﻿226 
  Dr. 
  0. 
  Ohree's 
  Notes 
  on 
  Thermometry 
  . 
  

  

  that 
  the 
  determination 
  of 
  the 
  zero 
  corresponding 
  to 
  the 
  

   temperature 
  — 
  39 
  c 
  C. 
  was 
  carried 
  out 
  exactly 
  in 
  the 
  way 
  

   approved 
  by 
  the 
  Bureau, 
  hut 
  any 
  failure 
  in 
  this 
  respect 
  

   would, 
  I 
  think, 
  be 
  more 
  likely 
  to 
  increase 
  than 
  reduce 
  the 
  

   apparent 
  difference. 
  

  

  Thus 
  the 
  experiments 
  with 
  the 
  Tonnelot 
  thermometer 
  

   show 
  that 
  if 
  the 
  Kevv 
  apparatus 
  gives 
  a 
  different 
  result 
  from 
  

   that 
  of 
  the 
  Bureau, 
  this 
  difference 
  tends 
  to 
  exaggerate 
  the 
  

   apparent 
  departure 
  of 
  the 
  natural 
  English 
  glass 
  scale 
  from 
  

   that 
  of 
  the 
  hydrogen 
  thermometer. 
  It 
  has 
  also 
  to 
  be 
  borne 
  

   in 
  mind 
  that, 
  as 
  stated 
  in 
  § 
  10, 
  an 
  English 
  glass 
  thermometer 
  

   treated 
  like 
  the 
  verre 
  dur 
  thermometer 
  on 
  a 
  movable 
  zero 
  

   method 
  would 
  give 
  a 
  reading 
  algebraically 
  higher, 
  and 
  so 
  

   nearer 
  that 
  of 
  the 
  hydrogen 
  thermometer, 
  than 
  when 
  treated 
  

   as 
  at 
  Kew 
  Observatory. 
  

  

  § 
  19. 
  It 
  may 
  appear 
  that 
  undue 
  space 
  has 
  been 
  given 
  to 
  

   the 
  question 
  of 
  the 
  temperature 
  answering 
  to 
  the 
  freezing- 
  

   point 
  of 
  mercury, 
  and 
  the 
  corresponding 
  reading 
  of 
  English 
  

   glass 
  thermometers. 
  Since 
  Balfour 
  Stewart's 
  determination, 
  

   however, 
  very 
  considerable 
  importance 
  has 
  attached 
  to 
  this 
  

   point. 
  For 
  many 
  years 
  it 
  was 
  not 
  unusual 
  to 
  treat 
  the 
  

   freezing-point 
  of 
  mercury 
  as 
  a 
  third 
  fixed 
  point 
  (taken 
  as 
  

   — 
  37 
  0, 
  9 
  F.) 
  on 
  the 
  scale 
  of 
  mercury 
  thermometers, 
  sub- 
  

   dividing 
  the 
  tube 
  below 
  32° 
  F. 
  by 
  reference 
  to 
  the 
  observed 
  

   readings 
  in 
  freezing 
  ice 
  and 
  freezing 
  mercury. 
  This 
  pro- 
  

   duced 
  a 
  thermometer 
  in 
  which 
  the 
  degree 
  volumes 
  below 
  

   32° 
  F. 
  were 
  equal 
  amongst 
  themselves 
  but 
  unequal 
  to 
  those 
  

   above 
  32° 
  F. 
  

  

  The 
  object, 
  of 
  course, 
  was 
  to 
  produce 
  a 
  mercury 
  thermo- 
  

   mometer 
  whose 
  readings 
  should 
  lie 
  nearer 
  those 
  of 
  the 
  air- 
  

   thermometer 
  than 
  if 
  it 
  were 
  divided 
  into 
  identically 
  equal 
  

   volumes 
  throughout 
  ; 
  and 
  so 
  would 
  give 
  better 
  results 
  when 
  

   no 
  corrections 
  were 
  applied, 
  other 
  than 
  for 
  observed 
  change 
  

   of 
  ice 
  reading. 
  This 
  practice, 
  no 
  doubt, 
  may 
  be 
  regarded 
  

   with 
  far 
  from 
  silent 
  contempt 
  by 
  the 
  present 
  generation 
  of 
  

   physicists; 
  but 
  to 
  our 
  ruder 
  ancestors, 
  who 
  thought 
  an 
  

   accuracy 
  of 
  from 
  0°'l 
  to 
  o, 
  2 
  F. 
  at 
  temperatures 
  below 
  32° 
  F. 
  

   something 
  on 
  which 
  to 
  congratulate 
  themselves, 
  the 
  idea 
  

   seemed 
  an 
  ingenious 
  one. 
  However 
  bad, 
  or 
  the 
  reverse, 
  the 
  

   idea 
  was, 
  the 
  fact 
  remains 
  that 
  a 
  good 
  many 
  mercury-thermo- 
  

   meters 
  divided 
  in 
  this 
  way 
  have 
  been 
  used 
  in 
  this 
  country 
  

   and 
  its 
  colder 
  dependencies. 
  

  

  Even 
  at 
  the 
  present 
  day 
  it 
  is 
  the 
  practice 
  of 
  English 
  

   opticians 
  to 
  take 
  — 
  37°*9 
  F. 
  as 
  a 
  fixed 
  point 
  in 
  the 
  con- 
  

   struction 
  of 
  spirit 
  thermometers 
  * 
  used 
  for 
  low-temperature 
  

  

  * 
  Of. 
  Balfour 
  Stewart's 
  'Beat,' 
  1st 
  edition, 
  Art. 
  26. 
  

  

  