242 



TRANSPORTATION OF DEBRIS BY RUNNING WATER. 



detaching device is placed at some point E, 

 the device possibly consisting of an open-rank 

 comb of elastic wire. The shaft should be 

 smooth and of uniform section, so that the 

 particles of de'bris near the drum may be 

 brought into actual contact with one another 

 by pressure of the debris above. That the 

 debris may not be caused to flow by excess of 

 moisture, it should be thoroughly drained be- 

 fore use. In a rough construction designed to 

 test the practicability of the apparatus the 

 drum surface was roughened by covering with 

 a wire screen, and this was found to secure the 

 delivery of the de'bris. 



Uniformity of feed having been provided, the 

 rhythms of transportation may be observed as 

 oscillations in the de'bris delivered at the out- 

 fall. Rhymths of slope may be studied, at 

 least initially, by observing changes in the 

 profile of the water surface. 



In the record of the Berkeley observations it 

 is not practicable fully to discriminate rhyth- 

 mic inequalities from those occasioned by 

 irregularities of de'bris feeding, but there is 

 reason to believe that several rhythms of differ- 

 ent period coexist. The shortest rhythms are 

 those connected with the dunes and antidunes, 

 and these are evidently associated with rhythms 

 of the flow of water. 



RHYTHM IN THE FLOW OF WATER. 



Reynolds, 1 treating of the flow of water 

 through tubes, distinguishes two modes of flow 

 as direct and sinuous. They are otherwise 

 called steady and turbulent. In direct flow the 

 filaments of current have simple lines, which 

 are straight and parallel if the walls of the con- 

 duit are straight and parallel. In sinuous flow 

 the filaments of current are neither simple nor 

 parallel and may be intricately convoluted. 

 The flow in tubes is direct for low velocities 

 and sinuous for high, the critical velocity vary- 

 ing inversely with the diameter and the rough- 

 ness of the tube and directly with the viscosity 

 of the water. It would follow from his gener- 

 alizations that the flow of such streams as were 

 used in our experiments would be sinuous, and 



i Reynolds, Osborne, An experimental investigation of the circum- 

 stances which determine whether the motion of water shall be direct or 

 sinuous and of the law of resistance in parallel channels: Eoy. Soc. 

 I'hilos. Trans., London, vol. 174, pp. 935-982, 1883. Also, The two 

 manners of motion of water: Roy. Inst. Great Britain Proc., 1884. 



its actual sinuosity was a matter of observa- 

 tion. To a large extent the curvature of the 

 flow lines was shown by the motions of minute 

 suspended particles, and when this evidence 

 was lacking it was still possible to infer diver- 

 sity of current from continual changes in the 

 configuration of the water surface. 



It is probable that all the diversities of flow 

 were rhythmic, for that is the nature of in- 

 equalities of motion developed by the inter- 

 action of constant forces; but in many cases 

 the rhythms were so numerous and so related 

 in period and other characters that their com- 

 bination gave the impression of irregularity. 

 In many other cases, however, some one 

 rhythm was dominant, and these cases appear 

 especially worthy of study. 



The cycle of movements constituting a 

 rhythm unit may be definitely related to space, 

 or to time, or to both space and time. The 

 pulsations of the water surface which ob- 

 structed our observations of water profile were 

 manifestations of time factors. The regular 

 sequence of dune crests was a manifestation 

 of a space factor. The two phenomena co- 

 existed. In the space interval from dune crest 

 to dune crest certain elements of motion were 

 constant. There was a large stationary vortex 

 in the hollow between the crests (fig. 10, p. 31), 

 and there might be stationary elements in the 

 configuration of the water surface. At the 

 same time the flow lines of the water at all 

 points swayed in direction and were affected 

 by variations of velocity, and these changes 

 were rhythmic with respect to time. Doubt- 

 less many of the fluctuations belonged to or 

 were associated with vortices which traveled 

 with the general current. It is conceivable, 

 or even probable, that the stationary features 

 and the traveling features were coordinated, so 

 that within the rhythmic space unit corres- 

 ponding to the dune interval there was a 

 cycle of variations of motion which was 

 rhythmic with respect to time. 



The rhythmic dunes marched slowly down the 

 trough and with them marched the intercrest 

 vortices and associated motions. To that ex- 

 tent the features of the bed controlled the fea- 

 tures of the current. Nevertheless the dunes 

 were not essential to the existence of a water 

 rhythm characterized by a definite space inter- 

 val. There was positive evidence that the dune 



