﻿1S4 
  OLDHAM: 
  GREAT 
  EARTHQUAKE 
  OF 
  1897. 
  

  

  13. 
  It 
  will 
  be 
  seen 
  that 
  the 
  beginning 
  of 
  the 
  seismic 
  disturbance, 
  as 
  recorded 
  

   by 
  this 
  instrument, 
  shows 
  that 
  it 
  was 
  somewhat 
  sudden 
  and 
  abrupt, 
  and 
  the 
  rise 
  

   to 
  the 
  maximum 
  after 
  the 
  commencement 
  was 
  within 
  3 
  to 
  4 
  minutes, 
  while, 
  the 
  

   falling 
  off, 
  as 
  shown 
  by 
  the 
  clear 
  trace, 
  appears 
  to 
  have 
  taken 
  place 
  gradually 
  and 
  

   steadily 
  till 
  normal 
  conditions 
  were 
  reached. 
  From 
  the 
  very 
  regular 
  trace 
  it 
  

   appears 
  that 
  the 
  time 
  of 
  vibration 
  of 
  the 
  magnet 
  must 
  bear 
  a 
  certain 
  relation 
  to 
  

   the 
  period 
  of 
  disturbance, 
  the 
  exact 
  time 
  of 
  vibration 
  being 
  some 
  even 
  multiple 
  of 
  

   submultiple 
  of 
  the 
  period, 
  otherwise 
  the 
  regularity 
  of 
  the 
  vibrations 
  would 
  be 
  

   disturbed 
  by 
  "interference* 
  " 
  

  

  14. 
  The 
  average 
  displacement 
  of 
  the 
  declination 
  East 
  or 
  West 
  during 
  the 
  4th 
  

   wave, 
  which 
  is 
  well 
  defined, 
  would 
  equal 
  that 
  due 
  to 
  a 
  change 
  of 
  '00034 
  C. 
  G. 
  S. 
  

   units 
  in 
  terms 
  of 
  force. 
  

  

  15. 
  If 
  by 
  producing 
  the 
  curves 
  on 
  each 
  side 
  we 
  get 
  the 
  probable 
  amplitude 
  of 
  

   the 
  most 
  intense 
  wave 
  where 
  the 
  two 
  curves 
  meet, 
  the 
  average 
  displacement 
  

   appears 
  to 
  be 
  *i6 
  inch, 
  equal 
  to 
  '00051 
  C. 
  G. 
  S- 
  units 
  in 
  terms 
  of 
  force, 
  and 
  this 
  

   shows 
  that 
  the 
  second 
  wave 
  was 
  the 
  most 
  intense. 
  

  

  16. 
  7 
  he 
  Horizontal 
  Force 
  Magnetograph. 
  — 
  The 
  trace 
  here 
  shows 
  the 
  disturb- 
  

   ance 
  in 
  the 
  Horizontal 
  Intensity, 
  which 
  appears 
  to 
  have 
  commenced 
  early, 
  about 
  

   2§ 
  minutes 
  after 
  sixteen, 
  while 
  the 
  instrument 
  shows 
  that 
  the 
  disturbance 
  did 
  not 
  

   end 
  before 
  36 
  minutes 
  past 
  sixteen. 
  This 
  must 
  be 
  due 
  to 
  the 
  extreme 
  sensitiveness 
  

   of 
  the 
  instrument. 
  The 
  sudden 
  displacement 
  of 
  the 
  curve 
  at 
  the 
  beginning 
  shows 
  

   that 
  the 
  vibrations 
  must 
  have 
  been 
  performed 
  about 
  a 
  displaced 
  zero 
  above 
  the 
  curve, 
  

   showing 
  increased 
  intensity 
  equal 
  to 
  about 
  "00004 
  C. 
  G. 
  S. 
  units, 
  and 
  the 
  action 
  

   appears, 
  therefore, 
  more 
  magnetic 
  than 
  mechanical, 
  though 
  both 
  actions 
  seem 
  to 
  

