﻿OF 
  TERRESTRIAL 
  MAGNETISM 
  AT 
  SEVERAL 
  PLACES, 
  ETC. 
  431 
  

  

  tions, 
  nor 
  proved 
  by 
  their 
  agreement 
  with 
  those 
  made 
  with 
  Nos. 
  1 
  and 
  

   3, 
  to 
  be 
  satisfactory, 
  we 
  have 
  thought 
  it 
  best 
  not 
  to 
  use 
  them 
  in 
  taking 
  

   our 
  mean. 
  We 
  have 
  placed 
  the 
  description 
  of 
  the 
  needle 
  here, 
  because 
  

   the 
  correction 
  for 
  temperature 
  was 
  observed, 
  and 
  we 
  are 
  enabled 
  to 
  use 
  

   the 
  results 
  in 
  their 
  bearing 
  upon 
  the 
  general 
  conclusions 
  in 
  regard 
  to 
  

   the 
  effect 
  of 
  heat 
  on 
  the 
  intensity 
  of 
  the 
  magnet 
  itself. 
  

  

  No. 
  3 
  was 
  of 
  the 
  Hansteen 
  model, 
  2.43 
  in 
  length, 
  and 
  .14 
  inch 
  in 
  

   diameter, 
  at 
  the 
  cylindric 
  part 
  ; 
  weighing, 
  with 
  its 
  pasteboard 
  stirrup, 
  

   78 
  grains. 
  For 
  this 
  needle, 
  we 
  are 
  indebted 
  to 
  our 
  friend, 
  professor 
  

   Henry, 
  of 
  Princeton, 
  by 
  whom 
  it 
  was 
  made 
  and 
  magnetized, 
  several 
  

   years 
  since. 
  Its 
  rate 
  is 
  slower 
  than 
  that 
  of 
  either 
  Nos. 
  1 
  or 
  2. 
  

  

  The 
  needles 
  were 
  kept 
  in 
  separate 
  boxes, 
  and 
  when 
  stationary, 
  or 
  car- 
  

   ried 
  from 
  place 
  to 
  place, 
  the 
  cases 
  containing 
  them 
  were 
  kept 
  as 
  far 
  as 
  

   possible 
  from 
  each 
  other, 
  and 
  from 
  iron 
  or 
  steel. 
  

  

  Observations 
  were 
  made 
  at 
  Philadelphia 
  and 
  West 
  Point, 
  with 
  a 
  

   fourth 
  needle, 
  C 
  of 
  second 
  series, 
  but 
  the 
  results 
  were 
  very 
  little 
  ac- 
  

   cordant. 
  This 
  we 
  attribute 
  to 
  its 
  small 
  mass, 
  and 
  the 
  rapidity 
  of 
  its 
  

   vibration. 
  So 
  materially 
  was 
  this 
  needle 
  affected 
  by 
  accidental 
  circum- 
  

   stances, 
  that 
  its 
  small 
  correction 
  for 
  temperature 
  not 
  only 
  was 
  masked 
  

   by 
  them, 
  but 
  was 
  even, 
  apparently, 
  negative 
  under 
  their 
  influence. 
  

   In 
  a 
  rarefied 
  medium 
  the 
  performance 
  of 
  this 
  needle 
  was 
  very 
  regular 
  

   and 
  satisfactory. 
  

  

  In 
  the 
  Second 
  Series 
  of 
  observations, 
  upon 
  which 
  we 
  principally 
  

   rely, 
  three 
  needles 
  were 
  used. 
  The 
  first, 
  A, 
  was 
  a 
  small 
  bar 
  2.83 
  

   inches 
  long, 
  and 
  .22 
  by 
  .14 
  of 
  an 
  inch 
  in 
  cross 
  section, 
  the 
  larger 
  

   dimension 
  being 
  vertical. 
  The 
  weight 
  of 
  the 
  bar, 
  and 
  of 
  its 
  suspend- 
  

   ing 
  stirrup 
  of 
  wire, 
  was 
  184 
  grains. 
  It 
  was 
  suspended 
  by 
  three 
  silk 
  

   worm 
  threads, 
  and 
  made, 
  when 
  in 
  a 
  medium 
  rarefied 
  to 
  between 
  three 
  

   and 
  a 
  half 
  and 
  three 
  inches 
  of 
  mercury, 
  three 
  hundred 
  vibrations, 
  be- 
  

   tween 
  the 
  semi-arcs 
  of 
  four 
  and 
  two 
  degrees. 
  A 
  small 
  black 
  line 
  on 
  a 
  

   white 
  ground, 
  upon 
  one 
  end 
  of 
  the 
  needle, 
  served 
  to 
  observe 
  its 
  passage 
  

   over 
  the 
  meridian, 
  when 
  oscillating. 
  

  

  The 
  second 
  needle, 
  B, 
  was 
  No. 
  3 
  of 
  the 
  first 
  series. 
  It 
  was 
  placed 
  in 
  

   a 
  pasteboard 
  stirrup, 
  which 
  made 
  its 
  moment 
  of 
  inertia 
  slightly 
  different 
  

   from 
  that 
  in 
  the 
  first 
  series. 
  It 
  made 
  about 
  two 
  hundred 
  and 
  fifty 
  

  

  VOL. 
  V. 
  — 
  5 
  L 
  

  

  