﻿XXli 
  OLDHAM: 
  GREAT 
  EARTHQUAKE 
  OF 
  1897. 
  

  

  69. 
  The 
  rate 
  of 
  transmission 
  is 
  obtained 
  from 
  observations 
  of 
  the 
  time 
  at 
  which 
  

   it 
  was 
  felt, 
  and 
  it 
  is 
  necessary 
  that 
  these 
  should 
  be 
  accurate 
  at 
  least 
  within 
  one 
  

   minute 
  of 
  time, 
  even 
  in 
  the 
  case 
  of 
  earthquakes 
  which 
  are 
  felt 
  over 
  a 
  large 
  area. 
  

   In 
  the 
  case 
  of 
  lesser 
  earthquakes 
  an 
  even 
  greater 
  degree 
  of 
  accuracy 
  is 
  required 
  

   if 
  the 
  results 
  are 
  to 
  be 
  of 
  any 
  value, 
  yet 
  even 
  here, 
  if 
  the 
  observations 
  are 
  

   sufficiently 
  numerous, 
  a 
  much 
  greater 
  degree 
  of 
  accuracy 
  in 
  the 
  result 
  can 
  

   be 
  obtained 
  than 
  the 
  errors 
  of 
  each 
  individual 
  observation 
  would 
  appear 
  to 
  

   justify. 
  

  

  70. 
  Another 
  necessary 
  precaution 
  is 
  to 
  compare 
  similar 
  phases 
  of 
  the 
  shocks, 
  

   and 
  in 
  practice 
  the 
  only 
  phase 
  which 
  can 
  be 
  identified, 
  with 
  even 
  an 
  approximate 
  

   degree 
  of 
  accuracy 
  at 
  different 
  places, 
  is 
  the 
  commencement. 
  When 
  a 
  sufficient 
  

   number 
  of 
  trustworthy 
  observations 
  are 
  available, 
  they 
  may 
  be 
  treated 
  either 
  by 
  

   means 
  of 
  the 
  mathematical 
  process 
  known 
  as 
  the 
  method 
  of 
  least 
  squares, 
  or 
  in 
  

   the 
  simpler 
  method 
  adopted 
  in 
  Chapter 
  III, 
  which 
  will 
  usually 
  give 
  results 
  as 
  

   accurate, 
  though 
  less 
  precise, 
  than 
  those 
  obtained 
  by 
  the 
  more 
  elaborate 
  method. 
  

  

  71. 
  The 
  replies 
  to 
  circulars 
  issued 
  after 
  the 
  earthquake 
  of 
  1897 
  showed 
  that 
  the 
  

   stoppage 
  of 
  clocks 
  is 
  popularly 
  supposed 
  to 
  be 
  a 
  particularly 
  accurate 
  method 
  of 
  

   recording 
  the 
  time 
  of 
  an 
  earthquake. 
  This, 
  however, 
  is 
  not 
  the 
  case; 
  not 
  only 
  is 
  

   there 
  no 
  means 
  of 
  determining 
  the 
  particular 
  phase 
  of 
  the 
  shock 
  to 
  which 
  the 
  

   stoppage 
  was 
  due, 
  but 
  in 
  some 
  cases 
  the 
  clock 
  may 
  not 
  stop 
  till 
  some 
  time 
  after 
  

   the 
  earthquake 
  has 
  ceased. 
  One 
  authentic 
  case 
  is 
  on 
  record 
  — 
  the 
  earthquake 
  of 
  

   24th 
  June 
  1870 
  at 
  Athens— 
  where 
  the 
  pendulum 
  of 
  an 
  astronomical 
  clock 
  was 
  

   observed 
  to 
  be 
  knocked 
  against 
  the 
  clock 
  case 
  by 
  the 
  earthquake, 
  yet 
  the 
  clock 
  

   only 
  stopped 
  some 
  five 
  minutes 
  after 
  the 
  earthquake 
  had 
  ceased. 
  

  

  72. 
  The 
  amplitude 
  of 
  the 
  wave 
  and 
  the 
  velocity 
  and 
  acceleration 
  of 
  the 
  wave- 
  

   particle 
  can 
  be 
  determined, 
  in 
  the 
  case 
  of 
  earthquakes 
  of 
  sufficient 
  violence 
  to 
  

   cause 
  damage, 
  by 
  observation 
  of 
  its 
  effects. 
  The 
  methods 
  and 
  formulae 
  in 
  use 
  

   are 
  sufficiently 
  explained 
  in 
  Chapter 
  V 
  and 
  in 
  Appendix 
  C. 
  

  

  VI. 
  — 
  Earthquake 
  effects, 
  

  

  73. 
  Cracks 
  in 
  the 
  walls 
  of 
  buildings 
  are 
  probably 
  in 
  large 
  part 
  due, 
  and 
  in 
  

   lesser 
  degree 
  fissures 
  in 
  the 
  earth, 
  to 
  the 
  bending 
  of 
  the 
  surface, 
  as 
  it 
  is 
  thrown 
  

   into 
  undulations 
  by 
  the 
  passage 
  of 
  the 
  earthquake 
  wave. 
  With 
  this 
  exception, 
  

   all 
  the 
  effects 
  of 
  an 
  earthquake, 
  whether 
  they 
  take 
  the 
  form 
  of 
  visible 
  damage 
  or 
  of 
  

   the 
  sensations 
  produced 
  at 
  the 
  time, 
  are 
  entirely 
  due 
  to 
  inertia. 
  

  

  74. 
  Everything 
  tends 
  to 
  remain 
  in 
  the 
  same 
  condition 
  of 
  rest 
  or 
  motion 
  in 
  which 
  

   it 
  is 
  at 
  any 
  moment, 
  and 
  is 
  only 
  by 
  the 
  action 
  of 
  an 
  external 
  force 
  that 
  a 
  change 
  

   can 
  be 
  produced. 
  A 
  pillar, 
  standing 
  upright, 
  has 
  no 
  tendency 
  to 
  fall 
  over, 
  

   but 
  when 
  its 
  base 
  is 
  suddenly 
  displaced, 
  as 
  by 
  the 
  motion 
  of 
  the 
  ground 
  in 
  an 
  

   earthquake, 
  the 
  mass 
  of 
  the 
  pillar 
  tends 
  to 
  maintain 
  its 
  original 
  position 
  and 
  a 
  

   strain 
  is 
  set 
  up 
  at 
  the 
  base. 
  If 
  this 
  strain 
  is 
  great 
  enough 
  the 
  pillar 
  will 
  be 
  tilted 
  

   up 
  on 
  the 
  edge 
  away 
  from 
  the 
  direction 
  In 
  which 
  the 
  base 
  was 
  moved, 
  and 
  if 
  the 
  

   movement 
  be 
  great 
  enough 
  the 
  pillar 
  will 
  topple 
  over. 
  

  

  75. 
  If, 
  from 
  the 
  form 
  of 
  the 
  body 
  affected, 
  it 
  is 
  not 
  free 
  to 
  fall 
  or 
  bend 
  in 
  

   the 
  direction 
  frcm 
  which 
  the 
  motion 
  starts, 
  the 
  velocity 
  of 
  movement 
  of 
  the 
  wave- 
  

  

  