﻿468 
  Mr. 
  J. 
  E. 
  Petavel 
  on 
  a 
  Recording 
  

  

  The 
  quantity 
  of 
  heat 
  dissipated 
  per 
  unit 
  of 
  cooling 
  surface 
  

   increases 
  with 
  the 
  temperature 
  interval 
  and 
  with 
  the 
  pressure 
  

   of 
  the 
  gas, 
  but 
  not 
  at 
  the 
  same 
  rate 
  as 
  the 
  latter 
  "*. 
  The 
  heat 
  

   developed, 
  on 
  the 
  other 
  hand, 
  is 
  simply 
  proportional 
  to 
  the 
  

   pressure. 
  

  

  By 
  increasing 
  the 
  pressure 
  from 
  1 
  to 
  70 
  atmospheres 
  we 
  

   increase 
  the 
  heat 
  generated 
  in 
  a 
  given 
  volume 
  70 
  times, 
  but 
  

   we 
  do 
  not 
  increase 
  the 
  rate 
  at 
  which 
  heat 
  is 
  dissipated 
  in 
  

   anything 
  like 
  the 
  same 
  ratio. 
  The 
  increase 
  of 
  efficiency 
  

   which, 
  in 
  the 
  case 
  of 
  gas-engines, 
  has 
  always 
  been 
  connected 
  

   with 
  the 
  use 
  of 
  high 
  initial 
  pressures, 
  is 
  mainly 
  due 
  to 
  this 
  

   cause. 
  It 
  is 
  also 
  to 
  some 
  extent 
  due 
  to 
  the 
  higher 
  tempe- 
  

   rature 
  obtained 
  and 
  to 
  the 
  smaller 
  dimensions 
  of 
  the 
  moving 
  

   parts. 
  

  

  One 
  more 
  point 
  deserves 
  attention. 
  It 
  will 
  be 
  noticed 
  that 
  

   0*05 
  second 
  after 
  firing 
  the 
  rate 
  of 
  rise 
  of 
  pressure 
  suddenly 
  

   increases, 
  and 
  becomes 
  over 
  nine 
  times 
  as 
  fast 
  as 
  before. 
  

   For 
  the 
  less 
  explosive 
  mixtures 
  this 
  change 
  in 
  curvature 
  does 
  

   not 
  occur, 
  the 
  curve 
  of 
  rise 
  of 
  pressure 
  being 
  similar 
  to 
  the 
  

   cooling-curve, 
  only 
  of 
  course 
  much 
  steeper. 
  It 
  is 
  worthy 
  of 
  

   note 
  that 
  the 
  change 
  of 
  curvature 
  occurs 
  when 
  the 
  gas 
  is 
  at 
  a 
  

   mean 
  temperature 
  about 
  equal 
  to 
  that 
  at 
  which 
  spontaneous 
  

   ignition 
  would 
  take 
  place. 
  A 
  similar 
  result 
  would 
  therefore 
  

   be 
  obtained 
  if 
  we 
  heated 
  the 
  gases 
  by 
  the 
  combustion 
  of 
  a 
  

   certain 
  portion 
  of 
  them 
  until 
  the 
  entire 
  bulk 
  was 
  at 
  the 
  

   " 
  flash-point 
  "; 
  the 
  combination 
  would 
  then 
  take 
  place 
  simul- 
  

   taneously 
  throughout 
  the 
  entire 
  mass, 
  resulting 
  in 
  an 
  almost 
  

   instantaneous 
  rise 
  to 
  the 
  maximum 
  temperature 
  and 
  pressure. 
  

   Explosives 
  may 
  be 
  divided 
  into 
  three 
  classes 
  — 
  instantaneous, 
  

   medium, 
  slow, 
  — 
  according 
  to 
  the 
  speed 
  at 
  which 
  the 
  maximum 
  

   pressure 
  is 
  attained. 
  The 
  first 
  class 
  comprises 
  fulminate 
  of 
  

   mercury, 
  nitro-glycerine, 
  &c. 
  : 
  the 
  duration 
  of 
  explosion 
  being- 
  

   equal 
  to 
  or 
  less 
  than 
  the 
  natural 
  period 
  of 
  vibration 
  of 
  solid 
  

   bodies, 
  the 
  term 
  pressure 
  hardly 
  applies, 
  the 
  phenomenon 
  

   being 
  not 
  static, 
  but 
  kinetic, 
  of 
  the 
  nature 
  of 
  an 
  impact 
  rather 
  

   than 
  a 
  pressure. 
  If 
  the 
  ignition 
  takes 
  place 
  in 
  a 
  closed 
  vessel, 
  

   a 
  certain 
  pressure 
  will 
  follow 
  the 
  combustion 
  ; 
  but 
  this 
  pres- 
  

   sure 
  is 
  no 
  measure 
  of 
  the 
  strains 
  exerted. 
  The 
  second 
  class 
  

   would 
  contain 
  such 
  explosives 
  as 
  ordinary 
  gunpowder, 
  cordite, 
  

   and 
  mixtures 
  of 
  hydrogen 
  or 
  coal-gas 
  with 
  oxygen 
  or 
  air. 
  

   The 
  time 
  of 
  rise 
  varying 
  according 
  to 
  circumstances 
  from 
  

   0*0001 
  to 
  O'l 
  second. 
  The 
  more 
  diluted 
  mixtures 
  of 
  hydrogen 
  

   and 
  of 
  coal-gas 
  take 
  their 
  place 
  in 
  the 
  third 
  category 
  with 
  

   speeds 
  from 
  0*1 
  second 
  to 
  2 
  seconds. 
  

  

  * 
  See 
  Phil. 
  Trans, 
  of 
  Eoyal 
  Soc. 
  vol. 
  cxcvii. 
  p. 
  229 
  (1901). 
  

  

  