MR. PALMER ON THE MOTIONS OF SHINGLE BEACHES. 569 



3rd. The motion of a current will not produce that order in which the pebbles are 

 found to lie, which order (as will be hereafter shown) may easily be distinguished as 

 the effect of the motion of the waves only. 



The direction of the waves is determined principally by the wind, the prevailing 

 direction of which on the coasts referred to is from the westward. Every breaker is 

 seen to drive before it the loose materials which it meets ; these are thrown up the 

 inclined plane on which they rest, and in a direction corresponding generally with 

 that of the breaker. In all cases we observe that the finer particles descend the whole 

 distance with the returning breaker, unless accidentally deposited in some interstices; 

 but we perceive that the larger pebbles return only a part of the distance ; and upon 

 further inspection we find 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 up the plane, and that 

 these are accompanied also by others, which had been previously deposited, but which 

 are in such cases disturbed by the waves ; and by a continued 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 indication of the de- 

 structive action. 



The particulars of the accumulative action, combined with that of progression, are 

 explained as follows. (See Plate XXVI. fig. 1.) 



Let A B C D 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 A a, 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 descends 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 represented by a! V V b\ &c., the distance in each step 

 of its descent being something less than in that of its ascent, until it has reached the 

 summit (/') 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 Ijefore ; 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 the line {a g, &c.), 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 



