﻿36 
  Mr. 
  C. 
  F. 
  Brush 
  on 
  the 
  Transmission 
  of 
  

  

  operation 
  to 
  confirm 
  results 
  before 
  reducing 
  the 
  pressure 
  by 
  

   the 
  pump. 
  

  

  Observations 
  were 
  thus 
  made 
  at 
  pressures 
  varying 
  from 
  

   atmospheric 
  down 
  to 
  the 
  best 
  vacuum 
  obtainable. 
  In 
  some 
  

   instances 
  many 
  series 
  of 
  observations 
  were 
  made 
  at 
  varying 
  

   pressures 
  all 
  within 
  the 
  last 
  millionth. 
  The 
  gauge 
  could 
  be 
  

   relied 
  upon 
  to 
  measure 
  these 
  small 
  pressures 
  with 
  very 
  great 
  

   accuracy. 
  But 
  it 
  was 
  difficult 
  to 
  maintain 
  them 
  long 
  at 
  any 
  

   exactly 
  constant 
  value 
  on 
  account 
  of 
  the 
  continual, 
  though 
  

   slight, 
  evolution 
  of 
  gas 
  from 
  the 
  glass 
  of 
  the 
  apparatus. 
  

  

  As 
  I 
  desired 
  only 
  comparative 
  results, 
  no 
  correction 
  was 
  

   made 
  for 
  the 
  probable 
  slight 
  inequalities 
  in 
  the 
  callibration 
  

   of 
  the 
  thermometer 
  ; 
  nor 
  for 
  heat 
  conducted 
  to 
  or 
  from 
  the 
  

   bulb 
  by 
  the 
  stem 
  ; 
  nor 
  for 
  the 
  change 
  of 
  zero-point 
  due 
  to 
  

   changing 
  external 
  pressure. 
  The 
  mercury 
  fell 
  exactly 
  to 
  

   zero 
  at 
  atmospheric 
  pressure, 
  and 
  about 
  one-fiftieth 
  of 
  a 
  

   degree 
  lower 
  at 
  no 
  pressure. 
  The 
  pressure 
  error 
  due 
  to 
  

   differences 
  of 
  capillary 
  depression 
  in 
  the 
  two 
  barometers 
  was 
  

   ascertained 
  at 
  high 
  exhaustions, 
  and 
  found 
  nearly 
  constant. 
  

   It 
  was 
  always 
  corrected. 
  The 
  different 
  gases 
  used 
  were 
  care- 
  

   fully 
  prepared 
  and 
  dried, 
  and 
  were 
  introduced 
  quite 
  free 
  from 
  

   any 
  admixture 
  with 
  air. 
  

  

  My 
  observations 
  have 
  extended 
  over 
  a 
  long 
  period, 
  and 
  

   are 
  far 
  too 
  voluminous 
  to 
  be 
  recorded 
  here 
  in 
  detail. 
  But 
  I 
  

   have 
  embodied 
  their 
  most 
  salient 
  features 
  in 
  a 
  series 
  of 
  

   curves 
  which 
  render 
  them 
  readily 
  apparent 
  to 
  the 
  eye. 
  In 
  

   these 
  curves 
  the 
  abscissas 
  represent 
  the 
  pressure, 
  and 
  the 
  

   ordinates 
  represent 
  the 
  rate 
  of 
  heat 
  transmission 
  through 
  the 
  

   gas, 
  from 
  the 
  thermometer 
  bulb 
  to 
  the 
  ice-cold 
  envelope. 
  

   The 
  rate 
  of 
  transmission 
  at 
  any 
  particular 
  pressure 
  is 
  expressed 
  

   by 
  the 
  reciprocal 
  of 
  the 
  number 
  of 
  seconds 
  required 
  for 
  the 
  

   temperature 
  to 
  fall 
  through 
  a 
  given 
  number 
  of 
  degrees. 
  For 
  

   convenience 
  of 
  scale 
  all 
  the 
  reciprocals 
  are 
  multiplied 
  by 
  500. 
  

  

  PL 
  II. 
  shows 
  the 
  curve 
  for 
  air. 
  The 
  heavy 
  line 
  represents 
  

   the 
  rate 
  of 
  cooling 
  from 
  15 
  degrees 
  to 
  10 
  degrees. 
  It 
  is 
  in 
  

   three 
  sections, 
  A, 
  B, 
  and 
  C. 
  Section 
  A 
  embraces 
  the 
  whole 
  

   range 
  of 
  pressure 
  from 
  nothing 
  to 
  atmospheric 
  ; 
  section 
  B 
  

   embraces 
  the 
  range 
  of 
  pressure 
  from 
  nothing 
  to 
  '01 
  of 
  atmo- 
  

   spheric 
  ; 
  and 
  section 
  C 
  embraces 
  the 
  range 
  of 
  pressure 
  from 
  

   nothing 
  to 
  "0001 
  of 
  atmospheric, 
  i. 
  e. 
  100 
  M. 
  (one 
  hundred 
  

   millionths). 
  Atmospheric 
  pressure 
  is 
  taken 
  at 
  760 
  millim. 
  

   Thus 
  it 
  will 
  be 
  seen 
  that 
  section 
  B 
  is 
  the 
  last 
  hundredth 
  of 
  A, 
  

   magnified 
  a 
  hundred 
  times 
  ; 
  and 
  section 
  C 
  is 
  the 
  last 
  

   hundredth 
  of 
  B, 
  magnified 
  a 
  hundred 
  times. 
  The 
  cross- 
  

   section 
  paper 
  on 
  which 
  these 
  curves 
  are 
  drawn 
  is 
  50 
  centi- 
  

   metres 
  wide 
  ; 
  hence 
  if 
  the 
  curve 
  were 
  completed 
  on 
  the 
  scale 
  

  

  