﻿PREFACE. 
  xxiYi 
  

  

  particle 
  will 
  be 
  communicated 
  to 
  it, 
  and, 
  when 
  the 
  next 
  semiphase 
  of 
  wave 
  motion 
  

   sets 
  in, 
  its 
  inertia, 
  or, 
  as 
  commonly 
  expressed, 
  momentum, 
  will 
  tend 
  to 
  make 
  ft 
  

   continue 
  to 
  move 
  with 
  the 
  direction 
  and 
  velocity 
  which 
  have 
  been 
  imparted 
  to 
  it. 
  

   A 
  strain 
  will 
  consequently 
  be 
  set 
  up, 
  just 
  as 
  in 
  the 
  case 
  considered 
  in 
  the 
  last 
  

   paragraph, 
  and 
  in 
  this 
  way 
  not 
  only 
  may 
  pillars 
  be 
  overthrown 
  but 
  the 
  ends 
  of 
  

   walls 
  may 
  be 
  shot 
  off, 
  or 
  the 
  soil 
  resting 
  on 
  a 
  hill 
  side, 
  or 
  even 
  the 
  face 
  of 
  a 
  cliff, 
  be 
  

   dislodged, 
  as 
  described 
  in 
  Chapter 
  VII. 
  

  

  76. 
  The 
  magnitude 
  of 
  the 
  strain 
  set 
  up 
  is 
  governed, 
  not 
  by 
  the 
  velocity, 
  but 
  by 
  

   the 
  acceleration 
  of 
  the 
  wave-particle. 
  This 
  is 
  easily 
  illustrated 
  experimentally. 
  

   Let 
  a 
  match-box, 
  preferably 
  full, 
  be 
  stood 
  on 
  end 
  on 
  a 
  sheet 
  of 
  paper 
  resting 
  on 
  a 
  

   table 
  ; 
  if 
  one 
  edge 
  of 
  the 
  paper 
  is 
  taken 
  hold 
  of, 
  the 
  paper 
  and 
  match-box 
  can 
  be 
  

   moved 
  over 
  the 
  table 
  and, 
  if 
  the 
  speed 
  of 
  movement 
  be 
  only 
  gradually 
  increased, 
  

   it 
  will 
  be 
  found 
  that 
  the 
  motion 
  may 
  become 
  quite 
  rapid 
  without 
  causing 
  the 
  

   match-box 
  to 
  overturn 
  ; 
  if, 
  on 
  the 
  other 
  hand, 
  the 
  sheet 
  of 
  paper 
  be 
  put 
  suddenly 
  

   in 
  motion, 
  the 
  match-box 
  will 
  fall 
  over 
  backwards, 
  or 
  if 
  after 
  having 
  been 
  put 
  in 
  

   rapid 
  motion 
  it 
  is 
  suddenly 
  stopped, 
  it 
  will 
  fall 
  over 
  forwards. 
  Similarly, 
  if 
  a 
  

   train 
  is 
  started 
  or 
  stopped 
  gently 
  it 
  is 
  very 
  difficult 
  for 
  a 
  passenger 
  to 
  recognise 
  

   the 
  moment 
  of 
  starting 
  or 
  stoppage, 
  but 
  if 
  an 
  unskilful, 
  or 
  careless, 
  driver 
  is 
  in 
  

   charge 
  of 
  the 
  engine, 
  the 
  stoppage, 
  or 
  starting, 
  may 
  be 
  a 
  jerk 
  that 
  is 
  very 
  notice- 
  

   able. 
  In 
  both 
  these 
  instances 
  it 
  is 
  not 
  the 
  rate 
  of 
  movement, 
  but 
  the 
  rapidity 
  of 
  

   change 
  of 
  rate 
  of 
  movement, 
  that 
  is 
  to 
  say, 
  the 
  acceleration, 
  which 
  produces 
  the 
  

   effect. 
  Similarly, 
  in 
  the 
  case 
  of 
  an 
  earthquake 
  the 
  sensation 
  produced, 
  or 
  the 
  

   damage 
  dene, 
  is 
  due 
  to 
  the 
  greater 
  or 
  less 
  suddenness 
  of 
  the 
  movement 
  and 
  not 
  

   to 
  the 
  actual 
  velocity 
  attained. 
  

