﻿104 
  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  194 
  3 
  

  

  about 
  a 
  single 
  polar 
  axis. 
  The 
  boiler 
  then 
  becomes 
  a 
  tube 
  of 
  small 
  

   diameter 
  lying 
  in 
  the 
  axis. 
  The 
  vacuum 
  sheath 
  is 
  an 
  elongated 
  

   Pyrex 
  thermos 
  bottle, 
  of 
  which 
  about 
  one-third 
  the 
  circumference 
  of 
  

   the 
  outer 
  surface 
  of 
  the 
  inner 
  wall 
  is 
  gold-plated. 
  The 
  mirror 
  is 
  

   a 
  rectangular 
  concave 
  cylindric 
  mirror, 
  of 
  parabolic 
  curvature, 
  whose 
  

   equation, 
  as 
  I 
  prefer 
  it, 
  is 
  y 
  2 
  = 
  ZGx. 
  The 
  mirror 
  is 
  long 
  and 
  narrow 
  

   and 
  rotates 
  about 
  its 
  focus, 
  the 
  polar 
  axis. 
  Being 
  long 
  compared 
  to 
  

   its 
  width, 
  the 
  deliberate 
  end-loss 
  of 
  light 
  at 
  the 
  solstices, 
  June 
  20 
  and 
  

   December 
  20, 
  from 
  neglecting 
  the 
  sun's 
  motion 
  in 
  declination, 
  is 
  not 
  

   serious, 
  and 
  this 
  loss 
  becomes 
  zero 
  at 
  the 
  equinoxes. 
  The 
  metallic 
  

  

  Figote 
  2. 
  — 
  Diagram 
  of 
  flash 
  boiler 
  (4), 
  in 
  vacuum 
  sheath 
  (1), 
  served 
  by 
  water 
  

   supply 
  (2), 
  governed 
  by 
  air 
  pressure 
  in 
  steam 
  chest 
  (7), 
  operating 
  extensible 
  

   chamber 
  (14), 
  governing 
  injector 
  (15). 
  First 
  arrangement. 
  

  

  boiler 
  tube 
  is 
  blackened 
  by 
  painting 
  with 
  a 
  suspension 
  of 
  lampblack 
  

   in 
  alcohol 
  with 
  a 
  slight 
  addition 
  of 
  shellac. 
  At 
  the 
  low 
  temperatures, 
  

   not 
  exceeding 
  200° 
  C, 
  this 
  paint 
  does 
  not 
  burn. 
  It 
  absorbs 
  about 
  95 
  

   percent 
  of 
  the 
  solar 
  radiation. 
  

  

  The 
  mirror 
  in 
  my 
  small 
  model 
  comprises 
  several 
  castings 
  of 
  alu- 
  

   minum, 
  machined 
  to 
  accurate 
  parabolic 
  curves 
  and 
  joined 
  by 
  L-string- 
  

   ers 
  of 
  duralumin 
  to 
  form 
  a 
  cradle. 
  To 
  this 
  cradle 
  are 
  attached 
  Alcoa 
  

   sheets 
  not 
  previously 
  bent, 
  and 
  held 
  down 
  by 
  narrow 
  metal 
  straps 
  

   screwed 
  through 
  the 
  sheets 
  to 
  the 
  parabolic 
  frames. 
  At 
  the 
  ends 
  the 
  

   mirror 
  frame 
  supports 
  steel 
  hanger 
  bars 
  which 
  carry 
  hollow 
  trun- 
  

   nions, 
  and 
  supports 
  above 
  a 
  counterbalance 
  bar 
  of 
  metal, 
  set 
  edgewise 
  

   to 
  the 
  beam 
  and 
  extending 
  from 
  end 
  to 
  end 
  of 
  the 
  mirror 
  as 
  a 
  stif- 
  

  

  