AND OTHER WATER WAVES 205 



the westwards until they reached a bed of pebbles 

 of the large size, when they interlock and cease 

 to travel readily ; and this, in^ his opinion, ex- 

 plained the accumulation of .large stones at 

 Chesilton. The observation, as 'far as a horizontal 

 current of shallow water is concerned, is correct. 

 It is a mode of motion readily seen in fine weather, 

 and with water of a depth of i or 2 inches. It 

 is a mistake to suppose that what is then seen 

 is a measure of the relative mobility of large and 

 small shingle in really rough weather, and nearer 

 the position where the wave breaks. Here, with 

 a depth of many inches, or a foot or two, and 

 with strong upward swirls, it is less easy to see 

 the motion of individual stones, but there is no 

 doubt that the smaller, loosened from their inter- 

 locking and lightened by the upward current, are 

 driven about much more freely than the large 

 stones. It is under such circumstances that most 

 pf the movement of shingle takes place. 



What, however, I particularly wish to point out 

 about Sir John Coode's theory of the Chesil Beach 

 is not so much the partial truth of the observa- 

 tion as the logical error of supposing it to be an 

 explanation of the origin of the observed grading. 



The logical error is this, that in order that the 

 stones shall travel in the required manner we have 



