﻿OLDHAM 
  : 
  GREAT 
  EARTHQUAKE 
  OF 
  1897. 
  

  

  directly 
  forwards 
  and 
  backwards 
  in 
  the 
  direction 
  of 
  travel 
  of 
  the 
  waves, 
  hence 
  they 
  

   are 
  sometimes 
  called 
  normal 
  waves. 
  

  

  13, 
  To 
  understand 
  the 
  nature 
  of 
  the 
  movement 
  in 
  a 
  wave 
  of 
  this 
  kind 
  let 
  o, 
  fig. 
  ii 
  

  

  represent 
  the 
  normal 
  position 
  of 
  

  

  a 
  molecule 
  when 
  undisturbed. 
  

  

  am 
  • 
  • 
  • 
  • 
  • 
  • 
  • 
  94, 
  Suppose 
  it 
  to 
  become 
  involved 
  in 
  

  

  a 
  wave 
  of 
  elastic 
  compression 
  

  

  travelling 
  in 
  a 
  straight 
  line 
  from 
  

  

  Fig. 
  ii. 
  Diagram 
  to 
  illustrate 
  the 
  successive 
  posi- 
  i 
  eft 
  to 
  right 
  . 
  then 
  start 
  j 
  n 
  g 
  from 
  o 
  

  

  tions 
  occupied 
  by 
  the 
  wave-particles 
  at 
  equal 
  inter- 
  . 
  .„ 
  , 
  

  

  vals 
  of 
  time 
  during 
  one 
  complete 
  undulation 
  of 
  a 
  it 
  will 
  move 
  outwards 
  passing 
  

  

  condensational 
  wave. 
  successively 
  through 
  the 
  positions 
  

  

  I, 
  2, 
  3, 
  till 
  at 
  4 
  it 
  reaches 
  the 
  

   extreme 
  limit 
  of 
  its 
  movement. 
  From 
  4 
  it 
  returns 
  with 
  an 
  ever-increasing 
  

   speed, 
  till 
  at 
  8 
  it 
  passes 
  on 
  through 
  its 
  original 
  position 
  and, 
  gradually 
  slowing 
  

   down, 
  comes 
  to 
  a 
  stop 
  at 
  12 
  ; 
  from 
  this 
  It 
  commences 
  its 
  return 
  journey 
  through 
  

   o 
  to 
  4 
  and 
  so 
  on. 
  The 
  movement 
  is 
  in 
  fact 
  just 
  like 
  that 
  of 
  the 
  bob 
  of 
  a 
  pendulum* 
  

   assuming 
  the 
  direction 
  of 
  travel 
  of 
  the 
  wave 
  to 
  be 
  that 
  of 
  the 
  swing 
  of 
  the 
  

   pendulum. 
  Returning 
  to 
  the 
  diagram 
  fig 
  ii, 
  the 
  distance 
  from 
  o 
  to 
  4 
  is 
  the 
  

   implitude 
  of 
  the 
  wave, 
  or 
  the 
  extreme 
  distance 
  which 
  the 
  wave-particle 
  reache 
  s 
  

   from 
  its 
  normal 
  position; 
  4 
  to 
  12 
  is 
  the 
  double 
  amplitude 
  or 
  range 
  of 
  motion 
  

   of 
  the 
  particle, 
  and 
  the 
  time 
  taken 
  by 
  the 
  wave-particle 
  in 
  travelling 
  from 
  o 
  out 
  to 
  4 
  

   and 
  back 
  through 
  12 
  to 
  o 
  is 
  the 
  period 
  of 
  the 
  wave. 
  

  

  14. 
  As 
  in 
  the 
  water 
  wave, 
  the 
  movement 
  of 
  the 
  wave-particle 
  may 
  be 
  divided 
  into 
  

   two 
  phases, 
  one 
  of 
  movement 
  in 
  the 
  direction 
  the 
  wave 
  is 
  travelling, 
  the 
  other 
  in 
  the 
  

   reverse 
  direction. 
  Each 
  of 
  these 
  can 
  again 
  be 
  divided 
  into 
  semiphases 
  according 
  

   as 
  the 
  movement 
  of 
  the 
  wave-particle 
  is 
  inwards, 
  towards, 
  or 
  outwards 
  from 
  its 
  

   normal 
  undisturbed 
  position. 
  

  

  15. 
  To 
  understand 
  what 
  is 
  meant 
  by 
  the 
  wave 
  length 
  in 
  a 
  wave 
  of 
  this 
  nature 
  

   we 
  must 
  consider 
  how 
  the 
  arrangement 
  of 
  the 
  molecules 
  is 
  affected 
  by 
  them. 
  In 
  

   fig. 
  iii, 
  let 
  o 
  represent 
  a 
  molecule 
  which 
  at 
  the 
  movement 
  is 
  passing 
  through 
  its 
  

  

  4 
  S 
  14- 
  1? 
  a 
  8 
  <? 
  4 
  2 
  

  

  — 
  s 
  •---# 
  © 
  »-• 
  e 
  * 
  e---« 
  » 
  •— 
  -» 
  • 
  • 
  «-— 
  -© 
  

  

  Fig. 
  iii. 
  Diagram 
  to 
  illustrate 
  the 
  distribution 
  of 
  matter 
  in 
  a 
  condensational 
  waves 
  

  

  normal 
  position 
  ; 
  the 
  next 
  molecule 
  behind 
  it, 
  the 
  wave 
  being 
  supposed 
  as 
  travelling 
  

   from 
  left 
  to 
  right, 
  is 
  in 
  a 
  slightly 
  more 
  advanced 
  stage 
  of 
  the 
  wave 
  motion, 
  the 
  next 
  

   still 
  more 
  so 
  and 
  so 
  on 
  till 
  we 
  come 
  to 
  one 
  where 
  the 
  wave-particle 
  has 
  reached 
  the 
  

   stage 
  marked 
  2 
  in 
  fig. 
  ii. 
  The 
  small 
  dot 
  numbered 
  2 
  in 
  fig. 
  ii. 
  may 
  be 
  taken 
  to 
  re- 
  

   present 
  the 
  normal 
  position 
  of 
  the 
  molecule 
  and 
  the 
  heavy 
  dot 
  its 
  position 
  at 
  the 
  

   instant 
  under 
  consideration. 
  Passing 
  still 
  further 
  backwards 
  along 
  the 
  wave 
  path 
  

   we 
  come 
  to 
  molecules 
  occupying 
  respectively 
  the 
  positions 
  4, 
  6, 
  and 
  8, 
  or 
  normal 
  

   and 
  behind 
  that 
  again 
  10, 
  12, 
  14, 
  and 
  so 
  on 
  to 
  position 
  o. 
  Now 
  if 
  the 
  momentary 
  

   disposition 
  of 
  the 
  dots 
  is 
  considered 
  they 
  will 
  be 
  seen 
  to 
  be 
  crowded 
  together 
  at 
  o 
  

   and 
  widely 
  separated 
  at 
  8 
  ; 
  that 
  is 
  to 
  say 
  there 
  are 
  definite 
  zones 
  in 
  the 
  substance 
  

   through 
  which 
  the 
  wave 
  is 
  travelling 
  where 
  its 
  material 
  is 
  alternately 
  condensed 
  

   and 
  rarified. 
  We 
  may 
  regard 
  the 
  point 
  of 
  maximum 
  condensation 
  as 
  analogous 
  

  

  