540 Mr. Scrope on the Ripple-Marks and 



superficial waves. It is easily imitated by agitating to and fro 

 a vessel of water, with a flat bottom, on which sand has been 

 strewed. 



To what depth superficial undulations affect water is a 

 problem yet unsolved, though the general opinion is, that they 

 do not ever extend beyond thirty or forty feet. The most vio- 

 lent movements of the surface water must be neutralized by its 

 inertia, and their lateral extension as they are propagated 

 downwards. But they will probably reach considerable depths 

 before they die away entirely, and will then, I conceive, subside 

 in precisely the sort of gentle and minute oscillations fitted to 

 produce the wrinkles in question in mud or sand, either in the 

 act of subsiding to the bottom, or stirred up there by the com- 

 mencement or increase of the agitation. 



There is an observation which, it strikes me, might perhaps 

 be applied to determine the depth to which the superficial un- 

 dulations of water are propagated; namely, by ascertaining 

 the depth of the water at the spot where the waves, approach- 

 ing a shore or shoal, begin to swell above their average height in 

 deep water, and to take the line of direction of the shoal or the 

 coast, instead of that which the wind impressed them with ori- 

 ginally. Supposing the British Channel wrinkled by waves 

 driven before a powerful westerly wind ; these waves, which in 

 mid-channel have their long axes directed due north and south, 

 will, as they near the coast on either side, but particularly the 

 shoaling coasts of Devon and Dorset, gradually take the line of 

 the shore, upon which each wave breaks at length in a direc- 

 tion nearly exactly parallel with all its sweeps. This alteration 

 of their original direction is, no doubt, impressed on them by 

 their reaction from the bottom, the resistance of which retards 

 the waves as soon as they come in contact with it, and gra- 

 dually compels them to assume its sweep. The reaction of 

 each oscillation from the bottom also causes it to rise by the 

 rebound higher at the surface, and hence the swell of each 

 wave as it nears the shore, and its beautiful curve and fall at 

 last, owing to its superficial movement outstripping that of the 

 lower part, which is retarded by the friction of the bottom. 

 A series of careful observations on the depth of the water at 



