﻿Disks 
  and 
  Rings 
  of 
  Metal. 
  255 
  

  

  n'est 
  le 
  dedans, 
  ainsi 
  qu'on 
  void 
  par 
  experience 
  en 
  ce 
  que 
  tous 
  

   les 
  corps 
  assez 
  petits, 
  quoyque 
  de 
  matiere 
  fort 
  pesante, 
  comme 
  

   sont 
  de 
  petites 
  aiguilles 
  d'acier, 
  peuvent 
  Hotter 
  et 
  estre 
  au-des- 
  

   sus, 
  lorsequ'elle 
  n'est 
  point 
  encore 
  diuisee, 
  aulieu 
  que 
  lorsqu'elle 
  

   l'est 
  ils 
  descendent 
  iusque 
  fonds 
  sans 
  s'arrester." 
  

  

  The 
  next 
  paper 
  on 
  the 
  subject 
  is 
  by 
  " 
  M, 
  Petit 
  le 
  Medecin," 
  

   published 
  in 
  the 
  Memoirs 
  of 
  the 
  Academy 
  of 
  Sciences, 
  Paris, 
  

   1731, 
  in 
  which 
  the 
  author 
  states 
  that 
  the 
  reason 
  why 
  a 
  metal 
  

   floats 
  on 
  water 
  is 
  that 
  it 
  is 
  kept 
  from 
  touching 
  the 
  water 
  by 
  a 
  

   film 
  of 
  air 
  which 
  coats 
  the 
  surfaces 
  of 
  all 
  metals. 
  I 
  think 
  that 
  

   M. 
  Petit 
  makes 
  good 
  his 
  statement, 
  for 
  he 
  shows 
  that 
  on 
  heat- 
  

   ing 
  a 
  metal 
  it 
  no 
  longer 
  floats, 
  but 
  sinks. 
  He 
  makes 
  no 
  men- 
  

   tion 
  of 
  having 
  previously 
  greased 
  the 
  wires 
  and 
  sheets 
  of 
  metal 
  

   with 
  which 
  he 
  experimented. 
  

  

  Pumford, 
  in 
  1807, 
  published 
  a 
  paper 
  in 
  the 
  Memoirs 
  of 
  the 
  

   Academy 
  of 
  Sciences, 
  of 
  Paris, 
  in 
  which 
  he 
  attempts 
  to 
  show 
  

   that 
  a 
  film 
  of 
  air 
  does 
  not 
  exist 
  on 
  floating 
  metals. 
  I 
  do 
  not 
  

   think 
  that 
  he 
  makes 
  good 
  his 
  opinion, 
  for 
  the 
  only 
  experiments 
  

   he 
  makes 
  to 
  sustain 
  it 
  are 
  those 
  in 
  which 
  water 
  is 
  covered 
  with 
  

   ether, 
  with 
  turpentine, 
  or 
  with 
  olive 
  oil, 
  and 
  small 
  spheres 
  of 
  tin 
  

   and 
  of 
  mercury, 
  allowed 
  to 
  fall 
  through 
  the 
  supernatant 
  liquid, 
  

   are 
  retained 
  by 
  the 
  surface 
  of 
  the 
  water, 
  which 
  takes 
  the 
  form 
  

   of 
  sacs, 
  that 
  support 
  and 
  partly 
  inclose 
  the 
  spheres. 
  

  

  In 
  1824 
  Pichard," 
  engineer 
  in 
  the 
  French 
  army, 
  thus 
  explains 
  

   the 
  floating 
  of 
  a 
  needle. 
  If 
  the 
  needle 
  be 
  not 
  wetted 
  then 
  it 
  

   sinks 
  in 
  a 
  depression 
  of 
  the 
  surface 
  of 
  the 
  water, 
  and 
  the 
  

   vacant 
  space 
  caused 
  by 
  this 
  depression 
  together 
  with 
  the 
  needle 
  

   forms 
  a 
  float. 
  When 
  the 
  specific 
  gravity 
  of 
  this, 
  as 
  a 
  whole, 
  is 
  

   less 
  than 
  that 
  ol 
  the 
  liquid, 
  its 
  weight 
  will 
  be 
  insufficient 
  to 
  

   overbalance 
  the 
  upward 
  hydrostatic 
  pressure 
  of 
  the 
  liquid, 
  and 
  

   it 
  will 
  float 
  upon 
  the 
  surface 
  of 
  the 
  latter. 
  He 
  then 
  refers 
  to 
  

   the 
  existence 
  of 
  an 
  elastic 
  pellicle, 
  as 
  held 
  by 
  some, 
  and 
  to 
  that 
  

   of 
  the 
  adherent 
  layer 
  of 
  air 
  as 
  held 
  by 
  others, 
  but 
  does 
  not 
  

   adopt 
  either, 
  or 
  propose 
  any 
  explanation 
  of 
  his 
  own 
  other 
  than 
  

   that 
  of 
  the 
  compound 
  float 
  formed 
  by 
  the 
  needle 
  and 
  the 
  empty 
  

   space 
  around 
  it 
  from 
  which 
  the 
  water 
  has 
  been 
  displaced. 
  

  

  Gaillieron,f 
  in 
  a 
  letter 
  to 
  De 
  la 
  Pive, 
  after 
  referring 
  to 
  the 
  

   opinion 
  of 
  some 
  physicists 
  that 
  a 
  film 
  of 
  air 
  adhering 
  to 
  the 
  

   floating 
  body 
  is 
  the 
  cause 
  of 
  the 
  depression 
  made 
  by 
  it 
  in 
  the 
  

   surface 
  of 
  the 
  liquid, 
  says 
  : 
  " 
  This 
  explanation 
  is 
  hypothetical, 
  

   not 
  even 
  reaching 
  a 
  high 
  degree 
  of 
  probability. 
  It 
  is 
  there- 
  

   fore 
  permissible 
  to 
  advocate 
  another 
  explanation, 
  that 
  of 
  Count 
  

   Pumford, 
  defining 
  once 
  for 
  all 
  what 
  is 
  understood 
  by 
  pellicle." 
  

  

  * 
  Considerations 
  sur 
  les 
  phenomeues 
  que 
  presentent 
  de 
  petites 
  aiguilles 
  a 
  coudre, 
  

   posees 
  doucement 
  et 
  dans 
  une 
  situation 
  horizontale, 
  sur 
  la 
  surface 
  d'une 
  eau 
  tran- 
  

   quille. 
  Biblioth. 
  Univ., 
  t. 
  xxv, 
  p. 
  213. 
  

  

  f 
  Sur 
  les 
  movements 
  de 
  certains 
  corps 
  flottants 
  sur 
  l'eau. 
  Bibl. 
  Univ., 
  vol. 
  xxvi, 
  

   p. 
  190. 
  1824. 
  

  

  