﻿252 
  OLDHAM: 
  GREAT 
  EARTHQUAKE 
  OF 
  1897. 
  

  

  ground 
  was 
  moved 
  bodily 
  backwards 
  and 
  forwards 
  in 
  a 
  direction 
  

   inclined 
  more 
  than 
  30 
  to 
  the 
  horizon. 
  

  

  A 
  movement 
  of 
  this 
  kind 
  need 
  not 
  be 
  accompanied 
  by 
  any 
  

   marked 
  surface 
  tilting, 
  and 
  any 
  way 
  the 
  tilts 
  would 
  be 
  too 
  rapid 
  for 
  

   the 
  slowly 
  moving 
  pendula 
  to 
  respond 
  largely 
  to 
  them 
  ; 
  consequently 
  

   the 
  effect 
  of 
  the 
  backward 
  and 
  forward 
  movement 
  on 
  the 
  pendula 
  

   would 
  be 
  much 
  greater 
  than 
  that 
  of 
  the 
  up 
  and 
  down 
  movement, 
  as 
  

   deduced 
  by 
  Prof. 
  Grablowitz 
  from 
  a 
  comparison 
  of 
  the 
  records 
  of 
  

   his 
  instruments, 
  

  

  In 
  the 
  second 
  phase 
  the 
  wave 
  period 
  was 
  longer 
  than 
  in 
  the 
  first, 
  

   and 
  there 
  are 
  indications 
  of 
  tilting 
  of 
  the 
  surface 
  of 
  the 
  ground. 
  

   This 
  phase 
  appears 
  to 
  be 
  due 
  to 
  the 
  waves 
  of 
  elastic 
  distortion, 
  in 
  

   which 
  the 
  movement 
  of 
  the 
  wave 
  particle 
  is 
  at 
  right 
  angles 
  to 
  the 
  

   direction 
  of 
  travel 
  of 
  the 
  wave. 
  As 
  demanded 
  by 
  theory 
  these 
  waves 
  

   travelled 
  at 
  a 
  slower 
  rate 
  than 
  the 
  waves 
  of 
  elastic 
  compression. 
  

  

  An 
  important 
  point 
  to 
  note 
  is 
  that 
  the 
  rate 
  of 
  propagation 
  of 
  the 
  

   wave 
  motion 
  of 
  the 
  second 
  phase, 
  5*3 
  kilometres 
  per 
  second, 
  is 
  in 
  

   close 
  agreement 
  with 
  what 
  is 
  regarded 
  as 
  the 
  normal 
  rate 
  of 
  

   propagation 
  of 
  the 
  preliminary 
  tremors, 
  corresponding 
  to 
  the 
  

   distance 
  between 
  Italy 
  and 
  the 
  epicentre. 
  That 
  cf 
  the 
  first 
  phase, 
  

   9 
  km., 
  per 
  second 
  is, 
  on 
  the 
  other 
  hand 
  very 
  much 
  in 
  excess. 
  Refer- 
  

   ence 
  has 
  already 
  been 
  made 
  to 
  the 
  fact 
  that 
  the 
  only 
  published 
  

   instance 
  of 
  a 
  three-fold 
  division 
  of 
  the 
  record, 
  similar 
  to 
  that 
  of 
  this 
  

   earthquake, 
  appears 
  to 
  be 
  the 
  Japanese 
  earthquake 
  of 
  1894, 
  but 
  too 
  

   much 
  importance 
  must 
  not 
  be 
  atached 
  to 
  this. 
  From 
  the 
  nature 
  of 
  

   the 
  case, 
  these 
  waves 
  would 
  only 
  be 
  recorded, 
  or 
  at 
  any 
  rate 
  only 
  

   separable, 
  in 
  the 
  records 
  of 
  heavy 
  pendula 
  of 
  the 
  Italian 
  type, 
  and 
  

   would 
  ordinarily 
  be 
  unrecorded 
  by 
  Darwin's 
  bifilar 
  pendulum, 
  or 
  if 
  

   recorded, 
  not 
  distinctly 
  separable 
  from 
  those 
  of 
  the 
  second 
  phase 
  on 
  

   account 
  of 
  the 
  slowness 
  of 
  travel 
  of 
  the 
  recording 
  surface 
  and 
  conse- 
  

   quent 
  compression 
  of 
  the 
  record. 
  It 
  is 
  therefore 
  possible 
  that 
  the 
  

   waves 
  of 
  elastic 
  compression 
  have 
  been 
  more 
  frequenty 
  recorded, 
  and 
  

   that 
  records 
  showing 
  a 
  three-phase 
  character 
  of 
  the 
  trace 
  are 
  com- 
  

   ( 
  252 
  ) 
  

  

  