﻿in 
  

  

  Toothed 
  Wheels 
  and 
  Rack-and-Pit 
  

  

  Gea 
  

  

  rs. 
  

  

  337 
  

  

  The 
  author's 
  de^HLce, 
  which 
  is 
  free 
  from 
  the 
  above 
  objections 
  

   is 
  as 
  follows 
  : 
  — 
  

  

  The 
  smaller 
  of 
  the 
  two 
  wheels 
  that 
  gear 
  together, 
  or 
  the 
  

   pinion-wheel 
  in 
  the 
  case 
  of 
  a 
  rack-and-pinion, 
  is 
  divided 
  in 
  

   half, 
  so 
  as 
  to 
  make 
  two 
  wheels 
  similar 
  to 
  the 
  original 
  one, 
  

   although 
  only 
  half 
  as 
  thick. 
  They 
  are 
  represented 
  at 
  C 
  

   and 
  D 
  in 
  the 
  fio-ure 
  — 
  the 
  teeth 
  not 
  beino- 
  shown. 
  Both 
  

  

  and 
  D 
  continue 
  to 
  gear 
  with 
  the 
  other 
  wheel 
  — 
  or 
  with 
  the 
  

   rack 
  — 
  as 
  before. 
  * 
  D 
  is 
  fixed 
  to 
  the 
  axle 
  F; 
  the 
  wheel 
  C, 
  the 
  

   sleeve 
  E, 
  and 
  the 
  arm 
  A 
  are 
  fixed 
  together 
  as 
  one 
  piece, 
  

   loose 
  on 
  the 
  axle. 
  The 
  arm 
  L 
  is 
  fixed 
  on 
  the 
  axle. 
  The 
  

   effect 
  of 
  this 
  arrangement 
  is 
  to 
  force 
  the 
  two 
  wheels 
  C 
  and 
  

   D 
  in 
  opposite 
  directions, 
  because 
  of 
  the 
  tension 
  of 
  the 
  

   springs 
  B_, 
  B. 
  Therefore 
  the 
  corresponding 
  teeth 
  on 
  C 
  and 
  D 
  

   turn 
  past 
  each 
  other 
  until 
  they 
  completely 
  fill 
  up 
  the 
  spaces 
  

   between 
  the 
  teeth 
  of 
  the 
  wheel 
  — 
  or 
  the 
  rack 
  — 
  with 
  which 
  

   they 
  gear 
  ; 
  and 
  hence 
  with 
  this 
  arrangement 
  there 
  can 
  be 
  na 
  

   backlash. 
  

  

  In 
  the 
  above 
  device 
  the 
  movement 
  in 
  one 
  direction 
  is 
  

   positive, 
  in 
  the 
  other 
  it 
  takes 
  place 
  through 
  the 
  intervention 
  

   of 
  the 
  springs 
  ; 
  which 
  therefore 
  must 
  be 
  of 
  sufficient 
  strength 
  

   to 
  transmit 
  the 
  force 
  without 
  yielding. 
  The 
  author 
  finds 
  

   that 
  in 
  the 
  case 
  of 
  scientific 
  instruments 
  this 
  condition 
  is 
  

   attained 
  without 
  difficulty, 
  a 
  comparatively 
  slight 
  strength 
  of 
  

   spring 
  being 
  sufficient. 
  

  

  The 
  author 
  has 
  had 
  the 
  device 
  for 
  some 
  time 
  in 
  use 
  in 
  

   connexion 
  with 
  the 
  rotation 
  of 
  the 
  Nicol's 
  prism 
  of 
  a 
  spectro- 
  

   photometer, 
  and 
  it 
  has 
  proved 
  very 
  successful. 
  

  

  * 
  The 
  following 
  is 
  the 
  way 
  in 
  which 
  the 
  author 
  made 
  use 
  of 
  this 
  

   device, 
  but 
  other 
  ways 
  might 
  be 
  used 
  to 
  suit 
  other 
  circumstances. 
  

  

  