   be 
  superimposed 
  upon 
  each 
  other. 
  At 
  $i 
  m 
  past 
  sixteen 
  the 
  vibrations 
  pass 
  the 
  limit 
  

   of 
  photographic 
  action 
  and 
  this 
  perhaps 
  marks 
  the 
  time 
  of 
  commencement 
  of 
  the 
  

   more 
  intense 
  oscillations. 
  The 
  trace 
  appears 
  faintly 
  at 
  about 
  19! 
  m 
  past 
  the 
  hour, 
  less 
  

   faintly 
  at 
  the 
  extreme 
  ends 
  of 
  the 
  amplitudes 
  than 
  at 
  the 
  middle*of 
  the 
  curve, 
  and 
  

   it 
  continues 
  faint 
  till 
  about 
  26 
  m 
  past 
  the 
  hour, 
  when 
  the 
  trace 
  suddenly 
  darkens 
  

   but 
  continues 
  disturbed 
  till 
  36 
  01 
  past 
  sixteen 
  when 
  the 
  normal 
  curve 
  is 
  resumed. 
  

   The 
  time 
  of 
  vibration 
  of 
  the 
  magnet 
  has 
  been 
  ascertained 
  to 
  be 
  8 
  s 
  , 
  and 
  obviously 
  

   as 
  this 
  factor, 
  together 
  with 
  the 
  period 
  of 
  disturbance, 
  must 
  influence 
  the 
  motion 
  

   of 
  the 
  magnet, 
  the 
  irregularity 
  in 
  the 
  trace 
  is 
  perhaps 
  to 
  a 
  considerable 
  extent 
  due 
  

   to 
  that 
  cause. 
  The 
  scale 
  value 
  of 
  the 
  original 
  trace 
  is 
  28' 
  67'' 
  for 
  an 
  inch 
  of 
  

   ta 
  ulation, 
  and 
  the 
  clear 
  length 
  of 
  suspension 
  of 
  the 
  magnet 
  is 
  about 
  12 
  inches. 
  

  

  17. 
  Vertical 
  Force 
  Magnetograph. 
  — 
  The 
  trace 
  shown 
  here 
  is 
  that 
  of 
  the 
  vertical 
  

   force 
  magnetograph. 
  As 
  pointed 
  out 
  elsewhere, 
  this 
  instrument 
  is 
  very 
  sensitive 
  

   to 
  shocks 
  resulting 
  in 
  dislocations 
  of 
  the 
  curve. 
  No 
  such 
  dislocation 
  is 
  detected 
  

   during 
  the 
  earthquake. 
  The 
  unusual 
  thickness 
  of 
  the 
  vibration 
  trace 
  shows 
  that 
  

   the 
  cause 
  of 
  the 
  disturbance 
  could 
  not 
  have 
  been 
  instantaneous, 
  but 
  must 
  have 
  

   acted 
  for 
  some 
  considerable 
  time, 
  for 
  vibration 
  of 
  the 
  above 
  amplitude, 
  if 
  caused 
  

   by, 
  say, 
  momentary 
  action 
  of 
  a 
  deflector, 
  would 
  die 
  out 
  within 
  i 
  a 
  minute, 
  while 
  

   the 
  movements 
  shown 
  in 
  trace 
  lasted 
  each 
  for 
  over 
  three 
  minutes. 
  The 
  first 
  wave 
  

   appears 
  to 
  have 
  commenced 
  the 
  disturbance 
  at 
  6? 
  m 
  , 
  attained 
  the 
  maximum 
  at 
  

   abou: 
  8 
  rn 
  , 
  and 
  ended 
  about 
  io 
  m 
  past 
  sixteen. 
  The 
  second 
  commenced 
  immediately 
  

   after 
  this, 
  reached 
  is 
  maximum 
  effect 
  at 
  about 
  I2 
  m 
  and 
  ended 
  at 
  i\ 
  m 
  past 
  four. 
  

  

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