﻿34 
  On 
  the 
  Theory 
  of 
  the 
  Greerjliouse. 
  

  

  Thus 
  of 
  the 
  heat 
  which 
  would 
  have 
  escaped 
  toward 
  tha 
  

  

  front 
  from 
  a 
  coverless 
  box 
  at 
  55° 
  C. 
  in 
  still 
  moist 
  air 
  at 
  15° 
  C, 
  

  

  197—70 
  

   the 
  salt 
  hinders 
  — 
  ^-^p= 
  — 
  = 
  65 
  per 
  cent, 
  as 
  much 
  as 
  the 
  glass. 
  

  

  Remembering, 
  however, 
  that 
  owing 
  to 
  its 
  higher 
  absorbing 
  

   power 
  for 
  long 
  wave 
  ra3's 
  the 
  glass 
  will 
  be 
  warmed 
  more 
  

   than 
  the 
  salt, 
  the 
  convection 
  loss 
  from 
  the 
  outside 
  of 
  the 
  

   warmed 
  cover 
  will 
  be 
  greater 
  for 
  glass 
  than 
  for 
  salt, 
  so 
  that 
  

   the 
  efficiency 
  of 
  a 
  salt 
  cover 
  may 
  be 
  much 
  more 
  than 
  65 
  per 
  

   cent, 
  of 
  that 
  of 
  one 
  of 
  glass. 
  The 
  convection 
  loss 
  from 
  the 
  

   front 
  of 
  the 
  cover 
  is 
  a 
  considerable 
  factor, 
  for 
  in 
  the 
  " 
  hot- 
  

   box 
  '' 
  tried 
  at 
  this 
  observatory 
  the 
  front 
  of 
  the 
  inner 
  glass 
  

   cover 
  became 
  too 
  hot 
  to 
  handle 
  and 
  often 
  cracked 
  with 
  the 
  heat. 
  

  

  In 
  view 
  of 
  these 
  figures 
  we 
  may 
  agree 
  with 
  Professor 
  

   Wood 
  that 
  a 
  salt 
  cover 
  * 
  is 
  nearly 
  as 
  efficient 
  as 
  a 
  glass 
  one 
  

   for 
  a 
  " 
  hot-box,^' 
  although 
  it 
  would 
  seem 
  strange 
  that 
  he 
  

   observed 
  no 
  difference 
  at 
  all. 
  Perhaps 
  in 
  spite 
  of 
  the 
  glass 
  

   filter 
  the 
  cover-glass 
  obstructed 
  the 
  entering 
  sun 
  rays 
  more 
  

   than 
  salt. 
  But 
  is 
  not 
  the 
  case 
  quite 
  different 
  with 
  a 
  planet 
  ? 
  

  

  Let 
  us 
  take 
  the 
  mean 
  temperature 
  of 
  the 
  earth'^s 
  surface 
  at 
  

   14° 
  C, 
  the 
  mean 
  effective 
  temperature 
  of 
  the 
  \a 
  ate 
  r- 
  vapour 
  

   layers 
  to 
  which 
  it 
  principally 
  radiates 
  as 
  0° 
  C, 
  the 
  tempera- 
  

   ture 
  of 
  space 
  as 
  —273° 
  C. 
  Then 
  the 
  rates 
  of 
  escape 
  of 
  heat 
  

   from 
  the 
  surface 
  by 
  radiation, 
  first 
  with 
  the 
  water-vapour 
  

   layer 
  interposed, 
  and, 
  second, 
  imagining 
  the 
  air 
  to 
  be 
  com- 
  

   pletely 
  transparent 
  to 
  earth 
  rays, 
  would 
  be 
  in 
  the 
  ratio 
  of 
  19 
  

   to 
  100 
  according 
  to 
  Planck's 
  formula. 
  It 
  is 
  very 
  difficult 
  to 
  

   estimate 
  how 
  fast 
  the 
  heat 
  of 
  the 
  earth's 
  surface 
  escapes 
  by 
  

   convection, 
  because 
  neither 
  the 
  difference 
  of 
  temperature 
  

   between 
  the 
  surface 
  and 
  the 
  air 
  nor 
  the 
  rate 
  of 
  motion 
  of 
  the 
  

   air 
  is 
  well 
  known. 
  But 
  if 
  for 
  the 
  sake 
  of 
  discussion 
  we 
  

   suppose 
  a 
  temperature 
  difference 
  of 
  10° 
  C. 
  and 
  a 
  velocity 
  of 
  

   3 
  metres 
  per 
  second, 
  the 
  rate 
  of 
  convection 
  loss 
  comes 
  out 
  only 
  

   0*54 
  as 
  great 
  as 
  the 
  rate 
  at 
  which 
  heat 
  would 
  escape 
  by 
  radiation 
  

   if 
  the 
  air 
  was 
  no 
  hindrance. 
  This 
  assumed 
  convection 
  loss 
  

   is 
  2*8 
  times 
  as 
  great, 
  on 
  the 
  other 
  hand, 
  as 
  the 
  estimated 
  rate 
  

   of 
  escape 
  of 
  heat 
  by 
  radiation 
  to 
  the 
  water- 
  vapour 
  layers 
  

   at 
  0° 
  0. 
  In 
  other 
  words, 
  according 
  to 
  this 
  estimate 
  the 
  con- 
  

   vection 
  is 
  the 
  main 
  agent 
  in 
  removing 
  heat 
  from 
  the 
  earth's 
  

   surface 
  as 
  things 
  are, 
  but 
  would 
  be 
  only 
  a 
  small 
  factor 
  if 
  the 
  

   air 
  was 
  transparent 
  to 
  long- 
  wave 
  raj's. 
  

  

  If 
  these 
  figures 
  represent 
  at 
  all 
  the 
  order 
  of 
  magnitudes 
  of 
  

   the 
  quantities 
  there 
  can 
  be 
  no 
  doubt, 
  I 
  think, 
  that 
  the 
  atmo- 
  

   sphere 
  is 
  important 
  as 
  a 
  trapping 
  agent 
  to 
  increase 
  the 
  earth^s 
  

   surface 
  temperature. 
  

  

  * 
  A 
  salt 
  cover, 
  however, 
  is 
  better 
  than 
  a 
  perfectly 
  transparent 
  one. 
  

  

  