﻿300 
  Dr. 
  C. 
  Chree's 
  Notes 
  on 
  Thermometry 
  . 
  

  

  Thus 
  if 
  the 
  temperature 
  r 
  be 
  absolutely 
  constant, 
  the 
  

   difference 
  between 
  t 
  and 
  t 
  tends 
  ultimately 
  to 
  vanish, 
  and 
  

   the 
  temperature 
  of 
  the 
  medium 
  may 
  be 
  obtained 
  directly, 
  to 
  

   any 
  required 
  degree 
  of 
  accuracy, 
  by 
  allowing 
  sufficient 
  time 
  

   to 
  elapse 
  before 
  reading 
  the 
  thermometer. 
  Usually, 
  how- 
  

   ever, 
  t 
  is 
  to 
  some 
  extent 
  variable 
  *", 
  and 
  to 
  arrive 
  at 
  a 
  

   sufficiently 
  exact 
  interpretation 
  of 
  the 
  readings 
  of 
  the 
  ther- 
  

   mometer 
  it 
  may 
  be 
  necessary 
  to 
  carefully 
  observe 
  the 
  rate 
  of 
  

   variation 
  of 
  t 
  and 
  employ 
  some 
  mathematical 
  analysis. 
  The 
  

   mere 
  presence 
  of 
  the 
  thermometer 
  tends 
  frequently 
  to 
  intro- 
  

   duce 
  variation 
  in 
  the 
  temperature 
  of 
  the 
  medium 
  surrounding 
  

   its 
  bulb. 
  When, 
  for 
  instance, 
  there 
  is 
  only 
  partial 
  immersion 
  

   in 
  a 
  liquid 
  bath 
  or 
  in 
  a 
  freezing 
  solution, 
  whose 
  temperature 
  

   differs 
  appreciably 
  from 
  that 
  of 
  the 
  room, 
  there 
  is 
  a 
  constant 
  

   transfer 
  of 
  heat 
  along 
  the 
  thermometer-stem, 
  which 
  influences 
  

   to 
  some 
  extent 
  the 
  temperature 
  recorded. 
  This 
  influence 
  

   depends 
  largely 
  on 
  the 
  character 
  of 
  the 
  thermometer 
  and 
  the 
  

   nature 
  of 
  the 
  bath. 
  

  

  The 
  phenomena 
  discussed 
  in 
  this 
  paragraph 
  have 
  an 
  

   important 
  bearing 
  on 
  the 
  accuracy 
  of 
  ordinary 
  thermometric 
  

   measurements, 
  which 
  is 
  perhaps 
  somewhat 
  apt 
  to 
  be 
  over- 
  

   looked 
  by 
  those 
  whose 
  experience 
  has 
  centred 
  in 
  laboratories 
  

   provided 
  with 
  elaborate 
  thermo-regulators. 
  

  

  Freezing-point 
  of 
  Water. 
  

  

  § 
  21. 
  Particulars 
  of 
  the 
  method 
  of 
  determining 
  (depressed 
  

   zero) 
  freezing-points 
  at 
  the 
  Bureau 
  International 
  will 
  be 
  found 
  

   in 
  Guillaume's 
  T1iermometrie 
  } 
  ^. 
  116. 
  The 
  object 
  is 
  to 
  reduce 
  

   the 
  temperature 
  to 
  0° 
  C. 
  as 
  rapidly 
  as 
  is 
  consistent 
  with 
  the 
  

   safety 
  of 
  the 
  thermometer. 
  The 
  ice 
  must 
  be 
  pure, 
  the 
  

   presence 
  of 
  the 
  least 
  trace 
  of 
  salt 
  being 
  especially 
  objection- 
  

   able. 
  It 
  should 
  be 
  very 
  finely 
  divided, 
  and 
  sufficiently 
  but 
  

   not 
  excessively 
  moist. 
  What 
  Guillaume 
  speaks 
  of 
  most 
  

   favourably 
  is 
  good 
  lake 
  ice 
  or 
  freshly 
  fallen 
  snow 
  moistened 
  

   with 
  pure 
  distilled 
  water. 
  He 
  quotes 
  the 
  following 
  results 
  

   so 
  observed 
  by 
  Pernet 
  : 
  — 
  

  

  * 
  The 
  case 
  where 
  t 
  13 
  a 
  linear 
  function 
  of 
  the 
  time 
  has 
  been 
  treated 
  

   recently 
  by 
  Grossmann 
  (Annalen 
  der 
  Mydroyvapliie 
  der 
  maritimen 
  

   Meteorologie, 
  vol. 
  x. 
  1897, 
  p. 
  483). 
  The 
  same 
  paper 
  contains 
  references 
  

   to 
  recent 
  -work 
  on 
  the 
  subject 
  and 
  also 
  a 
  variety 
  of 
  interesting- 
  experi- 
  

   mental 
  results. 
  

  

  