﻿194 
  ANNUAL, 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1922. 
  

  

  given 
  to 
  those 
  London 
  fogs 
  that 
  contain 
  only 
  a 
  moderate 
  amount 
  

   of 
  smoke 
  particles 
  — 
  just 
  enough 
  to 
  give 
  the 
  fog 
  a 
  distinctly 
  yel- 
  

   lowish 
  cast 
  ; 
  ice 
  fog, 
  or 
  " 
  frost 
  smoke," 
  the 
  fog 
  of 
  polar 
  seas, 
  caused 
  

   by 
  the 
  drifting 
  over 
  them 
  of 
  very 
  cold 
  air 
  ; 
  and 
  many 
  others, 
  mostly 
  

   of 
  less 
  interest 
  and 
  of 
  small 
  importance. 
  

  

  QUANTITY 
  OF 
  WATER 
  CONTAINED. 
  

  

  It 
  might 
  seem, 
  on 
  first 
  thought, 
  that 
  it 
  would 
  be 
  a 
  very 
  easy 
  thing 
  

   to 
  measure 
  the 
  amount 
  of 
  liquid 
  water 
  in 
  a 
  given 
  volume 
  of 
  fog, 
  but 
  

   this 
  amount 
  is 
  so 
  small 
  that 
  even 
  tolerably 
  accurate 
  measurements 
  of 
  

   it 
  require 
  much 
  care. 
  Nevertheless, 
  it 
  has 
  been 
  reliably 
  measured. 
  

   Thus, 
  in 
  the 
  course 
  of 
  an 
  official 
  ice 
  patrol 
  cruise 
  on 
  the 
  Seneca 
  about 
  

   the 
  southern 
  edge 
  of 
  the 
  Grand 
  Banks 
  of 
  Newfoundland, 
  that 
  is, 
  in 
  

   the 
  region 
  of 
  one 
  of 
  the 
  most 
  frequented 
  of 
  the 
  steamer 
  lanes, 
  Wells 
  

   and 
  Thuras 
  measured 
  the 
  water 
  content 
  of 
  a 
  dense 
  fog 
  that 
  occurred 
  

   on 
  the 
  evening 
  of 
  May 
  9, 
  1915. 
  In 
  this 
  fog 
  there 
  were 
  about 
  20,000 
  

   droplets 
  per 
  cubic 
  inch. 
  In 
  their 
  report 
  they 
  say 
  : 
  " 
  To 
  gain 
  some 
  

   idea 
  of 
  the 
  order 
  of 
  magnitude 
  of 
  the 
  quantities 
  involved 
  in 
  this 
  

   dense 
  fog, 
  assume 
  that 
  one 
  can 
  not 
  see 
  beyond 
  100 
  feet. 
  A 
  block 
  of 
  

   fog 
  3 
  feet 
  wide, 
  6 
  feet 
  high 
  and 
  100 
  feet 
  long 
  contains 
  less 
  than 
  one- 
  

   seventh 
  of 
  a 
  glass 
  of 
  liquid 
  water. 
  This 
  water 
  is 
  distributed 
  among 
  

   60 
  billion 
  drops." 
  

  

  Barely 
  one 
  good 
  swallow 
  ! 
  

  

  SIZE 
  AND 
  NUMBER 
  OF 
  PARTICLES. 
  

  

  The 
  size 
  of 
  fog 
  particles 
  is 
  easily 
  and 
  accurately 
  determined 
  by 
  

   looking 
  through 
  the 
  fog 
  at 
  a 
  point 
  (pinhole) 
  source 
  of 
  bright 
  light 
  

   and 
  noting 
  the 
  difference 
  in 
  direction 
  toward 
  center 
  and 
  circumfer- 
  

   ence 
  of 
  any 
  definite 
  one 
  of 
  the 
  rings 
  of 
  colored 
  light 
  seen 
  around 
  this 
  

   source 
  — 
  rings 
  of 
  precisely 
  the 
  same 
  nature 
  as 
  the 
  coronse 
  produced 
  

   by 
  thin 
  clouds 
  around 
  the 
  sun 
  and 
  moon. 
  This 
  angular 
  size, 
  or 
  dif- 
  

   ference 
  in 
  direction 
  between 
  center 
  and 
  circumference 
  of 
  any 
  one 
  of 
  

   these 
  rings, 
  increases 
  as 
  the 
  diameter 
  of 
  the 
  fog 
  particles 
  decreases, 
  

   and 
  in 
  such 
  known 
  and 
  definite 
  manner 
  that 
  when 
  either 
  is 
  given 
  the 
  

   other 
  can 
  at 
  once 
  be 
  accurately 
  calculated. 
  

  

  If 
  then, 
  we 
  know 
  the 
  size 
  of 
  the 
  fog 
  particle 
  and 
  if, 
  in 
  addition, 
  

   we 
  know 
  the 
  amount 
  of 
  liquid 
  water 
  in 
  a 
  given 
  volume 
  of 
  fog, 
  

   which, 
  as 
  explained 
  above, 
  can 
  be 
  measured, 
  it 
  obviously 
  is 
  only 
  a 
  

   matter 
  of 
  easy 
  arithmetic 
  to 
  find 
  the 
  number 
  of 
  such 
  particles 
  per 
  

   cubic 
  inch, 
  say, 
  or 
  any 
  other 
  given 
  volume. 
  

  

  These 
  sizes 
  and 
  numbers, 
  however, 
  are 
  surprising. 
  Everyone 
  

   knows, 
  of 
  course, 
  that 
  a 
  fog 
  droplet 
  is 
  small, 
  but 
  not 
  many, 
  per- 
  

   haps, 
  realize 
  that 
  2,500 
  of 
  them, 
  of 
  the 
  average 
  size, 
  placed 
  side 
  

   by 
  side 
  in 
  a 
  single 
  row 
  would 
  extend 
  only 
  one 
  inch, 
  or, 
  in 
  other 
  

  

  