﻿THE 
  UNFELT 
  EARTHQUAKE. 
  2$3 
  

  

  moner 
  than 
  the 
  published 
  references 
  suggest 
  ; 
  on 
  the 
  other 
  hand, 
  the 
  

   fact 
  that 
  the 
  rate 
  of 
  transmission 
  of 
  the 
  second 
  phase, 
  in 
  the 
  case 
  of 
  

   this 
  earthquake, 
  accords 
  with 
  what, 
  according 
  to 
  the 
  text-books, 
  should 
  

   be 
  regarded 
  as 
  a 
  normal 
  rate 
  of 
  transmission 
  of 
  the 
  preliminary 
  tremors, 
  1 
  

   seems 
  to 
  show 
  that 
  the 
  more 
  rapid 
  rate 
  of 
  transit 
  shown 
  by 
  the 
  

   disturbance 
  of 
  the 
  first 
  phase 
  is 
  exceptional, 
  or 
  at 
  least 
  unusual. 
  

  

  This 
  is 
  not 
  the 
  place 
  to 
  go 
  further 
  into 
  the 
  subject. 
  To 
  treat 
  it 
  

   satisfactorily 
  the 
  original 
  records 
  of 
  distant 
  earthquakes 
  preserved 
  in 
  

   the 
  observatories 
  of 
  Europe 
  require 
  a 
  careful 
  reexamination, 
  but 
  if 
  

   the 
  explanation 
  of 
  the 
  two 
  phases 
  of 
  preliminary 
  tremors 
  offered 
  is 
  

   accepted, 
  it 
  follows 
  that 
  the 
  waves 
  of 
  elastic 
  compression 
  die 
  

   out 
  sooner 
  than 
  those 
  of 
  elastic 
  distortion, 
  and 
  are 
  only 
  transmitted 
  

   to 
  great 
  distances 
  in 
  the 
  case 
  of 
  exceptionally 
  severe 
  earthquakes 
  ; 
  

   in 
  ordinary 
  cases 
  they 
  have 
  died 
  out 
  while 
  the 
  waves 
  of 
  elastic 
  

   distortion 
  remain 
  and 
  are 
  recorded 
  as 
  the 
  preliminary 
  tremors. 
  

   From 
  this 
  it 
  is 
  obvious 
  that 
  in 
  making 
  any 
  deduction 
  from 
  the 
  

   duration 
  of 
  these 
  preliminary 
  tremors, 
  it 
  is 
  necessary 
  to 
  be 
  certain 
  

   which 
  phase 
  they 
  begin 
  with, 
  whether 
  that 
  of 
  the 
  condensational 
  or 
  

   the 
  distortional 
  waves. 
  

  

  In 
  the 
  case 
  of 
  both 
  these 
  first 
  two 
  phases 
  of 
  the 
  records 
  we 
  have 
  a 
  

   rate 
  of 
  travel 
  much 
  greater 
  than 
  that 
  of 
  the 
  earthquake 
  which 
  was 
  

   felt, 
  an 
  increase 
  which 
  must 
  be 
  mainly 
  attributed 
  to 
  their 
  having 
  

   travelled 
  through 
  the 
  earth 
  at 
  depths 
  where 
  pressure 
  and 
  tempera- 
  

   ture 
  produced 
  profound 
  modifications 
  in 
  the 
  elasticity 
  of 
  the 
  rocks 
  

   they 
  travelled 
  through. 
  

  

  If 
  we 
  now 
  turn 
  to 
  the 
  large 
  undulations 
  which 
  are 
  estimated 
  by 
  

   Dr. 
  Agamennone 
  2 
  to 
  have 
  had 
  a 
  complete 
  period 
  of 
  22 
  seconds, 
  a 
  

   length 
  of 
  34 
  miles, 
  and 
  a 
  rise 
  and 
  fall 
  of 
  20 
  inches, 
  we 
  get 
  the 
  

  

  1 
  Prof. 
  Milne 
  gives 
  the 
  following 
  as 
  the 
  average 
  rate 
  of 
  transmission 
  of 
  the 
  preliminary 
  

   tremors 
  for 
  varying 
  distances 
  (Seismology, 
  8° 
  London, 
  1898, 
  p. 
  113). 
  

  

  Distance 
  from 
  origin 
  20 
  rate 
  of 
  travel 
  2 
  to 
  3 
  km. 
  per 
  sec. 
  

  

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  2 
  Boll. 
  Soc, 
  Sismol, 
  Ital. 
  IV, 
  pt. 
  i, 
  49 
  (1898). 
  

  

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  253 
  ) 
  

  

  