MR. PALMER ON THE MOTIONS OF SHINGLE BEACHES. 571 



became broken and dispersed as usual, yet they followed in such rapid succession, 

 that each wave rode over its predecessor while on its return, and thus produced a 

 continual downward current, which carried with it the pebbles that were disturbed. 

 That the pebbles were not removed far from the line of low water, would appear 

 from the fact, that on the subsiding of the swell, it being succeeded by a light breeze 

 of wind from the westward, the accumulation immediately commenced, and was 

 restored to its former quantity by the action of four tides. I have subsequently had 

 some favourable opportunities for making other observations on the effects produced 

 by different rates of succession of the waves, and particularly at Dover, during the 

 late gales, where the same actions were noticed. There I watched for an opportunity 

 of witnessing that rate of succession which exhibited the destructive and accumu- 

 lative actions in their smallest degrees ; and I observed, that when ten breakers 

 arrived in one minute, the destructive action was but just evinced ; and that when 

 only eight breakers arrived in the same period, the pebbles began to accumulate ; 

 which facts harmonized with my observations made at Sandgate and Folkestone, viz. 

 that the difference between the two actions was determined hy the rapidity in succession 

 of the waves upon the shore. 



In the description of the accumulative action, I have assumed the forces to be 

 directed obliquely with the line of coast, and have therefore necessarily included the 

 progressive motion ; but it remains to be explained in what manner the shingles are 

 carried forward while the destructive action is going on. 



It is known that the action and reaction of the waves give to the whole body of 

 the water, within a certain distance from the shore, an undulating motion. The 

 direction of this motion, when approaching the shore, will, to a certain degree, corre- 

 spond with that of the waves upon the surface, and the direction of the recoil will 

 also be affected in like manner ; therefore the pebbles that have been carried down 

 by the destructive action are moved forward through an angular course beneath the 

 water, until, by the excess of the impinging forces over those of the recoil, they are 

 again raised by the action of the water, and deposited where the destructive action 

 has ceased, or where, from local circumstances, it cannot occur. The circumstances 

 which are most unfavourable to the destructive action are those which least admit of 

 the constant downward under-current, — an inlet, or narrow arm of the sea, for ex- 

 ample. If we suppose a wave rolling through the mouth of an inlet, carrying with it 

 a charge of shingles, it does not break as upon an inclined plane, but is dispersed in 

 the general body of the water, which is comparatively quiescent ; and there being no 

 returning force, the shingle becomes deposited, and a bank is formed : and although 

 the destructive process would act upon that bank if it could attain a certain height, 

 yet the attainment of that height is prevented by the waves passing over it, and car- 

 rying with them, in succession, the shingles with which they are charged. 



