150 WAVES OF THE SEA 



of the waves' motion all detritus which they can 

 wholly or partly lift from the bottom, but cannot 

 maintain more than momentarily in suspension. I 

 will first suppose the waves to be symmetrical, 

 which is the condition least favourable to proving 

 my case, and I will begin by supposing the bottom 

 to be horizontal. The vertical currents are as 

 shown in the first figure ; /.., an upward current 

 begins when the trough passes and continues until 

 the arrival of the crest, after which subsidence 

 commences and continues until the next trough 

 arrives. 



The horizontal currents are as shown in the 

 second figure. The shoreward current begins in 

 the midst of the upward current. The material 

 which has been raised by the upward current is 

 caught thereby and heaved shoreward, and the 

 shoreward current, acting after the commencement 

 of the down current, continues to drive the material 

 forward during the first part of its period of sub- 

 sidence. The backward horizontal current hen 

 sets in when the water has been clarified from 

 all coarser and rapidly subsiding materials, so that 

 at its commencement it only finds fine material in 

 suspension, which it carries backwards. During 

 the last part of its period of action the backward 

 horizontal current is accompanied by an upward 



