AND OTHER WATER WAVES 157 



The Effect of Percolation to Promote the Building 

 of Beaches 



When I have watched the action of the breakers 

 upon a natural sloping platform of hard rock, cut 

 at the foot of a cliff, it has seemed to me that the 

 stones thrown forward by the breaker roll down 

 so freely in the backwash that a moderate slope 

 is sufficient to return to the breaker as many stones 

 as it rejects. 



It is otherwise, however, when the shore is 

 already covered with a sloping bank of shingle. 

 The wash from the breaker, rushing up this bank 

 in virtue of the speed with which it starts, travels 

 over its surface, filling only the interstices between 

 the upper layers of stones, and leaving the inter- 

 stices of the greater part of the bank above the 

 mean sea level unsaturated and void. When the 

 wash of the breaker has reached its limit and the 

 water begins to run downhill, a great part of it 

 subsides through the interstices of the shingle in- 

 stead of flowing as a surf ace -current, so that the 

 latter is weakened in two ways. There is the 

 diminution of momentum which lessens its power to 

 roll stones down, and secondly there is the diminu- 

 tion of depth which results in the larger of these 

 stones being only partly submerged. They are 



