﻿I76 
  OLDHAM: 
  GREAT 
  EARTHQUAKE 
  OF 
  1897. 
  

  

  travel 
  faster 
  at 
  greater 
  than 
  at 
  less 
  depths 
  and, 
  as 
  pointed 
  out 
  by 
  

   Dr. 
  A. 
  Schmidt, 
  1 
  of 
  Stuttgart, 
  one 
  consequence 
  of 
  this 
  would 
  be 
  that 
  

   the 
  wave 
  paths, 
  instead 
  of 
  being 
  straight 
  lines, 
  would 
  be 
  concave 
  up- 
  

   wards. 
  Another 
  consequence 
  would 
  be 
  to 
  throw 
  the 
  point 
  of 
  junc- 
  

   tion 
  of 
  the 
  two 
  contrary 
  curves 
  closer 
  to 
  the 
  centre 
  of 
  the 
  hodograph, 
  

   that 
  is 
  to 
  say, 
  the 
  focus 
  would 
  be 
  at 
  a 
  greater 
  depth 
  than 
  the 
  

   versed 
  sine 
  of 
  the 
  angle 
  contained 
  between 
  the 
  point 
  of 
  reversal 
  of 
  

   the 
  curve 
  and 
  the 
  centre. 
  

  

  If 
  then 
  the 
  curved 
  line 
  on 
  plate 
  XXXIX 
  represents 
  the 
  true 
  

   hodograph 
  or 
  time 
  curve, 
  the 
  depth 
  of 
  the 
  focus 
  must 
  be 
  more 
  than 
  

   10 
  miles. 
  But 
  it 
  has 
  already 
  been 
  shown 
  2 
  that 
  the 
  records 
  admit 
  of 
  

   another 
  and 
  totally 
  different 
  interpretation, 
  and 
  we 
  have 
  seen 
  that 
  the 
  

   dotted 
  line 
  is 
  that 
  which 
  more 
  probably 
  represents 
  the 
  true 
  hodo- 
  

   graph, 
  or 
  speed 
  curve, 
  of 
  the 
  earthquake, 
  and 
  is 
  in 
  accordance 
  with 
  

   what 
  is 
  probably 
  the 
  true 
  theory 
  of 
  the 
  propagation 
  of 
  the 
  earthquake. 
  

  

  Accepting 
  this 
  theory 
  and 
  the 
  corresponding 
  speed 
  curve 
  as 
  the 
  

   true 
  ones, 
  it 
  is 
  evident 
  that 
  the 
  time 
  observations 
  can 
  give 
  no 
  clue 
  to 
  

   the 
  depth 
  of 
  the 
  focus, 
  and 
  we 
  must 
  reject 
  the 
  estimate, 
  improbable 
  in 
  

   itself 
  of 
  10 
  miles. 
  This, 
  however, 
  leaves 
  us 
  without 
  any 
  definite 
  

   indication 
  of 
  the 
  depth, 
  and 
  we 
  have 
  to 
  attack 
  the 
  problem 
  by 
  the 
  

   aid 
  of 
  general 
  principles. 
  

  

  Assuming 
  the 
  earth 
  to 
  consist 
  of 
  highly 
  heated 
  matter, 
  and 
  to 
  be 
  

   cooling 
  down 
  by 
  a 
  slow 
  conduction 
  of 
  heat 
  outwards, 
  we 
  have 
  it 
  

   divided 
  into 
  three 
  portions. 
  First, 
  an 
  outermost 
  skin 
  of 
  rock 
  which 
  

   has 
  attained 
  a 
  constant 
  temperature 
  and 
  no 
  longer 
  cools 
  or 
  contracts 
  

   any 
  further 
  ; 
  second, 
  a 
  shell 
  of 
  rock 
  which 
  is 
  gradually 
  cooling 
  and 
  

   contracting; 
  and 
  third, 
  a 
  central 
  mass 
  which 
  has 
  not 
  yet 
  been 
  affected 
  

   by 
  the 
  loss 
  of 
  heat 
  from 
  the 
  surface. 
  Now, 
  the 
  loss 
  of 
  heat 
  by 
  the 
  

   second 
  of 
  these 
  zones 
  causes 
  a 
  general 
  reduction 
  of 
  bulk, 
  and 
  the 
  • 
  

   rock 
  composing 
  the 
  first 
  or 
  outermost 
  zone 
  is 
  thrown 
  into 
  a 
  state 
  of 
  

   compression. 
  The 
  materials 
  of 
  the 
  second 
  zone, 
  however, 
  are 
  con- 
  

   tracting 
  in 
  every 
  direction, 
  not 
  in 
  a 
  radial 
  direction 
  only, 
  but 
  also 
  

  

  1 
  Jahreshcft 
  Verein 
  f. 
  vaterl. 
  Natuikunde 
  in 
  Wurttemberg, 
  XLIV, 
  p. 
  25S 
  (18SS). 
  

  

  2 
  Supra, 
  p. 
  74. 
  

  

  ( 
  «76 
  ) 
  

  

  