﻿PREFACE. 
  xni 
  

  

  33. 
  The 
  greater 
  destruction 
  in 
  the 
  pleistoseismic 
  area, 
  as 
  well 
  as 
  the 
  com- 
  

   paratively 
  smaller 
  variations 
  in 
  the 
  amount 
  of 
  destruction, 
  are 
  probably 
  due 
  to 
  

   a 
  radical 
  difference 
  in 
  the 
  nature 
  of 
  the 
  motion 
  of 
  the 
  ground. 
  Above 
  the 
  focus 
  it 
  

   is 
  probable 
  that 
  the 
  ground 
  is 
  lifted 
  bodily 
  and 
  that, 
  besides 
  the 
  molecular 
  motion 
  

   of 
  the 
  elastic 
  waves, 
  there 
  is 
  a 
  certain 
  amount 
  of 
  molar 
  motion 
  of 
  the 
  whole 
  of 
  the 
  

   rocks 
  lying 
  between 
  the 
  focus 
  and 
  the 
  surface. 
  This 
  would 
  not 
  be 
  confined 
  to 
  

   the 
  region 
  vertically 
  above 
  the 
  focus, 
  but 
  would 
  extend 
  outwards 
  ; 
  what 
  the 
  exact 
  

   limit 
  may 
  be 
  is 
  not 
  known, 
  but 
  it 
  is 
  probable 
  that 
  an 
  angle 
  of 
  45 
  from 
  the 
  verti- 
  

   cal 
  would 
  include 
  all 
  that 
  was 
  so 
  affected. 
  For 
  this 
  mass 
  of 
  rock 
  the 
  term 
  core 
  

   of 
  the 
  earthquake 
  has 
  been 
  suggested, 
  and 
  it 
  affords 
  a 
  convenient, 
  and 
  apparently 
  

   true, 
  distinction 
  between 
  the 
  central 
  core 
  on 
  the 
  one 
  hand, 
  in 
  which 
  both 
  molar 
  and 
  

   molecular 
  movement 
  takes 
  place, 
  and 
  the 
  area 
  or 
  region 
  of 
  propagation 
  on 
  the 
  

   other, 
  in 
  which 
  we 
  have 
  only 
  elastic 
  waves, 
  set 
  up 
  by 
  the 
  earthquake 
  and 
  travelling 
  

   outwards 
  with 
  a 
  decreasing 
  energy 
  and 
  violence. 
  

  

  34. 
  The 
  term 
  seismic 
  area 
  was 
  applied 
  by 
  Mallet 
  to 
  the 
  region 
  over 
  which 
  the 
  

   earthquake 
  was 
  felt 
  ; 
  of 
  late 
  years, 
  when 
  the 
  improvement 
  of 
  instruments 
  for 
  

   recording: 
  earthquake 
  movements 
  has 
  rendered 
  it 
  possible 
  to 
  detect 
  the 
  disturbance 
  

   caused 
  by 
  a 
  great 
  earthquake 
  at 
  long 
  distances, 
  beyond 
  the 
  utmost 
  limit 
  at 
  

   which 
  it 
  can 
  be 
  recognised 
  without 
  the 
  assistance 
  of 
  these 
  instruments, 
  the 
  term 
  

   microseismic 
  has 
  been 
  applied 
  to 
  these 
  unfelt 
  movements 
  and 
  the 
  seismic 
  area 
  of 
  

   Mallet 
  has 
  sometimes 
  been 
  distinguished 
  as 
  macroseismic. 
  The 
  terms 
  seem 
  incon- 
  

   venient 
  and 
  hardly 
  appropriate, 
  for 
  some 
  of 
  the 
  waves 
  in 
  the 
  area 
  where 
  the 
  

   disturbance 
  would 
  be 
  described 
  as 
  microseismic 
  are 
  larger 
  in 
  size 
  than 
  any 
  of 
  

   those 
  in 
  the 
  macroseismic 
  area. 
  It 
  seems 
  consequently 
  best 
  to 
  retain 
  the 
  terms 
  

   seismic 
  area 
  in 
  its 
  original 
  sense 
  and 
  to 
  describe 
  the 
  movements 
  that 
  take 
  place 
  

   outside 
  it 
  as 
  cryptoseismic 
  reserving 
  microseismic 
  for 
  those 
  minute 
  earthquakes 
  

   which 
  nowhere 
  become 
  sensible 
  to 
  unassisted 
  observation. 
  

  

  35. 
  Two 
  more 
  terms 
  may 
  be 
  explained 
  with 
  advantage, 
  though 
  they 
  are 
  not 
  

   specially 
  seismological, 
  and 
  these 
  are 
  the 
  words 
  resultant 
  and 
  component. 
  If 
  a 
  body 
  

   is 
  acted 
  on 
  simultaneously 
  by 
  two 
  distinct 
  forces, 
  the 
  final 
  result 
  will 
  be 
  the 
  same 
  

   as 
  if 
  each 
  force 
  acted 
  singly 
  and 
  successivelv. 
  Where 
  the 
  two 
  forces 
  do 
  not 
  act 
  in 
  

   the 
  same 
  or 
  opposite 
  directions, 
  this 
  leads 
  to 
  the 
  construction 
  known 
  as 
  the 
  parallelo- 
  

   gram 
  of 
  forces; 
  in 
  fig. 
  vi 
  let 
  O 
  represent 
  a 
  body 
  acted 
  on 
  by 
  two 
  forces 
  in 
  the 
  

  

  directions 
  A 
  and 
  B, 
  and 
  let 
  O 
  A 
  and 
  OB 
  

   jJt 
  respectively 
  represent 
  the 
  magnitude 
  

   of 
  the 
  forces, 
  that 
  is, 
  the 
  distance 
  

   through 
  each 
  singly 
  would 
  displace 
  

   O 
  in, 
  say, 
  one 
  second 
  of 
  time 
  ; 
  com- 
  

   plete 
  the 
  parallelogram 
  O 
  A 
  R 
  B, 
  

   then 
  R 
  represents 
  the 
  position 
  which 
  

   O 
  will 
  occupy 
  at 
  the 
  end 
  of 
  a 
  second, 
  

   and 
  O 
  R 
  will 
  represent 
  the 
  direction 
  

   and 
  distance 
  that 
  O 
  will 
  travel 
  in 
  that 
  time. 
  In 
  other 
  words 
  O 
  R 
  represents 
  

   the 
  direction 
  and 
  magnitude 
  of 
  a 
  single 
  force, 
  called 
  the 
  resultant, 
  which 
  

   will 
  produce 
  the 
  same 
  result 
  as 
  the 
  two 
  separate 
  and 
  simultaneous 
  forces 
  O 
  A 
  and 
  

   O 
  B. 
  Similarly, 
  if 
  we 
  have 
  a 
  single 
  force 
  O 
  R, 
  it 
  may 
  be 
  regarded 
  as 
  composed 
  of 
  

   two 
  separate 
  forces 
  O 
  A 
  and 
  O 
  B 
  which 
  are 
  called 
  the 
  components. 
  Moreover, 
  as 
  

  

  ig. 
  VI. 
  

  

  