﻿254 
  OLDHAM 
  ; 
  GREAT 
  EARTHQUAKE 
  OF 
  1897. 
  

  

  result 
  that 
  the 
  rate 
  of 
  travel, 
  as 
  between 
  the 
  centre 
  and 
  Italy, 
  was 
  

   about 
  113 
  miles 
  per 
  minute, 
  or 
  2*98 
  kilometres 
  per 
  second, 
  which 
  is 
  

   practically 
  the 
  same 
  as 
  the 
  apparent 
  rate 
  of 
  travel 
  of 
  the 
  sensible 
  

   shock 
  along 
  the 
  surface 
  between 
  Calcutta 
  and 
  Bombay. 
  There 
  is 
  

   only 
  one 
  conclusion 
  possible 
  from 
  this, 
  that 
  these 
  undulations, 
  which 
  

   are 
  quite 
  characteristic 
  and 
  distinct 
  on 
  the 
  records, 
  were 
  not 
  due 
  

   to 
  waves 
  of 
  elastic 
  distortion, 
  as 
  has 
  been 
  assumed 
  by 
  some 
  

   observers, 
  and 
  did 
  not 
  travel 
  through 
  the 
  substance 
  of 
  the 
  earth, 
  but 
  

   along 
  its 
  surface. 
  Had 
  they 
  travelled 
  through 
  the 
  earth, 
  instead 
  of 
  

   along 
  its 
  surface, 
  it 
  is 
  impossible 
  to 
  conceive 
  that 
  their 
  rate 
  of 
  

   travel 
  would 
  not 
  have 
  been 
  affected 
  by 
  the 
  depth, 
  as 
  was 
  the 
  case 
  

   with 
  the 
  two 
  other 
  kinds 
  of 
  wave. 
  

  

  We 
  have 
  here 
  a 
  very 
  strong 
  confirmation 
  of 
  Professor 
  Milne's 
  

   suggestion 
  that 
  an 
  earthquake 
  sets 
  up 
  a 
  wave 
  motion 
  at 
  the 
  surface 
  

   which 
  travels 
  outwards 
  along 
  the 
  surface 
  at 
  a 
  practically 
  uniform 
  

   speed, 
  the 
  individual 
  waves 
  becoming 
  longer 
  and 
  slower 
  in 
  the 
  

   movement 
  of 
  the 
  wave 
  particle, 
  just 
  as 
  the 
  ripples 
  caused 
  by 
  a 
  stone 
  

   falling 
  into 
  a 
  pond 
  become 
  longer 
  and 
  less 
  pronounced 
  as 
  they 
  

   travel 
  outwards 
  from 
  the 
  centre. 
  

  

  One 
  thing 
  only 
  is 
  needed 
  to 
  complete 
  the 
  demonstration, 
  and 
  

   that 
  is 
  to 
  find 
  the 
  undulations 
  which 
  came 
  the 
  long 
  way 
  round 
  the 
  

   earth, 
  as 
  well 
  as 
  those 
  which 
  came 
  direct. 
  If 
  we 
  consider 
  what 
  

   happens 
  to 
  the 
  surface 
  undulations, 
  we 
  will 
  see 
  them 
  spreading 
  out- 
  

   wards 
  in 
  continually 
  widening 
  circles 
  till 
  they 
  pass 
  round 
  the 
  earth 
  

   and 
  converge 
  on 
  the 
  antipodes 
  of 
  the 
  epicentre. 
  Here 
  we 
  would 
  

   expect 
  to 
  find 
  an 
  instrument, 
  had 
  there 
  been 
  one 
  set 
  up, 
  giving 
  a 
  

   very 
  confused 
  record, 
  but 
  after 
  the 
  waves 
  cross 
  each 
  other 
  they 
  

   diverge 
  once 
  more, 
  in 
  widening 
  circles 
  as 
  they 
  travel 
  on 
  their 
  way, 
  

   and 
  in 
  due 
  time 
  we 
  should 
  find 
  the 
  trace 
  of 
  the 
  undulations 
  which 
  

   have 
  travelled 
  past 
  the 
  antipodes 
  of 
  the 
  centre 
  and 
  are 
  on 
  their 
  

   return 
  journey 
  round 
  the 
  other 
  side 
  of 
  the 
  world. 
  

  

  A 
  simple 
  calculation 
  will 
  show 
  that 
  if 
  the 
  rate 
  of 
  travel 
  of 
  these 
  

   surface 
  undulations 
  remained 
  constant, 
  the 
  return 
  wave 
  should 
  be 
  

   expected 
  about 
  10 
  minutes 
  past 
  2 
  P.M. 
  by 
  Greenwich 
  time. 
  

  

  (254) 
  

  

  