﻿634 
  Mr. 
  A. 
  E. 
  Tutton 
  on 
  the 
  

  

  Three 
  independent 
  determinations 
  were 
  carried 
  out 
  with 
  

   each 
  piece 
  of 
  porcelain, 
  each 
  occupying 
  seven 
  to 
  eight 
  hours. 
  

   The 
  method 
  followed 
  was 
  precisely 
  that 
  described 
  in 
  the 
  memoir 
  

   on 
  the 
  "Thermal 
  Deformation 
  of 
  the 
  Crystallized 
  Sulphates 
  

   of 
  Potassium, 
  Rubidium, 
  and 
  Caesium 
  " 
  (Phil. 
  Trans. 
  1899, 
  A. 
  

   vol. 
  cxcii. 
  p. 
  467). 
  

  

  Only 
  the 
  very 
  slightest 
  signs 
  of 
  inequality 
  of 
  expansion 
  

   were 
  observed, 
  as 
  indicated 
  by 
  a 
  just 
  perceptible 
  amount 
  of 
  

   rotation 
  of 
  the 
  bands, 
  nothing 
  like 
  so 
  much 
  as 
  is 
  described 
  by 
  

   Chappuis 
  for 
  the 
  fringes 
  of 
  the 
  annular 
  rim, 
  and 
  usually 
  it 
  

   was 
  only 
  temporary, 
  the 
  original 
  position 
  being 
  regained 
  

   before 
  the 
  close 
  of 
  each 
  half 
  of 
  the 
  determination, 
  after 
  

   constancy 
  at 
  the 
  two 
  higher 
  limiting 
  temperatures 
  in 
  the 
  

   neighbourhood 
  of 
  60° 
  and 
  120° 
  was 
  attained. 
  

  

  The 
  results 
  of 
  the 
  determinations, 
  together 
  with 
  the 
  ne- 
  

   cessary 
  data, 
  are 
  set 
  forth 
  in 
  the 
  tables 
  on 
  pp. 
  636, 
  637. 
  

  

  ~L 
  t 
  represents 
  the 
  length 
  of 
  the 
  specimen 
  of 
  porcelain, 
  as 
  

   measured 
  at 
  the 
  ordinary 
  temperature 
  by 
  the 
  author's 
  thick- 
  

   ness-measurer 
  (Phil. 
  Trans. 
  A, 
  vol. 
  cxci. 
  p. 
  337, 
  1898). 
  It 
  is 
  

   the 
  length 
  at 
  the 
  central 
  axis, 
  immediately 
  under 
  the 
  minute 
  

   silvered 
  ring 
  on 
  the 
  under 
  side 
  of 
  the 
  glass 
  cover-disk, 
  the 
  

   centre 
  of 
  which 
  is 
  the 
  point 
  of 
  reference 
  for 
  the 
  micrometric 
  

   measurement 
  of 
  the 
  initial 
  and 
  final 
  positions 
  of 
  the 
  bands 
  

   for 
  each 
  interval 
  of 
  temperature. 
  

  

  l 
  a 
  is 
  the 
  thickness 
  of 
  the 
  aluminium 
  disk 
  at 
  the 
  centre 
  of 
  

   reference. 
  

  

  I 
  represents 
  the 
  mean 
  length 
  of 
  the 
  three 
  platinum-iridium 
  

   screws, 
  or 
  vertical 
  distance 
  between 
  silver 
  ring 
  and 
  surface 
  of 
  

   platinum-iridium 
  table. 
  

  

  d 
  is 
  the 
  thickness 
  of 
  the 
  air-layer 
  at 
  the 
  centre 
  of 
  reference. 
  

  

  t 
  x 
  is 
  the 
  initial 
  temperature; 
  in 
  every 
  case 
  it 
  was 
  the 
  tem- 
  

   peratuie 
  of 
  the 
  room 
  (as 
  determined 
  by 
  the 
  thermometer 
  in 
  

   contact 
  with 
  the 
  tripod) 
  at 
  the 
  moment 
  of 
  commencing 
  the 
  

   determination, 
  about 
  6.30 
  a.m. 
  in 
  mid-winter, 
  after 
  the 
  appa- 
  

   ratus 
  had 
  been 
  left 
  16-17 
  hours 
  since 
  completing 
  on 
  the 
  

   previous 
  day 
  either 
  adjustment 
  or 
  a 
  determination. 
  It 
  was 
  

   considered 
  more 
  satisfactory 
  to 
  start 
  with 
  a 
  lower 
  limit 
  of 
  this 
  

   constant 
  character, 
  than 
  to 
  attempt 
  to 
  approach 
  nearer 
  zero 
  

   by 
  artificial 
  cooling, 
  as 
  is 
  the 
  method 
  at 
  the 
  Bureau 
  Inter- 
  

   national. 
  t 
  2 
  is 
  the 
  limiting 
  temperature 
  of 
  the 
  first 
  interval, 
  

   and 
  £ 
  3 
  that 
  of 
  the 
  second 
  interval. 
  t2~h 
  is 
  considered 
  the 
  

   first 
  interval, 
  / 
  3 
  — 
  t 
  x 
  the 
  second. 
  

  

  b 
  t 
  , 
  b 
  2i 
  and 
  b 
  3 
  are 
  the 
  corresponding 
  barometric 
  pressures, 
  

   required 
  for 
  computing 
  the 
  correction 
  to 
  the 
  observed 
  number 
  

   of 
  bands. 
  

  

  