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THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[May, 



rind that the distance to which each pebble returns bears some relation 

 to its dimensions. This process is an indication of the accumulative 

 action. 



But under some circumstances, depending on the wind, it is found 

 that pebbles of every dimension return with the breakers that forced 

 them np the plane, and that these are accompanied also by others, 

 which had been previously deposited, but which are in such case? dis- 

 turbed by the waves ; and by a continual repetition of the breakers 

 acting in this manner, the whole of the shingle previously accumulated 

 is immersed below the surface of the water. This process is an indi- 

 cation of the destructive action. 



The particulars of the accumulative action, combined with that of 

 progression, are explained as follows. (Fig. 1.) 



Fig. 1. 







■■'% 





Let AB CD be an inclined plane, representing that on which the 

 loose pebbles move. Suppose the wind to blow in such a direction 

 as to cause a wave to strike a pebble at A, in the direction of Aa, 

 and to the distance a up the plane, that point being the extent to 

 which the force can reach. Now here the wave breaks partly into 

 spray, and is dispersed in all directions; is partly absorbed, and de- 

 scends in a shallow form, which rapidly diminishes in its depth, so 

 that the pebble is soon left exposed, and therefore does not return the 

 whole distance with the water, but is left at rest at a' being at a 

 higher level than that from whence its motion commenced. 



With the rise of the tide the striking force is also elevated ; and 

 by the repetition of the operation described through the different 

 heights in succession, the further motion of the pebble will be repre- 

 sented by a' h' h' b', &:c., the distance in each step of its descent being 

 something less than in that of its ascent, until it has reached the sum- 

 mit/, determined by the height of the tide. Now if we suppose a 

 pebble of less dimensions than the former to be struck from the same 

 point, we shall find it raised as before ; but because its surface is 

 greater in proportion to its weight, and because from its less bulk it 

 remains longer immersed in the declining wave, it will descend further, 

 and follow tlie line a g, Sec, and will not be left at rest till it has 

 reached o. 



If, then, we suppose a pebble whose dimensions are less than either 

 of the former, it will be evident that the point at which that will arrive 

 on the highest level will be 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 suc- 

 cession 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 therefore their direction will also be the 

 same. 



In this action we observe a constant tendency to heap up and accu- 

 mulate 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 diffe- 

 rent 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 imjjinging and recoiling motions of the waves, bv 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 neighbourhood of Sandgate in Octo- 

 ber last. 



The accumulative action had been continued for a considerable time. 

 The numerous groins erected near Folkstone to impede the progress 

 of the beach, for the protection of the cliffs, 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 motioa 

 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 disappeared, including all that had been 

 collected by the groins at Folkstone. 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 

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

 succession, that each wave rode over its predecessor while on its re- 

 turn, 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 

 accumulative 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 ; w hich facts harmonized 

 with my observations made at Sandgate and Folkstone, viz. that the 

 difference beliteen the tno actions nas difermined by the rapidity in suc- 

 cession of the naves upon the shores. 



In the description of the accumulative action, 1 have assumed the 

 forces to be directed obliquely with the line of coast, and have there- 

 fore 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 re-action 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, correspond 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 mater, 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 ad- 

 mit of the constant downward under-current— an inlet, or narrow arm 

 of the sea, for example. 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 pre- 

 vented by the waves passing over it, and carrying with them, in suc- 

 cession, the shingles with wnich they are charged. 



Section 3. 

 In Fig. 2 is represented a section of the beach formed along the out- 

 side of Folkstone Harbour. This section was taken with great accu- 

 racy, 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 motion of the peb- 

 bles is carried on. Its slope is in the proportion of 1 to '.1, nearly, and 

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



