﻿F. 
  W. 
  Very 
  — 
  Solar 
  Radiation. 
  609 
  

  

  Art. 
  LIT. 
  — 
  Solar 
  Radiation 
  ; 
  by 
  Feaxk 
  W. 
  Yery. 
  

  

  There 
  are 
  two 
  modes 
  of 
  measuring 
  solar 
  radiation 
  which 
  

   may 
  be 
  called 
  the 
  actinometric 
  and 
  the 
  bolometric 
  modes. 
  

   The 
  former 
  measures 
  the 
  intensity 
  of 
  the 
  total 
  normal 
  radia- 
  

   tion 
  on 
  unit 
  surface 
  and 
  is 
  the 
  more 
  direct 
  of 
  the 
  two. 
  The 
  

   bolometric 
  method 
  traces 
  the 
  distribution 
  of 
  energy 
  in 
  the 
  

   solar 
  spectrum, 
  determines 
  the 
  wave-length 
  of 
  the 
  maximum 
  

   point 
  in 
  the 
  spectral 
  energy-curve, 
  and 
  infers 
  by 
  theory 
  the 
  

   temperature 
  and 
  radiating 
  power 
  of 
  the 
  solar 
  surface, 
  from 
  

   which 
  the 
  intensity 
  of 
  radiation 
  at 
  the 
  earth's 
  distance 
  is 
  at 
  

   once 
  known, 
  since 
  the 
  radiant 
  energy 
  diminishes 
  in 
  inverse 
  

   proportion 
  to 
  the 
  squares 
  of 
  sun's 
  radius 
  and 
  earth's 
  distance 
  

   which 
  on 
  the 
  average 
  is 
  given 
  by 
  the 
  formula 
  

  

  log 
  (R/D) 
  1 
  = 
  5-33535 
  — 
  10.* 
  

  

  Both 
  methods 
  require 
  a 
  knowledge 
  of 
  the 
  corrections 
  to 
  be 
  

   applied 
  for 
  instrumental 
  errors 
  and 
  for 
  absorption 
  by 
  the 
  earth's 
  

   atmosphere. 
  

  

  There 
  are 
  now 
  several 
  reliable 
  actinometers, 
  capable, 
  when 
  

   properly 
  handled, 
  of 
  giving 
  results 
  correct 
  to 
  1 
  or 
  2 
  per 
  cent. 
  

   Ub 
  fortunately, 
  some 
  of 
  these 
  instruments 
  may 
  give 
  readings 
  

   which 
  are 
  as 
  much 
  as 
  20 
  per 
  cent 
  in 
  error 
  when 
  inefficiently 
  

   manipulated, 
  or 
  imperfectly 
  corrected. 
  The 
  prime 
  necessity, 
  

   after 
  the 
  attainment 
  of 
  a 
  moderate 
  amount 
  of 
  skill, 
  is 
  that 
  the 
  

   observer 
  shall 
  make 
  a 
  thorough 
  study 
  of 
  the 
  theory 
  of 
  his 
  

   instrument, 
  become 
  familiar 
  with 
  the 
  necessary 
  precautions, 
  

   measure 
  the 
  instrumental 
  constants 
  with 
  the 
  greatest 
  attainable 
  

   precision, 
  and 
  apply 
  all 
  of 
  the 
  corrections 
  indicated 
  by 
  theory 
  

   which 
  have 
  a 
  sensible 
  magnitude. 
  

  

  The 
  spectrobolometric 
  method, 
  invented 
  by 
  Langley 
  and 
  

   now 
  perfected 
  by 
  the 
  observers 
  at 
  the 
  Smithsoniau 
  Institution, 
  

   has 
  corrections 
  which 
  are 
  more 
  complicated 
  than 
  those 
  of 
  the 
  

   actinometer, 
  but 
  with 
  reasonable 
  precautions 
  either 
  method 
  

   may 
  now 
  be 
  put 
  on 
  a 
  tolerably 
  safe 
  basis 
  as 
  far 
  as 
  the 
  instru- 
  

   mental 
  corrections 
  are 
  concerned. 
  

  

  The 
  corrections 
  for 
  atmospheric 
  interference 
  with 
  the 
  radi- 
  

   ant 
  transmission 
  are 
  in 
  an 
  altogether 
  different 
  category. 
  

   Hypotheses 
  and 
  mathematical 
  discussions 
  are 
  required, 
  and 
  

   when 
  all 
  is 
  clone 
  that 
  human 
  ingenuity 
  can 
  accomplish, 
  the 
  

   final 
  result 
  can 
  hardly 
  be 
  dignified 
  as 
  a 
  "measurement" 
  of 
  a 
  

   " 
  solar 
  constant," 
  but 
  must 
  be 
  conceded 
  to 
  be 
  scarcely 
  more 
  

  

  * 
  The 
  distance-factor 
  for 
  any 
  given 
  date 
  may 
  be 
  obtained 
  by 
  taking 
  the 
  

   semidiameter 
  of 
  the 
  sun 
  which 
  is 
  given 
  for 
  each 
  day 
  of 
  the 
  year 
  in 
  the 
  solar 
  

   ephemeris, 
  looking 
  out 
  the 
  log 
  sine 
  of 
  this 
  angle 
  and 
  doubling 
  it, 
  and 
  sub- 
  

   tracting 
  from 
  the 
  above 
  logarithm. 
  

  

  Am. 
  Jour. 
  Sci. 
  — 
  Fourth 
  Series, 
  Vol. 
  XXXVI, 
  No. 
  216. 
  — 
  December, 
  1913. 
  

   41 
  

  

  