﻿Dr. 
  G. 
  Chree's 
  Notes 
  on 
  Thermometry. 
  303 
  

  

  Boiling-point 
  of 
  Water, 
  

  

  § 
  22. 
  In 
  Balfour 
  Stewart's 
  ' 
  Heat 
  ' 
  (art. 
  xxi. 
  1st 
  ed.), 
  

   212° 
  F. 
  is 
  defined 
  as 
  the 
  temperature 
  of 
  steam 
  under 
  the 
  

   pressure 
  of 
  29*905 
  inches 
  of 
  mercury 
  reduced 
  to 
  32° 
  F. 
  at 
  

   the 
  latitude 
  of 
  London. 
  Balfour 
  Stewart 
  points 
  out 
  a 
  differ- 
  

   ence 
  between 
  212° 
  F. 
  and 
  100° 
  C, 
  defining 
  the 
  latter 
  as 
  the 
  

   temperature 
  of 
  steam 
  under 
  the 
  barometric 
  pressure 
  of 
  

   760 
  mm. 
  of 
  mercury 
  reduced 
  to 
  the 
  temperature 
  of 
  freezing 
  

   water 
  in 
  the 
  latitude 
  of 
  Paris. 
  The 
  Bureau 
  International, 
  

   however, 
  now 
  generally 
  recognized 
  as 
  the 
  central 
  authority, 
  

   have 
  substituted 
  latitude 
  45° 
  for 
  that 
  of 
  Paris 
  in 
  the 
  defi- 
  

   nition 
  *. 
  This 
  has 
  apparently 
  the 
  fortunate 
  result 
  of 
  bringing 
  

   212° 
  F. 
  and 
  100° 
  C. 
  into 
  almost 
  exact 
  agreement. 
  

  

  According 
  to 
  Kater's 
  determination, 
  

  

  760 
  mm. 
  = 
  29*9218 
  inches 
  ; 
  

   and 
  so 
  759-573 
  mm. 
  = 
  29*905 
  inches 
  ; 
  

  

  and, 
  according 
  to 
  Guillaume's 
  Thermometrie, 
  p. 
  325, 
  

   ] 
  Gravity 
  in 
  London 
  (Standards' 
  Office) 
  1 
  -*- 
  

  

  { 
  Gravity 
  in 
  latitude 
  45° 
  \ 
  = 
  1*0005815. 
  

  

  These 
  data 
  make 
  the 
  pressure 
  of 
  29*905 
  inches 
  of 
  mercury 
  

   in 
  London 
  equal 
  to 
  that 
  of 
  760*031 
  mm. 
  in 
  latitude 
  45° 
  ? 
  

   and 
  by 
  Kegnault's 
  table 
  (cf. 
  Guillaume's 
  Thermometrie, 
  

   p. 
  327) 
  the 
  boiling-point 
  corresponding 
  to 
  the 
  latter 
  pressure 
  

   is 
  100°-001 
  C. 
  

  

  Recent 
  comparisons 
  f 
  3 
  both 
  American 
  and 
  British, 
  give 
  

   apparently 
  for 
  the 
  ratio 
  (yard 
  /metre) 
  a 
  value 
  something 
  like 
  

   one 
  part 
  in 
  40,000 
  larger 
  than 
  Kater's, 
  and 
  it 
  is 
  uncertain 
  

   what 
  accuracy 
  is 
  assignable 
  to 
  values 
  of 
  g, 
  and 
  to 
  what 
  

   extent 
  it 
  presents 
  local 
  fluctuations. 
  Thus 
  the 
  closeness 
  of 
  

   the 
  above 
  agreement 
  between 
  212° 
  F. 
  and 
  100° 
  C. 
  may 
  not 
  

   be 
  absolutely 
  relied 
  on. 
  If 
  it 
  should 
  prove 
  that 
  g 
  has 
  a 
  daily 
  

   or 
  annual 
  variation 
  of 
  one 
  part 
  in 
  25,000, 
  an 
  uncertainty 
  of 
  

   the 
  order 
  0°*001 
  C. 
  would 
  exist 
  in 
  the 
  standard 
  boiling-point. 
  

  

  * 
  Guillaume's 
  Thermometrie, 
  p. 
  4. 
  

  

  t 
  A 
  preliminary 
  discussion 
  of 
  the 
  most 
  recent 
  comparison, 
  initiated 
  by 
  

   Mr. 
  Chaney, 
  will 
  be 
  found 
  on 
  pp. 
  37-40 
  of 
  the 
  Proces-Verbauv 
  for 
  

   1895 
  (Paris, 
  1896) 
  of 
  the 
  International 
  Committee 
  of 
  Weights 
  and 
  

   Measures. 
  Owing 
  to 
  the 
  absence 
  of 
  a 
  legally 
  defined 
  scale 
  of 
  tempera- 
  

   ture, 
  there 
  seems 
  some 
  uncertainty 
  as 
  to 
  the 
  interpretation 
  to 
  be 
  put 
  on 
  

   the 
  standard 
  temperature 
  62° 
  F. 
  At 
  this 
  point 
  the 
  verre 
  dur 
  and 
  

   hydrogen 
  scales 
  differ 
  by 
  about 
  o, 
  075 
  C. 
  

  

  Y2 
  

  

  