﻿THE 
  SEISMIC 
  FQGUS. 
  1 
  77 
  

  

  circumference 
  of 
  each 
  circle, 
  and 
  where 
  this 
  zone 
  passes 
  into 
  the 
  

   central 
  mass, 
  which 
  will 
  not 
  yield 
  to 
  its 
  contraction, 
  it 
  must 
  obviously 
  

   be 
  thrown 
  into 
  a 
  state 
  of 
  tension, 
  as 
  the 
  tyre 
  of 
  a 
  wheel, 
  which 
  i 
  s 
  

   put 
  on 
  hot, 
  is 
  thrown 
  into 
  a 
  state 
  of 
  tension, 
  while 
  it 
  compresses 
  the 
  

   framework 
  of 
  the 
  wheel. 
  

  

  From 
  this 
  it 
  appears 
  that 
  while 
  the 
  outermost 
  layers 
  of 
  the 
  earth 
  

   must 
  be 
  in 
  a 
  state 
  of 
  compression, 
  there 
  is 
  a 
  certain 
  depth 
  at 
  which 
  

   this 
  state 
  of 
  compression 
  is 
  replaced 
  by 
  one 
  of 
  tension, 
  and 
  somewhere 
  

   at 
  the 
  passage 
  of 
  the 
  one 
  to 
  the 
  other, 
  there 
  must 
  be 
  a 
  " 
  level 
  of 
  no 
  

   strain." 
  Now 
  it 
  is 
  evident 
  that 
  an 
  earthquake, 
  being 
  the 
  consequence 
  

   of 
  a 
  sudden 
  relief 
  of 
  strain, 
  can 
  originate 
  either 
  above 
  or 
  below, 
  

   but 
  not 
  at, 
  this 
  level 
  of 
  no 
  strain, 
  and, 
  practically, 
  we 
  may 
  say 
  above 
  

   only. 
  At 
  great 
  depths 
  both 
  pressure 
  and 
  temperature 
  increase, 
  and 
  

   the 
  effect 
  of 
  either 
  is 
  such 
  as 
  to 
  introduce 
  conditions 
  unfavourable 
  

   to 
  a 
  sudden 
  relief 
  of 
  strain. 
  Under 
  great 
  pressure 
  it 
  has 
  been 
  proved 
  

   experimentally 
  that 
  solids 
  flow 
  like 
  fluids 
  in 
  the 
  direction 
  of 
  

   least 
  pressure, 
  and 
  high 
  temperature 
  introduces 
  a 
  condition 
  of 
  plas- 
  

   ticity, 
  even 
  at 
  the 
  surface, 
  which 
  is 
  not 
  conducive 
  to 
  a 
  gradual 
  accu- 
  

   mulation 
  of 
  strain 
  and 
  a 
  sudden 
  giving 
  way. 
  

  

  For 
  this 
  reason 
  we 
  may 
  take 
  it 
  that 
  all 
  earthquakes 
  originate 
  

   above 
  the 
  level 
  of 
  no 
  strain 
  and, 
  what 
  is 
  more, 
  at 
  some 
  considerable 
  

   distance 
  above 
  that 
  level, 
  as 
  compared 
  with 
  its 
  depth 
  below 
  the 
  

   surface. 
  It 
  should, 
  therefore, 
  if 
  we 
  had 
  any 
  thoroughly 
  trustworthy 
  

   method 
  of 
  determining 
  the 
  depth 
  of 
  the 
  focus, 
  be 
  possible 
  to 
  determine 
  

   a 
  minimum 
  value 
  for 
  the 
  depth. 
  of 
  the 
  level 
  of 
  no 
  strain. 
  

  

  No 
  method 
  has, 
  however, 
  been 
  proposed 
  which 
  is 
  thoroughly 
  trust- 
  

   worthy, 
  and 
  instead 
  of 
  using 
  earthquakes 
  to 
  determine 
  the 
  depth 
  of 
  

   this 
  level 
  of 
  no 
  strain, 
  we 
  must 
  use 
  its 
  depth, 
  as 
  independently 
  cal- 
  

   culated, 
  to 
  fix 
  a 
  maximum 
  depth 
  for 
  the 
  focus 
  of 
  an 
  earthquake. 
  

   According 
  to 
  the 
  calculation 
  of 
  Rev. 
  O. 
  Fisher, 
  the 
  depth 
  of 
  the 
  level 
  

   of 
  no 
  strain 
  is 
  not 
  more 
  than 
  about 
  2 
  miles 
  from 
  the 
  surface. 
  1 
  Mr, 
  

   C. 
  Davison, 
  however, 
  has 
  pointed 
  out 
  that 
  this 
  estimate 
  is 
  based 
  on 
  

  

  1 
  Physics 
  of 
  the 
  Earth's 
  crust, 
  2nd 
  edition, 
  1889, 
  p. 
  98 
  : 
  Phil., 
  Mag 
  5th, 
  series, 
  XXXVIII, 
  

   »35 
  (1894). 
  

  

  N 
  

  

  I 
  i?7 
  

  

  