﻿186 
  OLDHAM: 
  GREAT 
  EARTHQUAKE 
  OF 
  1897, 
  

  

  a 
  copper 
  bar 
  for 
  the 
  magnet, 
  and 
  found 
  that 
  three 
  earthquakes, 
  which 
  

   affected 
  the 
  magnetograph, 
  did 
  not 
  affect 
  the 
  copper 
  bar. 
  But 
  

   no 
  certain 
  conclusion 
  can 
  be 
  drawn 
  from 
  this, 
  for, 
  as 
  pointed 
  out 
  by 
  

   Prof. 
  Agamennone, 
  1 
  the 
  mechanical 
  conditions 
  of 
  a 
  magnet 
  and 
  

   a 
  copper 
  bar, 
  on 
  which 
  the 
  earth's 
  magnetism 
  has 
  no 
  influence 
  

   are 
  so 
  different 
  that 
  they 
  could 
  not 
  be 
  expeced 
  to 
  respond 
  in 
  a 
  

   similar 
  manner, 
  even 
  to 
  a 
  purely 
  mechanical 
  disturbance 
  in 
  which 
  

   magnetism 
  had 
  no 
  part. 
  

  

  Turning 
  to 
  a 
  consideration 
  of 
  the 
  instruments 
  at 
  Bombay, 
  which 
  

   are 
  similar 
  to 
  those 
  at 
  any 
  other 
  magnetic 
  observatory, 
  we 
  have 
  three 
  

   types 
  devoted 
  to 
  recording, 
  (1) 
  vertical 
  force, 
  (2) 
  horizontal 
  force, 
  (3) 
  

   declination. 
  

  

  The 
  vertical 
  force 
  instrument 
  consists 
  of 
  a 
  bar 
  magnet 
  suspended 
  

   in 
  a 
  nearly 
  horizontal 
  position 
  on 
  two 
  knife-edges 
  which 
  allow 
  it 
  to 
  

   turn 
  in 
  a 
  vertical 
  plane, 
  but 
  not 
  to 
  turn 
  horizontally. 
  This 
  magnet 
  

   bears 
  a 
  small 
  mirror, 
  on 
  to 
  which 
  a 
  ray, 
  of 
  light 
  is 
  cast 
  from 
  a 
  lamp 
  

   and 
  reflected 
  back 
  on 
  to 
  a 
  slowly 
  travelling 
  band 
  of 
  sensitised 
  paper, 
  

   and 
  in 
  this 
  manner 
  a 
  record 
  is 
  kept 
  of 
  the 
  varying 
  inclination 
  of 
  the 
  

   magnet 
  due 
  to 
  variations 
  in 
  the 
  vertical 
  magnetic 
  force. 
  As 
  noted 
  

   by 
  Mr. 
  Moos, 
  the 
  centre 
  of 
  gravity 
  of 
  the 
  bar 
  does 
  not 
  coincide 
  with 
  

   that 
  of 
  the 
  knife-edges 
  ; 
  in 
  the 
  first 
  place 
  it 
  is 
  lower, 
  so 
  as 
  to 
  place 
  the 
  

   magnet 
  in 
  a 
  state 
  of 
  stable 
  equilibrium, 
  and 
  in 
  the 
  second 
  place 
  it 
  ha 
  s 
  

   to 
  be 
  a 
  short 
  distance 
  to 
  the 
  south 
  of 
  the 
  knife 
  edges 
  to 
  counteract 
  

   the 
  dip 
  and 
  keep 
  the 
  bar 
  approximately 
  horizontal. 
  As 
  a 
  consequence 
  

   the 
  effect 
  of 
  the 
  inertia 
  of 
  the 
  bar 
  would 
  tend 
  to 
  cause 
  a 
  disturbance 
  

   if 
  it 
  were 
  displaced 
  in 
  either 
  a 
  vertical 
  or 
  horizontal 
  direction 
  ; 
  and 
  

   if 
  the 
  disturbance 
  was 
  an 
  alternating 
  one, 
  and 
  the 
  period 
  corresponded 
  

   with 
  that 
  of 
  the 
  natural 
  swing 
  of 
  the 
  magnet, 
  these 
  disturbances, 
  

   might 
  accumulate 
  to 
  a 
  considerable 
  amount. 
  The 
  amount 
  of 
  the 
  

   displacement 
  of 
  the 
  centre 
  of 
  gravity 
  from 
  the 
  line 
  of 
  suspension 
  is, 
  

   however, 
  so 
  small, 
  that 
  this 
  effect 
  may 
  probably 
  be 
  rejected, 
  as 
  has 
  

   been 
  done 
  by 
  Mr. 
  Moos 
  in 
  explaining 
  the 
  observed 
  disturbances, 
  

  

  Atti. 
  ace. 
  R. 
  dei 
  Lincei, 
  4th 
  series, 
  VI, 
  pt. 
  i, 
  p. 
  21 
  (1890). 
  

  

  ( 
  '86 
  ) 
  

  

  