﻿Disks 
  and 
  Rings 
  of 
  Metal. 
  259 
  

  

  by 
  first 
  rubbing 
  them 
  with 
  ether 
  contained 
  in 
  a 
  soft 
  clean 
  rag. 
  

   The 
  ether 
  was 
  then 
  removed 
  with 
  absolute 
  alcohol, 
  and 
  the 
  

   latter 
  with 
  distilled 
  water. 
  The 
  disk 
  then 
  remained 
  under 
  

   cover 
  for 
  an 
  hour 
  or 
  more 
  before 
  experiments 
  were 
  made 
  

   with 
  it. 
  

  

  The 
  depth, 
  h, 
  to 
  which 
  a 
  disk 
  went 
  in 
  the 
  depressed 
  water 
  

   before 
  it 
  broke 
  through 
  its 
  surface 
  was 
  measured 
  as 
  follows 
  : 
  

   A 
  brass 
  rod 
  was 
  turned 
  so 
  that 
  it 
  fitted 
  neatly 
  into 
  the 
  cylinder 
  

   on 
  the 
  disk. 
  This 
  rod 
  was 
  attached 
  to 
  a 
  slide 
  moved 
  by 
  a 
  

   micrometer 
  screw 
  in 
  a 
  vertical 
  direction. 
  The 
  surface 
  of 
  the 
  

   disk 
  was 
  carefully 
  levelled. 
  The 
  disk 
  was 
  then 
  screwed 
  down 
  

   till 
  it 
  just 
  touched 
  the 
  surface 
  of 
  the 
  water. 
  That 
  the 
  plane 
  

   of 
  the 
  under 
  surface 
  of 
  the 
  disk 
  coincides 
  with 
  the 
  plane 
  of 
  

   the 
  water 
  is 
  known 
  by 
  examining 
  the 
  lower 
  surface 
  of 
  the 
  disk, 
  

   when 
  it 
  will 
  be 
  found 
  that 
  if 
  the 
  plane 
  of 
  the 
  disk 
  is 
  just 
  in 
  

   the 
  plane 
  of 
  the 
  water 
  there 
  is 
  a 
  uniform 
  illumination 
  over 
  the 
  

   surface 
  of 
  the 
  water, 
  but 
  if 
  the 
  surface 
  of 
  the 
  disk 
  is 
  the 
  least 
  

   below 
  or 
  above 
  the 
  surface 
  of 
  the 
  water 
  then 
  the 
  disk 
  will 
  be 
  

   surrounded 
  with 
  a 
  bright 
  border. 
  The 
  disk 
  is 
  now 
  lowered 
  by 
  

   the 
  micrometer 
  screw 
  till 
  it 
  breaks 
  through 
  the 
  water-surface. 
  

   The 
  instant 
  that 
  this 
  happens 
  is 
  not 
  as 
  sharply 
  marked 
  as 
  when 
  

   the 
  disk, 
  free 
  to 
  move 
  downwards, 
  breaks 
  through 
  the 
  surface 
  

   from 
  its 
  weight, 
  because 
  the 
  first 
  intimation 
  we 
  have 
  of 
  the 
  

   disk 
  having 
  reached 
  the 
  depth 
  when 
  rupture 
  of 
  the 
  depressed 
  

   surface 
  takes 
  place 
  is 
  that 
  the 
  latter 
  begins 
  to 
  leave 
  the 
  sharp 
  

   edge 
  of 
  the 
  disk 
  and 
  to 
  flow 
  inwards 
  over 
  its 
  surface. 
  Hence 
  

   h 
  cannot 
  be 
  measured 
  so 
  accurately 
  as 
  W. 
  This 
  is 
  shown 
  in 
  

   the 
  tables 
  of 
  experiments 
  on 
  disk 
  No. 
  1. 
  

  

  The 
  measure 
  of 
  h 
  given 
  in 
  the 
  tables 
  are 
  the 
  real 
  depths 
  to 
  

   which 
  the 
  disk 
  has 
  sunk 
  below 
  the 
  level 
  of 
  the 
  water 
  at 
  the 
  

   moment 
  of 
  rupture. 
  It 
  is 
  evident 
  that 
  as 
  the 
  disk 
  is 
  depressed 
  

   beneath 
  the 
  surface, 
  the 
  general 
  level 
  of 
  the 
  water 
  in 
  the 
  ves- 
  

   sel 
  rises, 
  and 
  that 
  the 
  larger 
  the 
  diameter 
  of 
  the 
  vessel 
  the 
  less 
  

   the 
  rise 
  of 
  the 
  water, 
  and 
  consequently 
  the 
  less 
  the 
  error 
  likely 
  

   to 
  be 
  made 
  in 
  the 
  computation. 
  The 
  vessel 
  used 
  in 
  these 
  

   experiments 
  has 
  a 
  diameter 
  of 
  IG'o 
  0111 
  . 
  By 
  adding 
  to 
  the 
  

   depression 
  of 
  the 
  disk, 
  as 
  measured 
  by 
  the 
  micrometer 
  screw, 
  

   the 
  computed 
  rise 
  of 
  the 
  water 
  in 
  the 
  vessel 
  we 
  obtain 
  A. 
  

  

  The 
  reason 
  why 
  we 
  take 
  W 
  and 
  h 
  at 
  the 
  moment 
  when 
  the 
  

   disk 
  breaks 
  through 
  the 
  surface 
  of 
  the 
  water 
  is 
  that 
  these 
  

   quantities 
  can 
  be 
  more 
  accurately 
  measured 
  at 
  the 
  moment 
  of 
  

   rupture 
  of 
  the 
  surface, 
  and 
  also 
  at 
  this 
  moment 
  W 
  corresponds 
  

   accurately 
  with 
  h. 
  

  

  