﻿2 
  2 
  2 
  OLDHAM 
  \ 
  GREAT 
  EARTHQUAKE 
  OF 
  1897. 
  

  

  ment 
  there 
  may 
  be 
  in 
  a 
  vertical 
  direction. 
  In 
  fig. 
  22 
  let 
  the 
  outer 
  

   circle 
  represent 
  the 
  ground 
  plan 
  of 
  a 
  cylindrical 
  column, 
  and 
  the 
  

   inner 
  circle 
  the 
  path 
  of 
  the 
  wave 
  particle, 
  that 
  is 
  to 
  say, 
  the 
  track 
  of 
  

   a 
  point 
  on 
  the 
  ground, 
  all 
  other 
  points 
  in 
  the 
  neighbourhood 
  describ- 
  

   ing 
  similar 
  circles. 
  At 
  N 
  the 
  movement 
  of 
  the 
  ground 
  is 
  eastwards, 
  

   causing 
  the 
  column 
  to 
  tilt 
  up 
  on 
  its 
  western 
  edge 
  ; 
  when 
  the 
  base 
  

   motion 
  has 
  progressed 
  so 
  that 
  the 
  particle 
  at 
  N 
  has 
  reached 
  E, 
  its 
  

   motion 
  is 
  southwards 
  and 
  the 
  column 
  will 
  be 
  tilted 
  on 
  to 
  its 
  northern 
  

   edge. 
  But 
  it 
  had 
  been 
  tilted 
  up 
  on 
  its 
  western 
  edge, 
  consequently 
  in 
  

   being 
  thrown 
  on 
  to 
  its 
  northern 
  it 
  will 
  be 
  revolved 
  slightly 
  in 
  a 
  

   negative 
  direction. 
  So 
  when 
  the 
  point 
  N 
  has 
  reached 
  S, 
  the 
  move- 
  

   ment 
  will 
  be 
  west 
  and 
  the 
  column 
  tilted 
  on 
  to 
  its 
  eastern 
  edge 
  and 
  

   again 
  revolved 
  slightly 
  in 
  a 
  negative 
  direction, 
  and 
  so 
  on. 
  For 
  

   simplicity 
  of 
  treatment 
  an 
  abrupt 
  change 
  of 
  motion 
  from 
  east 
  to 
  south 
  

   and 
  from 
  south 
  to 
  west 
  has 
  been 
  supposed, 
  but 
  the 
  effect 
  of 
  a 
  

   gradual 
  change 
  would 
  be 
  the 
  same, 
  a 
  continual 
  tendency 
  to 
  alter 
  the 
  

   point 
  on 
  which 
  the 
  column 
  was 
  tilted 
  and 
  to 
  cause 
  it 
  to 
  rotate 
  in 
  the 
  

   opposite 
  direction 
  to 
  the 
  course 
  of 
  the 
  wave 
  particle. 
  

  

  It 
  is 
  also 
  to 
  be 
  noticed 
  that 
  the 
  centre 
  of 
  the 
  column 
  will 
  de- 
  

   scribe 
  a 
  spiral 
  line 
  round 
  its 
  original 
  position, 
  as 
  each 
  change 
  of 
  tilt 
  

   will 
  carry 
  it 
  a 
  little 
  further 
  out, 
  but 
  its 
  final 
  position 
  will 
  be 
  closer 
  

   to 
  its 
  original 
  one 
  than 
  would 
  be 
  the 
  case, 
  had 
  the 
  same 
  amount 
  

   of 
  twisting 
  taken 
  place 
  by 
  a 
  single 
  tilt 
  on 
  one 
  edge, 
  and 
  a 
  si- 
  

   multaneous 
  sidelong 
  displacement. 
  In 
  other 
  words 
  the 
  virtual 
  

   radius 
  of 
  revolution 
  would 
  be 
  less 
  than 
  the 
  semidiameter 
  ; 
  

   being 
  thus 
  in 
  better 
  agreement 
  with 
  facts 
  than 
  either 
  the 
  third 
  or 
  

   fourth 
  hypothesis. 
  

  

  There 
  is 
  another 
  type 
  of 
  earthquake 
  motion, 
  more 
  common 
  than 
  

   that 
  required 
  by 
  the 
  preceding 
  explanation, 
  which 
  may 
  equally 
  well 
  

   give 
  rise 
  to 
  rotation. 
  This 
  form 
  may 
  be 
  diagrammatically 
  represented 
  

   in 
  fig. 
  23, 
  supposing 
  the 
  movement 
  to 
  start 
  from 
  1, 
  then 
  in 
  the 
  

   path 
  from 
  1 
  to 
  2 
  the 
  column 
  will 
  be 
  left 
  behind 
  and 
  caused 
  to 
  

   tilt 
  towards 
  1, 
  while 
  the 
  change 
  of 
  direction 
  between 
  1 
  and 
  2 
  may 
  

   ( 
  222 
  ) 
  

  

  