﻿Miscellaneous 
  Intelligence. 
  307 
  

  

  recognized: 
  (1) 
  condensational, 
  and 
  (2) 
  distortional 
  plane 
  waves, 
  

   traveling 
  by 
  brachi-stochronic 
  paths 
  through 
  the 
  earth, 
  and 
  (3) 
  

   elastic, 
  or 
  gravitational 
  elastic, 
  surface 
  waves, 
  traveling 
  round 
  

   the 
  surface 
  of 
  the 
  earth. 
  The 
  records 
  are, 
  however, 
  often 
  incom- 
  

   plete 
  by 
  the 
  omission 
  of 
  the 
  first 
  or 
  the 
  first 
  and 
  second 
  of 
  these 
  

   phases, 
  and 
  the 
  widely 
  divergent 
  estimates 
  of 
  the 
  apparent 
  rate 
  of 
  

   propagation 
  of 
  the 
  preliminary 
  tremors 
  are 
  largely 
  due 
  to 
  this. 
  — 
  

   Proc. 
  Roy. 
  Soc, 
  lxvi, 
  2, 
  1900. 
  

  

  3. 
  On 
  Earthquake 
  Sounds. 
  — 
  Dr. 
  C. 
  Davison 
  closes 
  an 
  inter- 
  

   esting 
  paper 
  in 
  a 
  recent 
  number 
  of 
  the 
  Philosophical 
  Magazine 
  

   with 
  the 
  following 
  discussion 
  as 
  to 
  the 
  origin 
  of 
  earthquake 
  

   sounds: 
  "In 
  any 
  earthquake 
  there 
  are 
  generally 
  three 
  well- 
  

   defined 
  stages 
  of 
  motion 
  — 
  the 
  preliminary 
  tremor, 
  the 
  principal 
  

   portion, 
  and 
  the 
  end 
  portion. 
  In 
  the 
  first, 
  the 
  characteristic 
  

   movements 
  are 
  small 
  in 
  amplitude 
  and 
  short 
  in 
  period 
  ; 
  the 
  

   second 
  and 
  most 
  active 
  part 
  of 
  the 
  disturbance 
  consists 
  chiefly 
  

   of 
  vibrations 
  of 
  larger 
  amplitude 
  and 
  longer 
  period; 
  and 
  these 
  

   are 
  followed 
  by 
  the 
  feeble 
  movements 
  which 
  constitute 
  the 
  end 
  

   portion. 
  Slow 
  undulations, 
  with 
  a 
  period 
  of 
  about 
  one-half 
  to 
  

   two 
  seconds, 
  may, 
  however, 
  be 
  present 
  in 
  all 
  three 
  stages 
  ; 
  while 
  

   the 
  ripples, 
  with 
  a 
  period 
  of 
  one-tenth 
  of 
  a 
  second 
  or 
  more, 
  may 
  

   be 
  superposed 
  on 
  the 
  slow 
  undulations 
  in 
  the 
  second 
  and 
  third 
  

   stages 
  as 
  well 
  as 
  in 
  the 
  first. 
  The 
  average 
  period 
  of 
  the 
  ripples, 
  it 
  

   is 
  important 
  to 
  notice, 
  is 
  slightly 
  greater 
  during 
  the 
  principal 
  

   portion 
  than 
  during 
  the 
  other 
  two 
  stages. 
  

  

  During 
  the 
  earlier 
  part 
  of 
  the 
  movement 
  there 
  is 
  a 
  distinct 
  

   increase 
  in 
  the 
  period 
  of 
  the 
  vibrations 
  ; 
  and 
  this 
  has 
  led 
  some 
  

   seismologists 
  (all 
  at 
  the 
  time 
  living 
  in 
  Japan) 
  to 
  suggest 
  that 
  the 
  

   first 
  recorded 
  tremors 
  may 
  be 
  the 
  successors 
  of 
  those 
  which 
  produce 
  

   the 
  preliminary 
  earihquake-sound. 
  The 
  observations 
  of 
  British 
  

   earthquakes 
  show, 
  however, 
  that 
  the 
  sound-vibrations 
  are 
  not 
  con- 
  

   fined 
  to 
  the 
  earlier 
  stage, 
  but 
  that, 
  like 
  the 
  ripples, 
  they 
  are 
  also 
  

   superposed 
  on 
  the 
  slow 
  undulations 
  which 
  form 
  the 
  main 
  feature 
  

   of 
  the 
  principal 
  and 
  end 
  portions 
  of 
  an 
  earthquake. 
  

  

  Some 
  of 
  the 
  phenomena 
  of 
  earthquake-sounds, 
  as 
  we 
  have 
  seen, 
  

   are 
  due 
  to 
  the 
  neighborhood 
  of 
  the 
  sound 
  to 
  the 
  varying 
  lower 
  

   limit 
  of 
  audibility. 
  There 
  are 
  others, 
  such 
  as 
  the 
  time 
  and 
  space- 
  

   relations 
  of 
  the 
  sound 
  and 
  shock, 
  lor 
  which 
  a 
  different 
  explana- 
  

   tion 
  is 
  required, 
  and 
  whose 
  origin 
  is 
  of 
  a 
  geological 
  rather 
  than 
  a 
  

   physical 
  character. 
  

  

  The 
  theory 
  which 
  follows 
  is 
  based 
  on 
  the 
  supposition 
  that 
  the 
  

   majority 
  of 
  non-volcanic 
  earthquakes 
  are 
  due 
  to 
  the 
  gradual, 
  but 
  

   intermittent, 
  growth 
  of 
  faults 
  ; 
  the 
  immediate 
  cause 
  of 
  the 
  dis- 
  

   turbance 
  being 
  the 
  friction 
  produced 
  by 
  the 
  slipping 
  and 
  rubbing 
  

   of 
  one 
  of 
  the 
  adjoining 
  rock-masses 
  over 
  and 
  against 
  the 
  other.* 
  

  

  * 
  The 
  principal 
  facts 
  in 
  favour 
  of 
  the 
  fault-slip 
  theory 
  are: 
  —(1) 
  The 
  elongated 
  

   forms 
  of 
  the 
  isoseismal 
  lines, 
  the 
  longer 
  axes 
  of 
  which 
  in 
  any 
  district 
  are, 
  as 
  a 
  

   rule, 
  either 
  parallel 
  or 
  perpendicular 
  to 
  one 
  another, 
  and 
  parallel, 
  or 
  nearly 
  so, 
  to 
  

   the 
  chief 
  lines 
  of 
  fault; 
  (2) 
  the 
  formation 
  of 
  fault-scarps 
  concurrently 
  with 
  

   violent 
  earthquakes; 
  (3) 
  the 
  impossibility 
  of 
  a 
  great 
  fault 
  growing 
  otherwise 
  

   than 
  by 
  an 
  almost 
  infinite 
  number 
  of 
  slips 
  ; 
  and 
  (4) 
  the 
  enormous 
  excess 
  of 
  the 
  

   number 
  of 
  earthquakes 
  over 
  the 
  number 
  of 
  faults 
  in 
  any 
  one 
  district. 
  

  

  