﻿THE 
  UNFELT 
  EARTHQUAKE. 
  251 
  

  

  travel, 
  whether 
  this 
  be 
  in 
  the 
  direction 
  of 
  diminishing 
  or, 
  as 
  

   observations 
  show, 
  of 
  increasing 
  the 
  speed. 
  

  

  The 
  study, 
  by 
  means 
  of 
  special 
  instruments, 
  of 
  the 
  earth 
  move- 
  

   ments 
  due 
  to 
  earthquakes 
  at 
  a 
  distance 
  from 
  their 
  place 
  of 
  origin, 
  

   shows 
  that 
  the 
  preliminary 
  tremors 
  begin 
  before 
  the 
  greater 
  undu* 
  

   lations, 
  and 
  that 
  the 
  period 
  through 
  which 
  they 
  last 
  is 
  longer 
  as 
  the 
  

   place 
  of 
  observation 
  is 
  more 
  remote 
  from 
  the 
  centre. 
  It 
  is 
  found 
  too 
  

   that 
  the 
  time 
  by 
  which 
  the 
  tremors 
  precede 
  the 
  greater 
  undulations 
  is 
  

   greater 
  than 
  that 
  which 
  is 
  due 
  to 
  the 
  difference 
  between 
  the 
  distance 
  

   in 
  a 
  straight 
  line 
  and 
  along 
  the 
  surface 
  of 
  the 
  earth 
  ; 
  in 
  other 
  words, 
  

   the 
  rate 
  of 
  propagation 
  of 
  the 
  tremors, 
  which 
  travel 
  through 
  the 
  

   earth, 
  is 
  more 
  rapid 
  the 
  greater 
  the 
  depth 
  of 
  the 
  wave 
  path 
  from 
  the 
  

   surface. 
  

  

  In 
  this 
  the 
  records 
  of 
  the 
  great 
  earthquake 
  present 
  no 
  peculiari- 
  

   ties, 
  but 
  the 
  marked 
  three-fold 
  character 
  of 
  the 
  disturbance 
  is 
  peculiar. 
  

   The 
  only 
  published 
  instance 
  of 
  a 
  similar 
  record, 
  with 
  which 
  

   I 
  am 
  acquainted, 
  is 
  that 
  of 
  the 
  Japanese 
  earthquake 
  of 
  1894, 
  1 
  and 
  the 
  

   meaning 
  of 
  these 
  three 
  phases 
  may 
  now 
  be 
  investigated. 
  

  

  The 
  first 
  phase 
  consisted 
  of 
  comparatively 
  rapid 
  waves, 
  whose 
  

   period 
  is 
  estimated 
  by 
  Dr. 
  Agamennone 
  2 
  as 
  not 
  more 
  than 
  about 
  '5s. 
  

   to 
  *8s. 
  Prof. 
  Grablowitz' 
  investigation 
  of 
  the 
  records 
  of 
  his 
  instru- 
  

   ments 
  has 
  already 
  been 
  referred 
  to, 
  and 
  his 
  conclusion 
  that 
  the 
  

   motion 
  in 
  this 
  phase 
  was 
  mainly 
  of 
  a 
  to 
  and 
  fro 
  nature, 
  unaccom- 
  

   panied 
  by 
  any 
  considerable 
  tilting 
  of 
  the 
  ground. 
  He 
  expressly 
  

   refers 
  to 
  the 
  wave 
  motion 
  in 
  this 
  phase 
  as 
  condensational, 
  and 
  the 
  

   must 
  reasonable 
  explanation 
  is 
  that 
  the 
  commencement 
  of 
  the 
  dis- 
  

   turbance 
  was 
  due 
  to 
  the 
  arrival 
  of 
  waves 
  of 
  elastic 
  compression. 
  If 
  

   these 
  travelled 
  in 
  a 
  straight 
  line 
  from 
  the 
  centre, 
  the 
  angle 
  of 
  

   emergence 
  would 
  be 
  about 
  30 
  , 
  if 
  the 
  wave 
  path 
  was 
  a 
  curved 
  one, 
  con- 
  

   vex 
  to 
  the 
  centre 
  of 
  the 
  earth, 
  it 
  would 
  be 
  higher. 
  That 
  is 
  to 
  say, 
  the 
  

  

  1 
  E. 
  von. 
  Rebeur 
  Paschwitz: 
  Europaische 
  Beobachtungen 
  des 
  grossen 
  japanischen 
  Erd- 
  

   bebens 
  vom. 
  22 
  Marz 
  1894, 
  und 
  des 
  venezolanischen 
  Erdbebens 
  vom. 
  28th 
  April 
  1894, 
  nebst 
  

   Untersuchungen 
  iiber 
  die 
  Fortplanzungsgeschwindigkeit 
  dieser 
  Erdbeben 
  ; 
  Peterman. 
  

   Mittheilungen, 
  1895, 
  pp. 
  13-21 
  and 
  39-42. 
  

  

  2 
  Boll. 
  Soc. 
  Sismol., 
  Ital. 
  IV, 
  pt. 
  i, 
  p. 
  48 
  (1898). 
  

  

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