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  ANN^UAL 
  EEPOET 
  SMITHSONIAN 
  INSTITUTION, 
  1927 
  

  

  modifying 
  of 
  view 
  necessitated 
  by 
  the 
  discovery 
  of 
  new 
  facts, 
  and 
  

   reveals 
  the 
  steps 
  by 
  which 
  we 
  have 
  so 
  gradually 
  and 
  laboriously 
  

   advanced 
  from 
  the 
  known 
  into 
  the 
  unlmown. 
  

  

  In 
  this 
  manner 
  we 
  should 
  learn 
  that 
  more 
  than 
  half 
  a 
  century 
  ago 
  

   hypothetical 
  explanations 
  of 
  climatic 
  change 
  were 
  put 
  forward 
  which, 
  

   essentially 
  and 
  in 
  principle, 
  Avere 
  almost 
  exhaustive 
  of 
  the 
  known 
  

   possibilities. 
  Sir 
  Charles 
  Lyell 
  sought 
  the 
  explanation 
  in 
  wide- 
  

   spread 
  and 
  radical 
  changes 
  in 
  the 
  distribution 
  of 
  land 
  and 
  sea. 
  

   Taking 
  the 
  present 
  continents, 
  he 
  showed 
  that, 
  without 
  any 
  alteration 
  

   of 
  size, 
  shape, 
  or 
  altitude, 
  great 
  climatic 
  differences 
  could 
  be 
  brought 
  

   about 
  by 
  grouping 
  these 
  land 
  masses, 
  first, 
  as 
  closely 
  as 
  possible 
  

   around 
  the 
  Equator, 
  and 
  subsequently 
  around 
  tlie 
  poles. 
  The 
  Vienna 
  

   geologists, 
  supported 
  by 
  the 
  eminent 
  astronomer, 
  Father 
  Secchi, 
  

   maintained 
  that 
  the 
  earth's 
  axis, 
  and 
  with 
  it 
  the 
  poles, 
  had 
  been 
  

   shifted, 
  bringing 
  certain 
  regions 
  which 
  were 
  formerly 
  Arctic 
  into 
  

   the 
  Temperate 
  Zone, 
  and 
  thus 
  changing 
  their 
  climate 
  very 
  completely. 
  

  

  Dr. 
  James 
  CroU, 
  of 
  the 
  Scottish 
  Geological 
  Survey, 
  published 
  his 
  

   Climate 
  and 
  Time 
  in 
  1875, 
  a 
  book 
  which 
  speedily 
  became 
  famous. 
  

   Croll 
  called 
  attention 
  to 
  the 
  fact 
  that 
  the 
  size 
  and 
  shape 
  of 
  the 
  earth's 
  

   orbit 
  were 
  not 
  constant, 
  but 
  subject 
  to 
  change, 
  which 
  resulted 
  in 
  

   periods 
  of 
  maximum 
  and 
  minimum 
  eccentricity. 
  He 
  contended 
  that 
  

   the 
  hemisphere 
  which 
  had 
  its 
  winter 
  in 
  aphelion 
  during 
  a 
  time 
  of 
  

   maximum 
  eccentricity 
  would 
  pass 
  through 
  a 
  period 
  of 
  glaciation. 
  

   On 
  this 
  hypothesis 
  glacial 
  periods 
  would 
  recur 
  alternatingly 
  be- 
  

   tween 
  the 
  Northern 
  and 
  Southern 
  Hemispheres 
  and 
  rhythmically 
  

   cver}^ 
  12,500 
  years. 
  

  

  I 
  have 
  not 
  been 
  able 
  to 
  learn 
  who 
  it 
  was 
  that 
  first 
  suggested 
  

   the 
  internal 
  heat 
  of 
  the 
  earth 
  as 
  the 
  cause 
  of 
  the 
  former 
  mild 
  and 
  

   uniform 
  climates, 
  and 
  the 
  gradual 
  loss 
  of 
  the 
  earth's 
  heat 
  by 
  radiation 
  

   as 
  having 
  brought 
  about 
  the 
  refrigeration 
  of 
  climate 
  which 
  has 
  led 
  

   to 
  the 
  present 
  order 
  of 
  things. 
  This 
  conception 
  is 
  nearly 
  as 
  old 
  as 
  

   geology 
  or 
  the 
  nebular 
  hypothesis 
  of 
  Laplace. 
  In 
  modified 
  form 
  

   it 
  has 
  lately 
  been 
  revived 
  by 
  Doctor 
  Knowlton. 
  

  

  Finally, 
  I 
  may 
  mention 
  Lord 
  Kelvin's^ 
  suggestion 
  that 
  the 
  cause 
  

   of 
  climatic 
  changes 
  on 
  the 
  earth 
  should 
  be 
  sought 
  in 
  fluctuations 
  of 
  

   the 
  sun's 
  activity, 
  a 
  suggestion 
  which 
  seemed 
  to 
  be 
  made 
  obvious 
  by 
  

   the 
  connection 
  between 
  the 
  weather 
  and 
  the 
  maxima 
  and 
  minima 
  of 
  

   the 
  sun-spot 
  periods. 
  Among 
  geologists 
  it 
  is, 
  perhaps. 
  Professor 
  

   Penck 
  who 
  has 
  most 
  strongly 
  championed 
  this 
  view, 
  for 
  it 
  is 
  he 
  who 
  

   has 
  most 
  clearly 
  brought 
  out 
  the 
  universal 
  nature 
  of 
  the 
  climatic 
  

   changes 
  in 
  the 
  Pleistocene. 
  

  

  ^The 
  hypothesis 
  of 
  solar 
  change 
  as 
  causing 
  climatic 
  changes 
  on 
  the 
  earth 
  was 
  adopted 
  

   by 
  Penck. 
  Prof. 
  H. 
  P. 
  Reid 
  kindly 
  pointed 
  out 
  tO' 
  me 
  the 
  fact 
  that 
  the 
  suggestion 
  was 
  

   originally 
  due 
  to 
  Lord 
  Kelvin. 
  

  

  