12 hours 24 minutes. Let the 

 wave move to the right. At the commencement of the motion 

 we have high-water, at 3 hours 6 minutes it is mid-water, at 

 6 hours 12 minutes low-water, at 9 hours 18 minutes mid- 

 water again, and at 12 hours 24 minutes again high-water. 



From o hours to 6 hours 12 minutes the particle moves 

 from its highest to its lowest level. This part of the vertical 

 movement is the ebb of the tide; from 6 hours 12 minutes to 

 12 hours 24 minutes the particle moves from its lowest to its 

 highest position this is the flood-tide. The vertical move- 

 ment is not felt as a stream, only the horizontal ; and this in 



