﻿PREFACE. 
  xxi 
  

  

  This 
  is 
  now 
  recognised 
  to 
  be 
  due 
  to 
  variations 
  in 
  the 
  rate 
  of 
  acceley 
  aiion 
  of 
  the 
  

   wave-particle, 
  and 
  this 
  might 
  be 
  used 
  as 
  a 
  measure 
  of 
  what 
  has 
  been 
  called 
  

   the 
  intensity, 
  but 
  that 
  the 
  degrees 
  of 
  the 
  scale 
  depend 
  to 
  some 
  extent 
  on 
  the 
  

   destructiveness 
  or 
  destruction 
  produced 
  by 
  the 
  earthquake, 
  while 
  the 
  accelera- 
  

   tion 
  is 
  more 
  properly 
  a 
  measure 
  of 
  what 
  Mendenhall 
  has 
  called 
  the 
  destructivity 
  

   or 
  power 
  to 
  destroy. 
  The 
  word 
  proposed 
  is, 
  however, 
  an 
  awkward 
  one, 
  and 
  the 
  

   simpler 
  word 
  violence 
  seems 
  more 
  suitable 
  ; 
  of 
  this 
  the 
  rate 
  of 
  acceleration 
  of 
  wave- 
  

   particle 
  is 
  a 
  measure, 
  and 
  in 
  proportion 
  as 
  this 
  increases 
  the 
  earthquake 
  

   becomes 
  more 
  distinctly 
  sensible 
  and 
  more 
  and 
  more 
  destructive. 
  Yet 
  the 
  

   destruction 
  actually 
  caused 
  must 
  not 
  be 
  taken 
  as 
  a 
  measure 
  of 
  the 
  violence 
  

   without 
  observing 
  proper 
  precautions, 
  for 
  it 
  is 
  determined 
  both 
  by 
  the 
  power 
  of 
  

   the 
  earthquake 
  to 
  destroy 
  and 
  the 
  power 
  of 
  the 
  object 
  to 
  resist 
  destruction. 
  

   This 
  is 
  equally 
  true 
  whether 
  we 
  have 
  to 
  deal 
  with 
  buildings 
  overthrown, 
  landslips 
  

   from 
  hills, 
  or 
  Assuring 
  of 
  the 
  ground, 
  and 
  consequently, 
  in 
  attempting 
  to 
  compare 
  

   the 
  violence 
  of 
  the 
  earthquake 
  at 
  two 
  different 
  places, 
  we 
  must 
  consider 
  not 
  only 
  

   the 
  effects 
  actually 
  produced, 
  but 
  the 
  particular 
  local 
  circumstances 
  governing 
  

   their 
  production. 
  

  

  65. 
  Bearing 
  these 
  considerations 
  in 
  mind 
  the 
  word 
  violence 
  may 
  well 
  be 
  used 
  

   instead 
  of 
  acceleration 
  in 
  all 
  those 
  cases 
  where 
  there 
  is 
  no 
  possibility 
  or 
  desire 
  of 
  

   giving 
  it 
  a 
  precise 
  numerical 
  value. 
  The 
  word 
  will, 
  in 
  fact, 
  be 
  used 
  in 
  the 
  follow- 
  

   ing 
  report 
  as 
  substitute 
  for 
  intensity, 
  the 
  latter 
  word 
  being 
  reserved 
  till 
  the 
  time 
  

   when, 
  as 
  the 
  means 
  of 
  seismological 
  observation 
  improve, 
  it 
  can 
  be 
  used 
  in 
  its 
  

   more 
  proper 
  sense. 
  

  

  V. 
  — 
  The 
  means 
  of 
  measuring 
  earthquakes. 
  

  

  66. 
  The 
  most 
  satisfactory 
  and 
  trustworthy 
  means 
  of 
  obtaining 
  the 
  values 
  of 
  

   the 
  elements 
  of 
  earthquake 
  motion 
  which 
  are 
  capable 
  of 
  measurement 
  is 
  that 
  of 
  

   instruments 
  specially 
  designed 
  for 
  the 
  purpose. 
  These 
  fall 
  into 
  three 
  classes 
  

   according 
  to 
  their 
  purpose; 
  seismoscopes, 
  which 
  merely 
  record 
  the 
  fact 
  of 
  an 
  earth- 
  

   quake, 
  of 
  sufficient 
  severity 
  to 
  affect 
  the 
  instrument, 
  having 
  occurred 
  ; 
  seis~ 
  

   mometers, 
  which 
  are 
  intended 
  to 
  measure 
  one 
  or 
  more 
  of 
  the 
  elements 
  of 
  the 
  

   earthquake 
  motion 
  ; 
  and 
  seismographs, 
  which 
  record 
  in 
  greater 
  or 
  less 
  detail- 
  

   the 
  actual 
  course 
  of 
  that 
  motion 
  from 
  the 
  commencement 
  to 
  the 
  close 
  of 
  the 
  

   shock. 
  

  

  67. 
  In 
  principle 
  the 
  instruments 
  maybe 
  classed 
  according 
  as 
  they 
  consist 
  of 
  

   (1) 
  a 
  mass 
  which 
  is 
  set 
  in 
  motion 
  by 
  the 
  earthquake 
  ; 
  (2) 
  a 
  mass 
  which 
  is 
  so 
  

   supported 
  as 
  to 
  remain 
  more 
  or 
  less 
  steady, 
  while 
  the 
  earth 
  moves 
  underneath 
  it 
  ; 
  

   and 
  (3) 
  an 
  arrangement 
  by 
  which 
  the 
  actual 
  tilting 
  of 
  the 
  surface 
  is 
  measured. 
  

   Two 
  or 
  more 
  of 
  these 
  may 
  be 
  combined 
  in 
  the 
  same 
  instrument, 
  and 
  on 
  one 
  or 
  

   more 
  of 
  these 
  principles 
  a 
  vast 
  number 
  of 
  instruments 
  of 
  varying 
  design 
  have 
  

   been 
  constructed 
  or 
  proposed. 
  To 
  attempt 
  a 
  description 
  of 
  them 
  here 
  would 
  

   occupy 
  too 
  much 
  space, 
  and 
  for 
  an 
  account 
  of 
  them 
  and 
  the 
  capacities 
  of 
  the 
  

   various 
  types 
  reference 
  should 
  be 
  made 
  to 
  Prof. 
  Milne's 
  two 
  books. 
  

  

  68. 
  Apart 
  from 
  special 
  instruments, 
  much 
  information 
  may 
  be 
  obtained 
  from 
  

   observations 
  made 
  at 
  the 
  time 
  of 
  the 
  earthquakes, 
  or 
  from 
  the 
  effect 
  produced 
  by 
  

   it 
  ; 
  in 
  the 
  case 
  of 
  very 
  violent 
  shocks 
  this 
  is 
  the 
  principal 
  source 
  of 
  information. 
  

  

  