﻿xxiv 
  OLDHAM: 
  GREAT 
  EARTHQUAKE 
  OF 
  1897. 
  

  

  that 
  at 
  which 
  the 
  hand 
  increases 
  its 
  rate 
  of 
  downward 
  movement. 
  Once, 
  however, 
  

   that 
  this 
  limit 
  is 
  passed 
  the 
  distance 
  to 
  which 
  the 
  stone 
  travels 
  through 
  the 
  air 
  is 
  

   governed 
  solely 
  by 
  its 
  initial 
  velocity 
  and 
  direction 
  of 
  movement, 
  and 
  if 
  any 
  two 
  of 
  

   these 
  three 
  is 
  known 
  the 
  other 
  can 
  be 
  calculated. 
  

  

  79. 
  Though 
  a 
  consideration 
  of 
  the 
  methods 
  of 
  construction, 
  which 
  should 
  be 
  

   adopted 
  to 
  prevent 
  or 
  lessen 
  the 
  damage 
  caused 
  by 
  earthquakes, 
  would 
  be 
  foreign 
  

   to 
  the 
  purpose 
  of 
  this 
  introduction, 
  it 
  may 
  be 
  pointed 
  out 
  that 
  the 
  foregoing 
  

   paragraphs 
  show 
  that 
  one 
  of 
  two 
  principles 
  should 
  be 
  used 
  as 
  a 
  guiding 
  one, 
  

   either 
  great 
  strength 
  or 
  great 
  lightness. 
  Seeing 
  that 
  the 
  strains 
  set 
  up 
  are 
  

   dependent 
  on 
  two 
  factors, 
  the 
  inertia 
  of 
  the 
  object 
  affected 
  and 
  the 
  acceleration 
  

   of 
  the 
  wave-particle, 
  we 
  must, 
  as 
  we 
  cannot 
  control 
  the 
  second, 
  either 
  diminish 
  the 
  

   first, 
  and 
  so 
  lessen 
  the 
  magnitude 
  of 
  the 
  strains, 
  or 
  increase 
  the 
  strength 
  of 
  the 
  

   structure 
  so 
  that 
  it 
  shall 
  be 
  able 
  to 
  withstand 
  the 
  strains 
  set 
  up 
  ; 
  in 
  other 
  words, 
  

   and 
  to 
  use 
  Prof. 
  Milne's 
  simile, 
  the 
  ideal 
  should 
  be 
  either 
  the 
  wicker 
  basket 
  or 
  the 
  

   steel 
  safe. 
  

  

  VII.— 
  The 
  causes 
  of 
  earthquakes. 
  

  

  80. 
  An 
  earthquake 
  may 
  be 
  produced 
  by 
  any 
  cause, 
  of 
  the 
  nature 
  of 
  a 
  shock 
  

   or 
  blow, 
  by 
  which 
  an 
  elastic 
  wave 
  or 
  waves 
  are 
  set 
  up 
  in 
  the 
  earth. 
  The 
  fall 
  of 
  a 
  

   mountain, 
  or 
  even 
  a 
  house, 
  the 
  explosion 
  of 
  a 
  mine 
  or 
  magazine, 
  or 
  the 
  work- 
  

   ing 
  of 
  a 
  large 
  steam 
  hammer, 
  may 
  set 
  up 
  a 
  disturbance 
  which 
  can 
  be 
  felt 
  as 
  a 
  

   earthquake 
  within 
  a 
  certain 
  distance 
  of 
  the 
  origin. 
  Excluding 
  these 
  disturbances, 
  

   which 
  originate 
  naturally 
  or 
  artificially 
  at 
  or 
  above 
  the 
  surface 
  of 
  the 
  ground, 
  

   and 
  restricting 
  the 
  term 
  to 
  those 
  natural 
  disturbances 
  which 
  originate 
  within 
  the 
  

   earth, 
  earthquakes 
  have 
  been 
  recognised 
  as 
  being 
  produced 
  by 
  causes 
  which 
  divide 
  

   them 
  into 
  three 
  distinct 
  classes, 
  namely, 
  (1) 
  Rockfall, 
  (2) 
  Volcanic, 
  (3) 
  Tectonic. 
  

  

  81. 
  The 
  rockfall 
  earthquakes 
  are 
  due 
  to 
  the 
  falling 
  in 
  of 
  the 
  roofs 
  and 
  sides 
  

   of 
  subterranean 
  hollows 
  or 
  caves. 
  They 
  are 
  feeble, 
  of 
  very 
  small 
  extent 
  and 
  may 
  

   often 
  be 
  only 
  noticeable 
  as 
  sounds, 
  and 
  not 
  as 
  sensible 
  shocks. 
  

  

  82. 
  The 
  volc%nic 
  earthquakes 
  are 
  those 
  due 
  to 
  the 
  activity 
  of 
  volcanoes, 
  with 
  

   which 
  may 
  be 
  included 
  earthquakes 
  due 
  to 
  the 
  rending 
  open 
  of 
  fissures 
  by 
  the 
  

   sudden 
  development 
  of 
  steam 
  under 
  high 
  pressure. 
  This 
  last 
  cause 
  has 
  been 
  

   inferred 
  in 
  the 
  case 
  of 
  some 
  earthquakes, 
  but 
  its 
  reality 
  has 
  never 
  been 
  

   proved 
  ; 
  volcanic 
  activity 
  is, 
  however, 
  well 
  known 
  to 
  be 
  associated 
  with 
  earth- 
  

   quakes, 
  and 
  these 
  may 
  sometimes 
  be 
  of 
  very 
  great 
  severity, 
  though 
  always 
  local 
  

   in 
  their 
  extent. 
  Within 
  a 
  very 
  moderate 
  distance 
  of 
  a 
  town 
  which 
  has 
  been 
  laid 
  

   in 
  ruins 
  the 
  shock 
  may 
  be 
  quite 
  insensible. 
  

  

  83. 
  To 
  the 
  class 
  of 
  tectonic 
  earthquakes 
  belongs 
  by 
  far 
  the 
  most 
  preponderating 
  

   majority 
  of 
  earthquakes 
  and 
  all 
  those 
  which 
  can 
  be 
  classed 
  as 
  really 
  great, 
  on 
  

   account 
  of 
  their 
  violence 
  or 
  extent. 
  They 
  may 
  be 
  regarded 
  as 
  invariably 
  due 
  

   to 
  the 
  sudden 
  relief 
  of 
  strain, 
  as 
  opposed 
  to 
  the 
  volcanic 
  earthquakes 
  properly 
  

   so 
  called, 
  which 
  may 
  be 
  regarded 
  as 
  due 
  to 
  the 
  sudden 
  development 
  of 
  strain. 
  

  

  84. 
  Leaving 
  on 
  one 
  side 
  all 
  question 
  of 
  ultimate 
  cause, 
  the 
  known 
  facts 
  of 
  

   geology 
  show 
  that 
  the 
  rocks 
  of 
  which 
  the 
  crust 
  of 
  the 
  earth 
  is 
  composed 
  have 
  

   been 
  subjected 
  to 
  great 
  deformations. 
  In 
  some 
  places 
  they 
  have 
  been 
  tilted, 
  bent 
  

   and 
  compressed 
  till 
  they 
  only 
  occupy 
  half 
  their 
  original 
  horizontal 
  extent; 
  in 
  

  

  