WAVES OF TRANSLATION 115 



breakers may share the characteristics of both the plunging and of 

 the spilling types, for while they resemble the former in their general 

 development, the break does not involve enough of the crest to lower 

 the wave form much (fig. 26). The wave then steepens again in its 

 further advance up the shoaling bottom, breaks partially for a second 

 time, and sometimes for even a third or a fourth time. Breakers of 

 this sort may be termed "intermittent" ; they are seen very commonly 

 during onshore storms. 



WAVES OF TRANSLATION 



In this connection, it is necessary to mention a very different sort 

 of wave that develops when a wave breaks some distance out, whether 

 on a gently sloping bottom or over the seaward margin of a submarine 

 terrace, for the mass of water that falls forward from it, and that is 

 suddenly added to the comparatively level water surface in advance of 

 it, often sets up a secondary wave of permanent form. Such a wave 

 consists of a crest with precipitous foaming front, and without any 

 trough, so that the water particles all move forward together — hence 

 its name, "wave of translation" (fig. 27). But it seems that the 

 foaming crests of this sort commonly seen almost always represent a 

 combination between these waves of translation and whatever rem- 

 nants of the original wave may still persist, for it is often easy to 

 see that the general advance of the bits of foam is combined with an 

 oscillating movement, forward and back. On a calm day there may 

 be anywhere from 1 to 4 or 5 lines of these combined crests, decreasing 

 in size shoreward, between the innermost heavy breaker and the beach. 

 And they may advance, unaltered, as a secondary, low surf for long 

 distances, perhaps for as much as a mile if the slope is gentle. They 

 are very characteristic in appearance, flat-topped, and with the dis- 

 tances between them many times longer than their own heights. 



When the form of a wave is largely destroyed in the process of 

 breaking, as it often is with a long swell in moderate weather, the only 

 other breaking waves between it and the beach may be one to several 

 lines of these small crests. But when the undulatory motion 

 of a wave continues after it first breaks, the combination*between it 

 and the waves of translation it produces may cause secondary breakers 

 several feet high, though never so large as the primary breakers farther 

 out. 



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