﻿314 
  Dr. 
  0. 
  Chree's 
  Notes 
  on 
  Thermometry. 
  

  

  The 
  change 
  of 
  external 
  pressure 
  in 
  balloon 
  ascents 
  or 
  in 
  

   mountain-climbing 
  merits 
  some 
  consideration. 
  At 
  20,000 
  feet 
  

   above 
  the 
  sea, 
  for 
  instance, 
  neglect 
  of 
  this 
  change 
  might 
  lead 
  

   to 
  an 
  error 
  of 
  0°'l 
  F. 
  in 
  the 
  calculated 
  boiling-point 
  of 
  a 
  

   thermometer, 
  and 
  thence 
  to 
  an 
  overestimate 
  of 
  some 
  50 
  feet 
  

   in 
  the 
  height. 
  

  

  A 
  more 
  important 
  case 
  arises 
  in 
  the 
  measurement 
  of 
  deep- 
  

   water 
  temperatures. 
  For 
  this 
  purpose 
  it 
  is 
  customary 
  to 
  use 
  

   a 
  special 
  pattern 
  of 
  " 
  deep-sea 
  " 
  thermometer, 
  in 
  which 
  

   complete 
  protection 
  from 
  external 
  pressure 
  is 
  aimed 
  at. 
  

   Occasionally, 
  however, 
  one 
  meets 
  with 
  instances 
  in 
  which 
  

   ordinary 
  unprotected 
  thermometers 
  have 
  been 
  used 
  to 
  take 
  

   readings 
  in 
  deep 
  wells. 
  In 
  one 
  case 
  which 
  came 
  under 
  my 
  

   notice 
  the 
  consequent 
  error 
  amounted 
  to 
  no 
  less 
  than 
  15° 
  F. 
  

   Analogous 
  results 
  would 
  follow 
  the 
  exposure 
  of 
  an 
  ordinary 
  

   thermometer 
  to 
  high 
  gaseous 
  pressures. 
  

  

  Emergent 
  Column. 
  

  

  §31. 
  Elementary 
  theory 
  assumes 
  all 
  the 
  glass 
  and 
  mercury 
  

   of 
  a 
  thermometer 
  to 
  be 
  at 
  one 
  temperature. 
  Even 
  in 
  well- 
  

   stirred 
  baths 
  in 
  physical 
  laboratories 
  this 
  is 
  rather 
  an 
  ideal 
  

   state 
  of 
  matters. 
  In 
  ordinar}' 
  use 
  there 
  is 
  often 
  an 
  appreciable, 
  

   sometimes 
  a 
  long, 
  mercury-column 
  exposed 
  to 
  a 
  temperature 
  

   differing 
  from 
  that 
  of 
  the 
  bulb. 
  This 
  is 
  especially 
  true 
  of 
  

   thermometers 
  employed 
  to 
  measure 
  the 
  temperature 
  of 
  a 
  

   liquid, 
  which 
  is 
  largely 
  in 
  excess 
  of 
  the 
  temperature 
  of 
  the 
  

   surrounding 
  air. 
  In 
  such 
  a 
  case, 
  if 
  a 
  long 
  mercury-column 
  

   be 
  emergent, 
  the 
  thermometer, 
  if 
  correct, 
  will 
  read 
  considerably 
  

   below 
  the 
  true 
  temperature 
  of 
  the 
  liquid. 
  A 
  direct 
  mathe- 
  

   matical 
  calculation 
  of 
  the 
  correction, 
  supposing 
  even 
  that 
  all 
  

   the 
  physical 
  data 
  were 
  known, 
  would 
  hardly 
  be 
  feasible, 
  

   unless 
  one 
  simplified 
  the 
  problem 
  by 
  making 
  jetsom 
  of 
  many 
  

   of 
  the 
  difficulties. 
  If 
  we 
  suppose 
  the 
  immersed 
  part 
  of 
  the 
  

   thermometer 
  unaffected 
  by 
  the 
  existence 
  of 
  the 
  emergent 
  

   part, 
  and 
  assume 
  the 
  whole 
  emergent 
  part, 
  glass 
  and 
  mercury, 
  

   to 
  be 
  at 
  one 
  temperature, 
  an 
  approximate 
  formula 
  for 
  the 
  

   correction 
  is 
  easily 
  obtained. 
  

  

  Suppose 
  that 
  the 
  liquid 
  reaches 
  to 
  the 
  division 
  t 
  ± 
  on 
  the 
  

   stem, 
  that 
  t 
  is 
  the 
  temperature 
  read 
  and 
  t' 
  the 
  temperature 
  

   assigned 
  to 
  the 
  emergent 
  column 
  ; 
  and 
  use 
  the 
  notation 
  of 
  §2, 
  

   x 
  being 
  the 
  correction 
  required. 
  The 
  emergent 
  mercury 
  

   would 
  at 
  0° 
  C. 
  have 
  the 
  volume 
  

  

  1 
  + 
  ajt' 
  + 
  a 
  2 
  t" 
  2 
  + 
  . 
  . 
  . 
  

  

  w-^i+Az+A/ 
  3 
  * 
  . 
  .: 
  

  

  