﻿VI 
  

  

  OLDHAM 
  : 
  GREAT 
  EARTHQUAKE 
  OF 
  1897. 
  

  

  specific 
  individual 
  particle 
  of 
  matter, 
  but 
  any 
  one 
  taken 
  at 
  random 
  at 
  a 
  chosen 
  

   place, 
  by 
  the 
  extent, 
  form, 
  and 
  rate 
  of 
  whose 
  movement 
  the 
  nature 
  of 
  the 
  wave 
  

   motion 
  at 
  that 
  place 
  is 
  defined. 
  

  

  3. 
  The 
  particles 
  referred 
  to 
  are 
  molecules, 
  the 
  ultimate 
  portions 
  of 
  matter 
  

   reached 
  t^ 
  continual 
  subdivision 
  till 
  a 
  limit 
  is 
  reached 
  where 
  no 
  further 
  division 
  is 
  

   possible 
  without 
  a 
  change 
  of 
  character 
  and 
  properties. 
  Hence 
  the 
  movement 
  in 
  

   a 
  wave 
  is 
  called 
  molecular, 
  while 
  a 
  bodily 
  displacement 
  of 
  the 
  water 
  or 
  other 
  

   substance 
  may 
  be 
  distinguished 
  as 
  molar. 
  When 
  a 
  kingfisher 
  dives 
  into 
  a 
  pool 
  it 
  

   makes 
  a 
  splash, 
  and 
  then 
  from 
  the 
  splash 
  a 
  series 
  of 
  concentric 
  ripples 
  travel 
  out- 
  

   wards 
  in 
  ever 
  widening 
  circles. 
  In 
  the 
  splash 
  the 
  movement 
  of 
  the 
  water 
  is 
  molar, 
  

   in 
  the 
  ripples 
  it 
  is 
  molecular. 
  So 
  too 
  the 
  general 
  movement 
  of 
  the 
  water 
  of 
  a 
  

   trout 
  stream 
  may 
  be 
  regarded 
  as 
  molar, 
  that 
  in 
  the 
  ripples 
  formed 
  by 
  the 
  rising 
  

   t-'out 
  as 
  molecular. 
  

  

  4. 
  Gravitational 
  waves. 
  — 
  Waves 
  are 
  of 
  several 
  kinds; 
  of 
  these 
  it 
  will 
  be 
  well 
  

   to 
  take 
  first 
  that 
  which 
  is 
  most 
  commonly 
  known, 
  such 
  as 
  may 
  be 
  seen 
  or 
  started 
  

   on 
  any 
  sheet 
  of 
  water. 
  In 
  these 
  the 
  course 
  of 
  the 
  wave-particle 
  is 
  elliptical, 
  or 
  

   circular, 
  as 
  is 
  diagrammatically 
  represented 
  in 
  fig. 
  i. 
  The 
  wave-particle 
  is 
  at 
  

   its 
  highest 
  point 
  as 
  the 
  crest 
  of 
  the 
  wave 
  passes 
  over 
  it, 
  and 
  at 
  its 
  lowest 
  when 
  

   the 
  trough 
  occupies 
  the 
  same 
  position. 
  "When 
  any 
  one 
  particle 
  is 
  at 
  its 
  highest 
  

   the 
  one 
  in 
  front 
  has 
  not 
  yet 
  risen 
  to 
  its 
  highest 
  point, 
  while 
  the 
  one 
  behind 
  has 
  

   begun 
  to 
  sink 
  again. 
  By 
  the 
  time 
  the 
  one 
  in 
  front 
  has 
  reached 
  its 
  highest 
  point 
  

   the 
  one 
  we 
  were 
  considering 
  has 
  begun 
  to 
  sink 
  ; 
  and 
  so 
  it 
  is 
  that 
  the 
  wave 
  travels. 
  

  

  Fig. 
  i. 
  Diagram 
  of 
  movement 
  of 
  wave 
  particle 
  in 
  a 
  gravitational 
  wave. 
  

  

  5. 
  Wave 
  motion 
  of 
  this 
  nature 
  is 
  quite 
  superficial, 
  and 
  where 
  waves'are 
  set 
  up 
  

   in 
  deep 
  water 
  it 
  is 
  only 
  that 
  near 
  the 
  surface 
  which 
  is 
  affected. 
  As 
  we 
  descend 
  

   from 
  the 
  surface 
  the 
  extent 
  of 
  movement 
  of 
  the 
  wave-particle 
  rapidly 
  diminishes, 
  

   the 
  shape 
  of 
  its 
  track 
  becomes 
  a 
  more 
  elongated 
  ellipse, 
  till, 
  at 
  a 
  comparative 
  short 
  

   distance 
  from 
  the 
  surface, 
  there 
  is 
  only 
  a 
  slight 
  to 
  and 
  fro 
  movement 
  which 
  

   becomes 
  less 
  and 
  less 
  till 
  it 
  vanishes. 
  

  

  6. 
  If 
  in 
  fig. 
  i 
  the 
  firm 
  and 
  broken 
  curved 
  lines 
  are 
  taken 
  to 
  represent 
  the 
  out- 
  

   lines 
  of 
  a 
  wave 
  when 
  its 
  crest 
  and 
  trough 
  respectively 
  are 
  over 
  the 
  same 
  place, 
  and 
  

   the 
  horizontal 
  dotted 
  line 
  the 
  normal 
  level 
  of 
  the 
  surface 
  of 
  the 
  water 
  ; 
  then 
  the 
  

   length 
  of 
  the 
  wave 
  is 
  the 
  distance 
  from 
  crest 
  to 
  crest 
  or 
  trough 
  to 
  trough. 
  The 
  

   distance 
  a 
  a 
  ' 
  is 
  the 
  amplitude 
  of 
  the 
  wave, 
  or 
  the 
  distance 
  the 
  wave-particle 
  moves 
  

   up 
  or 
  downwards 
  from 
  its 
  normal 
  level 
  ; 
  double 
  this, 
  or 
  a 
  ' 
  a 
  ' 
  is 
  the 
  height 
  of 
  the 
  

   wave 
  from 
  trough 
  to 
  crest. 
  

  

  7. 
  If 
  the 
  track 
  of 
  the 
  wave-particle 
  as 
  drawn 
  in 
  fig. 
  i 
  is 
  examined, 
  it 
  will 
  be 
  

   se^n 
  that 
  the 
  movement 
  can 
  be 
  divided 
  into 
  two 
  phases 
  : 
  one 
  of 
  forward, 
  and 
  one 
  of 
  

  

  