﻿696 
  Mr. 
  J. 
  R. 
  Boon 
  on 
  the 
  Effect 
  of 
  Pressure 
  on 
  the 
  

  

  were 
  introduced 
  as 
  shown 
  in 
  the 
  diagram. 
  A 
  wide 
  short- 
  

   circuiting 
  tap 
  S 
  also 
  joined 
  the 
  two 
  sides 
  o£ 
  the 
  gauge, 
  for 
  

   use 
  when 
  the 
  apparatus 
  was 
  being 
  pumped 
  out. 
  The 
  dis- 
  

   charge-tube 
  was 
  made 
  air-tight 
  with 
  picein. 
  The 
  discharging 
  

   point 
  P 
  was 
  sheathed 
  almost 
  to 
  its 
  end 
  with 
  glass 
  and 
  was 
  

   supported 
  by 
  a 
  thin 
  steel 
  rod 
  sliding 
  in 
  a 
  mercury-tight 
  

   stuffing-box, 
  B, 
  which 
  fitted 
  into 
  the 
  insulating 
  ebonite 
  

   plug. 
  

  

  Round 
  the 
  edge 
  of 
  the 
  plane, 
  but 
  insulated 
  from 
  it, 
  and 
  

   supported 
  by 
  a 
  slight 
  constriction 
  in 
  the 
  tube, 
  was 
  an 
  earthed 
  

   mercury 
  trough. 
  This 
  was 
  introduced 
  to 
  catch 
  any 
  current 
  

   due 
  to 
  discharge 
  down 
  the 
  sides 
  of 
  the 
  glass 
  which, 
  if 
  it 
  found 
  

   its 
  way 
  to 
  the 
  plane, 
  would 
  contribute 
  to 
  the 
  total 
  current 
  

   but 
  not 
  to 
  the 
  wind. 
  

  

  To 
  measure 
  z 
  Q 
  accurately 
  the 
  original 
  form 
  of 
  ring 
  electrode 
  

   used 
  by 
  Chattock 
  is 
  not 
  suitable, 
  as 
  it 
  is 
  not 
  possible 
  with 
  

   this 
  to 
  measure 
  z 
  accurately, 
  though 
  differences 
  in 
  z 
  can 
  be 
  

   measured. 
  An 
  attempt 
  was 
  made 
  to 
  replace 
  this 
  by 
  a 
  plane 
  

   consisting 
  of 
  a 
  series 
  of 
  layers 
  of 
  the 
  finest 
  mesh 
  copper 
  gauze, 
  

   but 
  it 
  was 
  found 
  that 
  the 
  flow 
  of 
  gas 
  through 
  this 
  was 
  not 
  

   viscous, 
  and 
  that 
  consequently 
  the 
  pressure 
  indicated 
  by 
  the 
  

   gauge 
  was 
  not 
  the 
  average 
  pressure 
  of 
  the 
  wind 
  on 
  the 
  plane. 
  

   As 
  in 
  the 
  later 
  work 
  of 
  Chattock 
  and 
  Tyndall, 
  a 
  plate 
  was 
  

   finally 
  made 
  from 
  a 
  sheet 
  of 
  zinc 
  by 
  perforating 
  it 
  with 
  fine 
  

   holes 
  by 
  means 
  of 
  a 
  needle. 
  It 
  is 
  necessary 
  in 
  such 
  a 
  plate 
  

   that 
  the 
  perforations 
  should 
  be 
  all 
  equal 
  and 
  equally 
  spaced, 
  

   and 
  that 
  the 
  flow 
  of 
  gas 
  (if 
  any) 
  through 
  the 
  plate 
  should 
  be 
  

   proportional 
  to 
  the 
  difference 
  of 
  pressure 
  producing 
  it. 
  The 
  

   differences 
  of 
  pressure 
  produced 
  on 
  the 
  two 
  sides 
  of 
  the 
  plate 
  

   were 
  so 
  small 
  that 
  this 
  last 
  condition 
  was 
  approximately 
  

   fulfilled. 
  The 
  plate 
  PI 
  was 
  connected 
  through 
  a 
  galvano- 
  

   meter 
  to 
  earth, 
  as 
  shown 
  in 
  the 
  figure. 
  

  

  Results. 
  

  

  The 
  measurements 
  of 
  wind-pressure 
  p 
  and 
  distance 
  between 
  

   electrodes 
  z 
  for 
  a 
  constant 
  current 
  have 
  been 
  carried 
  out 
  in 
  

   air 
  and 
  in 
  C0 
  2 
  over 
  a 
  range 
  of 
  atmospheric 
  pressure 
  from 
  

   76 
  cm. 
  to 
  20 
  cm., 
  and 
  the 
  results 
  are 
  collected 
  in 
  the 
  

   Curves 
  I., 
  II., 
  and 
  III. 
  (pp. 
  697, 
  698). 
  

  

  Curves 
  I. 
  and 
  II. 
  show 
  some 
  typical 
  p 
  and 
  z 
  results, 
  the 
  

   actual 
  results 
  plotted 
  being 
  those 
  for 
  negative 
  and 
  positive 
  

   discharge 
  in 
  air 
  for 
  a 
  current 
  of 
  1*24 
  x 
  10 
  ~ 
  6 
  amp. 
  (Different 
  

   scales 
  of 
  p 
  are 
  used 
  for 
  convenience 
  of 
  arrangement.) 
  From 
  

   such 
  curves 
  as 
  these 
  the 
  values 
  of 
  z 
  can 
  be 
  read 
  off 
  at 
  

   different 
  atmospheric 
  pressures 
  II. 
  

  

  