﻿270 
  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1922. 
  

  

  Second, 
  physical 
  theory, 
  based 
  on 
  well-known 
  laws 
  such 
  as 
  those 
  of 
  

   gravitation, 
  heat, 
  etc. 
  Third, 
  pure 
  speculation. 
  When 
  we 
  attempt 
  

   to 
  apply 
  these 
  methods 
  to 
  the 
  solar 
  system 
  we 
  find 
  a 
  complete 
  ab- 
  

   sence 
  of 
  any 
  observational 
  evidence 
  from 
  the 
  first 
  point 
  of 
  view, 
  be- 
  

   cause 
  we 
  have 
  no 
  stellar 
  systems 
  sufficiently 
  near 
  for 
  us 
  to 
  detect 
  

   planets 
  if 
  such 
  exist. 
  Thus 
  evolution 
  in 
  the 
  solar 
  system 
  is 
  mainly 
  

   a 
  mixture 
  of 
  physical 
  theory 
  and 
  speculation. 
  

  

  All 
  theories 
  of 
  evolution 
  use 
  the 
  idea 
  of 
  contraction 
  under 
  gravi- 
  

   tation, 
  which 
  in 
  general 
  causes 
  a 
  gain 
  of 
  heat 
  and 
  of 
  angular 
  velocity. 
  

   The 
  chief 
  differences 
  between 
  the 
  theories 
  consist 
  in 
  the 
  forms 
  of 
  

   matter 
  which 
  are 
  assumed 
  to 
  come 
  into 
  existence 
  under 
  the 
  operation 
  

   of 
  the 
  process 
  of 
  contraction. 
  Laplace 
  imagined 
  that 
  a 
  planetary 
  

   nebula 
  contracted 
  and 
  in 
  the 
  course 
  of 
  the 
  process 
  left 
  behind 
  rings 
  

   of 
  matter 
  which 
  later 
  condensed 
  into 
  planets. 
  Roche 
  showed 
  that 
  

   under 
  certain 
  conditions 
  matter 
  will 
  be 
  thrown 
  off 
  along 
  the 
  Equator. 
  

   G. 
  H. 
  Darwin 
  and 
  Poincare 
  developed 
  the 
  processes 
  of 
  fission 
  from 
  

   which 
  it 
  was 
  hoped 
  that 
  planetary 
  bodies 
  might 
  be 
  shown 
  to 
  have 
  

   developed 
  through 
  successive 
  divisions 
  of 
  the 
  central 
  body. 
  Later 
  

   workers 
  at 
  the 
  theory, 
  and 
  particularly 
  Jeans, 
  have 
  proved 
  that 
  this 
  

   hypothesis 
  is 
  very 
  improbable 
  for 
  planetary 
  evolution 
  on 
  account 
  of 
  

   the 
  fact 
  that 
  in 
  this 
  process 
  of 
  division 
  the 
  masses 
  should 
  be 
  of 
  the 
  

   same 
  order 
  of 
  magnitude 
  and 
  not, 
  as 
  in 
  the 
  case 
  of 
  the 
  planets, 
  of 
  

   very 
  different 
  orders 
  of 
  magnitude. 
  It 
  has, 
  however, 
  been 
  applied 
  

   with 
  considerable 
  success 
  to 
  the 
  evolution 
  of 
  close 
  double 
  stars. 
  

   Finally 
  there 
  are 
  the 
  tidal 
  hypotheses 
  in 
  which 
  the 
  matter 
  is 
  supposed 
  

   to 
  have 
  been 
  drawn 
  off 
  by 
  the 
  close 
  approach 
  of 
  some 
  second 
  body 
  

   which 
  later 
  moved 
  away. 
  Each 
  of 
  these 
  hypotheses 
  has 
  many 
  objec- 
  

   tions. 
  But 
  it 
  may 
  be 
  stated 
  that 
  from 
  these 
  points 
  of 
  view 
  we 
  can 
  

   learn 
  nothing 
  definite 
  or 
  even 
  approximate 
  about 
  the 
  age 
  of 
  the 
  earth. 
  

  

  Another 
  method 
  of 
  approach 
  is 
  through 
  observation 
  of 
  the 
  present 
  

   condition 
  of 
  the 
  bodies 
  in 
  the 
  solar 
  system. 
  For 
  evidence 
  we 
  have 
  

   eight 
  major 
  planets, 
  but 
  it 
  is 
  very 
  doubtful 
  whether 
  from 
  so 
  small 
  a 
  

   number 
  we 
  can 
  deduce 
  any 
  results 
  of 
  value. 
  In 
  fact, 
  it 
  is 
  now 
  well 
  

   known 
  that 
  differences 
  in 
  mass 
  may 
  produce 
  very 
  different 
  conse- 
  

   quences 
  in 
  the 
  history 
  of 
  bodies. 
  Thus 
  arguments 
  drawn 
  from 
  the 
  

   moon, 
  Mars, 
  Venus, 
  or 
  the 
  other 
  planets 
  have 
  never 
  inspired 
  very 
  

   much 
  confidence. 
  

  

  Still 
  another 
  method 
  is 
  a 
  consideration 
  of 
  the 
  present 
  condition 
  

   of 
  the 
  earth 
  combined 
  with 
  the 
  theory 
  of 
  contraction 
  and 
  subsequent 
  

   loss 
  of 
  heat. 
  Here 
  we 
  are 
  on 
  somewhat 
  firmer 
  ground, 
  since 
  we 
  have 
  

   many 
  observations 
  which 
  give 
  information 
  concerning 
  the 
  interior 
  

   condition 
  of 
  the 
  earth. 
  Amongst 
  these 
  may 
  be 
  mentioned 
  the 
  values 
  

   of 
  the 
  mean 
  density 
  and 
  the 
  surface 
  density, 
  the 
  phenomena 
  of 
  pre- 
  

   cession, 
  nutation, 
  etc., 
  the 
  measurements 
  of 
  earthquake 
  and 
  seismic 
  

  

  