﻿* 
  OLDHAM: 
  GREAT 
  EARTHQUAKE 
  OF 
  189?. 
  

  

  gravitational 
  waves. 
  Apart 
  from 
  this 
  there 
  are, 
  however, 
  as 
  has 
  been 
  shown 
  by 
  

   Lord 
  Rayleigh, 
  true 
  surface 
  undulations, 
  closely 
  simulating 
  gravitational 
  waves 
  

   but 
  propagated 
  elastically. 
  In 
  these 
  the 
  path 
  of 
  the 
  wave^particle 
  is 
  an 
  ellipse 
  

   whose 
  major 
  axis 
  is 
  vertical 
  and 
  from 
  one 
  and 
  a 
  half 
  to 
  twice 
  the 
  minor. 
  The 
  hori- 
  

   zontal 
  displacement 
  extends 
  but 
  a 
  short 
  Way 
  into 
  the 
  mass, 
  disappearing 
  at 
  from 
  

   one-eighth 
  to 
  one-fifth 
  of 
  the 
  wave-length 
  from 
  the 
  surface* 
  Below 
  this 
  there 
  is 
  only 
  

   a 
  vertical 
  movement. 
  It 
  does 
  not 
  appear 
  that 
  the 
  existence 
  of 
  waves 
  of 
  this 
  nature 
  

   has 
  ever 
  been 
  demonstrated, 
  and 
  they 
  have 
  certainly 
  not 
  as 
  yet 
  been 
  separated 
  

   in 
  the 
  complicated 
  disturbances 
  of 
  an 
  earthquake, 
  but 
  the 
  possibility 
  of 
  their 
  

   existence 
  may 
  be 
  taken 
  as 
  demonstrated, 
  and 
  time 
  and 
  the 
  collection 
  of 
  fuller 
  

   details 
  will 
  doubtless 
  lead 
  to 
  their 
  recognition. 
  

  

  20. 
  It 
  may 
  be 
  noted 
  here 
  that 
  whether 
  the 
  surface 
  undulations 
  noticed 
  in 
  great 
  

   earthquakes 
  are 
  of 
  the 
  nature 
  of 
  Lord 
  Rayleigh's 
  waves 
  or 
  not, 
  there 
  can 
  be 
  no 
  

   doubt 
  of 
  their 
  existence. 
  They 
  have 
  been 
  observed 
  too 
  often 
  for 
  this 
  to 
  be 
  doubted, 
  

   and 
  ample 
  proof 
  of 
  their 
  reality 
  will 
  be 
  found 
  in 
  the 
  pages 
  of 
  the 
  following 
  report. 
  

   It 
  must, 
  however, 
  also 
  be 
  noticed 
  that 
  there 
  are 
  apparently 
  two 
  distinct 
  types. 
  

   Firstly, 
  there 
  is 
  the 
  long, 
  low, 
  quick 
  travelling 
  wave, 
  which 
  first 
  begins 
  to 
  be 
  dis- 
  

   tinctly 
  noticed 
  in 
  the 
  area 
  where 
  the 
  earthquake 
  has 
  ceased 
  to 
  be 
  destructive, 
  

   or 
  where 
  the 
  destruction 
  wrought 
  has 
  become 
  much 
  less 
  than 
  in 
  the 
  epicentral 
  

   tract. 
  These 
  waves 
  cannot 
  be 
  seen 
  ; 
  they 
  are 
  recognised 
  only 
  by 
  a 
  successive 
  

   tilting 
  in 
  one 
  direction 
  and 
  then 
  in 
  the 
  opposite, 
  and 
  their 
  rate 
  of 
  travel, 
  100 
  to 
  

   120 
  miles 
  a 
  minute, 
  is 
  that 
  of 
  the 
  earthquake 
  shock. 
  Secondly, 
  there 
  are 
  the 
  

   much 
  shorter 
  and 
  steeper 
  sided 
  waves 
  which 
  can 
  be 
  seen 
  and 
  recognised 
  as 
  such 
  j 
  

   these 
  may 
  vary 
  in 
  length 
  from 
  20 
  feet 
  to 
  200 
  yards 
  and 
  in 
  height 
  from 
  a 
  few 
  

   inches 
  to 
  2 
  or 
  3 
  feet, 
  and 
  travel 
  at 
  a 
  rate 
  of 
  3 
  to 
  8 
  miles 
  an 
  hour. 
  

