﻿•466 
  Mr. 
  J. 
  E. 
  Petavel 
  on 
  a 
  Recording 
  

  

  With 
  regard 
  to 
  the 
  chronograph 
  there 
  is 
  but 
  little 
  to 
  be 
  

   said: 
  the 
  films 
  on 
  which 
  the 
  deflexions 
  are 
  photographically 
  

   recorded 
  are 
  wound 
  on 
  a 
  drum 
  which 
  is 
  kept 
  in 
  rapid 
  rotation 
  

   by 
  an 
  electric 
  motor 
  ; 
  the 
  usual 
  devices 
  being 
  used 
  to 
  regulate 
  

   and 
  measure 
  the 
  speed. 
  The 
  drum 
  is 
  inclosed 
  in 
  a 
  light-tight 
  

   box; 
  a 
  long 
  narrow 
  slit 
  (about 
  o 
  1 
  ^ 
  inch 
  in 
  width) 
  runs 
  the 
  entire 
  

   length 
  of 
  the 
  box 
  parallel 
  to 
  the 
  axis 
  o£ 
  rotation. 
  One 
  of 
  the 
  

   filaments 
  of 
  an 
  incandescent 
  lamp 
  is 
  focussed 
  by 
  the 
  mirror 
  

   on 
  to 
  this 
  slit, 
  forming 
  a 
  fine 
  straight 
  line 
  perpendicular 
  to 
  

   the 
  axis 
  of 
  rotation 
  and 
  to 
  the 
  slit. 
  The 
  sharp 
  point 
  of 
  light 
  

   thus 
  formed 
  on 
  the 
  film 
  moves 
  from 
  right 
  to 
  left 
  as 
  the 
  pressure 
  

   increases. 
  To 
  secure 
  the 
  quality 
  and 
  intensity 
  of 
  light 
  which 
  

   is 
  necessary, 
  the 
  lamp 
  is 
  run 
  at 
  twice 
  its 
  normal 
  voltage 
  at 
  

   the 
  moment 
  of 
  the 
  explosion. 
  

  

  To 
  avoid 
  the 
  blurring 
  of 
  the 
  zero-line 
  the 
  light 
  is 
  cut 
  off 
  an 
  

   instant 
  later, 
  and 
  the 
  zero 
  marked 
  in 
  when 
  the 
  products 
  of 
  

   the 
  explosion 
  have 
  had 
  ample 
  time 
  to 
  cool 
  to 
  atmospheric 
  

   temperature. 
  

  

  The 
  gauge 
  is 
  calibrated 
  by 
  hydraulic 
  pressure, 
  and 
  the 
  

   results 
  are 
  checked 
  by 
  comparison 
  with 
  the 
  values 
  of 
  the 
  

   maximum 
  pressure 
  obtained 
  by 
  the 
  statical 
  method 
  described 
  

   at 
  last 
  year's 
  Meeting 
  of 
  the 
  British 
  Association 
  *. 
  

  

  For 
  each 
  explosive 
  mixture 
  two 
  records 
  are 
  taken, 
  one 
  at 
  a 
  

   high 
  speed 
  giving 
  the 
  rise 
  of 
  the 
  pressure, 
  the 
  other 
  at 
  a 
  low 
  

   speed 
  giving 
  the 
  rate 
  of 
  cooling. 
  

  

  Records 
  obtained. 
  

  

  Two 
  typical 
  records 
  are 
  given 
  in 
  Plates 
  IX. 
  and 
  X.f; 
  they 
  

   refer 
  to 
  a 
  mixture 
  of 
  air 
  and 
  coal-gas 
  fired 
  at 
  an 
  initial 
  pressure 
  

   of 
  about 
  eleven 
  hundred 
  pounds 
  per 
  square 
  inch. 
  Oxygen 
  

   was 
  in 
  excess, 
  the 
  residual 
  gases 
  containing 
  about 
  3 
  per 
  cent, 
  

   of 
  it. 
  

  

  In 
  tracing 
  out 
  diagrams 
  like 
  the 
  above, 
  no 
  part 
  of 
  the 
  

   instrument 
  except 
  the 
  light 
  frame 
  carrying 
  the 
  mirror 
  moves 
  

   more 
  than 
  one 
  or 
  two 
  thousandths 
  of 
  an 
  inch. 
  In 
  spite, 
  

   therefore, 
  of 
  the 
  suddenness 
  of 
  the 
  rise 
  in 
  pressure 
  the 
  velocity 
  

   of 
  the 
  moving 
  parts 
  remains 
  small 
  and 
  the 
  usual 
  trouble 
  

   due 
  to 
  inertia 
  does 
  not 
  arise. 
  PI. 
  IX. 
  is 
  an 
  illustration 
  of 
  this 
  

   fact 
  ; 
  0*055 
  of 
  a 
  second 
  after 
  ignition 
  the 
  pressure 
  is 
  rising 
  

   at 
  the 
  rate 
  of 
  over 
  a 
  million 
  pounds 
  per 
  square 
  inch 
  per 
  second, 
  

   none 
  the 
  less 
  the 
  curve 
  turns 
  sharply 
  at 
  nearly 
  a 
  right 
  angle 
  

  

  * 
  See 
  Proceedings 
  of 
  the 
  Brit. 
  Assoc, 
  Bradford, 
  1900, 
  p. 
  655. 
  

  

  f 
  The 
  original 
  films 
  can 
  be 
  measured 
  to 
  about 
  one 
  thousandth 
  of 
  an 
  

   inch 
  ; 
  the 
  points 
  given 
  in 
  Plates 
  IX. 
  and 
  X. 
  have 
  lost 
  somewhat 
  ir. 
  

   sharpness, 
  but 
  are 
  still 
  sufficiently 
  good 
  to 
  give 
  a 
  fair 
  idea 
  of 
  the 
  records 
  

  

  