﻿Problems 
  of 
  Evaporation. 
  275 
  

  

  results. 
  Brown 
  and 
  Escombe 
  also 
  found 
  that 
  the 
  actual 
  rate 
  

   of 
  evaporation 
  varied 
  more 
  rapidly 
  than 
  the 
  radius 
  but 
  not 
  

   so 
  rapidly 
  as 
  the 
  area. 
  Thomas 
  and 
  Ferguson 
  find 
  that 
  when 
  

   the 
  surface 
  of 
  the 
  water 
  is 
  below 
  the 
  rim 
  of 
  the 
  containing 
  

   vessel 
  and 
  the 
  evaporation 
  is 
  assumed 
  proportional 
  to 
  a 
  n 
  , 
  

   where 
  n 
  is 
  a 
  constant 
  for 
  a 
  given 
  depth, 
  n 
  tends 
  to 
  2 
  when 
  

   the 
  depth 
  is 
  large. 
  

  

  The 
  evaporation 
  from 
  a 
  circular 
  cylinder, 
  wet 
  at 
  the 
  

   bottom 
  and 
  open 
  at 
  the 
  top, 
  may 
  be 
  considered 
  here. 
  80 
  

   long 
  as 
  the 
  depth 
  is 
  either 
  great 
  or 
  small 
  compared 
  with 
  the 
  

   radius, 
  we 
  can 
  assume 
  that 
  within 
  the 
  cylinder 
  the 
  layers 
  of 
  

   equal 
  concentration 
  are 
  parallel 
  to 
  the 
  base. 
  Let 
  I 
  be 
  the 
  

   depth, 
  a 
  the 
  radius, 
  and 
  R 
  the 
  rate 
  of 
  evaporation. 
  Let 
  the 
  

   concentration 
  at 
  the 
  bottom 
  be 
  V 
  , 
  and 
  at 
  the 
  top 
  Y 
  l 
  . 
  

   Then 
  inside 
  the 
  cylinder 
  the 
  condition 
  for 
  steady 
  flow 
  makes 
  

  

  ±\,=7T6rA:p 
  ^— 
  =z-ira 
  z 
  kp 
  ^ 
  . 
  

  

  Outside 
  the 
  cylinder, 
  if 
  the 
  air 
  is 
  at 
  rest, 
  the 
  vapour 
  is 
  

   practically 
  diffusing 
  from 
  one 
  side 
  of 
  a 
  disk 
  at 
  concentration 
  

   V 
  l9 
  and 
  therefore 
  the 
  rate 
  of 
  evaporation 
  is 
  AkpaVi 
  * 
  ; 
  this 
  

   also 
  must 
  be 
  equal 
  to 
  R, 
  else 
  the 
  concentration 
  at 
  the 
  mouth 
  

   would 
  be 
  changing. 
  Eliminating 
  Y 
  ± 
  we 
  find 
  at 
  once 
  

  

  U 
  = 
  7ra 
  2 
  kpV 
  /(l 
  + 
  %ira), 
  

  

  agreeing 
  with 
  a 
  result 
  of 
  Brown 
  and 
  Escombe 
  (loc. 
  cit. 
  

   p. 
  258). 
  

  

  If 
  a 
  strong 
  wind 
  is 
  blowing 
  over 
  the 
  top, 
  the 
  conditions 
  

   inside 
  will 
  be 
  unaltered, 
  but 
  at 
  the 
  top 
  we 
  shall 
  have 
  

  

  11 
  = 
  3*95 
  pYjQMa*)*, 
  

  

  giving, 
  after 
  elimination 
  of 
  V 
  l5 
  

  

  U== 
  P 
  Y 
  <>+ 
  (tto^ 
  + 
  3-95 
  (kua*)i) 
  ' 
  

   which 
  varies 
  with 
  I 
  in 
  the 
  way 
  found 
  by 
  Thomas 
  and 
  Ferguson. 
  

  

  Evaporation 
  from 
  the 
  surface 
  of 
  a 
  leaf 
  

  

  The 
  surface 
  of 
  a 
  leaf 
  consists 
  of 
  an 
  almost 
  impermeable 
  

   cuticle 
  perforated 
  by 
  a 
  large 
  number 
  of 
  small 
  holes, 
  called 
  

   stomatn, 
  through 
  which 
  respiration 
  and 
  absorption 
  of 
  carbon 
  

   dioxide 
  take 
  place. 
  It 
  is 
  a 
  matter 
  of 
  some 
  uncertainty 
  

  

  * 
  The 
  capacity 
  of 
  a 
  circular 
  disk 
  is 
  2a/ir, 
  and 
  therefore 
  that 
  of 
  one 
  

   side 
  of 
  it 
  is 
  a/71-. 
  

  

  