﻿1 
  8a 
  OLDHAM: 
  GREAT 
  EARTHQUAKE 
  OF 
  1897. 
  

  

  waves, 
  and, 
  therefore, 
  of 
  the 
  seismic 
  waves 
  was 
  inclined 
  more 
  to 
  " 
  East 
  to 
  West 
  " 
  

   than 
  to 
  "North 
  to 
  South." 
  From 
  the 
  mechanical 
  disturbance 
  of 
  the 
  magnets 
  of 
  

   the 
  two 
  instruments 
  Declination 
  and 
  Horizontal 
  Force, 
  which 
  are 
  suspended 
  at 
  

   right 
  angles 
  to 
  each 
  other, 
  the 
  more 
  pronounced 
  motion 
  observed 
  during 
  the 
  dis- 
  

   turbance 
  in 
  both, 
  was 
  from 
  East 
  to 
  West. 
  The 
  focal 
  distance 
  of 
  the 
  latter 
  instru- 
  

   ment 
  also 
  appears 
  to 
  be 
  slightly 
  affected, 
  due 
  to 
  motion 
  of 
  the 
  mirror 
  East 
  to 
  West. 
  

   It, 
  therefore, 
  strengthens 
  the 
  presumption 
  that 
  the 
  Direction 
  of 
  the 
  waves 
  must 
  

   have 
  had 
  a 
  strong 
  Easterly 
  component, 
  and 
  the 
  comparative 
  feeble 
  motion 
  of 
  

   both 
  the 
  magnets 
  parallel 
  to 
  North 
  and 
  South 
  direction, 
  shows 
  that 
  the 
  Northerly 
  

   component 
  was 
  present, 
  but 
  was 
  feeble. 
  We 
  shall 
  now 
  examine 
  each 
  trace 
  

   separately. 
  

  

  9. 
  Declination 
  Magnetograph.— 
  -The 
  photograph 
  shown 
  here 
  is 
  a 
  copy 
  (en- 
  

   larged 
  about 
  2\ 
  times) 
  of 
  the 
  trace 
  obtained 
  from 
  the 
  Declination 
  Magnetograph 
  

   at 
  the 
  Colaba 
  Observatory. 
  The 
  time 
  of 
  vibration 
  of 
  the 
  magnet 
  is 
  5*33 
  seconds, 
  

   and 
  the 
  original 
  trace 
  allows 
  of 
  an 
  accuracy 
  up 
  to 
  one 
  minute 
  in 
  the 
  determination 
  

   of 
  time. 
  The 
  scale 
  value 
  of 
  the 
  instrument 
  is 
  28' 
  72 
  for 
  an 
  inch 
  of 
  tabulation 
  in 
  the 
  

   original 
  curve. 
  The 
  curve 
  shows 
  a 
  part 
  of 
  the 
  trace 
  of 
  the 
  12th 
  June. 
  The 
  regular 
  

   break 
  seen 
  on 
  the 
  trace 
  is 
  due 
  to 
  a 
  metallic 
  fan, 
  which 
  automatically 
  cuts 
  off 
  light 
  

   every 
  two 
  hours 
  for 
  four 
  minutes 
  and 
  a 
  quarter. 
  The 
  middle 
  of 
  this 
  break, 
  there- 
  

   fore, 
  represents 
  i6 
  h 
  on 
  the 
  12th 
  June. 
  The 
  usual 
  sensitiveness 
  of 
  the 
  photographic 
  

   paper 
  used 
  at 
  this 
  Observatory 
  has 
  been 
  ascertained 
  by 
  experiments 
  with 
  a 
  steady 
  

   light, 
  and 
  found 
  that 
  an 
  exposure 
  of 
  about 
  4 
  s 
  to 
  the 
  usual 
  kerosine 
  burners 
  (of 
  about 
  

