﻿Value 
  of 
  the 
  Mechanical 
  Equivalent 
  of 
  Heat. 
  27 
  

  

  rotation, 
  whereas 
  in 
  the 
  second 
  method 
  a 
  motion 
  through 
  

   5 
  mm. 
  was 
  required 
  to 
  produce 
  the 
  same 
  result. 
  

  

  The 
  principal 
  foci 
  of 
  the 
  combination 
  were 
  also 
  found, 
  a 
  

   needle 
  being 
  used 
  as 
  object 
  in 
  the 
  first 
  method 
  and 
  to 
  locate 
  

   the 
  image 
  in 
  the 
  second 
  method. 
  The 
  distances 
  through 
  

   which 
  the 
  needle 
  could 
  be 
  moved 
  before 
  any 
  sensible 
  parallax 
  

   was 
  observed 
  were 
  2 
  mm. 
  and 
  4 
  mm. 
  respectively. 
  

  

  This 
  particular 
  combination 
  was 
  chosen 
  in 
  order 
  to 
  get 
  a 
  

   fairly 
  large 
  distance 
  between 
  the 
  nodal 
  points, 
  but 
  as 
  the 
  

   lenses 
  were 
  uncorrected 
  quite 
  a 
  small 
  rotation 
  caused 
  the 
  

   image 
  to 
  become 
  confused 
  owing 
  to 
  oblique 
  aberration. 
  

   A 
  further 
  test 
  was 
  therefore 
  made 
  with 
  a 
  15 
  in. 
  (38*1 
  cm.) 
  

   Hoss 
  photographic 
  lens 
  in 
  which 
  the 
  distance 
  between 
  the 
  

   nodal 
  points 
  was 
  only 
  1*6 
  cm. 
  The 
  following 
  results 
  were 
  

   obtained 
  for 
  the 
  ranges 
  of 
  adjustment 
  in 
  the 
  two 
  cases: 
  — 
  

  

  First 
  Method. 
  For 
  nodal 
  points 
  0'2 
  mm. 
  ; 
  for 
  principal 
  

   foci 
  3* 
  5 
  mm. 
  

  

  Second 
  Method. 
  For 
  nodal 
  points 
  1*5 
  mm.; 
  for 
  principal 
  

   foci 
  11 
  mm. 
  

  

  These 
  figures 
  must 
  be 
  taken 
  as 
  indicating 
  the 
  relative 
  

   rather 
  than 
  the 
  absolute 
  accuracies 
  of 
  the 
  two 
  methods, 
  

   since 
  the 
  nodal-slide 
  employed, 
  though 
  not 
  of 
  the 
  roughest, 
  

   had 
  no 
  fine 
  adjustments. 
  They 
  appear, 
  however^, 
  to 
  show 
  

   that 
  the 
  first 
  nul 
  method 
  is 
  in 
  every 
  respect 
  superior 
  to 
  the 
  

   second. 
  

  

  In 
  conclusion, 
  I 
  Mash 
  to 
  thank 
  my 
  colleague, 
  Mr. 
  J. 
  E. 
  

   Rycrof 
  t, 
  for 
  his 
  kindness 
  in 
  making 
  the 
  diagrams. 
  

  

  IV. 
  On 
  the 
  Value 
  of 
  the 
  Mechanical 
  Equivalent 
  of 
  Heat. 
  

   By 
  T. 
  Carlton 
  Sutton, 
  B.Sc* 
  

  

  THE 
  following 
  values 
  of 
  the 
  Mechanical 
  Equivalent 
  of 
  

   Heat 
  (for 
  references 
  see 
  Kaye 
  and 
  Laby's 
  ' 
  Tables' 
  

   and 
  Griffiths' 
  ' 
  Thermal 
  Measurement 
  of 
  Energy 
  ') 
  have 
  

   been 
  reduced 
  to 
  joules 
  per 
  mean 
  calorie 
  : 
  — 
  

  

  1813 
  Joule 
  4-173 
  

  

  1878 
  Rowland 
  4-184 
  

  

  1893 
  Griffiths 
  4-188 
  

  

  1894 
  Schuster 
  and 
  Gannon 
  4-185 
  

  

  1897 
  Reynolds 
  and 
  Moorbv 
  4'184 
  

  

  1899 
  Callendar 
  and 
  Barnes 
  4-184 
  

  

  1900 
  Griffiths 
  (deduced) 
  4-184 
  

  

  1906 
  Jager 
  and 
  Stein 
  wehr 
  4-188 
  

  

  1908 
  Cremieu 
  and 
  Rispail 
  4-189 
  

  

  1909 
  Barnes 
  (deduced) 
  4-185 
  

  

  1911 
  Bousfield 
  4-179 
  

  

  1914-5 
  Sutton-Henning 
  (see 
  below).. 
  4-185 
  

  

  * 
  Communicated 
  by 
  Principal 
  E. 
  H. 
  Griffiths. 
  

  

  