PHYSIOGRAPHY OF THE SEA-SHORE 33 



25-30 feet ; North Atlantic, 43 feet (maximum) ; North 

 Pacific, same ; Southern Ocean, 45-50 feet. Combined 

 waves may reach 50-65 feet. Swells seldom exceed 

 15-20 feet, and near the coast may be reduced to a few feet 

 only. Entering shallow water they seem to awaken to new 

 life, crowding closer together and increasing in height until 

 they break, at which time they may be anywhere from a 

 few feet up to 25 feet or more. Waves striking a vertical 

 rock surface are reflected back without breaking, the water 

 next the wall moving up and down through a vertical 

 distance to twice the original height of the wave. 



Regarding wave velocity, Johnson states that if a wave 

 be 400 feet long and 15 feet high the velocity will be about 

 45 feet per second in deep water. In shallow water waves 

 move less rapidly. 



In waves of translation the water particles move forward 

 as the wave passes, but do not exhibit a compensating back- 

 ward motion. While not important in the open sea, this 

 type is extensively developed in the shallow water along all 

 coasts, the waves of oscillation generated in deep water 

 becoming more or less completely transformed as they 

 approach the shore. Immediately before and after the 

 passage the surface of the water may be quite still. The 

 surface-water particles rise and move forward, descending 

 again to the original level in an advanced position, the paths 

 of the particles being semi-ellipses. The velocity depends 

 upon the depth of the water measured from the crest of the 

 wave. 



That this question of wave movements is not merely of 

 theoretical interest to us is evidenced by a theory of Hurst's 

 (1893) to account for the presence of the tentaculocysts of 

 medusae. According to this writer the tentaculocysts 

 function in a manner directly related to the water move- 

 ments produced by waves and, by their automatic response 

 thereto, serve to maintain the animal in the zone of optimum 

 tranquillity. 



An important factor in the work of waves is the effect 

 produced by stones, logs, blocks of ice, and other objects 



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