240 THE SEAS 



ward for the upper half of the back of the wave, and then 

 backward and down to the hollow of the trough (see Fig. 

 51). When these oscillatory waves approach the shallow 

 water of the coast their shape changes. They become 

 shorter and higher and the advancing front of the wave 

 steepens and the water particles now have a forward 

 motion. Eventually it becomes so steep and top-heavy 

 in front that it topples over in a mass of seething foam to 

 form a breaker and adds to the grandeur of the surf. 

 When the wind is off-shore water is blown off the top of the 

 breaking crest in the form of spray. 



There may be waves present, but no wind. Such waves 

 are known as ' swell," and they are waves that have 



Fig. 51. — Diagram illustrating the movement of a particle on the sea surface caused 

 by the passage of a wave. 1, 2, 3, etc., are the successive positions of the wave 

 crest and of a particle on the sea surface at the same time. 



travelled beyond the windswept area or have been left on 

 the face of the troubled ocean after a storm has passed 

 (Plate 83). A combination of swell and wind- waves can 

 often be seen. 



There are, besides, the so-called " tidal waves," the 

 products of land or submarine earthquakes. These may 

 be waves of very large dimensions produced by an earth- 

 quake on the sea floor ; or, when an earthquake occurs 

 in coastal regions, the sea may recede far beyond the normal 

 low-tide mark and then hurl itself with relentless fury back 

 on to the land, bringing death and destruction with it. 



