I20 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 in a Tidal Wave 

 IN Deep Water. 



water is at its greatest dépression 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 hâve high-water, at 3 hours 6 minutes it is mid-water, at 

 6 hoùrs 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 



