﻿The 
  Balance 
  as 
  a 
  Sensitive 
  Barometer. 
  135 
  

  

  IT. 
  The 
  same 
  rider. 
  

  

  10 
  determinations 
  alternately 
  left 
  and 
  right. 
  

  

  Mean 
  travel 
  of 
  rider 
  5"2713 
  mm. 
  

  

  Mean 
  deflexion 
  45'47 
  divisions. 
  

  

  Mean 
  value 
  for 
  40 
  divisions 
  ... 
  0*1681 
  mgm. 
  

   The 
  separate 
  determinations 
  ran^-e 
  between 
  

   40 
  divisions=0-1722 
  

   and 
  40 
  „ 
  =0-1632. 
  

  

  III. 
  Rider 
  German 
  silver 
  wire, 
  189*05 
  mgm. 
  

   7 
  determinations 
  alternately 
  left 
  and 
  right. 
  

  

  Mean 
  travel 
  of 
  rider 
  0*1764 
  mm. 
  

  

  Mean 
  deflexion 
  38*70 
  divisions. 
  

  

  Mean 
  value 
  for 
  40 
  divisions 
  ... 
  0*1691 
  mgm. 
  

   The 
  separate 
  determinations 
  range 
  between 
  

   40 
  divisions 
  = 
  0*1709 
  

   and 
  40 
  „ 
  =0*1654. 
  

  

  lY. 
  The 
  same 
  rider. 
  

  

  7 
  determinations 
  alternately 
  left 
  and 
  right. 
  

  

  Mean 
  travel 
  of 
  rider 
  0'3004 
  mm. 
  

  

  Mean 
  deflexion 
  64*84 
  divisions. 
  

  

  Mean 
  value 
  for 
  60 
  divisions 
  ... 
  0*2578 
  mgm. 
  

   The 
  separate 
  determinations 
  range 
  between 
  

   60 
  divisions 
  = 
  0*2612 
  

   and 
  60 
  „ 
  =0*2518. 
  

  

  XIII. 
  The 
  Balance 
  as 
  a 
  Sensitive 
  Barometer, 
  

   By 
  G. 
  W. 
  Todd, 
  ili.^^c* 
  

  

  IT 
  occurred 
  to 
  the 
  author 
  while 
  testing 
  with 
  Professor 
  

   Poyntiiig 
  the 
  accuracy 
  of 
  a 
  new 
  method 
  of 
  determining 
  

   the 
  sensibility 
  of 
  a 
  balance, 
  described 
  on 
  p. 
  132, 
  in 
  which 
  a 
  

   thin 
  rod 
  or 
  fibre 
  is 
  used 
  instead 
  of 
  the 
  usual 
  rider, 
  that 
  a 
  

   balance 
  might 
  with 
  proper 
  precautions 
  be 
  converted 
  into 
  

   a 
  very 
  delicate 
  barometer. 
  

  

  A 
  difference 
  in 
  volume 
  on 
  the 
  opposite 
  sides 
  of 
  a 
  balance 
  

   will 
  give 
  rise 
  to 
  motions 
  of 
  the 
  pointer 
  when 
  tlie 
  density 
  of 
  

   the 
  surrounding 
  air 
  changes. 
  Small 
  oscillations 
  of 
  the 
  

   pointer, 
  due 
  chiefly 
  to 
  convection 
  currents, 
  become 
  negligible 
  

   if 
  this 
  difference 
  in 
  volume 
  is 
  sufficiently 
  large. 
  Alterations 
  

   in 
  the 
  density 
  of 
  the 
  air 
  are 
  produced 
  by 
  changes 
  in 
  pressure, 
  

   in 
  temperature, 
  and 
  in 
  the 
  percentage 
  of 
  aqueous 
  vapour 
  in 
  

  

  * 
  Co 
  mmimicated 
  by 
  the 
  Physical 
  Society 
  : 
  read 
  June 
  25, 
  1909. 
  

  

  