570 MR. PALMER ON THE MOTIONS OF SHINGLE BEACHES. 



more distant still ; hence it follows that the distance travelled horizontally by the 

 pebbles during a tide will be in some proportion to their bulk, the specific gravities 

 being the same. 



(The pebbles do not in reality move in straight lines, but in a succession of curves; 

 the straight lines are assumed here, and in other parts of this paper, to simplify the 

 description.) 



I trust it is only necessary to remark, that if the wind continue to blow in the same 

 direction during the ebbing of the tide as through the flowing of it, the direction in 

 which the waves will strike the shore will be nearly the same, and the progress of the 

 pebbles will be urged by a similar action, and tlierefore their direction will also be 

 the same. 



In this action we observe a constant tendency to heap up and accumulate the 

 shingles ; and it is an interesting fact, that when the action has continued equally 

 through a tide, the pebbles are left in regular order, according to their dimensions, the 

 largest being uppermost, and the smallest at the bottom of the plane. I do not mean 

 to state that all the largest are at the top, or that all the smallest are at the bottom, 

 for it is evident that some of every size will be found at every level ; but that if an 

 equal measure (say half a peck) be taken from the different levels, the average of 

 each specimen will exhibit in regular order the various dimensions. 



The order in which the pebbles are thus found is, then, that by which the effect of 

 the waves is distinguished from that of a current, the effect of the latter consisting 

 only in its influence on the direction of the impinging and recoiling motions of the 

 waves, by which the motion of the beach may in a small degree be accelerated or 

 retarded. 



Section 2. 



In the illustration of that action of the sea which breaks down and removes an 

 accumulation, I propose referring to my observations in the order in which they were 

 made. My attention was first directed to this part of the subject in the neighbour- 

 hood of Sandgate in October last. 



The accumulative action had been continued for a considerable time. The nu- 

 merous groins erected near Folkestone to impede the progress of the beach, for the 

 protection of the cliff^s, had collected a bank of pebbles, which in some parts was five 

 feet in height. The wind had so much abated as to be scarcely perceptible, but the 

 sea had a motion denominated a ground swell. 



The waves approached the shore nearly at right angles with it ; but although in 

 rapid succession, their forces were very moderate. These circumstances continued 

 through five tides, by which time nearly the whole of the loose shingle had dis- 

 appeared, including all that had been collected by the groins at Folkestone. The 

 water being particularly clear, I was enabled to perceive distinctly the action upon 

 the pebbles, and their motion downwards. I observed, that although every wave 



