﻿12 
  On 
  the 
  Theory 
  of 
  Lubrication. 
  

  

  Fig. 
  3 
  is 
  a 
  sketch 
  of 
  a 
  suggested 
  arrangement 
  for 
  a 
  foot- 
  

   step. 
  The 
  white 
  parts 
  are 
  portions 
  of 
  an 
  original 
  plane 
  

  

  Tig. 
  3. 
  

  

  surface. 
  The 
  4 
  black 
  radii 
  represent 
  grooves 
  for 
  the 
  easy- 
  

   passage 
  of 
  lubricant. 
  The 
  shaded 
  parts 
  are 
  slight 
  depressions 
  

   of 
  uniform 
  depth, 
  such 
  as 
  might 
  be 
  obtained 
  by 
  etching 
  with 
  

   acid. 
  It 
  is 
  understood 
  that 
  the 
  opposed 
  surface 
  is 
  plane 
  

   throughout. 
  

  

  P.S. 
  Dec. 
  13. 
  — 
  In 
  a 
  small 
  model 
  the 
  opposed 
  pieces 
  were 
  

   two 
  pennies 
  ground 
  with 
  carborundum 
  to 
  a 
  fit. 
  One 
  of 
  

   them 
  — 
  the 
  stationary 
  one 
  — 
  was 
  afterwards 
  grooved 
  by 
  the 
  

   file 
  and 
  etched 
  with 
  dilute 
  nitric 
  acid 
  according 
  to 
  fig. 
  3, 
  

   sealing-wax, 
  applied 
  to 
  the 
  hot 
  metal, 
  being 
  used 
  as 
  a 
  

   " 
  resist." 
  They 
  were 
  mounted 
  in 
  a 
  small 
  cell 
  of 
  tin 
  plate, 
  

   the 
  upper 
  one 
  carrying 
  an 
  inertia 
  bar. 
  With 
  oil 
  as 
  a 
  lubricant 
  

   the 
  contrast 
  between 
  the 
  two 
  directions 
  of 
  rotation 
  was 
  very 
  

   marked. 
  

  

  Opportunity 
  has 
  not 
  yet 
  been 
  found 
  for 
  trying 
  polished 
  

   glass 
  plates, 
  such 
  as 
  are 
  used 
  in 
  optical 
  observations 
  on 
  

   " 
  interference." 
  In 
  this 
  case 
  the 
  etching 
  would 
  be 
  by 
  hydro- 
  

   fluoric 
  acid 
  *, 
  and 
  air 
  should 
  suffice 
  as 
  a 
  lubricant. 
  

  

  * 
  Compare 
  ' 
  Nature,' 
  vol. 
  lxiv. 
  p. 
  385 
  (1901) 
  ; 
  Scientific 
  Papers, 
  vol. 
  iv. 
  

   p. 
  546. 
  

  

  