﻿476 
  Mr. 
  F. 
  W. 
  Very 
  on 
  the 
  Greenhouse 
  

  

  house 
  effect, 
  using 
  the 
  term 
  in 
  a 
  very 
  wide 
  sense, 
  may 
  stiD 
  

   be 
  attained, 
  bnt 
  by 
  a 
  very 
  complicated 
  process. 
  

  

  If 
  we 
  apply 
  Poyn 
  ting's 
  equation 
  in 
  its 
  strictly 
  limited 
  

   sense 
  to 
  my 
  hot-box 
  experiment, 
  its 
  use 
  becomes 
  legitimate. 
  

   The 
  following 
  approximate 
  values 
  are 
  close 
  enough 
  for 
  

   illustration 
  : 
  Take 
  t 
  = 
  0S,a=0'2, 
  ^ 
  = 
  0, 
  fll 
  =l, 
  (0,16)* 
  = 
  1*744.. 
  

   Substituting 
  in 
  the 
  equation 
  

  

  <w«5+$£jfc? 
  • 
  • 
  • 
  • 
  (3> 
  

  

  n= 
  1-944/0*944= 
  2*06, 
  

  

  which 
  is 
  in 
  substantial 
  agreement 
  with 
  Professor 
  Poynting's 
  

   supposition 
  that 
  one 
  half 
  of 
  the 
  radiation 
  of 
  the 
  glass 
  goes 
  

   downward, 
  although, 
  of 
  course, 
  my 
  experiment 
  does 
  not 
  

   quite 
  fulfil 
  his 
  requirement 
  of 
  a 
  greenhouse 
  of 
  indefinite 
  

   extent. 
  

  

  In 
  the 
  application 
  of 
  the 
  formula 
  of 
  Arrhenius 
  to 
  the 
  cloud- 
  

   covered 
  half 
  of 
  the 
  earth's 
  surface, 
  Poynting 
  says 
  that 
  Lowell 
  

   *' 
  finds 
  that 
  this 
  half 
  only 
  receives 
  0*2 
  of 
  the 
  radiation 
  which 
  

   the 
  clear-sky 
  half 
  receives. 
  The 
  surface 
  temperature 
  under 
  

   cloud 
  should 
  therefore 
  be 
  only 
  ^/0'2 
  = 
  0'G7 
  of 
  that 
  under 
  

   clear 
  sky. 
  If 
  the 
  latter 
  is 
  300° 
  A., 
  the 
  former 
  is 
  only 
  about 
  

   200° 
  A. 
  Common 
  observation 
  contradicts 
  this 
  flatly, 
  for 
  the 
  

   difference 
  is 
  at 
  most 
  but 
  a 
  few 
  degrees/' 
  This 
  only 
  illustrates 
  

   the 
  inadequacy 
  of 
  a 
  consideration 
  of 
  the 
  problem 
  from 
  a 
  

   purely 
  radiant 
  aspect. 
  Suppose 
  that 
  the 
  clouds 
  are 
  so 
  dense 
  

   that 
  only 
  one 
  per 
  cent, 
  of 
  solar 
  radiation 
  reaches 
  the 
  surface. 
  

   Must 
  we 
  presume 
  that 
  the 
  temperature 
  will 
  immediately 
  fall 
  

   to 
  near 
  the 
  absolute 
  zero 
  ? 
  Not 
  at 
  all. 
  The 
  true 
  surface 
  

   temperature 
  cannot 
  be 
  found 
  unless 
  the 
  heat 
  received 
  be 
  

   linked 
  with 
  the 
  heat 
  lost. 
  This 
  Lowell 
  does, 
  obtaining 
  from 
  

   his 
  analysis 
  of 
  day 
  and 
  night 
  temperatures 
  on 
  the 
  earth 
  a 
  

   value 
  of 
  the 
  relative 
  emissivity 
  of 
  the 
  earth, 
  and 
  finding 
  that 
  

   only 
  about 
  16 
  per 
  cent, 
  of 
  the 
  heat 
  received 
  from 
  the 
  sun's 
  

   rays 
  in 
  the 
  day-time 
  is 
  lost 
  by 
  radiation 
  into 
  space 
  in 
  the 
  

   tw 
  T 
  enty-four 
  hours. 
  What 
  becomes 
  of 
  the 
  rest 
  ? 
  Obviously 
  

   if 
  so 
  much 
  heat 
  is 
  retained, 
  the 
  earth 
  must 
  be 
  getting 
  hotter. 
  

   The 
  answer 
  to 
  the 
  question 
  is 
  that 
  the 
  earth 
  does 
  get 
  hotter 
  in 
  

   summer, 
  but 
  the 
  extra 
  heat 
  is 
  lost 
  in 
  winter 
  ; 
  and 
  the 
  entire 
  

   process 
  requires 
  an 
  estimate 
  of 
  the 
  heat 
  transferred 
  from 
  the 
  

   summer 
  to 
  the 
  winter 
  hemisphere 
  by 
  the 
  winds 
  of 
  the 
  

   globe. 
  

  

  On 
  p. 
  749 
  (loc. 
  cit.) 
  Professor 
  Poynting 
  says 
  that 
  Lowell 
  

   " 
  pays 
  hardly 
  any 
  attention 
  to 
  the 
  ' 
  blanketing 
  effect,' 
  or, 
  as 
  

   I 
  prefer 
  to 
  call 
  it, 
  the 
  ' 
  greenhouse 
  effect 
  ' 
  of 
  the 
  atmosphere." 
  

  

  