﻿188 
  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1922. 
  

  

  (b) 
  The 
  temperature 
  of 
  the 
  evaporating 
  water. 
  The 
  higher 
  this 
  

   temperature 
  the 
  more 
  rapid 
  the 
  evaporation. 
  That 
  is 
  why 
  heating 
  

   things 
  hastens 
  their 
  drying. 
  

  

  (c) 
  The 
  velocity 
  of 
  the 
  wind 
  to 
  which 
  the 
  water 
  is 
  exposed. 
  This 
  

   explains 
  why 
  roads, 
  for 
  instance, 
  dry 
  so 
  much 
  quicker 
  when 
  the 
  

   winds 
  are 
  strong 
  than 
  when 
  they 
  are 
  light. 
  

  

  (d) 
  The 
  amount 
  of 
  moisture 
  already 
  in 
  the 
  atmosphere. 
  The 
  

   wash 
  on 
  the 
  clothesline, 
  as 
  every 
  housewife 
  knows, 
  dries 
  provokingly 
  

   slowly 
  during 
  cloudy, 
  muggy 
  weather. 
  

  

  All 
  the 
  other 
  factors 
  that 
  affect 
  the 
  rate 
  of 
  evaporation, 
  such 
  as 
  

   density 
  of 
  the 
  air, 
  saltiness 
  of 
  the 
  water, 
  etc., 
  generally 
  are 
  small 
  in 
  

   comparison 
  with 
  the 
  above, 
  and 
  need 
  not 
  here 
  be 
  considered 
  since 
  It 
  

   is 
  only 
  intended 
  to 
  make 
  perfectly 
  clear, 
  through 
  our 
  everyday 
  ex- 
  

   periences, 
  the 
  wide 
  occurrence 
  and 
  great 
  importance 
  of 
  evaporation. 
  

  

  SATURATION. 
  

  

  Although 
  evaporation, 
  as 
  just 
  stated, 
  is 
  so 
  very 
  general 
  there 
  are 
  

   conditions 
  under 
  which 
  things 
  will 
  not 
  dry, 
  nor 
  bodies 
  of 
  water 
  any 
  

   longer 
  decrease. 
  Thus 
  even 
  a 
  rather 
  small 
  amount 
  of 
  water 
  in 
  a 
  

   tightly 
  closed 
  bottle, 
  or 
  other 
  impervious 
  vessel, 
  remains 
  there 
  in- 
  

   definitely. 
  The 
  space 
  above 
  the 
  water 
  becomes 
  saturated, 
  we 
  say, 
  by 
  

   which 
  we 
  mean 
  that 
  it 
  gets 
  so 
  charged 
  with 
  water 
  vapor 
  that, 
  under 
  

   the 
  existing 
  conditions, 
  it 
  can 
  contain 
  no 
  more. 
  At 
  this 
  stage 
  the 
  net 
  

   evaporation 
  is 
  zero. 
  That 
  is, 
  the 
  amount 
  of 
  the 
  invisible 
  water 
  

   vapor 
  that 
  now 
  goes 
  back, 
  or 
  condenses, 
  into 
  the 
  liquid 
  stage 
  is 
  ex- 
  

   actly 
  equal 
  to 
  that 
  which, 
  in 
  the 
  same 
  time, 
  leaves 
  the 
  liquid 
  surface 
  

   and 
  becomes 
  invisible 
  vapor. 
  In 
  other 
  words, 
  at 
  this 
  stage, 
  however 
  

   rapid 
  the 
  interchange 
  between 
  liquid 
  and 
  vapor, 
  the 
  amount 
  of 
  each 
  

   remains 
  constant. 
  

  

  Numerous 
  careful 
  experiments 
  have 
  determined 
  very 
  closely 
  the 
  

   exact 
  weight 
  of 
  water 
  vapor 
  per 
  cubic 
  foot, 
  say, 
  when 
  the 
  space 
  in 
  

   question 
  is 
  saturated 
  at 
  any 
  given 
  temperature, 
  from 
  the 
  boiling 
  

   point 
  to 
  far 
  below 
  that 
  of 
  freezing. 
  And 
  these 
  experiments 
  show 
  

   two 
  facts 
  of 
  paramount 
  importance 
  in 
  the 
  formation 
  and 
  dissipation 
  

   of 
  clouds, 
  namely: 
  

  

  1. 
  The 
  amount 
  of 
  moisture 
  necessary 
  to 
  produce 
  saturation 
  in- 
  

   creases 
  rapidly 
  with 
  increase 
  of 
  temperature. 
  

  

  2. 
  The 
  amount 
  of 
  water 
  vapor 
  essential 
  to 
  saturation 
  is 
  not 
  ap- 
  

   preciably 
  affected 
  by 
  the 
  presence 
  or 
  absence 
  of 
  the 
  other 
  gases 
  of 
  

   the 
  normal 
  atmosphere. 
  

  

  It 
  is 
  true 
  that 
  even 
  in 
  technical 
  language 
  we 
  often 
  say 
  that 
  the 
  

   air 
  contains 
  such 
  or 
  such 
  an 
  amount 
  of 
  moisture, 
  as 
  though 
  the 
  pres- 
  

   ence 
  of 
  the 
  air 
  was 
  essential 
  to 
  the 
  existence 
  of 
  the 
  vapor, 
  or 
  as 
  

   though 
  the 
  air 
  acted 
  somehow 
  like 
  a 
  sponge 
  in 
  taking 
  up 
  water. 
  But, 
  

  

  