﻿PREFACE. 
  Xvil 
  

  

  support 
  to 
  the 
  hypothesis 
  that 
  the 
  preliminary 
  tremors 
  are 
  transmitted 
  through 
  

   the 
  earth, 
  as 
  almost 
  amounts 
  to 
  proof. 
  Were 
  they 
  transmitted 
  through 
  the 
  super- 
  

   ficial 
  crust 
  of 
  the 
  earth, 
  a 
  separation 
  of 
  the 
  two 
  classes 
  of 
  waves 
  would 
  be 
  im- 
  

   possible 
  for 
  the 
  reasons 
  given 
  in 
  para. 
  25, 
  but, 
  in 
  the 
  more 
  homogeneous 
  matter 
  

   constituting 
  the 
  central 
  core 
  of 
  the 
  globe, 
  such 
  a 
  separation 
  is 
  what 
  would 
  inevit- 
  

   ably 
  happen, 
  through 
  the 
  more 
  rapidly 
  travelling 
  condensational 
  waves 
  outracing 
  

   the 
  slower 
  travelling 
  distortion 
  al. 
  Moreover, 
  it 
  is 
  only 
  at 
  great 
  distances, 
  where 
  

   the 
  portion 
  of 
  the 
  wave 
  path 
  which 
  lies 
  within 
  the 
  surface 
  crust, 
  or, 
  say, 
  the 
  outermost 
  

   30 
  miles, 
  of 
  the 
  earth, 
  forms 
  but 
  a 
  small 
  fraction 
  of 
  the 
  total 
  wave 
  path, 
  that 
  such 
  

   separation 
  could 
  take 
  place. 
  If, 
  as 
  seems 
  probable, 
  this 
  hypothesis 
  stands 
  the 
  

   test 
  of 
  further 
  experience, 
  it 
  is 
  certain 
  that 
  in 
  the 
  majority 
  of 
  cases 
  the 
  preliminary 
  

   tremors 
  of 
  distant 
  earthquakes, 
  as 
  recorded 
  by 
  the 
  instruments 
  in 
  present 
  use, 
  

   are 
  those 
  due 
  to 
  the 
  distortional 
  waves, 
  those 
  due 
  to 
  the 
  condensational 
  being 
  

   ordinarily 
  too 
  feeble 
  to 
  be 
  recorded. 
  

  

  49. 
  Following 
  on 
  these 
  preliminary 
  tremors 
  come 
  the 
  great 
  surface 
  waves 
  

   so 
  well 
  shown 
  in 
  the 
  diagram 
  on 
  Plate 
  XLI. 
  which 
  have 
  expanded 
  from 
  a 
  length 
  

   of 
  2 
  to 
  3 
  miles 
  and 
  a 
  period 
  of 
  2 
  or 
  3 
  seconds 
  to 
  a 
  length 
  of 
  30 
  miles 
  and 
  a 
  period 
  of 
  

   25 
  seconds, 
  while 
  their 
  duration, 
  instead 
  of 
  being 
  a 
  few 
  minutes, 
  ranged 
  over 
  half 
  

   an 
  hour, 
  or, 
  if 
  the 
  slighter 
  and 
  less 
  distinctly 
  recorded 
  waves 
  are 
  included, 
  of 
  

   several 
  hours. 
  

  

  IV 
  \ 
  — 
  The 
  elements 
  of 
  earthquake 
  motion 
  which 
  can 
  be 
  measured. 
  

  

  There 
  are 
  certain 
  elements 
  of 
  the 
  motion 
  of 
  an 
  earthquake 
  which 
  can 
  be 
  

   measured 
  and 
  expressed 
  in 
  figures. 
  These 
  are 
  : 
  (i)the 
  rate 
  of 
  transmission 
  of 
  

   velocity 
  with 
  which 
  the 
  wave 
  motion 
  travels 
  from 
  place 
  to 
  place, 
  (2) 
  the 
  amplitude 
  

   (3) 
  the 
  period, 
  (4) 
  the 
  velocity 
  of 
  movement 
  of 
  the 
  wave-particle, 
  and 
  (5) 
  the 
  acce- 
  

   leration 
  of 
  the 
  wave-particle. 
  

  

  50. 
  The 
  rate 
  of 
  transmission 
  is 
  readily 
  determined 
  if 
  we 
  have 
  accurate 
  records 
  

   of 
  the 
  time 
  of 
  arrival 
  of 
  the 
  earthquake 
  at 
  different 
  places, 
  whose 
  distances 
  apart 
  

   are 
  known. 
  Theoretically, 
  if 
  the 
  time 
  of 
  arrival 
  at 
  three 
  places 
  is 
  known, 
  it 
  should 
  

   be 
  possible, 
  provided 
  the 
  times 
  are 
  not 
  simultaneous, 
  to 
  determine 
  the 
  rate 
  and 
  

   direction 
  of 
  travel 
  of 
  the 
  earthquake 
  wave. 
  In 
  practice, 
  however, 
  it 
  is 
  not 
  possible 
  

   to 
  obtain 
  records 
  of 
  sufficient 
  accuracy, 
  and 
  this 
  apart 
  from 
  any 
  fault 
  of 
  

   the 
  observations 
  or 
  records. 
  The 
  wave 
  motion 
  is 
  not 
  a 
  simple 
  one, 
  and 
  to 
  

   compare 
  the 
  times 
  it 
  would 
  be 
  necessary 
  to 
  be 
  certain 
  that^they 
  referred 
  to 
  one 
  

   and 
  the 
  same 
  phase 
  of 
  the 
  movement. 
  From 
  what 
  has 
  been 
  said 
  of 
  the 
  constant 
  

   variation 
  of 
  the 
  character 
  of 
  the 
  wave 
  motion 
  from 
  place 
  to 
  place, 
  it 
  will 
  be 
  seen 
  

   that 
  this 
  is 
  impossible 
  j 
  consequently, 
  it 
  is 
  only 
  possible, 
  in 
  practice, 
  to 
  obtain 
  a 
  fairly 
  

   accurate 
  estimate 
  by 
  combining 
  a 
  large 
  number 
  of 
  records 
  from 
  many 
  different 
  

   places. 
  

  

  51. 
  Another 
  element 
  of 
  uncertainty 
  in 
  all 
  determinations 
  of 
  the 
  rate 
  of 
  trans- 
  

   mission 
  of 
  the 
  earthquake 
  waves 
  lies 
  in 
  the 
  fact 
  that 
  all 
  we 
  can 
  obtain 
  by 
  direct 
  

   observation 
  is 
  the 
  apparent 
  rate 
  of 
  travel 
  along 
  the 
  surface 
  of 
  the 
  earth, 
  which 
  

   may 
  be 
  very 
  different 
  from 
  the 
  true 
  rate 
  of 
  travel 
  through 
  the 
  interior 
  of 
  the 
  earth. 
  

   In 
  fig. 
  v, 
  for 
  instance, 
  if 
  the 
  successive 
  circles 
  represent 
  homoseists 
  at 
  intervals 
  of 
  

   one 
  second, 
  then 
  the 
  true 
  distance 
  travelled 
  by 
  the 
  wave 
  in 
  the 
  first 
  second 
  after 
  

  

  