WIND CURRENTS AND WIND WAVES 147 



waves and long waves — that is, their lengths become great compared to 

 the depth of the water. The waves of long period and correspondingly 

 great wave length are transformed into long waves at a greater distance 

 from the coast than are those of short period. As a consequence, the 

 movement of the water particles of the longer waves will reach to the 

 bottom at a greater distance from the coast, although the height of these 

 waves may be smaller than that of the short-period waves. This circum- 

 stance may have considerable bearing on sand movement caused by 

 waves in shallow water. 



The breaking of the wave is partly due to friction, which is effective 

 when the water becomes so shallow that the motion reaches from the 

 surface to the bottom. In this case the lower portion of the wave will 

 be slowed down more than the upper portion and the deformation of 

 the wave will be accelerated. 



When a wave breaks near the shore, another wave type, known as a 

 wave of translation, may develop. This wave, which was discovered and 

 studied by Russell, is characterized by having only a crest, and no trough. 

 In such a wave the motion of water particles is only in the direction of 

 progress, and the water particles are therefore displaced forward as the 

 wave passes. It is formed when a mass of water is suddenly added to still 

 water, and may therefore be produced as the crest of a breaking wave 

 topples over and crashes down on the water surface in front. This wave 

 type is unimportant in the open sea, but may be prominent on a shallow 

 coast. 



Certain phenomena that appear to be associated with breakers are 

 not yet understood. The existence of undertow has not been satisfac- 

 torily explained and is doubted by some observers. The rip currents 

 which flow away from the coast through the breakers and which ma}^ 

 carry swimmers far out from the beach are probably associated with the 

 surface transport of water against the beach by the waves. 



Destructive Waves 



The destructive waves that occasionally inundate low-lying coasts 

 and cause enormous damage are commonly known as "tidal waves," 

 although they have nothing in common with the tides. The name 

 "tidal wave" has, however, become so firmly established in the English 

 language that the popular use will probably be continued in spite of the 

 unfortunate confusion to which it gives rise. Destructive waves are not 

 related to the tide-producing forces, but are caused by earthquakes or b}- 

 severe storms blowing against the coast. It is therefore necessary to 

 distinguish between earthquake waves and storm waves, since the former 

 are real waves and the latter are not even related to waves (p. 122). 



Waves in the sea caused by earthquakes are of two different types. 

 In the first place, a submarine earthquake may produce longitudinal 



