﻿PREFACE. 
  

  

  ON 
  SOME 
  GENERAL 
  PRINCIPLES 
  OF 
  SEISMOLOGY. 
  

  

  (The 
  memoirs 
  of 
  the 
  Geological 
  Survey 
  of 
  India 
  usually 
  assume 
  a 
  certain 
  

   previous 
  knowledge 
  of 
  the 
  subject 
  they 
  treat 
  of, 
  and 
  do 
  not 
  attempt 
  to 
  re- 
  

   capitulate 
  the 
  elementary 
  principles 
  which 
  will 
  be 
  found 
  in 
  any 
  text-book 
  of 
  geology. 
  

   In 
  the 
  case 
  of 
  the 
  earthquake 
  of 
  12th 
  June 
  1897, 
  which 
  has 
  attracted 
  more 
  general 
  

   attention 
  than 
  is 
  usually 
  given 
  to 
  the 
  labours 
  of 
  the 
  Geological 
  Survey, 
  more 
  

   elementary 
  details 
  than 
  customary 
  have 
  been 
  introduced, 
  but 
  even 
  so 
  there 
  are 
  

   some 
  points 
  which 
  it 
  is 
  necessary 
  to 
  understand, 
  and 
  of 
  which 
  a 
  sufficiently 
  exact 
  

   knowledge 
  is 
  by 
  no 
  means 
  common. 
  These 
  could 
  not 
  have 
  been 
  treated 
  in 
  the 
  

   body 
  of 
  the 
  report 
  without 
  great 
  interruption 
  to 
  its 
  continuity 
  and 
  the 
  introduction 
  

   of 
  a 
  large 
  amount 
  of 
  matter 
  which 
  would 
  only 
  embarrass 
  those 
  to 
  whom 
  it 
  was 
  

   already 
  familiar. 
  

  

  For 
  this 
  reason 
  an 
  introduction 
  dealing 
  with 
  certain 
  general 
  principles 
  has 
  

   been 
  prepared 
  and 
  printed 
  as 
  a 
  preface 
  to 
  the 
  report. 
  It 
  is 
  not 
  intended 
  as 
  an 
  

   introduction 
  to, 
  or 
  as 
  a 
  primer 
  of, 
  seismology 
  in 
  general, 
  but 
  only 
  of 
  such 
  parts 
  of 
  

   the 
  science 
  as 
  is 
  necessary 
  for 
  the 
  comprehension 
  of 
  the 
  following 
  report, 
  or 
  as 
  have 
  

   not 
  been 
  treated 
  with 
  sufficient 
  fullness 
  in 
  the 
  body 
  of 
  the 
  report. 
  Those 
  who 
  

   desire 
  to 
  pursue 
  the 
  matter 
  further 
  should 
  refer 
  to 
  the 
  two 
  volumes 
  of 
  the 
  Inter- 
  

   national 
  Science 
  Series, 
  ' 
  Earthquakes 
  'and 
  ' 
  Seismology 
  ■ 
  by 
  Prof. 
  J. 
  Milne, 
  f.r.s., 
  

   which 
  between 
  them 
  practically 
  cover 
  the 
  whole 
  range 
  of 
  the 
  science. 
  

  

  A 
  special 
  index 
  is 
  appended 
  to 
  this 
  preface 
  to 
  serve 
  the 
  purpose 
  of 
  a 
  glossary 
  

   of 
  the 
  technical 
  terms 
  used 
  in 
  the 
  report.) 
  

  

  /. 
  — 
  The 
  nature 
  of 
  wave 
  motion. 
  

  

  1. 
  When 
  a 
  stone 
  is 
  dropped 
  into 
  a 
  pool 
  of 
  water 
  a 
  series 
  of 
  concentric 
  ripples 
  

   are 
  formed, 
  which 
  travel 
  outwards 
  in 
  ever 
  widening 
  circles 
  till 
  they 
  reach 
  the 
  

   edge 
  of 
  the 
  pool, 
  or, 
  if 
  jt 
  is 
  large 
  enough, 
  gradually 
  die 
  out. 
  But 
  though 
  we 
  speak 
  

   of 
  the 
  ripples, 
  or 
  waves, 
  as 
  travelling, 
  it 
  is 
  important 
  to 
  remember 
  that 
  the 
  water 
  

   of 
  which 
  they 
  are 
  composed 
  does 
  not 
  travel 
  ; 
  each 
  separate 
  particle 
  of 
  it 
  moves 
  

   in 
  a 
  certain 
  orbit, 
  whose 
  size 
  and 
  shape 
  determines 
  that 
  of 
  the 
  wave, 
  but 
  ultimately 
  

   returns 
  to 
  its 
  original 
  position. 
  The 
  travel 
  of 
  the 
  wave 
  is 
  due 
  to 
  the 
  fact 
  that 
  all 
  

   the 
  particles 
  of 
  water 
  do 
  not 
  move 
  at 
  the 
  same 
  time, 
  each 
  one 
  starting 
  and 
  finishing 
  

   a 
  little 
  later 
  than 
  the 
  one 
  behind 
  and 
  a 
  little 
  earlier 
  than 
  the 
  one 
  in 
  front 
  of 
  it, 
  on 
  the 
  

   course 
  the 
  wave 
  is 
  travelling. 
  

  

  2. 
  The 
  shape 
  of 
  the 
  orbit 
  in 
  which 
  the 
  particles 
  of 
  water 
  travel 
  changes 
  

   gradually 
  as 
  the 
  wave 
  progresses, 
  as 
  may 
  be 
  seen 
  in 
  the 
  gradually 
  changing 
  

   form 
  of 
  the 
  waves 
  as 
  they 
  travel 
  outwards, 
  but 
  the 
  change 
  is 
  slow 
  and, 
  for 
  any 
  

   given 
  spot, 
  the 
  tracks 
  in 
  which 
  all 
  the 
  particles 
  of 
  water 
  move 
  may 
  be 
  regarded 
  

   as 
  similar, 
  and 
  the 
  nature 
  of 
  the 
  wave 
  motion 
  can 
  be 
  defined 
  by 
  the 
  shape 
  of 
  

   the 
  track 
  followed 
  by 
  each 
  particle 
  involved 
  in 
  the 
  wave. 
  Hence 
  comes 
  a 
  term 
  

   which 
  will 
  be 
  frequently 
  employed, 
  the 
  wave 
  particle 
  : 
  by 
  this 
  is 
  meant 
  no 
  

  

  