   4 
  candle 
  power 
  intensity) 
  used, 
  is 
  enough 
  for 
  action 
  ; 
  and 
  with 
  regard 
  to 
  impression 
  

   of 
  light 
  during 
  the 
  vibration 
  of 
  the 
  magnet, 
  it 
  is 
  found 
  that 
  oscillations 
  of 
  amplitude 
  

   of 
  3! 
  divisions 
  of 
  the 
  scale, 
  equal 
  to 
  a 
  displacement 
  of 
  about 
  28]minutes 
  of 
  declina- 
  

   tion 
  and 
  upwards, 
  are 
  not 
  recorded 
  ; 
  the 
  velocity 
  of 
  the 
  speck 
  of 
  light, 
  together 
  

   with 
  the 
  motion 
  of 
  the 
  paper, 
  a 
  litde 
  over 
  half 
  an 
  inch 
  in 
  one 
  hour, 
  precludes 
  

   photographic 
  action 
  unless 
  the 
  amplitudes 
  are 
  brought 
  within 
  the 
  above 
  limit, 
  and 
  

   naturally 
  the 
  first 
  impressions 
  produced 
  are 
  at 
  the 
  extreme 
  ends 
  of 
  the 
  amplitude, 
  

   where 
  the 
  velocity 
  is 
  a 
  minimum. 
  It 
  is 
  only 
  when 
  the 
  vibrations 
  fall 
  to 
  within 
  

   about 
  \\ 
  divisions 
  of 
  scale, 
  that 
  the 
  paper 
  shows 
  the 
  impression 
  of 
  light 
  in 
  the 
  

   middle 
  of 
  the 
  curve, 
  that 
  is, 
  where 
  the 
  velocity 
  is 
  a 
  maximum. 
  

  

  10. 
  It 
  is 
  assumed 
  in 
  what 
  follows, 
  that 
  every 
  seismic 
  wave 
  was 
  accompanied 
  

   by 
  a 
  temporary 
  disturbance 
  of 
  the 
  magnetic 
  circulation. 
  The 
  first 
  wave, 
  which 
  

   appears 
  to 
  have 
  just 
  commenced 
  the 
  disturbance 
  in 
  the 
  instrument 
  must 
  have 
  

   passed 
  Colaba 
  at 
  about 
  5 
  minutes 
  past 
  4. 
  The 
  seismic 
  disturbance 
  then 
  seems 
  

   to 
  have 
  grown 
  in 
  intensity, 
  and 
  about 
  7 
  minutes 
  past 
  the 
  hour 
  the 
  increased 
  

   amplitudes 
  (which 
  by 
  the 
  way, 
  it 
  must 
  be 
  noted, 
  were 
  first 
  performed 
  about 
  a 
  

   higher 
  zero 
  showing 
  a 
  decreased 
  easterly 
  declination, 
  movement 
  of 
  the 
  curve 
  in 
  

   the 
  downward 
  direction 
  showing 
  an 
  increase) 
  passed 
  the 
  limit 
  above 
  referred 
  to, 
  

   and 
  the 
  trace 
  is 
  entirely 
  lost. 
  Immediately 
  after 
  this, 
  however, 
  the 
  impulsive 
  force 
  

   appears 
  to 
  have 
  ceased, 
  and 
  the 
  damper 
  of 
  the 
  magnetograph 
  reducing 
  the 
  

   amplitudes 
  (the 
  logarithmic 
  decrement 
  has 
  been 
  ascertained 
  to 
  be 
  about 
  to 
  be 
  

   =•035), 
  bring 
  them 
  within 
  the 
  limit 
  of 
  photographic 
  action, 
  and 
  the 
  trace 
  is 
  just 
  

   photographed 
  at 
  about 
  9 
  minutes 
  past 
  sixteen. 
  This 
  marks 
  the 
  time 
  of 
  temporary 
  

   luli, 
  for 
  immediately 
  afterwards 
  the 
  trace 
  is 
  lost, 
  to 
  appear 
  faintly 
  again 
  but 
  more 
  

  

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