﻿266 
  ANNUAL 
  EEPORT 
  SMITHSONIAN 
  INSTITUTION, 
  192 
  9 
  

  

  have 
  been 
  reported 
  at 
  depths 
  of 
  about 
  75 
  miles, 
  250 
  miles, 
  750 
  miles, 
  

   and 
  1,100 
  miles. 
  All 
  of 
  these 
  four 
  breaks 
  require 
  further 
  study. 
  

   Their 
  estimated 
  depths 
  may 
  be 
  somewhat 
  changed, 
  and 
  other 
  dis- 
  

   continuities 
  may 
  be 
  discovered, 
  but 
  it 
  is 
  already 
  clear 
  in 
  a 
  general 
  

   way 
  how 
  the 
  earth 
  is 
  constituted 
  — 
  layer 
  on 
  layer. 
  There 
  is 
  good 
  

   evidence 
  that 
  the 
  core 
  and 
  layers 
  described 
  are 
  composed 
  of 
  matter 
  

   which 
  increases 
  in 
  density 
  as 
  the 
  depth 
  increases. 
  Hence, 
  so 
  far 
  as 
  

   the 
  great 
  body 
  of 
  the 
  earth 
  is 
  concerned, 
  it 
  is 
  built 
  stable, 
  and 
  convective 
  

   overturn 
  with 
  catastrophe 
  to 
  life 
  seems 
  impossible. 
  

  

  The 
  velocity 
  of 
  the 
  longitudinal 
  wave 
  in 
  the 
  earth's 
  core 
  has 
  been 
  

   measured. 
  The 
  value 
  obtained 
  is 
  appropriate 
  to 
  that 
  of 
  the 
  metallic 
  

   iron 
  of 
  the 
  meteorites. 
  However, 
  the 
  velocity 
  is 
  lower 
  than 
  that 
  

   expected 
  if 
  the 
  core 
  iron 
  were 
  crystalline 
  and 
  solid, 
  like 
  the 
  iron 
  of 
  our 
  

   museum 
  meteorites. 
  The 
  velocity 
  of 
  the 
  longitudinal 
  wave 
  suggests, 
  

   rather, 
  that 
  the 
  core 
  iron 
  is 
  fluid. 
  In 
  agreement 
  with 
  this 
  conclusion, 
  

   the 
  slower, 
  transverse 
  wave 
  seems 
  not 
  to 
  be 
  propagated 
  through 
  the 
  

   core; 
  we 
  have 
  learned 
  that 
  the 
  transverse 
  wave 
  can 
  not 
  persist 
  in 
  a 
  

   fluid. 
  If 
  further 
  research 
  corroborates 
  this 
  tentative 
  deduction 
  by 
  

   seismologists, 
  a 
  whole 
  set 
  of 
  new, 
  fascinating 
  problems 
  is 
  opened 
  up. 
  

  

  One 
  question 
  is 
  that 
  of 
  temperature. 
  The 
  pressure 
  on 
  the 
  core 
  iron 
  

   ranges 
  from 
  15,000,000 
  to 
  50,000,000 
  pounds 
  to 
  the 
  square 
  inch. 
  

   Under 
  such 
  colossal 
  pressures 
  the 
  iron 
  can 
  be 
  fluid 
  only 
  on 
  the 
  con- 
  

   dition 
  that 
  the 
  temperature 
  of 
  the 
  core 
  is 
  enormously 
  high 
  — 
  at 
  tens 
  of 
  

   thousands 
  of 
  degrees 
  centigrade. 
  Both 
  pressure 
  and 
  temperature 
  are 
  

   far 
  beyond 
  the 
  range 
  of 
  the 
  experimental 
  laboratory. 
  The 
  physical 
  

   state 
  of 
  the 
  core 
  iron 
  can 
  not 
  as 
  yet 
  be 
  described. 
  Is 
  it 
  a 
  liquid, 
  a 
  gas, 
  

   or 
  iron 
  in 
  a 
  ''state" 
  unknown 
  to 
  physics? 
  The 
  conditions 
  of 
  the 
  

   earth's 
  core 
  are 
  starlike. 
  From 
  their 
  study 
  can 
  physicists 
  of 
  the 
  

   future 
  tell 
  us 
  something 
  more 
  of 
  the 
  true 
  nature 
  of 
  the 
  stars? 
  If 
  they 
  

   can, 
  they 
  will 
  be 
  pretty 
  sure, 
  incidentally, 
  to 
  shed 
  new 
  light 
  on 
  the 
  

   structure 
  and 
  life 
  story 
  of 
  the 
  atoms; 
  for 
  the 
  secret 
  of 
  the 
  star 
  and 
  the 
  

   secret 
  of 
  the 
  atom 
  are 
  proving 
  to 
  be 
  part 
  of 
  a 
  single 
  problem, 
  the 
  ulti- 
  

   mate 
  nature 
  of 
  matter. 
  

  

  Again, 
  if 
  the 
  core 
  is 
  fluid, 
  it 
  is 
  infinitely 
  weak. 
  It 
  can 
  offer 
  no 
  

   permanent 
  resistance 
  to 
  forces 
  which 
  tend 
  to 
  distort 
  the 
  earth's 
  body. 
  

   Hence 
  other 
  questions 
  for 
  future 
  research: 
  Is 
  this 
  mobility 
  of 
  the 
  

   earth's 
  core 
  important 
  in 
  the 
  explanation 
  of 
  the 
  slow 
  upheavings 
  and 
  

   down-sinkings 
  of 
  great 
  areas 
  of 
  the 
  earth's 
  ''crust"? 
  Is 
  the 
  sensitive 
  

   core 
  involved 
  even 
  in 
  the 
  tumult 
  of 
  mountain 
  building? 
  No 
  one 
  can 
  

   now 
  tell, 
  but 
  speculate 
  we 
  must, 
  for 
  it 
  is 
  to-day's 
  speculation 
  that 
  

   leads 
  to 
  to-morrow's 
  science. 
  

  

  The 
  exact 
  nature 
  of 
  the 
  earth 
  shells 
  overlying 
  the 
  core 
  and 
  totaling 
  

   1,500 
  miles 
  in 
  thickness, 
  is 
  another 
  problem 
  for 
  the 
  future. 
  Pre- 
  

   sumably, 
  the 
  deepest 
  of 
  these 
  shells 
  is 
  a 
  more 
  rigid, 
  because 
  cooler, 
  

   chemical 
  equivalent 
  of 
  the 
  "fluid" 
  core, 
  but 
  it 
  is 
  not 
  yet 
  clear 
  how 
  

  

  