﻿2l8 
  OLDHAM: 
  GREAT 
  EARTHQUAKE 
  OF 
  1897. 
  

  

  of 
  gravity, 
  and 
  the 
  corner 
  on 
  which 
  the 
  object 
  is 
  tilted 
  

   not 
  being 
  in 
  the 
  same 
  line 
  with 
  the 
  direction 
  of 
  the 
  shock 
  

   and 
  to 
  the 
  object, 
  being 
  consequently 
  swung 
  round 
  on 
  

   that 
  angle. 
  

   In 
  discussing 
  these 
  theories 
  will 
  it 
  be 
  convenient 
  to 
  take 
  them 
  in 
  

   the 
  reverse 
  order 
  to 
  that 
  in 
  which 
  they 
  are 
  stated 
  above. 
  

  

  Commencing 
  with 
  Gray's, 
  it 
  will 
  be 
  noticed 
  that, 
  whereas 
  this 
  

  

  demands 
  that 
  the 
  rotation 
  should 
  be 
  round 
  one 
  of 
  the 
  corners 
  of 
  the 
  

  

  object 
  rotated, 
  a 
  study 
  of 
  the 
  plans 
  on 
  Plates 
  XXIV-XXVII 
  will 
  show 
  

  

  that 
  such 
  was 
  not 
  the 
  case 
  in 
  a 
  single 
  instance. 
  Besides 
  this 
  it 
  fails 
  

  

  to 
  account 
  for 
  one 
  of 
  a 
  number 
  of 
  monuments 
  all 
  similarly 
  oriented 
  

  

  being 
  rotated 
  positively 
  while 
  all 
  the 
  rest 
  were 
  turned 
  negatively. 
  1 
  

  

  Mallet's 
  second 
  theory 
  could 
  be 
  made 
  to 
  account 
  for 
  these 
  cases, 
  

  

  but 
  it 
  demands 
  that 
  the 
  rotation 
  should 
  be 
  

  

  ] 
  round 
  one 
  of 
  the 
  edges 
  of 
  the 
  surface 
  at 
  

  

  |C 
  which 
  it 
  took 
  place. 
  Now 
  every 
  case 
  of 
  

  

  rotation 
  may 
  be 
  explained 
  by 
  a 
  single 
  

  

  revolution 
  round 
  a 
  fixed 
  centre, 
  and 
  the 
  

  

  \ 
  radius 
  of 
  the 
  circle 
  in 
  which 
  the 
  centre 
  of 
  

  

  \ 
  gravity 
  may 
  be 
  supposed 
  to 
  have 
  revolved, 
  

  

  \ 
  in 
  order 
  to 
  bring 
  it 
  from 
  its 
  old 
  to 
  its 
  new 
  

  

  B 
  position, 
  is 
  easily 
  calculated. 
  

  

  To 
  determine 
  this 
  let 
  A 
  and 
  B, 
  fig. 
  21, 
  

   F,g,2! 
  * 
  be 
  the 
  old 
  and 
  new 
  positions 
  of 
  the 
  centre 
  

  

  of 
  gravity. 
  Then 
  the 
  required 
  centre 
  must 
  be 
  somewhere 
  on 
  the 
  

   line 
  which 
  bisects 
  A 
  B 
  at 
  right 
  angles, 
  for 
  it 
  must 
  be 
  equidistant 
  

   from 
  A 
  and 
  B. 
  But 
  in 
  moving 
  the 
  centre 
  of 
  gravity 
  from 
  A 
  to 
  B, 
  

   the 
  rotated 
  object 
  has 
  also 
  turned 
  on 
  its 
  axis 
  through 
  an 
  angle 
  which 
  

   we 
  will 
  call. 
  If 
  then 
  two 
  lines 
  are 
  drawn 
  from 
  a 
  point 
  C 
  so 
  that 
  

  

  1 
  The 
  plan? 
  on 
  Plate 
  XXXVIII 
  appear 
  at 
  first 
  sight 
  to 
  favour 
  this 
  hypothesis. 
  The 
  monu- 
  

   ments, 
  however, 
  are 
  so 
  low 
  in 
  proportion 
  to 
  their 
  base 
  that 
  it 
  is 
  very 
  doubtful 
  whether 
  any 
  tilting 
  

   took 
  place, 
  and 
  the 
  displacement 
  was 
  probably 
  due 
  to 
  the 
  vertical 
  components 
  of 
  the 
  shock, 
  

   and 
  was 
  of 
  the 
  nature 
  of 
  a 
  projection. 
  It 
  is 
  possible, 
  however, 
  that 
  these 
  cases 
  may 
  be 
  due 
  

   to 
  a 
  single 
  oscillation 
  of 
  greater 
  power 
  and 
  extent 
  than 
  the 
  rest, 
  for 
  the 
  possibility 
  of 
  the 
  

   action 
  involved 
  in 
  Gray's 
  explanation 
  cannot 
  be 
  doubted. 
  Though 
  it 
  cannot 
  be 
  accepted 
  as 
  

   a 
  complete 
  explanation. 
  

  

  ( 
  2l8 
  ) 
  

  

  