﻿426 
  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1929 
  

  

  of 
  the 
  Himalayas), 
  proves 
  that 
  a 
  great 
  change 
  took 
  place 
  in 
  the 
  early 
  

   centuries 
  of 
  the 
  Christian 
  era. 
  

  

  The 
  only 
  hypothesis 
  which 
  will 
  fit 
  all 
  the 
  facts 
  is 
  that 
  of 
  a 
  change 
  

   of 
  chmate 
  in 
  the 
  direction 
  of 
  greater 
  aridity 
  throughout 
  these 
  regions. 
  

   Except 
  in 
  Kashmir 
  the 
  change 
  brought 
  disaster. 
  Scores 
  of 
  once 
  

   prosperous 
  oases 
  were 
  abandoned 
  for 
  lack 
  of 
  water. 
  The 
  inhabitants 
  

   were 
  driven 
  away 
  in 
  waves 
  of 
  migration 
  to 
  confound 
  the 
  civilized 
  world. 
  

  

  A 
  rainfall 
  of 
  20 
  inches 
  a 
  year 
  in 
  Austraha 
  has 
  been 
  estimated 
  to 
  

   make 
  it 
  possible 
  to 
  keep 
  bOO 
  sheep 
  to 
  the 
  square 
  mile. 
  A 
  drop 
  to 
  13 
  

   inches 
  reduces 
  the 
  number 
  to 
  100; 
  and 
  10 
  inches 
  is 
  sufficient 
  for 
  only 
  

   10 
  sheep. 
  A 
  gradual 
  decrease 
  of 
  rainfall 
  in 
  the 
  steppes 
  of 
  Asia 
  would 
  

   naturally 
  lead 
  to 
  migrations 
  of 
  pastoral 
  nomads 
  from 
  the 
  drier 
  regions 
  

   to 
  those 
  which 
  offer 
  pasturage 
  for 
  their 
  flocks 
  and 
  herds. 
  As 
  the 
  

   steppes 
  became 
  drier 
  northern 
  and 
  central 
  Europe 
  were, 
  after 
  a 
  long 
  

   period 
  of 
  blighting 
  cold, 
  becoming 
  warmer 
  and 
  more 
  and 
  more 
  habita- 
  

   ble. 
  History 
  records 
  the 
  coming 
  of 
  horde 
  after 
  horde. 
  Nothing 
  

   could 
  stay 
  them. 
  Rome 
  and 
  Roman 
  civilization 
  fell 
  before 
  them. 
  

  

  Before 
  we 
  can 
  properly 
  estimate 
  the 
  influence 
  of 
  climatic 
  changes 
  

   upon 
  history, 
  it 
  is 
  necessary 
  to 
  investigate 
  the 
  nature 
  of 
  those 
  changes, 
  

   both 
  as 
  to 
  duration 
  and 
  origin, 
  as 
  w^ell 
  as 
  to 
  determine 
  the 
  reasons 
  for 
  

   supposing 
  that 
  climate 
  varies 
  uniformly 
  over 
  wide 
  areas. 
  

  

  Scientists 
  have 
  recognized 
  tw^o 
  chief 
  types 
  of 
  climatic 
  change. 
  

   The 
  first 
  is 
  that 
  of 
  the 
  glacial 
  periods; 
  the 
  second 
  is 
  that 
  of 
  the 
  36-year 
  

   cycle 
  discussed 
  by 
  Bruckner, 
  Clough, 
  and 
  others. 
  There 
  is 
  good 
  

   reason 
  to 
  believe 
  that 
  the 
  latter 
  is 
  applicable 
  to 
  the 
  whole 
  surface 
  of 
  

   the 
  globe. 
  During 
  a 
  cycle 
  there 
  are 
  two 
  extremes, 
  at 
  one 
  of 
  which 
  the 
  

   climate 
  of 
  continental 
  regions 
  is 
  to 
  a 
  greater 
  extent 
  cool 
  and 
  wet, 
  with 
  

   a 
  lower 
  barometric 
  pressure 
  and 
  relatively 
  frequent 
  storms 
  for 
  a 
  series 
  

   of 
  years; 
  at 
  the 
  other 
  it 
  is 
  comparatively 
  warm 
  and 
  dry, 
  with 
  higher 
  

   pressure 
  and 
  fewer 
  storms. 
  These 
  phenomena 
  are 
  most 
  pronounced 
  

   in 
  mid-continental 
  regions, 
  decline 
  towards 
  the 
  coasts, 
  and 
  are 
  

   sometimes 
  reversed 
  in 
  maritime 
  regions. 
  The 
  extremes 
  of 
  low 
  

   temperature 
  follow 
  periods 
  of 
  maximum 
  solar 
  activity 
  as 
  indicated 
  

   by 
  the 
  number 
  of 
  sun 
  spots 
  and 
  the 
  rapidity 
  with 
  which 
  they 
  are 
  

   formed. 
  The 
  periods 
  of 
  heaviest 
  rainfall 
  follow 
  those 
  of 
  lowest 
  

   temperature 
  at 
  intervals 
  of 
  a 
  few 
  years. 
  The 
  other 
  extremes 
  are 
  

   characterized 
  b\^ 
  diminished 
  solar 
  activity 
  followed 
  by 
  higher 
  tempera- 
  

   tures 
  and, 
  a 
  little 
  later, 
  by 
  scarcity 
  of 
  rainfall. 
  The 
  cycles 
  have 
  been 
  

   traced 
  back 
  by 
  Clough 
  to 
  about 
  300 
  a. 
  d., 
  but 
  only 
  the 
  data 
  of 
  the 
  last 
  

   century 
  can 
  be 
  accepted 
  as 
  approximately 
  accurate. 
  During 
  that 
  

   time, 
  it 
  may 
  be 
  noted, 
  neither 
  the 
  extremes 
  of 
  heat 
  nor 
  of 
  cold 
  have 
  

   shown 
  any 
  tendency 
  to 
  increase 
  in 
  intensity. 
  

  

  The 
  Bruckner 
  cycles 
  appear 
  to 
  differ 
  from 
  those 
  of 
  the 
  glacial 
  

   periods 
  only 
  in 
  degree 
  and 
  regularity. 
  The 
  effects 
  upon 
  glaciers, 
  

   rivers 
  and 
  lakes 
  are 
  of 
  precisely 
  the 
  same 
  nature 
  ; 
  and 
  the 
  distribution 
  

  

  