﻿On 
  the 
  Theory 
  of 
  the 
  Greenhouse. 
  33 
  

  

  From 
  an 
  interesting 
  paper 
  of 
  P. 
  Compan 
  * 
  it 
  may 
  be 
  seen 
  

   that 
  for 
  a 
  blackened 
  copper 
  ball 
  2 
  centimetres 
  in 
  diameter 
  

   cooling 
  from 
  a 
  temperature 
  of 
  55^ 
  C. 
  to 
  nearly 
  *' 
  black 
  '' 
  

   surroundings 
  at 
  0° 
  C, 
  the 
  rate 
  of 
  loss 
  of 
  heat 
  by 
  convection 
  

   in 
  still 
  dry 
  air 
  at 
  atmospheric 
  pressure 
  is 
  four- 
  thirds 
  as 
  rapid 
  

   as 
  the 
  simultaneous 
  loss 
  by 
  radiation. 
  In 
  a 
  breeze 
  of 
  

   3 
  metres 
  per 
  second 
  the 
  convection 
  loss 
  becomes 
  3 
  times 
  as 
  

   rapid 
  as 
  in 
  still 
  air, 
  or 
  4 
  times 
  as 
  rapid 
  as 
  the 
  loss 
  by 
  radia- 
  

   tion. 
  The 
  loss 
  of 
  heat 
  by 
  convection 
  alone 
  is 
  approximately 
  

   proportional 
  to 
  the 
  difference 
  of 
  temperatures 
  between 
  the 
  

   source 
  and 
  the 
  sink. 
  

  

  If 
  the 
  covers 
  had 
  been 
  absent 
  in 
  Professor 
  Wood's 
  experi- 
  

   ments, 
  the 
  boxes 
  would 
  have 
  been 
  exchanging 
  radiation 
  

   principally 
  with 
  the 
  w^ater 
  vapour 
  of 
  the 
  lower 
  atmosphere. 
  

   Experiments 
  of 
  Langley, 
  Rubens 
  and 
  Aschkinass^ 
  and 
  others 
  

   indicate 
  f 
  that 
  less 
  than 
  10 
  per 
  cent, 
  of 
  the 
  radiation 
  from 
  

   the 
  earth's 
  surface 
  can 
  penetrate 
  the 
  water 
  vapour 
  of 
  the 
  

   atmosphere 
  above 
  a 
  coast 
  station 
  like 
  Baltimore. 
  Hence 
  the 
  

   water 
  vapour 
  of 
  the 
  atmosphere 
  can 
  be 
  considered 
  as 
  practi- 
  

   cally 
  a 
  " 
  black 
  body 
  '"' 
  for 
  rays 
  of 
  great 
  wave-length. 
  The 
  

   effective 
  temperature 
  of 
  the 
  water- 
  vapour 
  layers 
  with 
  which 
  

   a 
  coverless 
  " 
  hot-box 
  " 
  would 
  have 
  been 
  exchanging 
  radiation 
  

   may 
  be 
  estimated 
  at 
  0° 
  0. 
  If 
  glass 
  is 
  interposed 
  the 
  radiation 
  

   is 
  entirely 
  cut 
  off. 
  If 
  a 
  rock-salt 
  plate 
  1 
  centimetre 
  thick 
  is 
  

   interposed 
  between 
  the 
  body 
  at 
  55° 
  C. 
  and 
  surroundings 
  at 
  

   0° 
  C, 
  the 
  absorption 
  of 
  the 
  plate 
  is 
  about 
  19 
  per 
  cent.f 
  and 
  

   the 
  reflexion 
  probably 
  nearly 
  10 
  per 
  cent, 
  more, 
  so 
  that 
  the 
  

   transmission 
  may 
  be 
  reckoned 
  at 
  about 
  70 
  per 
  cent. 
  

  

  In 
  combining 
  the 
  preceding 
  results 
  with 
  those 
  of 
  Compan 
  

   we 
  will 
  at 
  first 
  neglect 
  the 
  loss 
  of 
  heat 
  by 
  convection 
  from 
  

   the 
  outside 
  of 
  the 
  cover. 
  We 
  may 
  assume 
  the 
  temperature 
  

   of 
  the 
  air 
  just 
  outside 
  the 
  " 
  hot-box 
  '' 
  to 
  be 
  15° 
  C, 
  and 
  also 
  

   that 
  Newton's 
  law 
  of 
  cooling 
  is 
  applicable 
  to 
  the 
  convection 
  

   loss. 
  In 
  still 
  air 
  with 
  no 
  cover 
  the 
  rate 
  of 
  loss 
  of 
  heat 
  

   towards 
  the 
  front 
  by 
  convection 
  and 
  radiation 
  combined 
  is 
  

   proportional 
  to 
  : 
  

  

  t^x 
  133 
  + 
  100 
  = 
  197. 
  

   55 
  

  

  With 
  glass 
  cover 
  : 
  + 
  100 
  xO'OO 
  = 
  0. 
  

   With 
  salt 
  cover 
  : 
  + 
  100 
  X 
  70 
  = 
  70. 
  

  

  * 
  Aniiales 
  de 
  Chimie 
  et 
  de 
  Physique, 
  t. 
  xxvi. 
  pp. 
  488-574 
  (1902). 
  

  

  t 
  !See 
  Anuals, 
  Astrophvsical 
  Observatory, 
  Smithsonian 
  Institution, 
  

   vol. 
  ii. 
  ])p. 
  1()7-172. 
  

  

  X 
  See 
  Kayser's 
  Handbuch 
  d. 
  SiJectroscopie, 
  vol. 
  iv. 
  p. 
  485, 
  and 
  Planck's 
  

   formula 
  of 
  radiation, 
  

  

  Fhil. 
  Mag, 
  S. 
  6. 
  Vol. 
  18. 
  No. 
  103. 
  July 
  1909. 
  D 
  

  

  