﻿110 
  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1927 
  

  

  attempted 
  to 
  improve 
  on 
  these 
  first 
  results 
  by 
  constructing 
  a 
  lighter 
  

   system, 
  using 
  flies' 
  wings 
  to 
  prepare 
  the 
  vanes 
  of 
  the 
  instrument. 
  

   But 
  he 
  found 
  that 
  at 
  the 
  air 
  pressure 
  required 
  to 
  give 
  a 
  good 
  radiom- 
  

   eter 
  deflection, 
  the 
  system 
  was 
  damped 
  into 
  such 
  sluggishness 
  as 
  

   to 
  be 
  useless. 
  At 
  the 
  suggestion 
  of 
  Doctor 
  Anderson 
  of 
  Mount 
  

   Wilson 
  Observatory, 
  Doctor 
  Abbot 
  proposed 
  to 
  substitute 
  hydro- 
  

   gen 
  for 
  air, 
  hoping 
  to 
  get 
  equal 
  sensitiveness 
  and 
  much 
  less 
  damp- 
  

   ing. 
  As 
  hydrogen 
  would 
  be 
  contaminated 
  by 
  air 
  leakage, 
  or 
  cock 
  

   grease, 
  or 
  mercury 
  vapor 
  if 
  connected 
  with 
  an 
  air 
  pump, 
  as 
  usual, 
  he 
  

   proposed 
  to 
  seal 
  up 
  the 
  suspended 
  system 
  in 
  glass 
  like 
  an 
  X-ray 
  

   tube, 
  having 
  first 
  exhausted 
  the 
  glass 
  case 
  as 
  completely 
  as 
  possible, 
  

   and 
  filled 
  in 
  pure 
  hydrogen 
  (through 
  a 
  liquid 
  air 
  trap) 
  to 
  the 
  

   desired 
  pressure. 
  

  

  The 
  necessity 
  of 
  rotating 
  the 
  suspended 
  system 
  with 
  reference 
  to 
  

   the 
  glass 
  case 
  and 
  its 
  optical 
  windows 
  offered 
  difficulties. 
  However, 
  

   this 
  was 
  accomplished 
  by 
  including 
  within 
  the 
  case 
  a 
  train 
  of 
  gear- 
  

   ing 
  ending 
  in 
  a 
  little 
  horseshoe 
  magnet, 
  which 
  could 
  be 
  rotated 
  

   by 
  another 
  magnet 
  from 
  without. 
  The 
  reduction 
  of 
  speed 
  from 
  the 
  

   magnet 
  to 
  the 
  suspension 
  system 
  was 
  in 
  all 
  nearly 
  10,000-fold, 
  so 
  

   that 
  a 
  little 
  reversible 
  electric 
  motor, 
  with 
  cone 
  drive, 
  was 
  arranged 
  

   to 
  drive 
  the 
  outside 
  magnet 
  through 
  so 
  many 
  thousands 
  of 
  turns. 
  

   All 
  of 
  these 
  contrivances 
  were 
  constructed 
  by 
  Mr. 
  Kramer 
  under 
  

   Doctor 
  Abbot's 
  direction, 
  but 
  the 
  actual 
  expedition 
  to 
  Mount 
  Wilson 
  

   did 
  not 
  go 
  forward 
  until 
  July, 
  1927, 
  and 
  will 
  be 
  described 
  in 
  the 
  re- 
  

   port 
  of 
  1928. 
  It 
  may 
  be 
  worth 
  while 
  to 
  add, 
  however, 
  that 
  Mr. 
  

   Aldrich 
  tested 
  occasionally 
  for 
  10 
  months, 
  by 
  weighings, 
  the 
  evapo- 
  

   ration 
  of 
  a 
  large 
  surface 
  of 
  beeswax 
  laid 
  down 
  on 
  thin 
  mica. 
  The 
  

   loss 
  was 
  so 
  very 
  slight 
  that 
  this 
  substance 
  was 
  found 
  quite 
  suitable 
  

   to 
  fasten 
  the 
  parts 
  of 
  the 
  radiometer 
  siispension 
  without 
  fear 
  of 
  ap- 
  

   preciably 
  contaminating 
  the 
  hydrogen 
  by 
  mixture 
  of 
  its 
  heavy 
  mole- 
  

   cules. 
  

  

  (h) 
  Pyrheliometer. 
  — 
  Although 
  the 
  Calif 
  ornian 
  and 
  the 
  South 
  

   West 
  African 
  equipments 
  had 
  been 
  supplied 
  with 
  silver-disk 
  pyrheli- 
  

   omcters 
  Avith 
  very 
  long 
  vestibules 
  to 
  cut 
  down 
  the 
  effect 
  of 
  atmos- 
  

   pheric 
  radiation 
  immediately 
  surrounding 
  the 
  sun, 
  and 
  though 
  all 
  of 
  

   our 
  observatories 
  have 
  been 
  equipped 
  with 
  half-second 
  pendulums 
  to 
  

   reduce 
  error 
  in 
  time 
  observations 
  at 
  the 
  pyrheliometers, 
  yet 
  we 
  are 
  

   seeking 
  a 
  degree 
  of 
  accuracy 
  so 
  high 
  that 
  an 
  attempt 
  seemed 
  desirable 
  

   to 
  devise 
  a 
  new 
  type 
  of 
  pyrheliometer 
  in 
  which 
  errors 
  would 
  be 
  still 
  

   more 
  reduced. 
  This 
  instrument 
  was 
  not 
  entirely 
  completed 
  at 
  the 
  

   close 
  of 
  the 
  period 
  of 
  this 
  report, 
  and 
  its 
  performance 
  will 
  be 
  de- 
  

   scribed 
  in 
  the 
  report 
  for 
  1928. 
  

  

  (c) 
  Revision 
  of 
  ohservations.— 
  The 
  important 
  work 
  of 
  revision 
  of 
  

   s^oiar 
  radiation 
  measurements 
  mentioned 
  in 
  last 
  year's 
  report 
  was 
  

   prosecuted 
  vigorously 
  during 
  the 
  fiscal 
  year. 
  A 
  complete 
  re-reduc- 
  

  

  