﻿xiv 
  OLDHAM 
  : 
  GREAT 
  EARTHQUAKE 
  OF 
  1897. 
  

  

  the 
  distance 
  to 
  which 
  a 
  body, 
  free 
  to 
  move, 
  is 
  displaced 
  in 
  a 
  given 
  time 
  is 
  a 
  measure 
  

   of 
  the 
  force 
  affecting 
  it, 
  the 
  same 
  words 
  are 
  applied 
  in 
  a 
  similar 
  way 
  to 
  motion, 
  

   which 
  may 
  be 
  compounded 
  into 
  a 
  single 
  resultant, 
  or 
  resolved 
  into 
  one 
  or 
  more 
  

   components. 
  

  

  The 
  components 
  are 
  usually, 
  but 
  not 
  necessarily, 
  considered 
  to 
  be 
  at 
  right 
  

   angles 
  to 
  each 
  other, 
  this 
  being 
  purely 
  a 
  matter 
  of 
  convenience 
  or 
  convention. 
  

  

  36. 
  Though 
  possibly 
  self-explanatory 
  it 
  may 
  be 
  well 
  to 
  point 
  out 
  the 
  sense 
  in 
  

   which 
  the 
  words 
  wave 
  front 
  and 
  wave 
  path 
  are 
  used. 
  The 
  wave 
  front 
  is 
  the 
  same 
  

   as 
  a 
  homoseist 
  ; 
  it 
  is 
  the 
  limit 
  to 
  which, 
  at 
  a 
  given 
  moment, 
  the 
  wave 
  motion 
  has 
  

   reached. 
  In 
  a 
  perfectly 
  homogeneous 
  medium 
  this 
  is 
  everywhere 
  at 
  the 
  same- 
  

   distance 
  from 
  the 
  focus, 
  but 
  if 
  the 
  medium 
  is 
  like 
  the 
  material 
  of 
  which 
  the 
  earth 
  

   is 
  composed, 
  such 
  that 
  the 
  wave 
  motion 
  is 
  transmitted 
  more 
  rapidly 
  in 
  some 
  

   directions 
  than 
  in 
  others, 
  the 
  distance 
  of 
  the 
  wave 
  front 
  from 
  the 
  focus 
  will 
  vary 
  

   from 
  place 
  to 
  place. 
  The 
  wave 
  path 
  is 
  a 
  line 
  along 
  which 
  the 
  wave 
  motion 
  may 
  be 
  

   supposed 
  to 
  travel. 
  There 
  are 
  an 
  infinite 
  number 
  of 
  such 
  lines 
  radiating 
  from 
  

   the 
  focus, 
  and 
  each 
  cuts 
  the 
  successive 
  wave 
  fronts 
  always 
  at 
  right 
  angles. 
  From 
  

   this 
  it 
  follows 
  that 
  if 
  the 
  wave 
  motion 
  is 
  propagated 
  with 
  equal 
  rapidity 
  in 
  every 
  

   direction 
  the 
  wave 
  paths 
  will 
  be 
  straight 
  lines, 
  but 
  if 
  the 
  wave 
  motion 
  is 
  propagated 
  

   more 
  rapidly 
  in 
  some 
  directions 
  than 
  in 
  others, 
  the 
  wave 
  paths 
  will 
  bend 
  round, 
  so 
  

   that 
  they 
  shall 
  always 
  form 
  a 
  right 
  angle 
  with 
  the 
  varying 
  direction 
  of 
  the 
  wav& 
  

   front. 
  

  

  ///. 
  — 
  Nature 
  of 
  earthquake 
  motion.. 
  

  

  From 
  the 
  earliest 
  times 
  the 
  fact 
  that 
  there 
  are 
  distinct 
  types 
  of 
  earthquake 
  

   motion 
  has 
  been 
  recognised 
  in 
  Italy. 
  Earthquakes 
  are 
  there 
  divided 
  into 
  four 
  

   classes, 
  orizzontale 
  or 
  horizontal, 
  in 
  which 
  only 
  a 
  to 
  and 
  fro 
  motion 
  is 
  felt 
  ; 
  onduloso 
  

   or 
  undulating, 
  in 
  which 
  the 
  ground 
  appears 
  to 
  be 
  thrown 
  into 
  waves 
  and 
  the 
  

   sensation 
  is 
  like 
  that 
  of 
  a 
  ship 
  rocking 
  on 
  a 
  gentle 
  sea 
  ; 
  sussultorio 
  or 
  palpitating^ 
  

   in 
  which 
  there 
  is 
  a 
  distinct 
  vertical 
  or 
  up 
  and 
  down 
  motion 
  ; 
  and 
  vorticoso 
  or 
  vorti- 
  

   cose. 
  The 
  justification 
  of 
  this 
  last 
  distinction, 
  which 
  is 
  based 
  on 
  the 
  effect 
  of 
  the 
  

   earthquake, 
  has 
  been 
  questioned 
  but 
  it 
  appears 
  sound 
  ; 
  the 
  subject 
  is 
  discussed 
  at 
  

   length 
  in 
  Chapter 
  XIV 
  of 
  the 
  following 
  report. 
  

  

  38. 
  Though 
  these 
  distinctions, 
  which 
  are 
  popular 
  rather 
  than 
  scientific, 
  have 
  

   long 
  been 
  recognised, 
  it 
  is 
  only 
  comparatively 
  recently 
  that 
  instruments 
  have 
  been 
  

   devised 
  by 
  which 
  the 
  actual 
  movement 
  of 
  the 
  ground 
  has 
  been 
  traced. 
  Of 
  the 
  

   exact 
  nature 
  of 
  the 
  movement 
  within 
  the 
  core 
  of 
  a 
  really 
  destructive 
  earthquake, 
  or 
  

   even 
  of 
  that 
  same 
  earthquake 
  outside 
  the 
  core 
  so 
  long 
  as 
  it 
  retains 
  a 
  high 
  degree 
  

   of 
  destructive 
  power, 
  nothing 
  detailed 
  is 
  yet 
  known. 
  In 
  these 
  circumstances 
  the 
  

   instruments 
  and 
  their 
  records 
  are 
  alike 
  lost 
  in 
  the 
  ruins 
  of 
  the 
  building 
  which 
  

   contained 
  them. 
  

  

  39. 
  Beyond 
  this 
  region 
  of 
  extreme 
  destruction, 
  but 
  where 
  the 
  shock 
  might 
  still 
  

   be 
  described 
  as 
  destructive, 
  records 
  have 
  been 
  obtained. 
  It 
  is 
  found 
  that, 
  the 
  

   earthquake 
  sets 
  in 
  with 
  minute 
  rapid 
  preliminary 
  tremors 
  which 
  increase 
  in 
  

   amplitude 
  and 
  period 
  till 
  they 
  suddenly 
  give 
  way 
  to 
  one 
  or 
  two 
  very 
  much 
  

   larger 
  movements, 
  and 
  these 
  again 
  are 
  succeeded 
  by 
  a 
  series 
  of 
  waves 
  of 
  longer, 
  

   and 
  longer 
  period. 
  

  

  