﻿x\iii 
  OLDHAM: 
  GREAT 
  EARTHQUAKE 
  OF 
  1897. 
  

  

  the 
  earthquake 
  reaches 
  the 
  surface 
  is 
  represented 
  by 
  A'A 
  while 
  the 
  distance 
  it 
  

   would 
  appear 
  to 
  have 
  travelled 
  along 
  the 
  surface 
  is 
  represented 
  by 
  E 
  A; 
  

   Similarly, 
  B'B 
  and 
  A 
  B 
  represent 
  the 
  true 
  and 
  apparent 
  distances 
  travelled 
  in 
  the 
  

   next 
  second, 
  and 
  so 
  on. 
  

  

  52. 
  It 
  will 
  be 
  seen 
  from 
  this 
  that 
  not 
  only 
  will 
  the 
  apparent 
  rate 
  of 
  travel 
  along 
  

   ths 
  . 
  urface 
  be 
  different 
  from 
  the 
  true 
  rate 
  of 
  travel 
  through 
  the 
  earth, 
  but 
  that 
  it 
  

   will 
  vary 
  as 
  the 
  distance 
  from 
  the 
  epicentre 
  varies. 
  This 
  will 
  be 
  true 
  of 
  the 
  waves 
  

   which 
  are 
  transmitted 
  through 
  the 
  earth 
  whether 
  the 
  wave 
  paths 
  be 
  straight 
  lines, 
  

   as 
  represented 
  in 
  fig. 
  v, 
  or 
  curved, 
  as 
  is 
  more 
  probable. 
  In 
  the 
  case 
  of 
  those 
  

   great 
  earthquakes 
  which 
  have 
  been 
  exhaustively 
  examined, 
  no 
  such 
  variation 
  has 
  

   been 
  recognised 
  ; 
  on 
  the 
  contrary, 
  the 
  rate 
  of 
  transmission 
  seems 
  to 
  be 
  practically 
  

   uniform, 
  and 
  it 
  is 
  this 
  which 
  has 
  led 
  to 
  the 
  conclusion 
  that, 
  in 
  the 
  case 
  of 
  great 
  

   earthquakes 
  at 
  least, 
  the 
  movement 
  which 
  can 
  be 
  felt 
  is 
  due 
  to 
  waves 
  propa- 
  

   gated 
  along 
  the 
  surface, 
  and 
  that 
  in 
  their 
  case 
  the 
  apparent 
  and 
  true 
  velocities 
  

   are 
  identical. 
  In 
  the 
  case 
  of 
  the 
  preliminary 
  tremors, 
  on 
  the 
  other 
  hand, 
  the 
  

   apparent 
  rate 
  of 
  transmission 
  varies 
  largely 
  according 
  to 
  the 
  distance 
  from 
  the 
  

   epicentre, 
  and 
  this 
  leads 
  to 
  the 
  conclusion 
  that 
  they 
  are 
  transmitted 
  through 
  the 
  

   earth, 
  and 
  that 
  in 
  their 
  case 
  the 
  true 
  and 
  apparent 
  velocities 
  are 
  not 
  the 
  same. 
  

  

  53. 
  The 
  amplitude 
  of 
  the 
  wave 
  has 
  already 
  been 
  explained 
  (paras. 
  6, 
  13, 
  18) 
  ; 
  

   it 
  is 
  usually 
  expressed 
  in 
  millimetres 
  or 
  in 
  inches 
  ; 
  double 
  the 
  amplitude 
  is 
  the 
  

   range 
  of 
  motion 
  of 
  the 
  wave-particle, 
  or 
  more 
  correctly 
  would 
  be, 
  if 
  the 
  motion 
  

   was 
  a 
  simple 
  harmonic 
  one. 
  The 
  error 
  introduced 
  in 
  practice 
  by 
  assuming 
  the 
  

   amplitude 
  to 
  be 
  half 
  the 
  range 
  of 
  motion 
  is, 
  however, 
  in 
  all 
  probability 
  immaterial. 
  

