﻿466 
  Mr. 
  F. 
  W. 
  Very 
  on 
  the 
  Greenhouse 
  

  

  was 
  exposed 
  to 
  the 
  sun's 
  rays 
  in 
  an 
  enclosure 
  protected 
  by 
  

   three 
  layers 
  of 
  thin 
  glass, 
  separated 
  by 
  air-spaces, 
  I 
  obtained 
  

   a 
  reading 
  o£ 
  104° 
  C, 
  the 
  shade 
  temperature 
  in 
  the 
  air 
  being 
  

   35° 
  C. 
  The 
  altitude 
  o£ 
  the 
  sun 
  was 
  50°, 
  and 
  the 
  solar 
  radiation 
  

   was 
  1*30 
  small 
  calories 
  per 
  square 
  centimetre 
  per 
  minute 
  

   (obtained 
  with 
  a 
  mercury 
  pyrheliometer). 
  The 
  temperature 
  

   o£ 
  the 
  superficial 
  layer 
  o£ 
  the 
  soil 
  was 
  not 
  taken, 
  but 
  is 
  usually 
  

   about 
  10° 
  higher 
  than 
  the 
  air 
  temperature 
  for 
  the 
  given 
  

   altitude 
  of 
  the 
  sun. 
  Admitting 
  that 
  a 
  layer 
  of 
  soil 
  about 
  

   1 
  cm. 
  deep 
  was 
  maintained 
  at 
  a 
  temperature 
  o£ 
  45° 
  C. 
  by 
  

   the 
  sun's 
  rays, 
  the 
  retention 
  of 
  heat 
  was 
  augmented 
  by 
  three 
  

   layers 
  of 
  glass 
  in 
  the 
  ratio 
  

  

  /104 
  + 
  273V-1-975 
  

   II5T273.)- 
  1975 
  - 
  

  

  This 
  already 
  approaches 
  the 
  theoretical 
  duplicate 
  ratio, 
  and 
  

   yet 
  nothing 
  has 
  been 
  allowed 
  for 
  heat-trapping 
  by 
  the 
  atmo- 
  

   sphere 
  which, 
  with 
  a 
  relative 
  humidity 
  of 
  42 
  per 
  cent., 
  must 
  

   have 
  been 
  considerable. 
  Hence 
  the 
  ground 
  temperature 
  was 
  

   probably 
  higher 
  than 
  45° 
  C. 
  

  

  If 
  we 
  take 
  the 
  excess 
  of 
  surface 
  soil-temperature 
  over 
  air- 
  

   temperature 
  twice 
  as 
  large, 
  making 
  the 
  surface 
  temperature 
  

   35 
  + 
  20 
  = 
  55° 
  C, 
  the 
  augmentation 
  by 
  the 
  three 
  layers 
  of 
  

   glass 
  was 
  

  

  V55 
  + 
  273/ 
  

  

  Even 
  this 
  gives 
  a 
  smaller 
  heat-retaining 
  power 
  to 
  the 
  atmo- 
  

   sphere 
  than 
  it 
  usually 
  possesses 
  ; 
  but 
  except 
  on 
  mountains 
  

   the 
  excess 
  of 
  surface 
  temperature 
  is 
  seldom 
  greater 
  than 
  20° 
  C. 
  

   Be 
  this 
  as 
  it 
  may, 
  the 
  observation 
  certainly 
  shows 
  that 
  a 
  solar 
  

   radiation 
  of 
  1*3 
  cal./cm. 
  2 
  min. 
  is 
  competent 
  to 
  produce 
  a 
  tem- 
  

   perature 
  above 
  that 
  of 
  boiling 
  water 
  ; 
  and 
  if 
  the 
  solar 
  constant 
  

   is 
  as 
  large 
  as 
  3'1 
  cal./cm. 
  2 
  min., 
  which 
  I 
  believe 
  to 
  be 
  probable, 
  

   the 
  sun 
  can 
  give 
  at 
  the 
  earth's 
  distance 
  a 
  temperature 
  of 
  %°, 
  

   where 
  

  

  X 
  4 
  : 
  (104 
  + 
  273) 
  4 
  = 
  3-l:l'3 
  ; 
  

   or 
  x 
  = 
  468°A. 
  

  

  Langley 
  * 
  obtained 
  at 
  his 
  mountain 
  camp 
  on 
  Mt. 
  Whitney, 
  

   in 
  a 
  double 
  hot-box 
  of 
  his 
  design, 
  an 
  excess 
  of 
  98°*5 
  C. 
  above 
  

   shade 
  temperature. 
  No 
  simultaneous 
  actinometer 
  readings 
  

   were 
  made 
  ; 
  but 
  by 
  interpolation 
  from 
  those 
  of 
  other 
  days 
  

  

  * 
  S. 
  P. 
  Langley, 
  " 
  Researches 
  on 
  Solar 
  Heat," 
  Professional 
  Papers 
  of 
  

   the 
  Signal 
  Service, 
  No. 
  15, 
  Chap. 
  18, 
  p. 
  166. 
  

  

  