174 



WORK OF THE OCEAN 



it rises and falls and moves forward and backward, describing an 

 orbit in a vertical plane. If the passing wave is a swell, the orbit 

 of the particle is a circle or an ellipse; but in the case of a wind-wave 

 the orbit is not closed, for in such a wave the water, as well as the 

 undulation, moves forward. On the crest of the wind-wave each 

 particle of water moves forward, and in the trough it moves less 

 rapidly backward, and the excess of the forward movement over 

 the backward gives the water a slight advance. As a result of this 

 advance, the upper part of the water is carried forward with refer- 

 ence to that below, in the direction toward which the wind blows. 

 The waves of any considerable or long-continued wind, therefore, 



generate a surface movement 

 in the direction of the wind. 

 Wave motion is prop- 

 agated downward indefi- 

 nitely, but the amount of mo- 

 tion diminishes rapidly with 

 increasing depth (Fig. 176). 

 Engineering operations have 

 shown that submarine struc- 



Fig. 176. Diagram illustrating the de- 

 creasing size of orbits of water particles in a 

 wave, with increasing depth. 



tures are little disturbed at 

 depths of five meters in the 

 Mediterranean, and eight 

 meters in the Atlantic. On the other hand, debris as coarse as 

 gravel, which is transported by rolling on the bottom, may be 

 carried out to depths of 50 feet, and sometimes even to 150 feet. 

 Fine sediment, like silt, is disturbed at still greater depths, for 

 ripple-marks, which indicate agitation of the water, are said to 

 have been found at depths of 100 fathoms. 



When a wave approaches a shelving shore, its habit is changed. 

 The velocity of the undulation is diminished, while the velocity of 

 the advancing particle of water in the crest is increased; the wave- 

 length, measured from trough to trough, is diminished, and the 

 wave-height is increased; the crest becomes acute, with the front 

 steeper than the back, and these changes culminate in the breaking 

 of the crest when the undulation proper ceases. Waves of a 

 given height break in about the same depth of water, and the line 

 along which incoming waves break is the line of breakers. The 

 line of breakers is in deepest water and farthest from shore when 

 the waves are strongest. The return of the water thrown forward 



