﻿THE 
  BOMBAY 
  MAGNETiC 
  OBSERVATORY. 
  I 
  87 
  

  

  but 
  there 
  is 
  another 
  way 
  in 
  which 
  the 
  disturbance 
  of 
  the 
  trace 
  may 
  

   be 
  accounted 
  for 
  on 
  mechanical 
  grounds 
  alone. 
  If 
  the 
  instrument 
  

   as 
  a 
  whole 
  is 
  tilted, 
  the 
  lamp 
  and 
  recording 
  paper 
  are 
  raised 
  or 
  

   lowered, 
  but 
  the 
  magnet 
  retains 
  its 
  original 
  inclination, 
  and 
  

   the 
  position 
  of 
  the 
  spot 
  of 
  light 
  on 
  the 
  sensitive 
  paper 
  is 
  con- 
  

   sequently 
  shifted. 
  This 
  is, 
  in 
  fact, 
  exactly 
  what 
  happened, 
  and 
  it 
  

   would 
  be 
  possible 
  to 
  explain 
  the 
  appearances 
  produced 
  by 
  a 
  gentle 
  

   undulation, 
  or 
  alternate 
  up 
  and 
  down 
  tilting, 
  of 
  the 
  recording 
  

   apparatus 
  while 
  the 
  magnet 
  remained 
  steady. 
  The 
  absence 
  of 
  any 
  

   interruption 
  in 
  the 
  curve, 
  in 
  the 
  case 
  of 
  an 
  instrument 
  so 
  extraordi- 
  

   narily 
  sensitive 
  to 
  the 
  slightest 
  shock 
  as 
  this 
  is 
  described 
  to 
  be, 
  cannot 
  

   be 
  held 
  to 
  militate 
  against 
  such 
  an 
  explanation, 
  as 
  it 
  would 
  not 
  be 
  

   exposed 
  to 
  anything 
  of 
  the 
  nature 
  of 
  a 
  shock. 
  

  

  The 
  horizontal 
  force 
  instrument 
  consists 
  of 
  a 
  magnet 
  suspended 
  

   by 
  two 
  silk 
  fibres, 
  fastened 
  about 
  £ 
  inch 
  apart 
  from 
  each 
  other 
  on 
  the 
  

   magnet, 
  and 
  about 
  12 
  inches 
  long. 
  The 
  upper 
  attachment 
  of 
  these 
  

   fibres 
  can 
  be, 
  and 
  is, 
  turned 
  round, 
  till 
  the 
  magnet, 
  instead 
  of 
  point- 
  

   ing 
  north 
  and 
  south, 
  points 
  east 
  and 
  west 
  ; 
  in 
  the 
  case 
  of 
  the 
  Bombay 
  

   instrument, 
  the 
  upper 
  pair 
  of 
  suspension 
  points 
  are 
  turned 
  through 
  

   about 
  seven-eighths 
  of 
  two 
  right 
  angles. 
  As 
  a 
  consequence 
  of 
  

   this 
  mode 
  of 
  suspension 
  the 
  magnet 
  is 
  very 
  sensitive, 
  and 
  a 
  move" 
  

   ment, 
  either 
  horizontal 
  or 
  vertical, 
  or 
  a 
  tilting 
  in 
  any 
  direction 
  except 
  

   that 
  at 
  right 
  angles 
  to 
  the 
  upper 
  pair 
  of 
  suspension 
  points, 
  will 
  

   affect 
  the 
  two 
  fibres 
  unequally 
  and 
  cause 
  the 
  needle 
  to 
  twist 
  more 
  

   or 
  less 
  till 
  it 
  resumes 
  a 
  position 
  of 
  equilibrium. 
  Here 
  again 
  we 
  find 
  

   that 
  the 
  appearances 
  might 
  be 
  accounted 
  for 
  by 
  purely 
  mechanical 
  

   causes, 
  and 
  the 
  earlier 
  commencement 
  and 
  later 
  cessation 
  of 
  the 
  

   disturbances 
  might 
  be 
  accounted 
  for 
  by 
  the 
  greater 
  sensitiveness 
  of 
  

   the 
  instrument 
  to 
  purely 
  mechanical 
  disturbance 
  from 
  the 
  passing 
  

   earthquake-waves. 
  

  

  The 
  declination 
  instrument 
  consists 
  of 
  a 
  simple 
  free 
  swinging 
  bar 
  

   magnet 
  suspended 
  by 
  a 
  branch 
  of 
  silk 
  fibres 
  and 
  free 
  to 
  swing 
  in 
  

   any 
  direction. 
  Here, 
  as 
  in 
  the 
  vertical 
  force 
  instrument, 
  a 
  slight 
  

   displacement 
  of 
  the 
  centre 
  of 
  gravity 
  is 
  necessary 
  to 
  counteract 
  the 
  

  

  ( 
  ^7 
  ) 
  

  

  