﻿464 
  Mr. 
  F. 
  W. 
  Very 
  on 
  the 
  Greenhouse 
  

  

  reflect 
  as 
  well 
  as 
  a 
  more 
  perfect 
  one. 
  In 
  the 
  present 
  case, 
  

   the 
  mirror 
  must 
  be 
  supposed 
  to 
  have 
  the 
  peculiar 
  quality 
  o£ 
  

   being 
  transparent 
  to 
  the 
  ingoing 
  radiation, 
  and 
  a 
  perfect 
  

   reflector 
  of 
  the 
  outgoing 
  rays 
  ; 
  but 
  with 
  such 
  a 
  mirror, 
  if 
  it 
  

   were 
  possible 
  to 
  obtain 
  it, 
  there 
  would 
  be 
  no 
  possibility 
  of 
  

   stopping 
  the 
  accumulation 
  of 
  heat 
  short 
  of 
  a 
  temperature 
  at 
  

   which 
  penetrating 
  rays 
  of 
  solar 
  quality 
  are 
  emitted. 
  

  

  With 
  absorbent, 
  instead 
  of 
  reflective 
  layers, 
  the 
  tempe- 
  

   rature 
  can 
  only 
  increase 
  until, 
  in 
  the 
  steady 
  state, 
  the 
  roof 
  

   radiates 
  out 
  as 
  much 
  as 
  the 
  sun 
  sends 
  in. 
  But 
  since 
  the 
  roof 
  

   then 
  radiates 
  greatly 
  inward 
  to 
  the 
  background, 
  the 
  latter 
  is 
  

   receiving 
  the 
  direct 
  solar 
  rays 
  as 
  before 
  there 
  was 
  a 
  roof, 
  and 
  

   also 
  an 
  equal 
  amount 
  of 
  returned 
  radiation. 
  Hence 
  the 
  

   thermal 
  effect 
  is 
  doubled. 
  The 
  maximum 
  heat-trapping 
  effect 
  

   is 
  reached 
  when 
  the 
  inmost 
  layer 
  of 
  the 
  roof 
  and 
  the 
  back- 
  

   ground 
  reach 
  the 
  same 
  temperature, 
  or 
  when 
  equal 
  amounts 
  

   of 
  energy 
  are 
  interchanged 
  between 
  these 
  surfaces 
  according 
  

   to 
  Prevost's 
  law 
  of 
  exchanges. 
  The 
  condition 
  is 
  represented 
  

   in 
  Poynting's 
  equation 
  for 
  the 
  greenhouse 
  effect, 
  

  

  R_ 
  t 
  + 
  a/n 
  * 
  m 
  

  

  IS 
  t! 
  + 
  (n-l)ajn 
  ' 
  W 
  

  

  if 
  fc=l, 
  a 
  = 
  0, 
  £i=0, 
  a 
  x 
  — 
  l, 
  and 
  n 
  = 
  2, 
  when 
  R/S 
  = 
  2. 
  This 
  is 
  

   of 
  course 
  the 
  maximum 
  increment 
  of 
  thermal 
  energy 
  which 
  

   it 
  is 
  possible 
  to 
  get 
  by 
  means 
  of 
  a 
  greenhouse 
  provided 
  simply 
  

   with 
  an 
  absorbent 
  covering. 
  The 
  more 
  perfect 
  our 
  heat- 
  

   trapping 
  device 
  becomes, 
  the 
  nearer 
  will 
  be 
  the 
  approach 
  to 
  

   this 
  ratio. 
  If 
  the 
  number 
  of 
  chambers 
  is 
  not 
  indefinitely 
  

   great, 
  n 
  must 
  be 
  a 
  little 
  larger 
  than 
  2, 
  because 
  each 
  suc- 
  

   cessive 
  chamber 
  in 
  the 
  outward 
  direction 
  is 
  at 
  a 
  little 
  lower 
  

   temperature, 
  and 
  radiation 
  outward, 
  being 
  towards 
  a 
  colder 
  

   surface, 
  exceeds 
  radiation 
  inwards. 
  

  

  Having 
  demonstrated 
  the 
  properties 
  of 
  our 
  ideal 
  glass 
  

   house, 
  I 
  might 
  leave 
  the 
  subject 
  with 
  the 
  remark 
  that 
  at 
  first 
  

   blush 
  neither 
  it 
  nor 
  Professor 
  Poynting's 
  greenhouse 
  bears 
  

   the 
  remotest 
  resemblance 
  to 
  our 
  actual 
  complex 
  world. 
  But 
  

   if 
  the 
  temperature 
  of 
  an 
  ideal 
  planet 
  all 
  depends 
  on 
  hypo- 
  

   theses, 
  let 
  us 
  introduce 
  into 
  our 
  discussion 
  assumptions 
  

   which 
  make 
  for 
  warmth 
  rather 
  than 
  cold 
  ; 
  because 
  evidence 
  

   drawn 
  from 
  astronomical 
  observations 
  indicates 
  that 
  there 
  is 
  

   abundant 
  warmth 
  at 
  the 
  surfaces 
  of 
  the 
  more 
  distant 
  planets. 
  

   I 
  shall 
  endeavour 
  to 
  show 
  that 
  the 
  higher 
  temperatures 
  agree 
  

   better 
  w 
  r 
  ith 
  facts 
  of 
  observation 
  and 
  with 
  theory. 
  

  

  * 
  t 
  and 
  a 
  denote 
  the 
  transmission 
  and 
  absorption 
  of 
  solar 
  rays 
  (S), 
  and 
  

   t 
  x 
  , 
  a 
  x 
  the 
  same 
  for 
  ground 
  radiation 
  (E), 
  while 
  l/» 
  is 
  the 
  fraction 
  of 
  

   glass 
  radiation 
  sent 
  downward. 
  

  

  