﻿iSS 
  OLDHAM: 
  GREAT 
  EARTHQUAKE 
  OF 
  1 
  897. 
  

  

  magnetic 
  dip, 
  but 
  the 
  displacement 
  is 
  so 
  small 
  that 
  the 
  centre 
  

   of 
  gravity 
  lies 
  practically 
  within 
  the 
  area 
  of 
  the 
  small 
  bundle 
  of 
  

   fibres 
  from 
  which 
  the 
  magnet 
  is 
  suspended, 
  and 
  it 
  is 
  difficult 
  

   to 
  understand 
  how 
  an 
  angular 
  swing 
  could 
  be 
  imparted 
  to 
  the 
  

   magnet 
  by 
  its 
  inertia, 
  while 
  a 
  tilting 
  of 
  the 
  instrument 
  as 
  a 
  

   whole 
  would 
  have 
  no 
  effect 
  in 
  producing 
  an 
  angular 
  displacement 
  of 
  

   the 
  needle. 
  The 
  only 
  form 
  of 
  mechanical 
  displacement 
  which 
  would 
  

   account 
  for 
  the 
  effect 
  of 
  the 
  earthquake 
  on 
  the 
  trace, 
  is 
  a 
  twisting 
  

   of 
  the 
  instrument 
  as 
  a 
  whole, 
  or 
  a 
  side-long 
  motion 
  of 
  the 
  recording 
  

   paper 
  relative 
  to 
  the 
  magnet. 
  As 
  in 
  the 
  case 
  of 
  the 
  vertical 
  force 
  

   instrument 
  when 
  tilted, 
  the 
  magnet 
  would 
  retain 
  its 
  original 
  position, 
  

   and 
  the 
  spot 
  of 
  light 
  would 
  reach 
  the 
  paper 
  on 
  one 
  side 
  o 
  r 
  

   other 
  of 
  the 
  position 
  properly 
  due 
  to 
  that 
  of 
  the 
  magnet. 
  

   Apart, 
  however, 
  from 
  such 
  twisting 
  of 
  the 
  instrument 
  as 
  a 
  whole 
  

   in 
  a 
  horizontal 
  direction, 
  it 
  is 
  difficult 
  to 
  conceive 
  any 
  explanation, 
  

   other 
  than 
  magnetic, 
  to 
  account 
  for 
  the 
  angular 
  displacement 
  of 
  

   the 
  needle. 
  

  

  I 
  know 
  of 
  no 
  means 
  of 
  directly 
  testing 
  whether 
  there 
  was 
  any 
  

   such 
  horizontal 
  twisting 
  of 
  the 
  instrument. 
  The 
  vertical 
  force 
  

   instrument 
  gives 
  no 
  check, 
  for, 
  the 
  motion 
  of 
  the 
  paper 
  being 
  in 
  a 
  

   horizontal 
  direction, 
  the 
  only 
  effect 
  of 
  such 
  displacement 
  would 
  be 
  

   to 
  cause 
  a 
  slight 
  uncertainty 
  in 
  the 
  time 
  determination, 
  but 
  the 
  

   amount 
  of 
  the 
  angular 
  displacement 
  of 
  the 
  declination 
  needle 
  re- 
  

   corded— 
  exceeding 
  as 
  it 
  does 
  28' 
  of 
  arc 
  — 
  makes 
  it 
  improbable 
  that 
  any 
  

   such 
  displacement 
  took 
  place 
  ; 
  moreover, 
  though 
  it 
  would 
  be 
  im- 
  

   possible 
  to 
  recognise 
  the 
  effect 
  of 
  a 
  small 
  horizontal 
  displacement 
  on 
  

   the 
  trace 
  of 
  the 
  vertical 
  force 
  instrument, 
  it 
  is 
  difficult 
  to 
  believe 
  that 
  

   so 
  large 
  a 
  horizontal 
  angular 
  displacement 
  as 
  half 
  a 
  degree 
  of 
  arc 
  

   would 
  not 
  introduce 
  anomalies 
  in 
  the 
  trace, 
  which 
  it 
  would 
  be 
  

   impossible 
  to 
  overlook. 
  

  

  There 
  is 
  one 
  supposition 
  by 
  which 
  a 
  large 
  disturbance 
  of 
  this 
  

   needle 
  might 
  be 
  explained, 
  and 
  that 
  is 
  a 
  succession 
  of 
  small 
  alternating 
  

   angular 
  twists, 
  whose 
  period 
  was 
  either 
  identical 
  with, 
  or 
  a 
  small 
  

  

  ( 
  «88 
  ) 
  

  

  