172 WAVES OF THE SEA 



steeply sloping shore of a shingle beach at the 

 highest level of the tide we can at any time see 

 that strong rushes of water are poured back in 

 a shallow sheet to the foot of the breakers. It is 

 not difficult to understand that the effect on the 

 bottom water just beyond the breaker of this strong 

 intermittent current will be to diminish the velocity 

 of the bottom current under the crests and increase 

 the seaward velocity of the current under the 

 troughs. 



The more the shoreward bottom current is 

 checked under the crests, the deeper will be the 

 water in which the wave will break. 



It is well established that wind at sea causes 

 a current which is strong at the surface, but of 

 which the intensity decreases rapidly with depth. 

 Thus, quite apart from any slight translation of 

 water which may be theoretically deduced for 

 trochoidal waves, there is a surface drift in a storm 

 at sea which has the following effect upon the 

 oscillations of a particle of water at the surface, 

 viz., at the end of each complete oscillation it has 

 advanced in the direction of the wave when the 

 wave is running before the wind. Thus, in an 

 on-shore gale there is a surface drift, which takes 

 place in jerks, towards the shore, and this goes on 

 as long as the gale endures. But the level of the 



