﻿X-EAYING 
  THE 
  EARTH 
  DALY 
  263 
  

  

  the 
  deadly 
  earthquake 
  strikes. 
  Man 
  is 
  learning 
  to 
  harness 
  for 
  his 
  

   inquiring 
  use 
  the 
  very 
  wrath 
  of 
  the 
  earth; 
  the 
  tremblings 
  of 
  our 
  

   vibrant 
  globe 
  are 
  used 
  to 
  "X-ray" 
  the 
  deep 
  interior. 
  

  

  When 
  with 
  his 
  hands 
  one 
  bends 
  a 
  stick 
  until 
  it 
  breaks, 
  the 
  sudden 
  

   snap 
  sends 
  vibrations, 
  often 
  painful, 
  along 
  muscle, 
  bone, 
  and 
  nerve 
  of 
  

   the 
  arms. 
  The 
  "strain" 
  of 
  the 
  stick 
  is 
  relieved 
  by 
  fracture, 
  and 
  the 
  

   elastic 
  energy 
  accumulated 
  in 
  the 
  stick 
  during 
  the 
  bending 
  is 
  largely 
  

   converted 
  into 
  the 
  energy 
  of 
  wave 
  motion. 
  In 
  a 
  somewhat 
  similar 
  

   way 
  the 
  rocks 
  of 
  the 
  earth's 
  crust 
  have 
  been, 
  and 
  now 
  are 
  being, 
  

   strained; 
  every 
  day, 
  somewhere, 
  they 
  are 
  snapping 
  and 
  sending 
  out 
  

   elastic 
  waves 
  from 
  one 
  or 
  more 
  centers. 
  The 
  passage 
  of 
  these 
  waves 
  

   in 
  the 
  earth 
  we 
  call 
  an 
  earthquake, 
  a 
  seismic 
  disturbance. 
  

  

  Each 
  heavy 
  shock 
  creates 
  waves 
  of 
  several 
  kinds. 
  The 
  kind 
  which 
  

   travels 
  fastest 
  is 
  like 
  a 
  sound 
  wave; 
  it 
  is 
  propagated 
  by 
  the 
  alternation 
  

   of 
  compression 
  and 
  rarefaction 
  in 
  the 
  rocks. 
  The 
  particles 
  of 
  the 
  

   rocks 
  here 
  vibrate 
  to 
  and 
  fro, 
  in 
  the 
  direction 
  of 
  wave 
  motion, 
  that 
  

   is, 
  along 
  the 
  wave 
  path 
  or 
  wave 
  "ray." 
  Waves 
  of 
  this 
  type, 
  techni- 
  

   cally 
  called 
  longitudinal 
  waves, 
  can 
  pass 
  from 
  rock 
  into 
  the 
  fluid 
  of 
  

   ocean, 
  lake, 
  or 
  atmosphere, 
  and 
  if 
  the 
  vibrations 
  are 
  frequent 
  and 
  

   energetic 
  enough, 
  are 
  heard 
  with 
  the 
  unaided 
  ear. 
  Somewhat 
  slower 
  

   is 
  a 
  second 
  kind 
  of 
  wave 
  which 
  follows 
  nearly 
  the 
  same 
  path 
  in 
  the 
  

   rocks, 
  but 
  is 
  distinguished 
  by 
  the 
  fact 
  that 
  now 
  the 
  rock 
  particles 
  

   vibrate 
  at 
  right 
  angles 
  to 
  the 
  direction 
  of 
  propagation. 
  Waves 
  of 
  the 
  

   second 
  type, 
  called 
  transverse 
  waves, 
  are 
  analogous 
  to 
  waves 
  of 
  

   light. 
  Unlike 
  the 
  latter, 
  however, 
  the 
  transverse 
  seismic 
  waves 
  are 
  

   propagated 
  in 
  solids 
  only 
  and 
  can 
  not 
  pass 
  through 
  a 
  liquid 
  or 
  gas. 
  

  

  These 
  two 
  kinds 
  of 
  waves, 
  longitudinal 
  and 
  transverse, 
  each 
  radiat- 
  

   ing 
  from 
  the 
  center 
  of 
  shock, 
  correspond 
  after 
  a 
  fashion 
  to 
  the 
  X 
  rays 
  

   used 
  by 
  the 
  surgeon 
  for 
  exploring 
  the 
  deep 
  inside 
  of 
  the 
  human 
  body. 
  

   Similarly, 
  the 
  deep 
  inside 
  of 
  the 
  earth 
  is 
  being 
  explored 
  with 
  the 
  two 
  

   kinds 
  of 
  seismic 
  (earthquake) 
  waves, 
  waves 
  whose 
  diverging 
  paths, 
  or 
  

   "rays, 
  "plunge 
  right 
  down 
  into 
  the 
  vast 
  interior 
  of 
  the 
  globe 
  and 
  

   emerge, 
  with 
  their 
  messages, 
  thousands 
  of 
  miles 
  from 
  the 
  center 
  of 
  

   shock. 
  The 
  longitudinal 
  waves 
  emerge 
  even 
  at 
  the 
  antipodes. 
  

  

  A 
  major 
  earthquake 
  has 
  enormous 
  energy. 
  At 
  and 
  near 
  the 
  center 
  

   of 
  shock 
  it 
  shatters 
  the 
  works 
  of 
  man 
  and 
  may 
  rupture 
  the 
  very 
  hills 
  

   and 
  mountain 
  sides. 
  As 
  each 
  wave 
  front 
  spreads 
  into 
  the 
  earth, 
  the 
  

   intensity 
  of 
  the 
  vibration 
  falls 
  very 
  rapidly, 
  so 
  that 
  not 
  many 
  hun- 
  

   dreds 
  of 
  miles 
  from 
  the 
  center 
  the 
  heaviest 
  shock 
  can 
  not 
  be 
  felt 
  by 
  a 
  

   human 
  being. 
  Much 
  less 
  can 
  he, 
  at 
  the 
  "other 
  side" 
  of 
  the 
  globe, 
  

   feel 
  the 
  impact 
  of 
  a 
  wave 
  which 
  has 
  plunged 
  to 
  a 
  depth 
  of 
  a 
  thousand 
  

   miles 
  or 
  more 
  and 
  emerges 
  under 
  his 
  feet. 
  

  

  In 
  order 
  to 
  watch 
  and 
  time 
  accurately 
  each 
  wave, 
  as 
  its 
  ray 
  emerges 
  

   on 
  the 
  "other 
  side," 
  highly 
  sensitive 
  instruments 
  are 
  used. 
  These 
  

   wonderful 
  instruments, 
  called 
  seismographs, 
  magnify 
  the 
  motion 
  of 
  the 
  

  

  