  

  21. 
  Whatever 
  may 
  be 
  the 
  nature 
  of 
  the 
  laws 
  which 
  govern 
  the 
  movement 
  of 
  

   the 
  wave-particle 
  in 
  these 
  surface 
  undulations 
  it 
  is 
  hardly 
  possible 
  that 
  they 
  can 
  

   be 
  the 
  same 
  in 
  the 
  case 
  of 
  waves 
  which 
  differ 
  so 
  much 
  in 
  size 
  and 
  rate 
  of 
  move- 
  

   ment 
  as 
  the 
  two 
  forms 
  of 
  surf 
  ace 
  wa 
  ve 
  referred 
  to 
  in 
  the 
  preceding 
  paragraph 
  

   This 
  conclusion 
  appears 
  to 
  be 
  supported 
  by 
  the 
  absence 
  of 
  any 
  passage 
  from 
  

   the 
  one 
  to 
  the 
  other, 
  but 
  on 
  this 
  point 
  further 
  observation 
  is 
  required. 
  

  

  22. 
  Complication 
  of 
  waves. 
  — 
  If 
  the 
  earth 
  were 
  a 
  perfectly 
  homogeneous 
  globe, 
  

   then, 
  supposing 
  elastic 
  waves 
  of 
  one 
  of 
  the 
  three 
  classes 
  were 
  set 
  up 
  by 
  a 
  dis- 
  

   turbance 
  at 
  any 
  part 
  of 
  it, 
  they 
  would 
  be 
  propagated 
  without 
  change. 
  If 
  the 
  

   waves 
  of 
  all 
  three 
  classes 
  were 
  set 
  up, 
  they 
  would 
  separate 
  as 
  they 
  travelled, 
  the 
  

   waves 
  of 
  elastic 
  compression, 
  travelling 
  at 
  the 
  greatest 
  speed, 
  would 
  outrace 
  the 
  

   others 
  ; 
  then 
  at 
  a 
  gradually 
  increasing 
  distance 
  behind 
  would 
  come 
  the 
  waves 
  of 
  

   elastic 
  distortion, 
  while 
  lagging 
  more 
  and 
  more 
  behind 
  would 
  come 
  the 
  elastic 
  

   surface 
  waves, 
  which 
  not 
  only 
  travel 
  slowest, 
  but 
  have 
  to 
  travel 
  round 
  the 
  surface 
  

   of 
  the 
  globe, 
  while 
  the 
  others 
  can 
  take 
  a 
  short 
  cut 
  through 
  it. 
  

  

  23. 
  In 
  the 
  case 
  of 
  the 
  disturbances 
  which 
  are 
  registered 
  by 
  sufficiently 
  deli- 
  

   cate 
  instruments 
  at 
  a 
  long 
  distance 
  from 
  the 
  place 
  they 
  start 
  from, 
  and 
  have 
  

   consequently 
  travelled 
  through 
  the 
  comparatively 
  homogeneous 
  central 
  core 
  of 
  

   the 
  earth, 
  something 
  like 
  this 
  has 
  been 
  observed. 
  In 
  the 
  case 
  too 
  of 
  artificial 
  

   disturbances, 
  such 
  as 
  are 
  produced 
  by 
  explosion 
  of 
  gun-cotton, 
  something 
  of 
  the 
  

   kind 
  has 
  been 
  observed 
  where 
  the 
  disturbances 
  have 
  been 
  measured 
  at 
  compara- 
  

   tively 
  small 
  distances 
  and 
  have 
  travelled 
  through 
  fairly 
  homogeneous 
  material. 
  

  

  