﻿430 
  ON 
  THE 
  RELATIVE 
  HORIZONTAL 
  INTENSIIES 
  

  

  derably 
  beyond 
  what 
  could 
  be 
  claimed 
  for 
  the 
  mere 
  determination 
  of 
  

   the 
  horizontal 
  intensities 
  at 
  the 
  several 
  places 
  of 
  observation. 
  

  

  These 
  and 
  other 
  circumstances 
  render 
  it 
  imperative 
  upon 
  us 
  to 
  give 
  

   the 
  observations 
  in 
  detail, 
  and 
  will, 
  we 
  trust, 
  excuse 
  to 
  the 
  Society 
  the 
  

   length 
  of 
  our 
  memoir. 
  

  

  OF 
  THE 
  INSTRUMENTS. 
  

  

  The 
  needles 
  used 
  in 
  the 
  First 
  Series 
  for 
  horizontal 
  intensities 
  were 
  

   of 
  the 
  form 
  originally 
  adopted 
  by 
  professor 
  Hansteen, 
  namely, 
  cylinders 
  

   terminated 
  by 
  cones. 
  They 
  were 
  three 
  in 
  number, 
  and 
  of 
  different 
  

   masses. 
  These 
  were 
  in 
  turn 
  suspended 
  by 
  one 
  or 
  more 
  fibres 
  of 
  silk- 
  

   worm's 
  thread 
  in 
  a 
  small 
  box 
  which 
  protected 
  them 
  from 
  the 
  air, 
  and 
  

   at 
  the 
  bottom 
  of 
  which 
  was 
  placed 
  a 
  graduated 
  circle. 
  Two 
  threads 
  

   were 
  fastened 
  vertically 
  against 
  two 
  small 
  glass 
  windows 
  in 
  the 
  sides 
  

   of 
  the 
  Box, 
  and 
  in 
  a 
  plane 
  passing 
  through 
  the 
  centre 
  of 
  the 
  divided 
  

   circle. 
  These 
  threads, 
  when 
  brought 
  into 
  the 
  plane 
  of 
  the 
  magnetic 
  

   meridian, 
  served 
  to 
  observe 
  the 
  passage 
  of 
  the 
  ends 
  of 
  the 
  needle 
  across 
  

   this 
  plane. 
  Three 
  levelling 
  screws 
  were 
  attached 
  to 
  the 
  bottom 
  of 
  the 
  

   box. 
  The 
  method 
  of 
  adjusting 
  for 
  the 
  level 
  of 
  the 
  needle, 
  and 
  of 
  the 
  

   box, 
  for 
  placing 
  the 
  line 
  of 
  suspension 
  in 
  the 
  direction 
  of 
  the 
  threads, 
  

   and 
  bringing 
  them 
  into 
  the 
  meridian, 
  and 
  for 
  centring 
  the 
  needle, 
  are 
  

   too 
  simple 
  to 
  need 
  any 
  particular 
  description. 
  The 
  needles 
  were 
  placed 
  

   in 
  small 
  stirrups, 
  and 
  were 
  raised, 
  usually, 
  about 
  half 
  an 
  inch 
  above 
  the 
  

   bottom 
  of 
  the 
  box. 
  

  

  Needle 
  No. 
  1, 
  the 
  largest, 
  was 
  three 
  inches 
  long, 
  and 
  .22 
  in 
  diame- 
  

   ter 
  in 
  the 
  cylindrical 
  part. 
  It 
  was 
  placed 
  in 
  a 
  brass 
  stirrup, 
  and 
  sus- 
  

   pended 
  by 
  two 
  fibres 
  of 
  silkworm's 
  thread. 
  This 
  needle 
  was 
  most 
  

   steady 
  in 
  its 
  vibrations, 
  and 
  did 
  not 
  lose 
  any 
  appreciable 
  part 
  of 
  its 
  

   magnetism 
  while 
  in 
  use, 
  though 
  it 
  was 
  softer 
  than 
  No. 
  3. 
  

  

  No. 
  2 
  was 
  of 
  the 
  same 
  material 
  as 
  No. 
  1. 
  Its 
  length 
  was 
  2.53 
  inches, 
  

   and 
  diameter 
  of 
  the 
  cylinder 
  .22 
  inches 
  ; 
  weighing, 
  with 
  its 
  stirrup, 
  

   203 
  grains. 
  Both 
  of 
  these 
  needles 
  were 
  made 
  at 
  West 
  Point, 
  and 
  

   they 
  were 
  not 
  highly 
  charged. 
  

  

  The 
  observations 
  made 
  with 
  the 
  latter 
  needle, 
  No. 
  2, 
  were 
  few 
  in 
  

   number, 
  and 
  they 
  were 
  neither 
  considered 
  at 
  the 
  time 
  of 
  the 
  observa- 
  

  

  