﻿iSo 
  OLDHAM 
  : 
  GREAT 
  EARTHQUAKE 
  OF 
  1897 
  

  

  Chapter 
  XI 
  —THE 
  RECORDS 
  OF 
  THE 
  BOMBAY 
  

   MAGNETIC 
  OBSERVATORY, 
  

  

  At 
  Bombay, 
  though 
  the 
  earthquake 
  was 
  not 
  felt, 
  it 
  left 
  its 
  

   impress, 
  as 
  has 
  already 
  been 
  mentioned 
  (Chapter 
  IV), 
  on 
  the 
  records 
  

   of 
  the 
  magnetic 
  instruments 
  in 
  the 
  Observatory 
  at 
  Colaba. 
  

  

  The 
  instruments 
  affected 
  were 
  the 
  declination, 
  horizontal 
  force, 
  

   and 
  vertical 
  force 
  magnetographs, 
  and 
  the 
  barograph. 
  Reproductions 
  

   of 
  the 
  traces 
  of 
  the 
  first 
  three 
  will 
  be 
  found 
  on 
  plate 
  XL, 
  and 
  the 
  

   following 
  description, 
  extracted 
  from 
  the 
  account 
  published 
  by 
  

   Mr. 
  N. 
  A. 
  F. 
  Moos, 
  1 
  Director 
  of 
  the 
  Observatory, 
  in 
  his 
  report 
  for 
  

   the 
  year 
  1896, 
  may 
  with 
  advantage 
  be 
  repeated 
  here: 
  — 
  

  

  3. 
  It 
  must 
  be 
  noted 
  here, 
  that 
  the 
  disturbance 
  fortunately 
  occurred 
  just 
  after 
  

   16 
  hours— 
  the 
  time 
  of 
  hourly 
  eye 
  observation^ 
  ; 
  a 
  valuable 
  opportunity, 
  therefore, 
  

   was 
  secured, 
  and 
  it 
  became 
  possible 
  to 
  note 
  and 
  study 
  the 
  peculiar 
  behaviour 
  of 
  

   the 
  instruments 
  during 
  their 
  disturbances, 
  some 
  of 
  which 
  by 
  the 
  rapidity 
  of 
  their 
  

   movements 
  could 
  not 
  have 
  been 
  photographed. 
  There 
  may 
  be 
  some 
  doubt 
  as 
  to 
  

   whether 
  the 
  disturbance 
  in 
  the 
  Magnetographs 
  was 
  due 
  to 
  mechanical 
  or 
  magnetic 
  

   action, 
  but 
  no 
  such 
  doubt 
  can 
  exist 
  for 
  the 
  Barograph. 
  The 
  disturbance 
  of 
  this 
  

   instrument 
  must 
  hare 
  been 
  caused 
  by 
  distortional 
  waves, 
  or 
  due 
  to 
  tilting 
  of 
  the 
  

   instrument. 
  It 
  will 
  be 
  noticed 
  that 
  the 
  maximum 
  effect 
  of 
  the 
  disturbance, 
  as 
  

   photographed 
  in 
  the 
  Barograph 
  trace, 
  followed 
  the 
  largest 
  wave 
  shown 
  by 
  Declina» 
  

   tion 
  and 
  Vertical 
  Force 
  Magnetographs 
  by 
  about 
  one 
  minute. 
  

  

  4. 
  With 
  regard 
  to 
  the 
  disturbed 
  trace 
  of 
  the 
  Declination 
  Magnetograph 
  on 
  the 
  

   other 
  hand, 
  it 
  is 
  difficult 
  to 
  conceive 
  how 
  condensational 
  or 
  distortional 
  waves 
  could 
  

   affect 
  thejsuspended 
  magnet 
  under 
  an 
  exhausted 
  receiver 
  so 
  as 
  to 
  set 
  up 
  large 
  

   vibrations 
  in 
  it, 
  simulating 
  magnetic 
  action. 
  The 
  magnet 
  was 
  disturbed 
  mechani- 
  

   cally, 
  no 
  doubt, 
  by 
  the 
  seismic 
  waves, 
  and 
  these 
  disturbances 
  were 
  typical 
  of 
  

   mechanical 
  action. 
  The 
  three 
  motions 
  (besides 
  the 
  vibratory 
  motion) 
  observed 
  in 
  

   the 
  Declinometer 
  at 
  the 
  time 
  of 
  disturbance, 
  showed 
  (1) 
  motion 
  of 
  the 
  whole 
  magnet 
  

   parallel 
  to 
  itself 
  (East 
  and 
  West), 
  (2) 
  motion 
  of 
  the 
  magnet 
  as 
  a 
  whole 
  (North 
  and 
  

   South), 
  and 
  (3) 
  the 
  slight 
  bobbing 
  motion 
  of 
  the 
  ends 
  of 
  the 
  magnet 
  up 
  and 
  down. 
  

   But 
  neither 
  of 
  these 
  motions 
  would 
  show 
  increase 
  of 
  the 
  scale 
  reading, 
  nor 
  would 
  

   vibratory 
  motion, 
  if 
  set 
  up 
  by 
  mechanical 
  causes, 
  continue 
  for 
  some 
  time 
  to 
  show 
  

  

  1 
  A 
  short 
  note 
  on 
  the 
  disturbance 
  of 
  the 
  magnetical 
  and 
  meteorological 
  instruments 
  at 
  the 
  

   Colaba 
  observatory 
  during 
  the 
  earthquake 
  of 
  12th 
  June 
  1897. 
  Magnetical 
  and 
  Meteorological 
  

   Observations 
  made 
  at 
  the 
  Government 
  Observatory, 
  Bombay. 
  1896 
  : 
  4° 
  Bombay, 
  1897* 
  

   Appendix, 
  pp. 
  (i)_f 
  7 
  ). 
  

  

  ( 
  180 
  ) 
  

  

  