﻿PREFACE. 
  X! 
  

  

  24. 
  In 
  the 
  case 
  of 
  earthquake 
  waves 
  in 
  the 
  region 
  where 
  the 
  shock 
  is 
  severe 
  

   enough 
  to 
  be 
  felt, 
  nothing 
  of 
  the 
  sort 
  has 
  been 
  recognised 
  and 
  nothing 
  of 
  the 
  sort 
  

   is 
  to 
  be 
  expected, 
  seeing 
  how 
  heterogeneous 
  are 
  the 
  materials 
  of 
  which 
  the 
  outer 
  

   crust 
  ot 
  the 
  earth 
  is 
  composed. 
  

  

  25. 
  The 
  rate 
  at 
  which 
  elastic 
  waves 
  are 
  transmitted 
  depends 
  only 
  partly 
  on 
  

   the 
  elasticity, 
  whether 
  that 
  measured 
  by 
  the 
  bulk 
  modulus, 
  or 
  the 
  rigidity 
  ; 
  the 
  

   density 
  of 
  the 
  materials 
  is 
  also 
  a 
  controlling 
  factor. 
  A 
  wave 
  may, 
  consequently, 
  

   preserve 
  the 
  same 
  rate 
  of 
  propagation, 
  even 
  when 
  it 
  passes 
  from 
  one 
  medium 
  to 
  

   another 
  of 
  different 
  density 
  and 
  elasticity, 
  provided 
  that 
  the 
  changes 
  are 
  in 
  exactly 
  

   such 
  proportion 
  that 
  the 
  new 
  density 
  combined 
  with 
  the 
  new 
  elasticity 
  give 
  the 
  

   same 
  rate 
  of 
  propagation 
  as 
  before. 
  Usually, 
  however, 
  there 
  will 
  be 
  a 
  change 
  in 
  

   th 
  e 
  rate 
  of 
  propagation 
  ; 
  and 
  when 
  a 
  wave 
  of 
  either 
  elastic 
  compression 
  or 
  distor- 
  

   tion 
  passes 
  from 
  one 
  medium 
  into 
  another, 
  in 
  which 
  it 
  has 
  a 
  different 
  rate 
  of 
  pro- 
  

   pagation, 
  not 
  only 
  is 
  its 
  direction 
  of 
  travel 
  changed, 
  in 
  other 
  words, 
  not 
  only 
  

   does 
  it 
  undergo 
  refraction, 
  but 
  it 
  is 
  also 
  split 
  up 
  and 
  partially- 
  converted 
  into 
  

   elastic 
  wave 
  motion 
  of 
  the 
  other 
  kind. 
  

  

  26. 
  Seeing 
  how 
  very 
  heterogeneous 
  are 
  the 
  materials 
  of 
  which 
  the 
  earth's 
  crust 
  

   is 
  composed 
  and 
  how 
  frequently 
  waves 
  travelling 
  through 
  it 
  must 
  pass 
  from 
  one 
  

   medium 
  to 
  another 
  in 
  which 
  they 
  have 
  a 
  different 
  rate 
  of 
  travel, 
  it 
  is 
  easy 
  to 
  see 
  

   that, 
  even 
  if 
  the 
  motion 
  in 
  an 
  earthquake 
  wave 
  was 
  simple 
  to 
  start 
  with, 
  it 
  would 
  

   soon 
  become 
  extremely 
  complicated. 
  

  

  //. 
  — 
  Definition 
  of 
  seismo 
  logical 
  terms. 
  

  

  Having 
  treated 
  of 
  the 
  various 
  possible 
  forms 
  of 
  wave 
  motion, 
  we 
  are 
  ready 
  

   to 
  go 
  on 
  to 
  a 
  consideration 
  of 
  the 
  observed 
  facts 
  of 
  earthquake 
  movements, 
  but 
  

   before 
  doing 
  so 
  it 
  will 
  be 
  necessary 
  to 
  explain 
  the 
  meaning 
  of 
  certain 
  special 
  

   terms 
  which 
  will 
  be 
  frequently 
  used. 
  

  

  8* 
  

  

  Fig. 
  v. 
  Diagiam 
  in 
  explanation 
  of 
  seismological 
  terms. 
  

  

  27. 
  Whatever 
  be 
  the 
  cause 
  of 
  an 
  earthquake, 
  the 
  disturbance 
  starts 
  from 
  some 
  

   place 
  within 
  the 
  crust 
  of 
  the 
  earth. 
  This 
  may 
  be 
  so 
  small 
  that 
  it 
  can 
  be 
  consi- 
  

   dered 
  as 
  a 
  point, 
  or 
  more 
  usually 
  have 
  considerable 
  extent. 
  Whatever 
  its 
  "size 
  or 
  

   form, 
  it 
  is 
  known 
  as 
  the 
  centrum 
  or 
  centre, 
  or, 
  if 
  regarded 
  from 
  another 
  point 
  of 
  

   View, 
  as 
  the 
  focus 
  of 
  the 
  earthquake. 
  For 
  convenience 
  we 
  will 
  assume 
  it 
  to 
  be 
  a 
  

   point, 
  F, 
  in 
  fig. 
  v 
  , 
  which 
  represents, 
  diagram 
  matically, 
  a 
  vertical 
  section 
  through 
  the 
  

  

  