﻿630 
  F. 
  W. 
  Very 
  — 
  Solar 
  Radiation. 
  

  

  uate 
  the 
  air 
  mass 
  of 
  the 
  dust 
  layer. 
  No 
  satisfactory 
  mode 
  of 
  

   estimating 
  the 
  dust 
  factor 
  has 
  yet 
  been 
  devised, 
  and 
  the 
  sug- 
  

   gested 
  separation 
  of 
  the 
  dust 
  depletion 
  from 
  the 
  air 
  depletion, 
  

   though 
  theoretically 
  necessary 
  and 
  much 
  to 
  be 
  desired, 
  is 
  not 
  

   yet 
  on 
  a 
  practicable 
  basis. 
  Mr. 
  Abbot* 
  characterizes 
  the 
  

   method 
  as 
  arbitrary, 
  a 
  criticism 
  which 
  is 
  partially 
  justified 
  since, 
  

   unfortunately, 
  the 
  data 
  for 
  a 
  reliable 
  estimate 
  of 
  the 
  amount 
  of 
  

   dust 
  in 
  the 
  air, 
  and 
  the 
  depletion 
  which 
  it 
  exercises, 
  are 
  both 
  

   regrettably 
  deficient 
  ; 
  but 
  to 
  ignore 
  the 
  dust 
  factor 
  entirely 
  and 
  

   to 
  omit 
  even 
  an 
  imperfect 
  discrimination 
  of 
  this 
  important 
  

   variable 
  is 
  equally 
  arbitrary. 
  One 
  great 
  merit 
  of 
  winter 
  obser- 
  

   vations 
  in 
  high 
  continental 
  latitudes 
  is 
  the 
  reduction 
  of 
  atmos- 
  

   pheric 
  dust 
  to 
  a 
  minimum 
  over 
  extensive 
  snow 
  fields, 
  and 
  for 
  

   this 
  reason 
  such 
  measures 
  are 
  the 
  best 
  we 
  have 
  for 
  determining 
  

   the 
  solar 
  constant.f 
  The 
  sole 
  justification 
  for 
  the 
  total 
  omis- 
  

   sion 
  of 
  any 
  dust 
  factor 
  in 
  the 
  present 
  reduction 
  is 
  the 
  absence 
  

   of 
  observations 
  on 
  which 
  even 
  an 
  approximate 
  estimate 
  of 
  its 
  

   amount 
  may 
  be 
  founded. 
  Increasing 
  pressure 
  of 
  water 
  vapor 
  

   is 
  apt 
  to 
  be 
  accompanied 
  by 
  a 
  growing 
  mistiness 
  of 
  the 
  air, 
  

   which 
  is 
  of 
  the 
  nature 
  of 
  a 
  dry 
  fog 
  and 
  implies 
  an 
  increment 
  

   of 
  the 
  action 
  of 
  presumably 
  pre-existing 
  dust 
  particles 
  of 
  exces- 
  

   sive 
  fineness 
  which 
  are 
  enlarged 
  and 
  become 
  evident 
  when 
  serv- 
  

   ing 
  as 
  nuclei 
  of 
  condensation. 
  This 
  is 
  probably 
  the 
  reason 
  why 
  

   the 
  dust 
  depletion 
  may 
  be 
  represented 
  to 
  a 
  certain 
  extent 
  as 
  a 
  

   function 
  of 
  the 
  vapor 
  pressure, 
  which 
  is 
  tacitly 
  done 
  when 
  

   only 
  the 
  latter 
  is 
  considered, 
  as 
  in 
  the 
  present 
  examples. 
  

  

  The 
  value 
  of 
  about 
  3^ 
  cal./sq. 
  cm. 
  min. 
  which 
  is 
  given 
  by 
  

   the 
  mean 
  of 
  the 
  reduced 
  Mt. 
  Wilson 
  observations 
  must 
  not 
  be 
  

   taken 
  as 
  an 
  independent 
  determination 
  of 
  the 
  solar 
  constant. 
  

   Other 
  and 
  better 
  methods 
  must 
  be 
  employed 
  to 
  get 
  a 
  reliable 
  

   value 
  of 
  this 
  quantity. 
  The 
  sole 
  merits 
  of 
  the 
  present 
  reduction 
  

   are 
  that 
  it 
  permits 
  the 
  approximate 
  utilization 
  of 
  measures 
  which 
  

   would 
  be 
  of 
  no 
  value 
  without 
  some 
  mode 
  of 
  supplying 
  the 
  lost 
  

   radiation, 
  and 
  that 
  it 
  justifies 
  the 
  assumption 
  that 
  the 
  radiant 
  

   depletion 
  is 
  most 
  intimately 
  connected 
  with 
  the 
  amount 
  of 
  

   aqueous 
  vapor 
  in 
  the 
  atmosphere. 
  

  

  In 
  spite 
  of 
  imperfections, 
  the 
  considerable 
  number 
  of 
  obser- 
  

   vations 
  averaged 
  justifies 
  some 
  general 
  conclusions. 
  The 
  use 
  

   of 
  the 
  chart 
  very 
  nearly 
  corrects 
  for 
  the 
  direct 
  effect 
  of 
  water 
  

   vapor 
  and 
  for 
  some 
  of 
  its 
  indirect 
  influence, 
  although 
  the 
  curves 
  

   might 
  be 
  improved 
  by 
  data 
  from 
  tropical 
  regions 
  of 
  large 
  vapor- 
  

   content. 
  The 
  very 
  marked 
  midday 
  depression 
  of 
  solar 
  radiation, 
  

   which 
  results 
  from 
  the 
  diurnal 
  change 
  in 
  atmospheric 
  trans- 
  

   missive 
  quality 
  and 
  whictj. 
  increases 
  with 
  the 
  amount 
  of 
  aqueous 
  

  

  * 
  Annals 
  Smithsonian 
  Observatory, 
  vol. 
  ii, 
  p. 
  119. 
  

  

  f 
  Compare 
  my 
  "Criterion 
  of 
  Accuracy," 
  etc., 
  already 
  cited, 
  and 
  especially 
  

   the 
  notable 
  work 
  by 
  Savelief 
  , 
  which 
  is 
  there 
  described. 
  

  

  