﻿198 
  Bucher 
  — 
  Origin 
  of 
  Ripples, 
  and 
  

  

  imposed 
  on 
  the 
  ordinary 
  rippling, 
  obviously 
  caused 
  by 
  

   the 
  obstruction 
  offered 
  by 
  the 
  stones. 
  In 
  a 
  similar 
  way 
  

   he 
  saw 
  a 
  small 
  new 
  dune 
  produced 
  on 
  the 
  lee 
  side 
  of 
  a 
  

   large 
  dune 
  within 
  one 
  year. 
  60 
  In 
  an 
  experimental 
  trough 
  

   he 
  produced 
  the 
  same 
  effect 
  by 
  inserting 
  a 
  3-inch 
  (7-6 
  cm.) 
  

   partition 
  and 
  exposing 
  the 
  sand 
  to 
  the 
  wind. 
  61 
  First 
  the 
  

   ordinary 
  ripples 
  with 
  4 
  in. 
  (10 
  cm.) 
  wave-length 
  formed; 
  

   but 
  gradually 
  a 
  series 
  of 
  larger 
  ripples 
  developed 
  quite 
  

   independently 
  of 
  the 
  smaller 
  ones, 
  with 
  an 
  amplitude 
  of 
  

   about 
  3 
  inches 
  and 
  length 
  ranging 
  from 
  18 
  to 
  21 
  inches 
  

   (average 
  about 
  50 
  cm.). 
  These 
  remained 
  stationary 
  

   while 
  the 
  smaller 
  ripples 
  continued 
  their 
  march 
  towards 
  

   the 
  lee 
  side, 
  — 
  a 
  clear 
  indication 
  of 
  their 
  dependence 
  on 
  

   the 
  immovable 
  obstacle. 
  

  

  Many 
  of 
  the 
  cases 
  on 
  record 
  of 
  wind-formed 
  ripples 
  

   of 
  exceptional 
  wave-length 
  probably 
  were 
  formed 
  in 
  an 
  

   analogous 
  way 
  ; 
  for 
  instance, 
  the 
  heavy 
  rippling 
  observed 
  

   by 
  King 
  in 
  the 
  col 
  between 
  two 
  adjoining 
  crescent 
  dunes, 
  

   the 
  tapering 
  ends 
  of 
  the 
  barchanes 
  forming 
  the 
  obstacles. 
  

  

  2. 
  Eolian 
  dunes. 
  

  

  The 
  chief 
  obstacle 
  in 
  the 
  way 
  of 
  a 
  satisfactory 
  discus- 
  

   sion 
  of 
  the 
  fundamental 
  principles 
  underlying 
  the 
  forma- 
  

   tion 
  of 
  dunes, 
  lies 
  in 
  the 
  fact 
  that 
  it 
  is 
  very 
  difficult 
  to 
  

   form 
  a 
  quantitative 
  estimate 
  of 
  the 
  role 
  played 
  by 
  secon- 
  

   dary 
  factors 
  in 
  the 
  production 
  of 
  the 
  final 
  shape 
  of 
  each 
  

   dune 
  observed. 
  Such 
  factors 
  are 
  for 
  instance 
  : 
  

  

  (a.) 
  The 
  presence 
  of 
  obstacles, 
  especially 
  of 
  vegeta- 
  

   tion, 
  which 
  may 
  give 
  rise 
  to 
  a 
  dune 
  and 
  influence 
  its 
  final 
  

   height 
  and 
  shape. 
  

  

  (b.) 
  The 
  presence 
  of 
  moisture 
  within 
  sandy 
  surfaces, 
  

   especially 
  within 
  dunes. 
  

  

  (c.) 
  The 
  changing, 
  at 
  shorter 
  or 
  longer 
  intervals, 
  of 
  

   the 
  direction 
  of 
  the 
  wind. 
  

  

  (d.) 
  The 
  changing 
  of 
  the 
  average 
  or 
  the 
  maximum 
  

   velocity 
  of 
  the 
  wind. 
  

  

  (e.) 
  Changes 
  in 
  the 
  supply 
  of 
  sand, 
  as 
  to 
  quantity 
  or 
  

   material. 
  

  

  (/.) 
  The 
  traveling 
  of 
  dunes, 
  in 
  consequence 
  of 
  which 
  

   a 
  dune 
  may 
  be 
  observed 
  under 
  conditions 
  quite 
  different 
  

   from 
  those 
  under 
  which 
  it 
  came 
  into 
  existence. 
  

  

  00 
  p. 
  195; 
  excellent 
  photograph 
  opp. 
  p. 
  200. 
  

  

  01 
  To 
  prevent 
  the 
  fine 
  dune 
  sand 
  used 
  from 
  being 
  blown 
  away, 
  it 
  was 
  

   sprinkled 
  with 
  coarser 
  grains. 
  On 
  all 
  ripples 
  formed 
  under 
  the 
  lee 
  of 
  an 
  

   obstacle, 
  the 
  large 
  grains 
  concentrated 
  on 
  the 
  crest, 
  which 
  was 
  sometimes 
  

   entirely 
  covered 
  by 
  them 
  (p. 
  199). 
  

  

  