﻿13G 
  Mr. 
  a. 
  W. 
  Todd 
  on 
  the 
  

  

  it. 
  By 
  eliminating 
  the 
  effects 
  o£ 
  any 
  two 
  of 
  these 
  the 
  

   variations 
  in 
  the 
  other 
  may 
  be 
  obtained. 
  Thus 
  the 
  balance 
  

   might 
  be 
  used 
  either 
  as 
  a 
  barometer, 
  thermometer, 
  or 
  hygro- 
  

   meter. 
  

  

  Experiments 
  were 
  carried 
  out 
  with 
  the 
  balance 
  mentioned 
  

   in 
  the 
  previous 
  paper. 
  A 
  sealed 
  copper 
  sphere, 
  about 
  760 
  c.c, 
  

   was 
  suspended 
  from 
  one 
  arm 
  and 
  counterpoised 
  with 
  brass 
  

   weights 
  placed 
  in 
  the 
  opposite 
  scale-pan. 
  The 
  movements 
  of 
  

   the 
  pointer 
  were 
  observed 
  by 
  means 
  o£ 
  a 
  small 
  '^ 
  doubly 
  sus- 
  

   pended 
  " 
  mirror 
  which 
  reflected 
  a 
  distant 
  scale 
  into 
  a 
  tele- 
  

   scope. 
  One 
  division 
  in 
  this 
  telescope 
  corresponded 
  to 
  a 
  rise 
  

   or 
  fall 
  in 
  the 
  atmospheric 
  pressure 
  o£ 
  the 
  order 
  of 
  '002 
  mm. 
  

   of 
  mercury. 
  

  

  If 
  W, 
  V 
  are 
  the 
  weight 
  and 
  volume 
  of 
  the 
  sphere 
  and 
  

   W?; 
  similar 
  quantities 
  for 
  the 
  counterpoise, 
  we 
  have, 
  assuming 
  

   that 
  the 
  difference 
  in 
  volumes 
  of 
  the 
  opposite 
  sides 
  of 
  the 
  

   balance 
  itself 
  is 
  nil 
  and 
  that 
  the 
  air 
  is 
  dry, 
  

  

  whereat; 
  = 
  atm. 
  press, 
  in 
  mm., 
  D 
  = 
  density 
  of 
  air 
  at 
  0° 
  C. 
  

   and 
  7G0 
  mm., 
  A 
  = 
  abs. 
  temp. 
  

  

  97^ 
  1 
  

   ■•• 
  W-W' 
  = 
  (V-.)Dp|^^. 
  

  

  If 
  the 
  air 
  is 
  moist 
  and 
  the 
  pressure 
  of 
  aqueous 
  vapour 
  is 
  

   /mm., 
  

  

  where 
  d 
  = 
  dens, 
  of 
  wat. 
  vap. 
  at 
  0° 
  C. 
  and 
  760 
  mm. 
  

  

  Suppose 
  there 
  is 
  a 
  change 
  ^W 
  when 
  there 
  are 
  changes 
  

   "bp, 
  B/, 
  BA, 
  then 
  

  

  With 
  the 
  balance-case 
  d 
  

  

  n, 
  

  

  or 
  

  

  BW 
  = 
  (V_.)|S[B.?-M5i] 
  

  

  ^ 
  760A 
  - 
  „^ 
  P^ 
  K 
  

  

  