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  299 
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  XVII. 
  — 
  On 
  the 
  Weight 
  of 
  Aqueous 
  Vapour 
  which 
  is 
  condensed 
  on 
  a 
  Cold 
  Surface, 
  

   under 
  given 
  conditions. 
  By 
  James 
  Dalmahoy, 
  Esq. 
  

  

  (Read 
  3d 
  March 
  1851.) 
  

  

  In 
  the 
  accompanying 
  tables 
  are 
  contained 
  the 
  results 
  of 
  some 
  experiments 
  

   respecting 
  the 
  rate 
  at 
  which 
  aqueous 
  vapour 
  condenses 
  on 
  a 
  cold 
  surface. 
  

  

  These 
  results 
  are 
  not 
  so 
  consistent 
  as 
  could 
  be 
  desired, 
  but 
  having 
  been 
  ob- 
  

   tained 
  by 
  a 
  definite 
  and 
  carefully-conducted 
  process, 
  they 
  may 
  claim 
  at 
  least 
  to 
  

   be 
  received 
  as 
  approximately 
  true. 
  

  

  In 
  planning 
  the 
  experiments, 
  it 
  was 
  assumed 
  that 
  c=m 
  (/"—/'"), 
  where 
  c 
  is 
  

   the 
  weight 
  of 
  moisture 
  which 
  is 
  condensed 
  on 
  a 
  surface 
  of 
  given 
  area 
  in 
  a 
  given 
  

   time 
  ; 
  f" 
  the 
  tension 
  of 
  vapour 
  at 
  the 
  dew-point 
  ; 
  /"' 
  its 
  tension 
  at 
  the 
  tempera- 
  

   ture 
  of 
  the 
  condensing 
  surface 
  ; 
  m 
  a 
  coefficient 
  varying 
  with 
  the 
  velocity 
  of 
  the 
  

   current 
  of 
  air. 
  

  

  It 
  was 
  found, 
  however, 
  in 
  the 
  course 
  of 
  the 
  experiments, 
  that 
  the 
  coefficient 
  m 
  

   was 
  not 
  constant 
  for 
  calm 
  air, 
  as 
  was 
  at 
  first 
  supposed. 
  The 
  results 
  under 
  this 
  state 
  

   of 
  the 
  air 
  indicated 
  that 
  the 
  formula 
  should 
  be 
  changed 
  to 
  c=M 
  (t—t" 
  r 
  ) 
  (/"-/'"), 
  

   in 
  which 
  t 
  is 
  the 
  temperature 
  of 
  the 
  air 
  ; 
  t'" 
  the 
  temperature 
  of 
  the 
  condensing 
  

   surface 
  ; 
  M 
  a 
  constant 
  coefficient; 
  and/",/""', 
  and 
  c 
  have 
  their 
  former 
  values. 
  

  

  The 
  object 
  of 
  the 
  experiments 
  in 
  connection 
  with 
  these 
  formulae 
  was 
  to 
  deter- 
  

   mine 
  the 
  values 
  of 
  the 
  coefficients 
  m 
  and 
  M 
  from 
  those 
  of 
  their 
  equivalent 
  ex- 
  

  

  C 
  A 
  c 
  

  

  pressions 
  j„ 
  _.,„ 
  ana 
  / 
  t 
  _^„\ 
  , 
  «_™y 
  

  

  As 
  it 
  would 
  have 
  been 
  difficult, 
  if 
  not 
  impossible, 
  to 
  have 
  kept 
  the 
  condensing 
  

   surface 
  steadily 
  at 
  any 
  low 
  temperature 
  except 
  that 
  of 
  melting 
  ice, 
  the 
  value 
  of 
  

   /'" 
  throughout 
  the 
  experiment 
  was 
  equal 
  to 
  0-2 
  inch, 
  the 
  tension 
  of 
  vapour 
  at 
  

   32° 
  Fahr. 
  

  

  In 
  each 
  experiment, 
  therefore, 
  there 
  were 
  only 
  two 
  things 
  to 
  be 
  determined, 
  

   namely,/" 
  the 
  tension 
  of 
  vapour 
  at 
  the 
  dew-point, 
  and 
  c 
  the 
  weight 
  of 
  condensed 
  

   moisture. 
  

  

  The 
  value 
  of/ 
  " 
  was 
  deduced 
  by 
  means 
  of 
  Dr 
  Apjohn's 
  formula, 
  from 
  the 
  indi- 
  

   cations 
  of 
  the 
  wet 
  and 
  dry 
  bulb 
  thermometers. 
  The 
  data 
  and 
  results 
  connected 
  

   with 
  this 
  quantity 
  will 
  be 
  found 
  in 
  Table 
  L, 
  from 
  the 
  fifth 
  to 
  the 
  eighth 
  column 
  

   inclusive. 
  The 
  instruments 
  used 
  were 
  two 
  standard 
  thermometers, 
  made 
  by 
  

   Messrs 
  Adie 
  and 
  Son, 
  of 
  Edinburgh. 
  

  

  The 
  value 
  of 
  c 
  was 
  ascertained 
  by 
  a 
  simple 
  process, 
  which, 
  however, 
  as 
  the 
  

   success 
  of 
  the 
  experiments 
  entirely 
  depended 
  on 
  it, 
  must 
  be 
  described 
  in 
  detail. 
  

  

  VOL. 
  XX. 
  PART 
  II. 
  4 
  M 
  

  

  