vary widely with wind action, period, and bottom conditions, ranging 

 from a few inclies to many feet. Wave crests in deep water may be 

 inclined at an angle with the shore line, parallel to the shore at the 

 plmige point. Several wave trains of different periods and heights 

 may simultaneously approach the shore from different directions, 

 giving rise to a beat frequency in the breakers — this is the basis for 

 the common observation that waves "break in sets." Under some 

 conditions the wave energy may not be completely dissipated in the 

 breakers and the plunge and an appreciable portion may return sea- 

 ward in the form of a reflected wave. In all cases the observed wava 

 pattern may be considered as the result of superposition of primary 

 and reflected waves of different wave trains, each periodic and of 

 approximately constant amplitude. Accordingly, the first step in 

 the analysis of oscillatory waves in water is to study the behavior 

 of single wave trains of uniform period and amplitude as they progress 

 over horizontal or sloping bottoms. 



Although the motion in a breaking or plimging wave is too com- 

 pUcated for present methods of analysis, the essential periodic char- 

 acter of the waves is retained and the motion is oscillatory, in the 

 broad sense of the term, from deep water to the limit of uprush. It 

 should be noted that the plunge of the wave is the final break before 

 the uprush on the beach. 



The creation of osciUatory waves is probably the result of wind 

 traction on the water surface. Once disturbed, pressure differences 

 develop on the leeward and windward sides of the crest and permit a 

 direct transfer of energy from the wind to the water through the 

 medium of normal as well as tangential stresses. The details of the 

 mechanism by which a small wave builds up into a large one should 

 be investigated experimentally, but for the present it is sufficient to 

 note that energy is continually transferred from the atmosphere to 

 the surface layers of the ocean and that the waves thus formed pro- 

 vide the mechanism which transfers this energy to the shore line. 

 From theory and experiment it is known that ocean waves in deep 

 water travel great distances without appreciable loss of energy by 

 internal friction. It is evident that on the average the waves 

 observed at any point along the shore will be more controUed by 

 distant than local winds. 



The form of oscfllatory waves depends upon the ratio of height to 

 length and upon the local wind effect. Very long, low sweUs from 

 distant storms are almost, but not exactly, sinusoidal in deep water. 

 As they are telescoped by the retarding effect of a shoaling bottom, 

 the crests become sharper while the troughs flatten and the form is 

 then approximately that of a trochoid. Short waves under the in- 

 fluence of local wind are usually assymetrical or may be trochoidal. 



