542 



Mr. Scrope on the Ripple-Marks and 



As the wave a falls, and that marked b rises, the particles of 

 water that lie between a and x (the axis of oscillation) move 

 laterally towards b ; so that when b is at its utmost elevation, 

 the particles that were at x are now at b, those that were at 6 

 at y, &c. , while those that were at a are at x, those that were 

 at y at a. As 6 falls and a rises, the particles return again, 

 those that were at b to x, those that were at x moving to a. 

 Thus there is a continual oscillation of particles of water along 

 the arcs axb, &c. in the direction of the dotted lines. In the 

 case of superficial waves, floating objects will appear to vibrate 

 backwards and forwards between a and x, and x and b. But 

 these oscillations are not merely superficial : they are propa- 

 gated to greater or less depths in vertical columns, or rather 

 wedges, whose axes of vibration are represented by the dotted 

 lines ex, dy, &c. In the oscillations of the lower stratum of water, 

 the sand, fuci, or other objects that are held in suspension 

 near the bottom, or are easily moved along it, will also be seen 

 to vibrate across the axes of oscillation. The shaded part of 

 the diagram shews the simplest form of sand-ripple formed by 

 this vibratory action of the water in contact with the bottom, 

 the ridges of the wrinkles corresponding to the axes of oscilla- 

 tion, or intervals between the contiguous oscillations ; there 

 the water is comparatively still, and the sand consequently 

 either deposits itself or remains undisturbed, while the inter- 

 vening hollows are scooped out by the motion of the water. 



If the alternate vibrations are not equal in force, but the 

 movement is more powerful in one direction than in another, 

 as will be the case with waves breaking on a shore, or those 

 formed in a stream of running water, the ridge will be pro- 

 duced on one side of the ripple, that towards which the 

 strongest oscillations move, as in the diagram below ; so that 

 each ripple will have one steep and one gentle slope. Where 



