﻿Theory 
  and 
  Planetary 
  Temperatures. 
  477 
  

  

  Ii 
  is 
  true 
  that 
  Professor 
  Lowell 
  does 
  not 
  consider 
  the 
  green- 
  

   house 
  effect 
  analytically 
  and 
  obviously, 
  but 
  it 
  is 
  nevertheless 
  

   implicitly 
  contained 
  in 
  his 
  deduction 
  o£ 
  the 
  heat 
  retained, 
  

   obtained 
  by 
  the 
  method 
  o£ 
  day 
  and 
  night 
  averages. 
  The 
  

   method 
  does 
  not 
  specify 
  whether 
  the 
  heat 
  is 
  lost 
  by 
  radiation 
  

   •or 
  by 
  some 
  more 
  circuitous 
  process 
  ; 
  and 
  thus 
  it 
  would 
  not 
  

   be 
  precise 
  to 
  label 
  the 
  retaining 
  power 
  of 
  the 
  atmosphere 
  a 
  

   " 
  greenhouse 
  effect 
  " 
  without 
  giving 
  a 
  somewhat 
  wider 
  inter- 
  

   pretation 
  to 
  this 
  name. 
  If 
  it 
  be 
  permitted 
  to 
  extend 
  the 
  

   meaning 
  of 
  the 
  term 
  to 
  cover 
  a 
  variety 
  of 
  processes 
  which 
  

   lead 
  to 
  identical 
  results, 
  the 
  deduction 
  of 
  the 
  loss 
  of 
  surface 
  

   heat 
  by 
  comparison 
  of 
  day 
  and 
  night 
  temperatures 
  is 
  directly 
  

   concerned 
  with 
  this 
  wider 
  " 
  greenhouse 
  effect/" 
  The 
  method 
  

   employed 
  by 
  Professor 
  Lowell 
  is 
  a 
  good 
  one, 
  for 
  it 
  dispenses 
  

   with 
  a 
  detailed 
  knowledge 
  which 
  we 
  may 
  hope 
  to 
  possess 
  

   eventually, 
  but 
  which, 
  when 
  found, 
  cannot 
  be 
  incompatible 
  

   with 
  the 
  conclusions 
  deduced 
  by 
  the 
  less 
  circumstantial 
  

   method. 
  Thus 
  the 
  objection 
  which 
  Professor 
  Poynting 
  makes 
  

   to 
  the 
  method 
  where 
  he 
  says 
  that 
  Lowell 
  " 
  neglects 
  both 
  

   the 
  surface 
  radiation 
  reflected 
  down 
  again 
  and 
  the 
  radia- 
  

   tion 
  downwards 
  of 
  the 
  energy 
  absorbed 
  by 
  the 
  atmosphere," 
  

   seems 
  to 
  me 
  to 
  be, 
  on 
  the 
  contrary, 
  one 
  of 
  the 
  strong 
  points 
  

   in 
  its 
  favour 
  ; 
  because 
  for 
  its 
  use 
  it 
  is 
  not 
  necessary 
  that 
  we 
  

   should 
  know 
  these 
  details. 
  The 
  transmission 
  by 
  cloud 
  for 
  

   solar 
  and 
  terrestrial 
  radiation 
  may 
  not 
  be 
  the 
  same 
  ; 
  but 
  if 
  a 
  

   complex 
  of 
  radiant 
  and 
  convectional 
  processes 
  which 
  cannot 
  

   be 
  represented 
  by 
  so 
  simple 
  a 
  formula 
  as 
  that 
  of 
  Arrhenius, 
  

   gives 
  a 
  virtual 
  equality 
  of 
  depletions 
  under 
  cloudy 
  or 
  clear 
  

   sky, 
  which 
  can 
  be 
  represented 
  by 
  the 
  statement 
  that, 
  

   though 
  less 
  heat 
  is 
  received 
  under 
  cloud, 
  less 
  is 
  also 
  lost 
  and 
  

   in 
  about 
  the 
  same 
  proportion, 
  the 
  precise 
  part 
  assigned 
  to 
  

   each 
  fraction 
  of 
  the 
  process 
  does 
  not 
  affect 
  the 
  argument. 
  

  

  Reverting 
  to 
  my 
  simile 
  of 
  the 
  bundle 
  of 
  mirrors, 
  trans- 
  

   mitting 
  some 
  rays 
  and 
  perfectly 
  reflecting 
  others, 
  may 
  it 
  not 
  

   be 
  possible 
  to 
  realize 
  it 
  ? 
  It 
  is 
  known 
  that 
  there 
  are 
  some 
  

   transparent 
  substances, 
  such 
  as 
  quartz, 
  which 
  have 
  bands 
  of 
  

   metallic 
  reflexion 
  in 
  the 
  infra-red 
  spectrum. 
  If 
  there 
  were 
  

   gases 
  in 
  which 
  such 
  bands 
  were 
  sufficiently 
  numerous 
  in 
  the 
  

   infra-red 
  without 
  too 
  seriously 
  encroaching 
  upon 
  the 
  visible 
  

   part 
  of 
  the 
  spectrum, 
  it 
  would 
  be 
  possible 
  to 
  obtain 
  almost 
  any 
  

   increment 
  of 
  temperature 
  at 
  the 
  surface 
  of 
  a 
  planet 
  covered 
  

   by 
  such 
  selectively 
  reflecting 
  gases. 
  

  

  The 
  major 
  planets 
  have 
  atmospheres 
  containing 
  unknown 
  

   and 
  highly 
  absorbent 
  gases 
  which 
  produce 
  bands 
  in 
  the 
  visible 
  

   spectrum, 
  increasing 
  in 
  strength 
  with 
  the 
  distances 
  of 
  the 
  

   planets 
  from 
  the 
  sun. 
  These 
  bands 
  invade 
  the 
  red 
  end 
  of 
  

  

  