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  OLDHAM: 
  GREAT 
  EARTHQUAKE 
  OF 
  1897. 
  

  

  number 
  of 
  instances 
  of 
  tombs 
  that 
  had 
  been 
  twisted, 
  he 
  goes 
  on 
  td 
  

   say 
  1 
  : 
  — 
  

  

  "Although 
  fully 
  recognizing 
  the 
  sufficiency 
  of 
  two 
  transverse 
  shocks 
  to 
  produce 
  

   the 
  effects 
  which 
  have 
  been 
  observed 
  in 
  Yokohama, 
  I 
  will 
  offer 
  an 
  explanation 
  of 
  

   this 
  phenomenon 
  which 
  was 
  first 
  suggested 
  to 
  me 
  by 
  my 
  colleague 
  Mr 
  Gray, 
  

   which 
  appears 
  to 
  be 
  simpler 
  than 
  any 
  with 
  which 
  I 
  am 
  acquainted. 
  

  

  If 
  any 
  columnar-like 
  object, 
  for 
  example 
  a 
  prism 
  of 
  which 
  the 
  basal 
  section 
  is 
  

   represented 
  by 
  A, 
  B, 
  C, 
  D 
  (see 
  fig. 
  20) 
  receives 
  a 
  shock 
  at 
  right 
  angles 
  to 
  B 
  C, 
  

   there 
  will 
  be 
  a 
  tendency 
  for 
  the 
  inertia 
  of 
  the 
  body 
  to 
  cause 
  it 
  to 
  overturn 
  on 
  the 
  

   edge 
  B 
  C. 
  If 
  the 
  shock 
  were 
  at 
  right 
  angles 
  to 
  D 
  C, 
  the 
  tendency 
  would 
  be 
  to 
  

   overturn 
  on 
  the 
  edge 
  D 
  C. 
  If 
  the 
  shock 
  were 
  in 
  the 
  direction 
  of 
  the 
  diagonal 
  C 
  A, 
  

   the 
  tendency 
  would 
  be 
  to 
  overturn 
  on 
  the 
  point 
  C. 
  Let 
  us, 
  however, 
  now 
  suppose 
  

   the 
  impulse 
  to 
  be 
  in 
  some 
  direction 
  E 
  G, 
  where 
  G 
  is 
  the 
  centre 
  of 
  gravity 
  of 
  the 
  body. 
  

   For 
  simplicity 
  we 
  may 
  imagine 
  the 
  overturning 
  effect 
  to 
  be 
  an 
  impulse 
  given 
  through 
  

   G 
  in 
  an 
  opposite 
  direction, 
  that 
  is, 
  in 
  the 
  direction 
  G 
  E. 
  This 
  force 
  will 
  tend 
  to 
  tip 
  

   or 
  make 
  the 
  body 
  bear 
  heavily 
  on 
  C, 
  and 
  at 
  the 
  same 
  time 
  to 
  whirl, 
  round 
  C 
  as 
  an 
  

   axis, 
  the 
  direction 
  of 
  turn 
  being 
  in 
  the 
  direction 
  of 
  the 
  hands 
  of 
  a 
  watch. 
  If, 
  how- 
  

   ever, 
  the 
  direction 
  of 
  impulse 
  had 
  been 
  E 
  — 
  G, 
  then 
  although 
  the 
  turning 
  would 
  still 
  

   have 
  been 
  round 
  C, 
  the 
  direction 
  would 
  have 
  been 
  opposite 
  to 
  that 
  of 
  the 
  hands 
  o-f 
  

   a 
  watch. 
  

  

  Fig. 
  20. 
  Diagram 
  to 
  illustrate 
  Gray's 
  theory 
  of 
  rotation. 
  

  

  To 
  put 
  these 
  statements 
  in 
  another 
  form 
  imagine 
  G 
  E, 
  to 
  be 
  resolved 
  into 
  twc* 
  

   components, 
  one 
  of 
  them 
  along 
  G 
  C 
  and 
  the 
  other 
  at 
  right 
  angles 
  G 
  F. 
  Here 
  the 
  

   component 
  of 
  the 
  direction 
  G 
  C 
  tends 
  to 
  make 
  the 
  body 
  tip 
  on 
  C 
  s 
  whilst 
  the 
  

   other 
  component 
  along 
  G 
  F 
  causes 
  revolution. 
  

  

  Similarly 
  G 
  E 
  may 
  be 
  resolved 
  into 
  its 
  two 
  components 
  G 
  C 
  and 
  G 
  F, 
  the 
  

   latter 
  being 
  the 
  one 
  tending 
  to 
  cause 
  revolution. 
  

  

  From 
  this 
  we 
  see 
  that 
  if 
  a 
  body 
  has 
  a 
  rectangular 
  section 
  so 
  long 
  as 
  it 
  is 
  acted 
  

   upon 
  by 
  a 
  shock 
  which 
  is 
  parallel 
  to 
  its 
  sides 
  or 
  to 
  its 
  diagonals, 
  there 
  ought 
  not 
  to 
  

  

  1 
  J. 
  Milne. 
  The 
  Earthquake 
  in 
  Japan 
  of 
  February 
  22nd, 
  iSSo. 
  Trans. 
  Seismol. 
  Soc, 
  Japar^ 
  

   I., 
  Ft. 
  II, 
  pp. 
  33 
  35(1880). 
  

  

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