﻿Theory 
  and 
  Planetary 
  lemperatures. 
  475 
  

  

  great 
  altitudes. 
  Any 
  extra 
  warmth 
  in 
  the 
  higher 
  layers 
  of 
  

   the 
  atmosphere 
  must 
  have 
  some 
  effect 
  in 
  raising 
  the 
  tempe- 
  

   rature 
  of 
  lower 
  ones. 
  Heat 
  transferred 
  from 
  earth 
  to 
  air 
  is 
  

   partly 
  lost 
  as 
  radiation 
  from 
  air 
  and 
  cloud 
  before 
  being- 
  

   returned 
  to 
  earth. 
  Whether 
  the 
  loss 
  is 
  counterbalanced 
  by 
  

   heat 
  from 
  absorption 
  of 
  solar 
  rays 
  in 
  the 
  upper 
  air 
  brought 
  

   down 
  to 
  earth 
  in 
  descending 
  air-currents 
  as 
  an 
  accession 
  to 
  

   the 
  potential 
  temperature 
  which 
  is 
  represented 
  by 
  the 
  work 
  

   stored 
  up 
  in 
  a 
  previous 
  expansion, 
  it 
  might 
  be 
  difficult 
  to 
  

   determine 
  ; 
  but 
  quite 
  a 
  large 
  part 
  of 
  the 
  band 
  absorption 
  

   may 
  be 
  indirectly 
  useful 
  in 
  enhancing 
  surface 
  temperatures. 
  

   Professor 
  Lowell 
  cuts 
  the 
  Gordian 
  knot 
  by 
  an 
  ingenious 
  

   method 
  which 
  dispenses 
  with 
  a 
  knowledge 
  of 
  these 
  data. 
  

   Before 
  passing 
  to 
  it, 
  I 
  wish 
  to 
  consider 
  what 
  Professor 
  

   Poynting 
  calls 
  the 
  " 
  greenhouse 
  effect 
  " 
  in 
  planetary 
  tempe- 
  

   ratures 
  a 
  little 
  more 
  in 
  detail. 
  

  

  In 
  Poynting's 
  equation 
  for 
  the 
  atmospheric 
  greenhouse 
  

   effect,, 
  1/n 
  is 
  the 
  fraction 
  of 
  the 
  radiant 
  energy 
  absorbed 
  by 
  

   the 
  air 
  which 
  is 
  returned 
  downwards. 
  Of 
  n 
  Professor 
  

   Poynting 
  says: 
  " 
  I 
  do 
  not 
  see 
  how 
  to 
  estimate 
  the 
  distribution 
  

   of 
  the 
  radiation 
  from 
  the 
  air 
  between 
  the 
  upward 
  stream 
  into 
  

   space 
  and 
  the 
  downward 
  stream 
  to 
  the 
  surface." 
  This 
  being 
  

   so, 
  I 
  propose 
  to 
  substitute 
  values 
  for 
  the 
  other 
  factors 
  which 
  

   are 
  less 
  problematical 
  and 
  determine 
  n. 
  Following 
  Lowell, 
  

   Professor 
  Poynting 
  has 
  assumed 
  a 
  1 
  = 
  0*5, 
  a 
  = 
  0*325, 
  ^ 
  = 
  0'5, 
  

   t 
  = 
  0-4:2. 
  From 
  (1) 
  we 
  obtain 
  

  

  _ 
  ^R 
  + 
  aS 
  ,- 
  

  

  fS-fJEt-axR 
  W 
  

  

  Let 
  S 
  = 
  3 
  cal./cm. 
  2 
  min. 
  = 
  2*1 
  X 
  10 
  6 
  ergs/sec, 
  which 
  distri- 
  

   buted 
  over 
  the 
  whole 
  earth 
  becomes 
  one 
  fourth 
  of 
  this, 
  or 
  

   5*25 
  x 
  10 
  5 
  ergs/sec. 
  Taking 
  the 
  mean 
  temperature 
  of 
  the 
  

   whole 
  earth 
  as 
  6= 
  288° 
  A., 
  

  

  R 
  = 
  o-6> 
  4 
  = 
  5-32 
  x 
  10~ 
  5 
  X 
  288 
  4 
  = 
  3-66x 
  10 
  5 
  ergs/sec. 
  

  

  Substituting 
  these 
  values 
  in 
  (2), 
  »=2 
  , 
  43, 
  that 
  is, 
  less 
  

   than 
  half 
  of 
  the 
  atmospheric 
  radiation 
  comes 
  downwards 
  ; 
  

   but 
  the 
  determination 
  probably 
  has 
  little 
  or 
  no 
  significance, 
  

   because 
  the 
  storing 
  effect 
  of 
  the 
  atmosphere 
  is 
  not 
  a 
  purely 
  

   radiant 
  effect. 
  In 
  fact 
  the 
  radiation 
  from 
  the 
  atmosphere 
  

   to 
  the 
  earth 
  is 
  comparatively 
  small, 
  as 
  I 
  have 
  shown 
  in 
  my 
  

   "Atmospheric 
  Radiation." 
  Although 
  much 
  of 
  the 
  energy 
  

   of 
  terrestrial 
  radiation 
  absorbed 
  by 
  the 
  air 
  comes 
  back 
  to 
  the 
  

   ground, 
  it 
  does 
  so 
  largely 
  by 
  modes 
  other 
  than 
  radiant, 
  such 
  

   as 
  the 
  condensation 
  of 
  water- 
  vapour 
  into 
  rain. 
  The 
  green- 
  

  

  