CHAPTER V 



On the depth to which wave-agitation extends, and on the 

 transport of fine mud The action of waves to drive 

 shingle shorewards The effect of percolation to promote 

 the building-up of beaches The movement of sand by 

 waves The undertow. 



On the Depth to which W<ave Agitation Extends, 

 and on the Transport of Fine Mud 



WHEN the surface of the deep sea is in waves, the 

 agitation extends to some depth. Eor our notion 

 of what goes on below the surface we have to 

 depend mainly upon theory, observations being 

 almost wholly wanting. According to theory, if 

 there be one set of regular trochoidal waves 

 running, the repeated motion of any particle of 

 surface-water is a vertical circle. 



In the upper half of the circle the motion of 

 the particle is forwards i.e., in the direction of 

 advance of the wave. In the lower half it is back- 

 ,wards. Thus the return flow of the wave-water 

 in a travelling wave is not a precise reversal pf 



9 141 



