﻿X-KAYING 
  THE 
  EARTH 
  DALY 
  265 
  

  

  the 
  world 
  supply. 
  With 
  electrical, 
  magnetic, 
  and 
  gravitational 
  

   methods 
  — 
  all 
  products 
  of 
  the 
  "unpractical" 
  man 
  of 
  "pure" 
  science 
  — 
  

   valuable 
  indications 
  of 
  hidden 
  metal-bearing 
  ores 
  are 
  secured, 
  and 
  

   the 
  expense 
  of 
  exploration 
  by 
  bore 
  holes 
  and 
  shafts 
  is 
  greatly 
  reduced. 
  

   Seismological 
  methods 
  promise 
  to 
  be 
  adaptable 
  to 
  this 
  kind 
  of 
  detec- 
  

   tive 
  work. 
  Conquering 
  the 
  difficulties 
  that 
  still 
  remain, 
  future 
  

   research 
  should 
  make 
  this 
  branch 
  of 
  geophysics, 
  even 
  in 
  the 
  search 
  

   after 
  metals, 
  pay 
  for 
  its 
  upkeep 
  many 
  times 
  over. 
  

  

  The 
  depths 
  of 
  the 
  ocean 
  are 
  now 
  being 
  quickly 
  and 
  accurately 
  

   measured 
  by 
  the 
  echo 
  of 
  sound 
  waves 
  from 
  the 
  bottom 
  of 
  the 
  sea. 
  

   This 
  method, 
  incomparably 
  more 
  rapid 
  and 
  less 
  expensive 
  than 
  the 
  

   old 
  one 
  by 
  sounding 
  line, 
  is 
  based 
  on 
  a 
  principle 
  fundamental 
  in 
  

   seismology. 
  With 
  variation 
  of 
  detail, 
  "sonic" 
  sounding, 
  the 
  use 
  of 
  

   waves 
  reflected 
  from 
  underlying 
  rock, 
  is 
  employed 
  to 
  measure 
  the 
  

   thicloiess 
  of 
  glaciers. 
  

  

  Thus, 
  the 
  Hintereisferner 
  glacier 
  of 
  the 
  Alps 
  has 
  recently 
  been 
  

   proved 
  to 
  be 
  830 
  feet 
  thick 
  in 
  the 
  middle. 
  When, 
  with 
  the 
  similar 
  

   use 
  of 
  explosion 
  shocks 
  and 
  the 
  seismograph, 
  the 
  thickness 
  of 
  the 
  

   Antarctic 
  and 
  Greenland 
  ice 
  caps 
  are 
  measured, 
  we 
  shall 
  have 
  precious 
  

   data 
  for 
  guiding 
  thought 
  on 
  the 
  conditions 
  of 
  North 
  America 
  and 
  

   Europe 
  during 
  the 
  glacial 
  period. 
  Furthermore, 
  we 
  could 
  then 
  esti- 
  

   mate 
  how 
  far 
  the 
  sea 
  level 
  was 
  everywhere 
  lowered 
  when 
  the 
  water 
  

   of 
  these 
  ice 
  caps 
  was 
  abstracted 
  from 
  the 
  ocean 
  and 
  piled 
  up, 
  solid, 
  

   on 
  the 
  land. 
  

  

  But 
  from 
  depths 
  far 
  greater 
  than 
  glacier 
  floor, 
  ocean 
  floor, 
  or 
  

   mineral 
  deposit, 
  come 
  the 
  messages 
  from 
  nature's 
  earthquakes. 
  

   A 
  few 
  illustrations 
  of 
  success 
  in 
  detecting 
  the 
  anatomy 
  of 
  a 
  planet 
  

   will 
  show 
  the 
  real 
  majesty 
  of 
  the 
  questions 
  and 
  answers 
  that 
  already 
  

   inspire 
  the 
  all-too-few 
  workers 
  in 
  the 
  new 
  science 
  of 
  geophysics. 
  

  

  One 
  of 
  the 
  outstanding 
  seismological 
  discoveries 
  of 
  recent 
  years 
  is 
  

   the 
  shelled 
  character 
  of 
  our 
  planet. 
  At 
  the 
  center, 
  and 
  outward 
  to 
  

   a 
  little 
  more 
  than 
  one-half 
  of 
  its 
  radius, 
  the 
  earth 
  is 
  homogeneous 
  in 
  

   high 
  degree. 
  This 
  so-called 
  "core" 
  is 
  surrounded 
  by 
  successive 
  shells 
  

   or 
  layers 
  of 
  material. 
  Each 
  shell, 
  out 
  to 
  a 
  level 
  about 
  30 
  miles 
  from 
  

   the 
  surface, 
  is 
  relatively 
  homogeneous, 
  and 
  its 
  material 
  differs 
  from 
  

   that 
  of 
  the 
  shell 
  above 
  or 
  below, 
  as 
  well 
  as 
  from 
  the 
  material 
  of 
  the 
  

   central 
  core. 
  The 
  contacts 
  between 
  the 
  shells 
  and 
  between 
  the 
  deep- 
  

   est 
  shell 
  and 
  the 
  core 
  are 
  technically 
  called 
  "discontinuities." 
  

  

  The 
  discontinuity, 
  or 
  break 
  of 
  material, 
  at 
  the 
  surface 
  of 
  the 
  core 
  

   is 
  one 
  of 
  the 
  most 
  remarkable 
  of 
  all. 
  It 
  is 
  located 
  at 
  a 
  level 
  about 
  

   1,500 
  miles 
  below 
  the 
  earth's 
  surface, 
  nearly 
  2,500 
  miles 
  from 
  its 
  

   center. 
  A 
  second 
  principal 
  break, 
  found 
  only 
  under 
  the 
  continents 
  

   and 
  larger 
  islands 
  — 
  and 
  thus 
  representing 
  only 
  parts 
  of 
  a 
  complete 
  

   earth 
  shell 
  — 
  is 
  situated 
  at 
  the 
  average 
  depth 
  of 
  about 
  30 
  miles. 
  

   Other 
  discontinuities, 
  limiting 
  complete 
  shells 
  of 
  the 
  earth's 
  body 
  

  

  