572 



MR. PALMER ON THE MOTIONS OF SHINGLE BEACHES. 



Section 3. 



In Plate XXVII. is represented a section of the beach formed along the outside of 

 Folkestone Harbour. This section was taken with great accuracy, after the ground 

 swell before referred to had removed most of the loose pebbles from it ; so that the 

 section may be considered as representing the plane upon which the progressive mo- 

 tion of the pebbles is carried on. Its slope is in the proportion of 1 to 9, nearly, and 

 (with the exception of that part near the summit where there remained a bank of 

 pebbles beyond the reach of the previous tides,) the surface of the plane corresponds 

 vei7 nearly with a straight line, which, considering that it is a natural formation, is a 

 fact worthy of notice. 



I think this plane may be considered as representing the average dimensions and 

 inclinations of the surfaces over which the beach travels along this coast, and I have 

 therefore generally assumed such an one for the present purposes. Upon such an 

 inclination, the loose pebbles are in contact with each other; and although their 

 depth upon the plane is constantly varying, yet, for the sake of conveying a general 

 idea, we may assume the average to be about six inches, extending between high- and 

 low-water marks. When, however, the plane is less inclined, the same quantity of 

 beach is spread over a larger surface, and its depth is diminished ; and the pebbles 

 are in some places so far separated as to exhibit the appearance of a diminished 

 quantity. In Plate XXVI. fig. 2. this is illustrated geometrically. 



Let A B represent a plane on which all the pebbles are in contact, C B a plane 

 considerably more inclined. If, from the centre of each pebble on the plane A B, a 

 horizontal line be drawn to the plane C B, the position of the pebbles on the latter 

 will be respectively at the various points of intersection. 



Section 4. 



There are numerous points on the coast at which the line of beach is apparently 

 intercepted and its continuity destroyed, and the rock washed bare. Having suffi- 

 cient evidence that the motion of the beach was continuous, I thought it important 

 to ascertain in what manner the pebbles escaped past those places, and was happy in 

 finding, upon investigation, that a valuable deduction could be made. 



In the description of the accumulative action, it was remarked that the waves 

 having struck the pebbles upwards, became dispersed, and were incapable of return- 

 ing them to the level from which they were forced. But I now observed that the sur- 

 face of the rock, being very irregular, constituted numerous channels ; so that the 

 waves, instead of returning in a dispersed and weakened form, moved back in columns, 

 which were of sufficient power to return every pebble that had been thrown up ; and 

 as these channels offered no impediment to the angular progressive motion of the 

 pebbles, it was more rapid than on the ordinary plane surface. Here, then, was 

 pointed out by nature a principle on which the shingles might be hastened forward, 



