﻿F. 
  II 
  Bigelow 
  — 
  Variations 
  of 
  the 
  Magnetic 
  Needle. 
  255 
  

  

  certain 
  meridians 
  for 
  the 
  hours, 
  and 
  supposing 
  the 
  permanent 
  

   pole 
  to 
  take 
  up 
  its 
  position 
  from 
  one 
  meridian 
  to 
  another, 
  there 
  

   was 
  finally 
  collected 
  upon 
  these 
  meridians 
  representing 
  a 
  series 
  

   of 
  local 
  hour 
  angles, 
  now 
  referred 
  to 
  the 
  sun 
  as 
  if 
  the 
  earth 
  

   had 
  ceased 
  to 
  rotate 
  on 
  its 
  axis, 
  an 
  exhibition 
  of 
  what 
  exists 
  

   over 
  the 
  globe 
  at 
  the 
  same 
  instant 
  of 
  time. 
  

  

  The 
  result 
  is 
  most 
  interesting 
  and 
  gratifying, 
  but 
  I 
  can 
  only 
  

   indicate 
  now 
  what 
  could 
  be 
  elaborated 
  by 
  a 
  mass 
  of 
  computa- 
  

   tions. 
  It 
  is 
  difficult 
  to 
  convey 
  any 
  view 
  of 
  the 
  complicated 
  

   system 
  of 
  lines 
  of 
  force 
  produced 
  by 
  inserting 
  a 
  magnetized 
  or 
  

   polarized 
  sphere 
  in 
  a 
  field 
  of 
  force, 
  supposing 
  the 
  sphere 
  at 
  

   rest 
  ; 
  if 
  it 
  rotates 
  it 
  is 
  much 
  more 
  troublesome. 
  These 
  refer- 
  

   ences, 
  however, 
  may 
  be 
  cited 
  : 
  Sir 
  W. 
  Thomson 
  in 
  § 
  xxxii, 
  p. 
  

   486, 
  Papers 
  on 
  Electrostatics 
  and 
  Magnetism, 
  illustrates 
  some 
  

   of 
  the 
  forms 
  produced 
  in 
  the 
  case 
  of 
  symmetry, 
  that 
  is, 
  the 
  

   axis 
  of 
  polarization 
  being 
  parallel 
  to 
  the 
  field 
  ; 
  in 
  article 
  434, 
  

   Vol. 
  II, 
  Maxwell's 
  Electricity 
  and 
  Magnetism 
  a 
  similar 
  illus- 
  

   tration 
  is 
  found 
  ; 
  in 
  article 
  436 
  of 
  the 
  same 
  is 
  given 
  an 
  ex- 
  

   ample 
  of 
  the 
  sphere 
  being 
  placed 
  at 
  an 
  angle 
  to 
  the 
  field. 
  

   The 
  mathematical 
  treatment 
  of 
  these 
  cases, 
  when 
  once 
  the 
  

   constants 
  involved 
  are 
  known, 
  leads 
  to 
  certain 
  typical 
  lines 
  of 
  

   force 
  entering 
  the 
  sphere 
  at 
  definite 
  angles 
  corresponding 
  to 
  

   the 
  latitude 
  of 
  the 
  point.- 
  Furthermore 
  when 
  the 
  sphere 
  is 
  

   rotated 
  the 
  whole 
  system 
  recedes 
  through 
  an 
  angle 
  depending 
  

   on 
  the 
  constants, 
  as 
  indicated 
  in 
  the 
  Bulletin. 
  My 
  model 
  gives 
  

   the 
  angles 
  and 
  directions 
  corresponding 
  to 
  such 
  a 
  system, 
  if 
  we 
  

   take 
  the 
  radiant 
  sunlight 
  as 
  the 
  uniform 
  field 
  of 
  force, 
  direc- 
  

   ted 
  positive 
  towards 
  the 
  sun. 
  The 
  entering 
  and 
  emerging 
  

   forces 
  are 
  on 
  the 
  respective 
  sides 
  of 
  the 
  earth, 
  and 
  the 
  whole 
  

   system 
  is 
  receding 
  by 
  about 
  twenty-three 
  degrees. 
  The 
  peculiar 
  

   form 
  of 
  the 
  polar 
  station 
  lines 
  and 
  the 
  inclination 
  of 
  their 
  

   planes 
  of 
  action 
  to 
  the 
  meridians 
  is 
  well 
  displayed. 
  The 
  sta- 
  

   tions 
  all 
  over 
  the 
  world 
  bear 
  the 
  same 
  testimony. 
  The 
  action 
  

   of 
  the 
  coronal 
  field 
  is 
  entirely 
  similar 
  but 
  not 
  strong 
  enough 
  

   to 
  appear 
  on 
  the 
  model. 
  The 
  separation 
  of 
  the 
  two 
  fields 
  is 
  

   merely 
  a 
  question 
  of 
  close 
  computation. 
  

  

  There 
  remains 
  one 
  more 
  important 
  point. 
  The 
  positive 
  

   direction 
  of 
  the 
  earth's 
  permanent 
  magnetism 
  is 
  from 
  the 
  

   north 
  towards 
  the 
  south 
  side 
  of 
  the 
  ecliptic 
  y, 
  the 
  uniform 
  

   field 
  is 
  positive 
  towards 
  the 
  sun 
  z 
  9 
  the 
  motion 
  of 
  the 
  earth 
  in 
  

   its 
  orbit 
  x, 
  is 
  perpendicular 
  to 
  the 
  field. 
  If 
  these 
  are 
  taken 
  as 
  

   the 
  usual 
  rectangular 
  oc 
  9 
  y 
  9 
  % 
  9 
  they 
  form 
  a 
  consistent 
  positive 
  or 
  

   right-handed 
  cyclic 
  system. 
  In 
  a 
  word, 
  the 
  permanent 
  mag- 
  

   netic 
  condition 
  of 
  the 
  earth 
  may 
  be 
  principally 
  due 
  to 
  the 
  

   orbital 
  motion 
  of 
  the 
  earth 
  through 
  the 
  radiant 
  field 
  of 
  sun- 
  

   light. 
  The 
  rotation 
  of 
  the 
  earth 
  on 
  its 
  axis 
  causes 
  a 
  modifica- 
  

   tion 
  of 
  the 
  direction 
  of 
  the 
  axis 
  of 
  polarization, 
  by 
  diminishing 
  

  

  