﻿Theory 
  and 
  Planetarij 
  Temperatures. 
  479 
  

  

  in 
  the 
  two 
  bodies 
  considered. 
  Thus 
  we 
  cannot 
  compare 
  the 
  

   greenhouse 
  effect 
  for 
  the 
  earth 
  and 
  Neptune 
  ; 
  but 
  Mars 
  and 
  

   the 
  earth 
  appear 
  to 
  resemble 
  each 
  other 
  so 
  nearly 
  that 
  

   Professor 
  Lowell's 
  method 
  for 
  obtaining 
  the 
  surface 
  tempe- 
  

   rature 
  of 
  Mars 
  is 
  appropriate. 
  

  

  In 
  this 
  method 
  as 
  applied 
  to 
  the 
  earth, 
  it 
  is 
  assumed 
  that 
  

   clouds 
  transmit 
  20 
  per 
  cent, 
  of 
  terrestrial 
  radiation 
  from 
  a 
  

   surface 
  at 
  sea-level, 
  and 
  clear 
  air 
  50 
  per 
  cent., 
  values 
  which 
  

   are 
  close 
  enough 
  as 
  approximations. 
  The 
  mean 
  of 
  these 
  

   being 
  0*35, 
  if 
  y 
  is 
  the 
  radiant 
  energy 
  received 
  at 
  the 
  earth's 
  

   surface, 
  and 
  e 
  the 
  relative 
  emission 
  of 
  radiation 
  from 
  that 
  

   surface, 
  the 
  heat 
  retained 
  is 
  equal 
  to 
  that 
  radiated 
  from 
  the 
  

   surface, 
  or 
  

  

  a6 
  i 
  = 
  cr(2§Sy=y(l— 
  0'35e) 
  (4) 
  

  

  Lowell 
  introduces 
  into 
  this 
  equation 
  different 
  values 
  obtained 
  

   by 
  observation 
  of 
  the 
  loss 
  of 
  temperature 
  by 
  nocturnal 
  

   radiation 
  with 
  clear 
  sky, 
  and 
  also 
  under 
  a 
  cloudy 
  sky; 
  and 
  

   thus 
  obtains 
  for 
  e 
  the 
  value, 
  e 
  = 
  0*4634, 
  whence 
  

  

  o-(288) 
  4 
  = 
  0-838,?/. 
  

  

  By 
  the 
  ratio 
  of 
  retained 
  heat 
  deduced 
  from 
  my 
  hot-box 
  

   experiment, 
  when 
  compared 
  with 
  a 
  possible 
  maximum 
  re- 
  

   tention 
  of 
  heat 
  of 
  2 
  to 
  1, 
  we 
  get 
  

  

  l-744/2-000 
  = 
  0-872z/. 
  

  

  We 
  may 
  conclude 
  that 
  something 
  like 
  85 
  per 
  cent, 
  of 
  the 
  

   heat 
  received 
  from 
  the 
  sun's 
  rays 
  is 
  retained 
  at 
  the 
  earth's 
  

   surface 
  through 
  the 
  greenhouse 
  effect 
  of 
  the 
  atmosphere. 
  

  

  For 
  the 
  whole 
  earth 
  with 
  its 
  moist 
  tropics, 
  and 
  including 
  

   marginal 
  zones 
  where 
  the 
  solar 
  rays 
  pass 
  through 
  a 
  great 
  

   thickness 
  of 
  absorbent, 
  the 
  band 
  absorption 
  is 
  probably 
  

   about 
  0*4. 
  If 
  0*7 
  of 
  the 
  solar 
  radiations 
  are 
  reflected 
  from 
  

   the 
  hemisphere 
  exposed 
  to 
  the 
  sun's 
  rays, 
  and 
  if 
  of 
  the 
  

   (1 
  — 
  0*7) 
  x 
  (1 
  — 
  0'4) 
  = 
  18 
  received, 
  85 
  per 
  cent, 
  are 
  retained, 
  

   the 
  temperatures 
  involved 
  become 
  according 
  to 
  the 
  fourth- 
  

   power 
  law 
  

  

  <r(2$8)*-±=<r(0 
  t 
  ) 
  4 
  x 
  0*18x0-85 
  

  

  = 
  <r(0„) 
  4 
  x 
  0*153, 
  

   whence 
  0,=46O° 
  A. 
  

  

  In 
  the 
  last 
  part 
  of 
  his 
  paper, 
  Professor 
  Poynting 
  makes 
  

   " 
  an 
  attempt 
  to 
  represent 
  the 
  effect 
  of 
  day 
  and 
  night 
  on 
  

   the 
  temperature 
  of 
  the 
  earth""; 
  but 
  as 
  he 
  neglects 
  the 
  con- 
  

   duction 
  of 
  the 
  ground, 
  and 
  assumes 
  that 
  "the 
  surface 
  has 
  

   reached 
  an 
  equilibrium 
  between 
  receipt 
  and 
  expenditure 
  of 
  

   radiation," 
  a 
  condition 
  which 
  is 
  never 
  attained 
  on 
  the 
  earth, 
  

   since 
  also 
  he 
  considers 
  that 
  a 
  large 
  amount 
  of 
  radiation 
  is 
  

   absorbed 
  by 
  the 
  air, 
  "nearly 
  1 
  with 
  the 
  setting 
  sun," 
  which 
  

  

  