  

  77. 
  There 
  is 
  another 
  way 
  in 
  which 
  solid 
  bodies 
  may 
  be 
  affected 
  by 
  the 
  earth- 
  

   quake 
  wave, 
  which 
  depends, 
  not 
  on 
  the 
  acceleration, 
  but 
  on 
  the 
  velocity 
  of 
  the 
  

   wave-particle. 
  If, 
  instead 
  of 
  a 
  tall 
  pillar, 
  we 
  take 
  the 
  case 
  of 
  a 
  flat 
  stone 
  lying 
  on 
  

   the 
  ground, 
  it 
  is 
  obvious 
  that 
  any 
  overturning 
  is 
  impossible. 
  The 
  motion 
  of 
  the 
  

   wave-particle 
  will 
  be 
  more 
  or 
  less 
  completely 
  imparted 
  to 
  the 
  stone, 
  and 
  when 
  the 
  

   next 
  semiphase 
  of 
  the 
  wave 
  motion 
  sets 
  in, 
  the 
  inertia 
  or 
  momentum 
  of 
  the 
  

   stone 
  will 
  tend 
  to 
  make 
  it 
  continue 
  moving 
  with 
  the 
  direction 
  and 
  velocity 
  which 
  

   has 
  been 
  imparted 
  to 
  it. 
  If 
  the 
  momentum 
  imparted 
  be 
  more 
  than 
  sufficient 
  to 
  

   overcome 
  the 
  resistance 
  of 
  the 
  attachment 
  of 
  the 
  stone 
  to 
  its 
  support, 
  it 
  will 
  be 
  

   projected 
  through 
  the 
  air, 
  and 
  the 
  distance 
  to 
  which 
  it 
  is 
  projected 
  is 
  dependent 
  on 
  

   the 
  direction 
  and 
  velocity 
  with 
  which 
  it 
  started 
  on 
  its 
  way. 
  Yet, 
  even 
  here, 
  

   acceleration 
  is 
  not 
  without 
  its 
  influence, 
  for, 
  where 
  a 
  body 
  is 
  projected 
  upwards, 
  

   whether 
  vertically 
  or 
  in 
  a 
  slanting 
  direction, 
  the 
  acceleration 
  of 
  the 
  wave-particle 
  

   must 
  at 
  least 
  exceed 
  that 
  due 
  to 
  the 
  action 
  of 
  gravitation, 
  or 
  the 
  attraction 
  of 
  the 
  

   earth 
  on 
  a 
  free 
  falling 
  body 
  near 
  its 
  surface.* 
  

  

  78. 
  This 
  statement 
  may 
  easily 
  be 
  illustrated 
  by 
  placing 
  a 
  small 
  stone 
  in 
  the 
  

   upturned 
  palm 
  of 
  an 
  open 
  hand. 
  If 
  the 
  hand 
  is 
  raised 
  and 
  then 
  lowered 
  slowly 
  

   the 
  stone 
  will 
  not 
  leave 
  it, 
  but 
  if 
  the 
  movement 
  is 
  made 
  quicker 
  and 
  more 
  quickly 
  

   a 
  limit 
  will 
  be 
  reached 
  when 
  the 
  stone 
  leaves 
  the 
  hand 
  and 
  rises 
  in 
  the 
  air 
  ; 
  this 
  

   means 
  that 
  the 
  rate 
  at 
  which 
  the 
  stone 
  increases 
  its 
  velocity 
  of 
  fall 
  is 
  less 
  than 
  

  

  1 
  This 
  vaiies 
  slightly 
  from 
  place 
  to 
  place, 
  but 
  may 
  be 
  taken 
  as 
  about 
  32 
  ft. 
  per 
  second 
  per 
  

   second. 
  

  

  