Joly — Formation of Sand-ripples. 329 



Observation of the spacing of the ripples on inclined surfaces 

 also supported the foregoing explanation. Thus I found that on 

 inclines facing the wind the wave-length was shortened, and on 

 inclines away from the wind, the wave-length was lengthened 

 beyond the normal as observed upon the flat. Obviously in the 

 first case the trajectory of the sand particle is shortened, in the 

 second lengthened, owing to the inclination of the ground. 



Of course, it will be seen that ripples can only form — according 

 to this explanation — when the wind is moving above a certain 

 velocity. When this velocity is attained (and what this must be 

 will depend upon the density and coarseness of the sand), there is 

 necessarily instability, as any roughness then initiates the flight 

 and convection of grains, and thus the beginnings of ripples, which 

 will increase in height till the transport is a maximum for the 

 wind-force, and they are removed from the crests just as fast 

 as they accumulate there by projection and rolling. 



C C2 



It would appear that in a precisely similar manner steadily 

 moving water must give rise to sand-ripples, provided that 

 the velocity is above a certain limit. This minor limit to the 

 velocity will, of course, be less than that required for sand- 

 ripples due to wind, owing to the greater "transporting power" 

 of water. 



In both cases there would seem to be reasons to expect that 

 under very various rates of movement of the fluid medium (above 

 the minor limit) the height and wave-length of the ripple would 

 approximate to a certain average. In fact, if the velocity is great, 

 the crest is pushed over and levelled; and the trajectory is 

 diminished in throw by its origination at a lower level, while it 

 is increased in throw due to the greater transporting power. 

 Hence there are conditions which would appear to confer upon 

 the path of the particle a certain average length, from which 



