﻿474 
  Mr. 
  F. 
  W. 
  Very 
  on 
  the 
  Greenhouse 
  

  

  be 
  further 
  increased 
  in 
  a 
  summation 
  for 
  the 
  entire 
  exposed 
  

   hemisphere, 
  becoming 
  0*690. 
  This 
  value 
  is 
  too 
  small, 
  because 
  

   the 
  reflexion 
  from 
  dust 
  is 
  to 
  a 
  certain 
  extent 
  independent 
  of 
  

   the 
  air 
  mass 
  ; 
  but 
  it 
  represents 
  fairly 
  well 
  the 
  reflective 
  

   depletion 
  for 
  the 
  entire 
  spectrum. 
  

  

  Although 
  the 
  above 
  is 
  only 
  a 
  single 
  observation 
  which 
  

   would 
  have 
  to 
  be 
  repeated 
  on 
  many 
  different 
  occasions 
  to 
  

   give 
  a 
  thoroughly 
  reliable 
  result, 
  it 
  is 
  at 
  least 
  something- 
  

   better 
  than 
  a 
  guess. 
  To 
  this 
  depletion 
  must 
  be 
  added 
  the 
  band 
  

   absorption 
  which 
  chiefly 
  affects 
  the 
  infra-red 
  spectrum, 
  and 
  

   may 
  be 
  taken 
  for 
  moderate 
  altitudes 
  above 
  the 
  horizon 
  in 
  

   temperate 
  latitudes 
  as 
  0*3 
  x 
  2/3 
  = 
  0*2, 
  but 
  which 
  is 
  probably 
  

   twice 
  as 
  great 
  for 
  the 
  entire 
  earth 
  with 
  its 
  moist 
  tropics. 
  

  

  The 
  radiation 
  received 
  from 
  a 
  zenith 
  sun 
  is 
  (A— 
  B) 
  xt 
  r 
  , 
  

   where 
  A 
  is 
  the 
  solar 
  constant, 
  and 
  B 
  is 
  the 
  band 
  absorption. 
  

   If 
  A 
  is 
  3 
  calories 
  per 
  square 
  centimetre 
  per 
  minute, 
  

  

  (A-BX. 
  = 
  (3*0-0*6)x0-56 
  = 
  l*34cal., 
  

  

  a 
  quantity 
  which 
  is 
  not 
  often 
  exceeded 
  at 
  sea-level. 
  

  

  In 
  regard 
  to 
  selective, 
  reflective 
  depletion 
  of 
  the 
  solar 
  rays, 
  

   it 
  is 
  clear 
  that 
  radiation 
  of 
  every 
  wave-length 
  undergoes 
  more 
  

   or 
  less 
  superficial 
  reflexion 
  at 
  the 
  outside 
  surface 
  of 
  suspended 
  

   particles 
  of 
  dust, 
  ice 
  crystals, 
  &c. 
  This 
  differs 
  in 
  no 
  wise 
  

   from 
  the 
  reflexion 
  at 
  the 
  surface 
  of 
  a 
  mirror, 
  although 
  if 
  the 
  

   particles 
  be 
  many-faceted, 
  the 
  rays 
  will 
  be 
  reflected 
  in 
  many 
  

   directions 
  and 
  may 
  not 
  be 
  distinguishable 
  from 
  the 
  diffuse 
  

   reflexion 
  from 
  internal 
  bubbles 
  or 
  other 
  discontinuities 
  in 
  a 
  

   transparent 
  bat 
  turbid 
  medium. 
  This 
  surface 
  reflexion 
  is 
  a 
  

   general 
  one, 
  or 
  in 
  the 
  visible 
  spectrum 
  we 
  should 
  call 
  it 
  a 
  

   colourless 
  one. 
  If 
  the 
  rays 
  penetrate 
  a 
  little 
  way 
  into 
  the 
  sub- 
  

   stance 
  before 
  being 
  reflected, 
  certain 
  rays 
  may 
  be 
  absorbed, 
  

   giving 
  selective 
  reflexion, 
  which 
  in 
  the 
  visible 
  spectrum 
  we 
  

   call 
  coloured 
  reflexion. 
  Melloni 
  coined 
  a 
  special 
  word 
  for 
  the 
  

   selection 
  of 
  invisible 
  rays, 
  labelling 
  it 
  " 
  thermochrose," 
  but 
  

   the 
  distinction 
  is 
  not 
  needed. 
  Finally, 
  molecules 
  and 
  particles 
  

   whose 
  dimensions 
  are 
  small 
  relatively 
  to 
  the 
  wave-length 
  

   give 
  selective 
  diffraction 
  which 
  varies 
  progressively 
  with 
  the 
  

   wave-length. 
  I 
  have 
  used 
  the 
  word 
  depletion 
  to 
  cover 
  losses 
  

   by 
  all 
  of 
  these 
  processes 
  without 
  individual 
  specification. 
  Any 
  

   part 
  of 
  the 
  solar 
  radiation 
  which 
  is 
  reflected 
  back 
  into 
  space 
  

   can 
  have 
  no 
  effect 
  on 
  planetary 
  temperature. 
  This 
  precept 
  is 
  

   so 
  obvious 
  that 
  it 
  seems 
  strange 
  that 
  it 
  should 
  be 
  overlooked. 
  

  

  In 
  considering 
  the 
  effect 
  on 
  climate 
  it 
  is 
  not 
  so 
  easy 
  to 
  

   decide 
  how 
  we 
  ought 
  to 
  treat 
  the 
  band 
  absorption. 
  So 
  far 
  as 
  

   absorption 
  occurs 
  in 
  the 
  upper 
  air, 
  it 
  raises 
  its 
  temperature, 
  

   and 
  this 
  the 
  more 
  on 
  account 
  of 
  the 
  low 
  density 
  of 
  the 
  air 
  at 
  

  

  