﻿432 
  ON 
  THE 
  RELATIVE 
  HORIZONTAL 
  INTENSITIES 
  

  

  vibrations 
  between 
  the 
  semi-arcs 
  of 
  six 
  and 
  two 
  degrees, 
  in 
  a 
  medium 
  

   rarefied 
  to 
  between 
  three 
  and 
  a 
  half 
  and 
  three 
  inches 
  of 
  mercury. 
  

  

  The 
  third 
  needle, 
  C, 
  was 
  the 
  fourth 
  of 
  the 
  first 
  series. 
  It 
  made, 
  be- 
  

   tween 
  the 
  semi-arcs 
  of 
  six 
  and 
  two 
  degrees, 
  three 
  hundred 
  vibrations, 
  

   at 
  the 
  pressure 
  just 
  referred 
  to. 
  The 
  length 
  of 
  this 
  needle 
  was 
  2.36 
  

   inches. 
  The 
  diameter 
  of 
  the 
  cylindrical 
  part 
  .14 
  of 
  an 
  inch, 
  and 
  the 
  

   weight 
  of 
  the 
  needle 
  and 
  paper 
  stirrup 
  in 
  which 
  it 
  was 
  hung 
  72 
  

   grains. 
  

  

  The 
  apparatus 
  in 
  which 
  these 
  needles 
  were 
  oscillated, 
  consisted 
  of 
  a 
  

   small 
  jar, 
  furnished 
  with 
  a 
  brass 
  cap, 
  screwing 
  into 
  a 
  ring 
  cemented 
  

   around 
  the 
  mouth 
  of 
  the 
  jar. 
  Attached 
  to 
  this 
  cap 
  was 
  a 
  siphon 
  gauge 
  

   to 
  show 
  the 
  pressure 
  within 
  ; 
  a 
  lateral 
  tube, 
  or 
  passage, 
  from 
  the 
  jar, 
  

   terminated 
  by 
  a 
  screw, 
  served 
  to 
  apply 
  a 
  small 
  syringe 
  for 
  the 
  purpose 
  

   of 
  exhausting 
  the 
  jar. 
  The 
  tube 
  was 
  closed 
  by 
  a 
  valve 
  of 
  oiled 
  silk, 
  

   which 
  acted 
  as 
  one 
  of 
  the 
  valves 
  of 
  the 
  air-pump. 
  A 
  metallic 
  stem, 
  

   passing 
  through 
  a 
  collar 
  of 
  leathers, 
  occupied 
  the 
  centre 
  of 
  the 
  plate 
  ; 
  

   and 
  the 
  needles, 
  being 
  suspended 
  by 
  a 
  hook 
  at 
  the 
  end 
  of 
  this 
  stem, 
  

   were 
  raised 
  or 
  lowered 
  by 
  means 
  of 
  it, 
  as 
  occasion 
  required. 
  A 
  

   scale 
  for 
  measuring 
  the 
  arcs 
  of 
  vibration 
  was 
  fastened 
  around 
  the 
  exte- 
  

   rior 
  of 
  the 
  jar. 
  The 
  smallness 
  of 
  these 
  arcs 
  rendered 
  minute 
  accuracy 
  

   in 
  their 
  measurement 
  of 
  no 
  importance. 
  

  

  A 
  jar 
  of 
  the 
  requisite 
  form 
  not 
  being 
  at 
  hand, 
  one 
  of 
  ordinary 
  height 
  

   in 
  proportion 
  to 
  its 
  diameter, 
  was 
  partly 
  filled 
  with 
  cement, 
  so 
  as 
  to 
  

   diminish 
  the 
  space 
  within. 
  The 
  height 
  of 
  the 
  part 
  of 
  the 
  jar 
  in 
  which 
  

   the 
  needle 
  vibrated, 
  was 
  about 
  three 
  and 
  a 
  half 
  inches, 
  and 
  its 
  diameter 
  

   nearly 
  the 
  same. 
  The 
  needles 
  swung 
  about 
  .6 
  of 
  an 
  inch 
  from 
  the 
  

   cement 
  floor. 
  This 
  jar 
  was 
  particularly 
  adapted 
  to 
  needle 
  A. 
  

  

  A 
  thermometer 
  placed 
  within 
  the 
  jar 
  completed 
  this 
  apparatus. 
  

   The 
  parts 
  being 
  readily 
  detached, 
  the 
  whole 
  was 
  very 
  portable. 
  

  

  METHODS 
  OF 
  OBSERVATION. 
  

  

  In 
  the 
  commencement 
  we 
  adopted 
  the 
  method 
  used 
  by 
  Captain 
  Sa- 
  

   bine, 
  and 
  described 
  in 
  the 
  Royal 
  Society's 
  Transactions, 
  London, 
  for 
  

   1828. 
  We 
  have 
  subsequently 
  adopted 
  another 
  method, 
  which 
  not 
  

   only 
  saves 
  much 
  labour, 
  but 
  is, 
  we 
  think, 
  quite 
  as 
  unexceptionable 
  in 
  

  

  