﻿464 
  

  

  Mr. 
  J. 
  E. 
  Petavel 
  on 
  a 
  Recording 
  

  

  corresponding 
  to 
  the 
  elastic 
  compression 
  of 
  the 
  material 
  of 
  

   which 
  the 
  spring 
  is 
  made. 
  This 
  motion 
  is 
  transmitted 
  to 
  the 
  

   exterior 
  by 
  the 
  rod 
  R. 
  

  

  Fig. 
  1. 
  — 
  Diagrammatic 
  Representation 
  of 
  the 
  Recording 
  Manometer. 
  

  

  The 
  lever 
  L 
  supporting 
  the 
  mirror 
  rests 
  on 
  the 
  fulcrum 
  F 
  

   at 
  " 
  3 
  } 
  '; 
  it 
  is 
  kept 
  against 
  the 
  knife-edge 
  " 
  2 
  " 
  of 
  R 
  by 
  the 
  

   tension 
  of 
  the 
  wire 
  W. 
  The 
  wire 
  W 
  is 
  of 
  considerable 
  length, 
  

   and 
  it 
  is 
  stretched 
  to 
  near 
  its 
  limit 
  of 
  elasticity. 
  The 
  lever 
  L 
  

   can 
  therefore 
  follow 
  the 
  small 
  advance 
  of 
  the 
  rod 
  R 
  without 
  

   greatly 
  diminishing 
  the 
  tension 
  of 
  the 
  wire 
  W. 
  

  

  The 
  mirror 
  focusses 
  a 
  point 
  source 
  of 
  light 
  on 
  to 
  a 
  

   rapidly 
  revolving 
  cylinder, 
  thus 
  recording 
  on 
  a 
  magnified 
  

   scale 
  the 
  motion 
  of 
  the 
  piston 
  P. 
  It 
  is 
  not 
  impossible 
  that 
  an 
  

   indicator 
  of 
  this 
  type 
  would 
  work 
  in 
  practice 
  ; 
  but 
  the 
  de- 
  

   flexion 
  of 
  the 
  mirror, 
  and 
  therefore 
  the 
  scale 
  of 
  the 
  records 
  

   obtained, 
  would 
  be 
  much 
  too 
  small. 
  To 
  increase 
  the 
  deflexions 
  

   three 
  modifications 
  are 
  necessary 
  : 
  the 
  spring 
  S 
  must 
  be 
  made 
  

   longer, 
  the 
  ratio 
  of 
  its 
  cross-sectional 
  area 
  to 
  that 
  of 
  the 
  piston 
  

   must 
  be 
  decreased, 
  and 
  the 
  knife-edges 
  " 
  2 
  " 
  and 
  " 
  3 
  " 
  brought 
  

   closer 
  together. 
  

  

  In 
  fig. 
  2 
  the 
  design 
  of 
  the 
  actual 
  instrument 
  is 
  given, 
  the 
  

   lettering 
  being 
  the 
  same 
  as 
  in 
  the 
  previous 
  figure. 
  

  

  By 
  means 
  of 
  the 
  thread 
  U 
  the 
  gauge 
  screws 
  into 
  the 
  ex- 
  

   plosion-chamber, 
  C 
  being 
  flush 
  with 
  the 
  inside 
  surface 
  ; 
  an 
  

   air-tight 
  joint 
  is 
  formed 
  by 
  the 
  ring 
  D 
  pressing 
  against 
  a 
  flat 
  

  

  