Hunt — On the Action of Waves on 8ea-heaches, 8^c. 287 



the ordinary sea waves of oscillation. A simple experiment 

 in the artificial formation of ripple-mark illustrates incident- 

 ally the action of waves on a shoal. The experiment, as I have 

 often made it, consists in placing an inverted flower pan in an 

 experimental tank, and causing- artificial waves of regular period to 

 roll over it, and act on sand strewn upon its surface, or on a pane 

 of glass resting on it. At first sight the waves seem to have no 

 power to wash the sand off the pan, so little is the translatory 

 motion in any direction. Occasionally such a motion can be de- 

 tected. On one occasion, when experimenting with sand strewn on 

 a gelatine film ^ (a convenient method for preserving the ripple- 

 marks when made), I observed the sand-ripples evinced a tendency 

 to travel forwards. On investigation, I found that the resultant 

 forward current over the bank arose from the balance between the 

 primary and secondary wave-currents being disturbed by the 

 greater depth of water at the sides of the bank. I have since 

 found that an inward current over a raised surface can be trans- 

 formed into an outward current by raising the sand at the sides of 

 the bank, and thus converting the quondam bank into a depression. 

 In the case of waves of oscillation we have seen that the water 

 below the level of repose travels to meet the advancing wave-crests, 

 and with the waves when above that level. Thus, in the case of 

 a sandbank just level with the surface of the water the whole of the 

 wave-action affecting it must be forwards, as during the backward 

 flow the surface of the bank, ex hypothesis will be high and dry. In 

 such a case, however, the forward current cannot be fairly described 

 as a wave of translation ; it is but one half of a wave of oscillation, 

 whose other half has been applied elsewhere. The result of such 

 unbalanced action will be the depression of all the bank to a limi- 

 ted depth below the surface of the water, unless the sand be accumu- 

 lated by currents faster than the waves can dislodge it. Marine 

 shoals and sandbanks are commonly the joint production of wave- 

 currents and tidal- currents. The neutral wave-currents keep the 

 component parts in motion, whilst cross- tidal- currents accumulate 

 them. Occasionally an important sandbank results from these 

 combined effects ; more commonly the result is a collection of 

 dead shells, or shell-sand, known only to the trawler-man or dredger. 



An ordinary photograpHe dry plate. 



