﻿Dr. 
  C. 
  Chree's 
  Notes 
  on 
  Thermometry. 
  305 
  

  

  accuracy 
  even 
  of 
  the 
  order 
  0°*005 
  C* 
  in 
  the 
  absolute 
  value 
  of 
  

   a 
  boiling-point 
  determination 
  would 
  appear 
  highly 
  problem- 
  

   atical. 
  It 
  is 
  not 
  unlikely, 
  however, 
  that 
  many 
  of 
  the 
  discre- 
  

   pancies 
  arose 
  in 
  great 
  part 
  from 
  changes 
  in 
  the 
  portable 
  

   barometers 
  during 
  transit. 
  Barometers, 
  even 
  with 
  contracted 
  

   tubes, 
  are 
  awkward 
  things 
  to 
  carry 
  by 
  rail, 
  and 
  trustworthy 
  

   results 
  can 
  only 
  be 
  obtained 
  after 
  the 
  transported 
  instrument 
  

   has 
  been 
  hung 
  up 
  for 
  a 
  considerable 
  time. 
  Not 
  unlikely 
  

   Prof. 
  Mohn's 
  | 
  suggestion, 
  viz. 
  to 
  compare 
  barometers 
  by 
  the 
  

   aid 
  of 
  a 
  travelling 
  hyposometric 
  thermometer, 
  might 
  prove 
  a 
  

   good 
  one, 
  at 
  least 
  in 
  the 
  case 
  of 
  stations 
  where 
  appliances 
  

   exist 
  for 
  observing 
  both 
  boiling- 
  and 
  freezing-points 
  with 
  the 
  

   highest 
  degree 
  of 
  accuracy. 
  

  

  § 
  24. 
  There 
  are 
  other 
  sources 
  of 
  uncertainty 
  in 
  boiling- 
  

   point 
  determinations. 
  Every 
  thermometer 
  has 
  a 
  time-lag, 
  

   and 
  so 
  probably 
  has 
  every 
  barometer 
  ; 
  hence 
  simultaneous 
  

   readings 
  of 
  thermometer 
  and 
  barometer 
  may 
  not 
  really 
  cor- 
  

   respond 
  to 
  simultaneous 
  temperature 
  and 
  pressure 
  phenomena. 
  

   The 
  columns 
  of 
  wide-tube 
  barometers, 
  in 
  which 
  the 
  lag 
  is 
  

   least, 
  generally 
  pulsate 
  slightly 
  even 
  to 
  the 
  eye, 
  unless 
  under 
  

   exceptionally 
  steady 
  conditions 
  of 
  pressure. 
  Under 
  normal 
  

   conditions, 
  one 
  may 
  expect 
  changes 
  of 
  several 
  thousandths 
  of 
  

   an 
  inch 
  in 
  barometric 
  pressure 
  during 
  the 
  time 
  occupied 
  by 
  a 
  

   boiling-point 
  determination. 
  

  

  In 
  the 
  paper 
  already 
  referred 
  to, 
  Mr. 
  Griffiths 
  raises 
  the 
  

   question 
  whether 
  the 
  temperature 
  of 
  steam 
  accommodates 
  

   itself 
  immediately 
  to 
  the 
  barometric 
  pressure 
  when 
  the 
  latter 
  

   changes. 
  On 
  his 
  p. 
  230 
  he, 
  however, 
  expresses 
  the 
  opinion 
  

   that 
  a 
  table 
  on 
  his 
  p. 
  23.1 
  i( 
  will 
  . 
  . 
  . 
  convince 
  the 
  most 
  sceptical 
  

   that 
  the 
  temperature-lag 
  of 
  the 
  steam 
  may 
  be 
  disregarded." 
  

   I 
  am 
  not 
  sure 
  his 
  argument 
  will 
  be 
  universally 
  conceded, 
  as 
  

   the 
  data 
  — 
  consisting 
  of 
  observed 
  boiling-points 
  by 
  a 
  platinum 
  

   thermometer, 
  and 
  boiling-points 
  calculated 
  from 
  simultaneous 
  

   readings 
  of 
  a 
  barometer 
  in 
  the 
  course 
  of 
  one 
  afternoon 
  — 
  

   might 
  have 
  been 
  advanced, 
  with 
  perhaps 
  equal 
  force, 
  in 
  

   support 
  of 
  the 
  contention 
  that 
  barometers 
  and 
  platinum 
  

   thermometers 
  are 
  instruments 
  free 
  from 
  lag. 
  The 
  uncertain- 
  

   ties 
  arising 
  from 
  lag 
  are 
  usually 
  small, 
  but 
  still 
  it 
  is 
  desirable 
  

  

  * 
  Since 
  writing 
  the 
  above, 
  I 
  have 
  observed 
  it 
  stated 
  on 
  p. 
  102 
  of 
  the 
  

   Reichsanstalt's 
  Wiss. 
  Abhandl. 
  vol. 
  i. 
  that 
  their 
  determination 
  of 
  the 
  

   fundamental 
  interval 
  of 
  a 
  certain 
  Tonnelot 
  thermometer 
  differed 
  by 
  

   •005 
  0. 
  from 
  that 
  of 
  the 
  Bureau 
  International. 
  It 
  is 
  significantly 
  added 
  

   that 
  "005 
  C. 
  answers 
  to 
  a 
  difference 
  of 
  *14 
  mm. 
  in 
  the 
  barometer-reading 
  

   at 
  the 
  boiling-point. 
  

  

  f 
  Report 
  of 
  the 
  International 
  Meteorological 
  Conference, 
  Paris, 
  1896, 
  

   pp. 
  76-78. 
  

  

  