  

  54. 
  The 
  period 
  of 
  the 
  wave 
  has 
  also 
  been 
  explained, 
  and 
  need 
  not 
  be 
  further 
  

   treated. 
  It 
  is 
  usually 
  expressed 
  in 
  seconds 
  of 
  time. 
  

  

  55. 
  The 
  velocity 
  of 
  movement 
  of 
  the 
  wave-particle 
  is 
  a 
  quantity 
  which 
  varies 
  

   from 
  moment 
  to 
  moment. 
  Each 
  particle 
  as 
  it 
  starts 
  from 
  a 
  position 
  of 
  rest 
  com- 
  

   mences 
  moving 
  slowly, 
  then 
  moves 
  more 
  and 
  more 
  rapidly, 
  till 
  it 
  attains 
  its 
  

   maximum 
  velocity, 
  after 
  which 
  this 
  decreases 
  till 
  the 
  particle 
  once 
  more 
  comes 
  

   to 
  rest 
  before 
  commencing 
  its 
  return 
  movement, 
  in 
  which 
  the 
  same 
  succession 
  of 
  

   a 
  gradual 
  increase 
  of 
  the 
  rate 
  of 
  movement 
  up 
  to 
  a 
  maximum 
  and 
  a 
  sub- 
  

   sequent 
  gradual 
  decrease 
  takes 
  place. 
  The 
  velocity 
  of 
  movement 
  at 
  any 
  moment 
  

   is 
  expressed 
  in 
  feet, 
  inches, 
  or 
  millimetres 
  per 
  second, 
  and 
  does 
  not 
  mean 
  that 
  the 
  

   particle 
  actually 
  travels 
  so 
  many 
  feet 
  in 
  one 
  second, 
  but 
  that, 
  if 
  it 
  were 
  to 
  continue 
  

   moving 
  at 
  the 
  same 
  rate 
  for 
  a 
  whole 
  second, 
  it 
  would 
  then 
  have 
  travelled 
  the 
  

   given 
  number 
  of 
  feet, 
  inches, 
  or 
  millimetres. 
  

  

  56. 
  As 
  a 
  rule, 
  we 
  are 
  only 
  concerned 
  with 
  the 
  maximum 
  velocity 
  cf 
  the 
  wave- 
  

   particle, 
  that 
  is 
  to 
  say, 
  the 
  most 
  rapid 
  rate 
  of 
  movement 
  attained 
  ; 
  like 
  all 
  the 
  lesser 
  

   velocities 
  of 
  wave-particle 
  it 
  is 
  only 
  momentarily 
  attained, 
  but 
  is 
  the 
  only 
  one 
  to 
  

   which 
  a 
  numerical 
  value 
  can 
  be 
  given 
  which 
  is 
  independent 
  of 
  time. 
  The 
  lesser 
  

   values 
  vary 
  from 
  moment 
  to 
  movement 
  and 
  from 
  zero 
  to 
  the 
  maximum 
  ; 
  but 
  it 
  is 
  

   obvious 
  that 
  there 
  can 
  be 
  but 
  one 
  maximum 
  value, 
  and 
  that 
  this 
  value 
  can 
  be 
  given, 
  

   and 
  remains 
  true, 
  apart 
  from 
  any 
  statement 
  of 
  the 
  exact 
  instant 
  at 
  which 
  it 
  was 
  

   attained. 
  

  

  57. 
  The 
  acceleration, 
  or 
  rate 
  of 
  change 
  of 
  velocity, 
  of 
  the 
  wave-particle 
  is 
  one 
  

   of 
  the 
  most 
  important 
  elements 
  to 
  be 
  determined, 
  for 
  on 
  this 
  depends 
  the 
  violence 
  

   and 
  destructiveness 
  of 
  the 
  earthquake. 
  As 
  explained, 
  in 
  para. 
  55, 
  the 
  velocity 
  

  

  