June 1, 1896.1 



KNOWLEDGE 



139 



agitation ia so violent in the Eace as to render it dangerous 

 for small vessels ; and in tempestuous weather, when the 

 West channel stream is running to the eastward, the whole 

 space between Portland and the Shambles is one sheet of 

 broken water." 



In the older " Sailing Directions for the English 

 Channel " (1835), Captain White, R.N., gives a very 

 explicit account of tlie causes which produce the standing 

 waves in the principal races and overfalls of the Channel. 

 The following passages are condensed from this book : — 

 "Off Scilly, S.E. and South of St. Agnes Island, there is 

 great rippling or overfaU, between IV. hours flood and 

 II. hours ebb, occasioned by the confluence of two streams 



of tide there at that period Off the Lizard, south 



of the Stag Rocks, there is always an extensive rippling on 

 both streams of tide ; but this is chiefly occasioned by the 

 unevenness of the ground. There is another extensive 

 race, or rippling, to the S.E. of the Lizard (off Black 

 Head), occasioned by the confluence of the tides. Off" the 

 Start there is considerable rippUng, occasioned by the 

 confluence of the fair-channel tide with the in-shore 

 streams between Start Point and Dartmouth and within 

 the Skerries. The Portland Race is caused by the rocky 

 ledge which projects somewhat more than a mile in a 

 southerly direction from the Bill ; both sides whereof being 

 remarkably steep, the consequent transition from deep to 

 shoal water is very sudden. The overfalls off St. Alban 

 Head are chiefly caused by the unevenness of the ground. 

 The overfalls off St. Catherine and Dunnose are also partly 

 caused by the various sudden transitions from deep to 

 shoal water in that neighbourhood." 



A group of standing waves may originate from a hole in 

 the bed of a stream as well as from a hump on the bottom. 

 It depends upon the velocity and depth of the stream 

 whether the water will be raised or depressed over the 

 hole ; in a rapid shallow stream it is depressed over the 

 hole. Hollows worked in the bottom of a stream flowing 

 across a sandy beach frequently originate a train of 

 standing waves. These holes are, I think, generally 

 formed by the action of an eddy, or upward swirl in 

 the water. The sand which rolls along the bottom of 

 the stream keeps filling in the hole on the up-stream 

 side, but the vortex scours away the sand on the lee side, 

 so that the hole moves down stream. With it, of course, 

 the group of standing waves to leeward moves down 

 stream also. Another way in which standing waves are 

 formed in these streams is by the convergence near the 

 middle of the stream of ridges of water maintained by 

 the pressure of the current upon the opposite banks. 

 This gives rise to waves of very little lateral extension, 

 but often of considerable heiglit at their middle part. 

 These likewise move down stream as the banks are con- 

 tinually eaten into by the currents. The most interesting 

 thing about these groups of waves is that under each is a 

 group of sand tvann, which, when we look straight down 

 upon the stream, are often more conspicuous objects than 

 the corrugations on the surface of the clear water. Fig. 1 

 shows the kind of locality in which to study such sand 

 waves, and the kindred phenomenon of Ripple-Mark. 

 Fig. 3 shows a water wave and sand wave, the dimensions 

 being h-om actual measurements. It will be noticed that 

 the wave is deeper over the crests than over the troughs, 

 the amplitude of the water waves being greater than that 

 of the sand waves. In order to watch standing water 

 waves car\Tng ridge and furrow in a sandy bottom, I have 

 strewn sand on the lee side of a stone fixed on the hard 

 bottom of a stream. To the leeward of the stone were 

 standing water waves, and as the sand was rolled down by 

 the current it was rapidly carved into ridge and furrow of 



symmetrical form. In this case the stone was fixed, and 

 the positions of the corrugations of the water surface 

 were also fixed relatively to the banks of the stream. The 

 grains of sand could be watched rolling rapidly over the 

 farrowed bottom, but the ridges and furrows in the sand 

 maintained their positions unaltered as long as the supply 

 of sand was kept up. This means that the sand wave is 

 travelling against the stream with the same velocity as 

 that with which the sand grains travel with the stream. 



In the case of sand waves under a group of standing 

 waves which are moving down stream, the sand ridges 

 move down stream also, keeping pace with the corrugations 

 of the surface. The movement of the sand wave can be 

 watched, and wUl repay the trouble of watching. If a 

 small flat stone be dropped into a furrow or trough, it is 

 speedily buried by the surface sand which rolls down upon 

 it from the proceeding crest on the weather side. It is 

 always the surface sand that moves, the lower layers 



Fig. 3. — Waves of Flowing Water and Rolling Sand, 



Fig. 4. —Wares of Flowing Water and Flying Sand. 



having to be uncovered before they roll freely. Soon the 

 stone is under the crest of a sand wave, presently it 

 emerges on the up-stream side as the crest moves on. 

 These are waves of rolliii;/ sand in which the top layer 

 moves ; but it is not alv.-ays the same grains of sand which 

 are at the top. The grains at the surface of the weather 

 slope of a ridge are carried over to the trough, and are 

 speedily buried by the grains of the next layer brought from 

 the weather slope of the ridge ; and so the motion of the 

 ridge continues, the grains moving very quickly, but the 

 form of the sand progressing slowly because the ridge con- 

 tains many layers of sand grains. The down -stream 

 movement of the ridges is the motion of the (jroup of 

 sand waves, and the velocity of the ridges relatively to the 

 bank is the group velocity. Above the compact but 

 mobile sand which forms these waves the water is clear : 

 there is no flying or floatmg sand. 



Further down the stream, when the water is flowing 

 strongly, matters are different. The stream gathers sand 

 as it goes, largely by erosion of the banks, and, as we 

 near the sea, the sand is looser. The sand flies along in a 

 turbid current of sand and water, sometimes covered by 

 clear water, but often rendering the whole stream turbid. 

 At those places where the stream narrows and the waters 

 rush together from the sides, or where two minor channels 

 converge, or whore pressure-ridges from impact of water with 

 the sides of the channel meet, the water is thrown into 

 standing waves, close under which sand waves are formed. 

 These waves move »/i stircim : yet their origin is generally 

 in the action of the water upon sand which is constantly 

 being carried down stream. Even if they originate from a 



