﻿182 
  Bucher 
  — 
  Origin 
  of 
  Ripples, 
  and 
  

  

  ment 
  was 
  set 
  in 
  motion 
  at 
  the 
  next 
  rising 
  of 
  the 
  river, 
  

   and 
  thus 
  offers 
  a 
  good 
  evidence 
  for 
  the 
  instability 
  of 
  the 
  

   meta-ripple 
  form 
  at 
  higher 
  velocities. 
  

  

  The 
  ripples 
  of 
  the 
  tidal 
  flats 
  probably 
  are 
  such 
  "meta- 
  

   ripples," 
  but 
  they 
  present 
  more 
  complicated 
  conditions 
  

   than 
  those 
  observed 
  along 
  streams. 
  Once 
  formed, 
  they 
  

   are 
  not 
  easily 
  destroyed, 
  since 
  only 
  the 
  greatest 
  velocity 
  

   attained 
  during 
  high 
  tide 
  can 
  erase 
  them. 
  This 
  however, 
  

   is 
  limited 
  to 
  such 
  a 
  short 
  period 
  of 
  time, 
  that 
  it 
  usually 
  

   is 
  unable 
  to 
  remove 
  them. 
  

  

  Correspondingly, 
  Cornish 
  could 
  not 
  only 
  identify 
  the 
  

   same 
  sand-waves 
  after 
  successive 
  tides, 
  but 
  even 
  found 
  

   them 
  better 
  developed 
  with 
  increasing 
  tides, 
  while 
  they 
  

   rapidly 
  decayed 
  when 
  the 
  neap 
  tides 
  approached. 
  

  

  Except 
  at 
  the 
  time 
  of 
  their 
  first 
  formation, 
  they 
  behave 
  

   like 
  large 
  current-ripples. 
  It 
  is, 
  therefore, 
  to 
  be 
  expected 
  

   that 
  they 
  also 
  show 
  the 
  interference 
  pattern 
  so 
  common 
  

   in 
  current-ripples, 
  which 
  is 
  characteristic 
  of 
  the 
  action 
  

   of 
  vortices. 
  This 
  pattern 
  was 
  indeed 
  repeatedly 
  ob- 
  

   served 
  by 
  Cornish, 
  for 
  instance, 
  on 
  the 
  Severn 
  Shoals. 
  35 
  

   Here 
  the 
  ebb 
  current 
  runs 
  nearly 
  at 
  right 
  angles 
  to 
  the 
  

   flood-current. 
  "When 
  it 
  increases 
  in 
  velocity 
  after 
  the 
  

   slack 
  water 
  period, 
  eddies 
  must 
  form 
  in 
  the 
  horizontal 
  

   as 
  well 
  as 
  in 
  the 
  vertical 
  plane 
  on 
  every 
  bend 
  of 
  the 
  

   rather 
  sinuous 
  sand-waves, 
  causing 
  a 
  convergence 
  of 
  cur- 
  

   rents 
  on 
  the 
  weather 
  and 
  the 
  lee 
  side 
  and 
  prolonging 
  it 
  

   across 
  the 
  trough, 
  thus 
  producing 
  cross-bars 
  in 
  the 
  

   troughs, 
  which 
  form 
  strings 
  of 
  pools, 
  in 
  which 
  salmon 
  

   are 
  often 
  found 
  impounded 
  between 
  tides. 
  That 
  inter- 
  

   ference 
  patterns 
  are 
  rarely 
  seen 
  in 
  tidal 
  meta-ripples 
  is 
  

   probably 
  due 
  only 
  to 
  the 
  rare 
  occurrence 
  of 
  ebb 
  and 
  flow 
  

   currents 
  running 
  at 
  an 
  angle 
  to 
  each 
  other, 
  not 
  to 
  any 
  

   inherent 
  difficulty 
  of 
  formation. 
  

  

  III. 
  Oscillation 
  Ripples. 
  

  

  1. 
  The 
  first 
  appearance 
  of 
  oscillation 
  ripples. 
  

  

  Darwin, 
  in 
  his 
  classical 
  investigation 
  "on 
  the 
  forma- 
  

   tion 
  of 
  Ripple 
  marks'' 
  describes 
  the 
  first 
  appearance 
  of 
  

   oscillation 
  ripples 
  in 
  his 
  experimental 
  trough 
  30 
  as 
  fol- 
  

   lows: 
  

  

  "When 
  a 
  very 
  small 
  quantity 
  of 
  sand 
  is 
  sprinkled 
  in 
  

  

  85 
  1901, 
  p. 
  188 
  and 
  fig. 
  18, 
  plate 
  1. 
  

  

  ri0 
  This 
  experiment 
  can 
  easily 
  be 
  repeated 
  with 
  almost 
  any 
  vessel 
  rocked 
  

   to 
  and 
  fro. 
  

  

  