﻿F. 
  H. 
  Bigelow 
  — 
  Variations 
  of 
  the 
  Magnetic 
  Needle. 
  253 
  

  

  Apart 
  from 
  the 
  obvious 
  proof 
  of 
  the 
  preglacial 
  age 
  of 
  these 
  

   gravels 
  and 
  sands 
  afforded 
  by 
  the 
  superposition 
  of 
  the 
  drift 
  

   upon 
  them, 
  the 
  character 
  of 
  the 
  till 
  affords 
  a 
  further 
  proof 
  of 
  

   the 
  same 
  thing. 
  If 
  the 
  sands 
  and 
  gravels 
  occupied 
  the 
  region 
  

   before 
  the 
  ice 
  invasion, 
  they 
  should 
  have 
  made 
  their 
  contribu- 
  

   tion 
  to 
  the 
  drift. 
  This 
  they 
  have 
  done, 
  and 
  so 
  generously 
  that 
  

   at 
  many 
  points 
  and 
  over 
  considerable 
  areas 
  the 
  character 
  of 
  the 
  

   drift 
  has 
  been 
  in 
  very 
  large 
  measure 
  determined 
  by 
  this 
  contri- 
  

   bution. 
  

  

  To 
  the 
  arguments 
  adduced 
  in 
  the 
  article 
  referred 
  to 
  above 
  

   for 
  the 
  Pre-pleistocene 
  reference 
  of 
  the 
  Orange 
  Sand 
  gravels 
  

   and 
  sands, 
  must 
  now 
  be 
  added 
  the 
  further 
  arguments 
  of 
  (1) 
  

   superposition 
  of 
  the 
  earlier 
  glacial 
  drift 
  upon 
  them, 
  and 
  (2) 
  

   the 
  contribution 
  of 
  these 
  sands 
  and 
  gravels 
  to 
  this 
  drift. 
  

  

  Subsequent 
  to 
  the 
  writer's 
  determination 
  of 
  the 
  existence 
  of 
  

   Pre-pleistocene 
  material 
  in 
  the 
  region 
  indicated, 
  reference 
  to 
  

   the 
  reports 
  of 
  the 
  Illinois 
  Geological 
  Survey 
  revealed 
  the 
  fact 
  

   that 
  in 
  the 
  reports 
  on 
  Pike 
  and 
  Hancock 
  counties, 
  the 
  Illinois 
  

   geologists 
  had 
  made 
  note 
  of 
  the 
  fact 
  that 
  ferruginous 
  flint 
  

   gravels 
  occur 
  beneath 
  the 
  drift 
  in 
  these 
  counties, 
  and 
  that 
  

   they 
  had 
  further 
  correlated 
  them 
  with 
  the 
  gravels 
  in 
  the 
  south- 
  

   ern 
  part 
  of 
  the 
  state. 
  To 
  them, 
  therefore, 
  belongs 
  the 
  credit 
  

   of 
  the 
  first 
  recognition 
  of 
  these 
  gravels, 
  as 
  wholly 
  distinct 
  from 
  

   the 
  drift* 
  

  

  Aet. 
  XXV. 
  — 
  Note 
  on 
  the 
  Causes 
  of 
  the 
  Variations 
  of 
  the 
  

   Magnetic 
  Needle 
  ; 
  by 
  Professor 
  Feank 
  H. 
  Bigelow. 
  

  

  In 
  May, 
  1890, 
  I 
  published 
  in 
  Bulletin 
  No. 
  18 
  of 
  the 
  U. 
  S. 
  

   Scientific 
  Expedition 
  to 
  West 
  Africa, 
  a 
  preliminary 
  statement 
  

   of 
  a 
  new 
  theory 
  of 
  terrestrial 
  magnetism 
  which 
  had 
  been 
  con- 
  

   ceived 
  in 
  order 
  to 
  account 
  for 
  the 
  observed 
  variations 
  of 
  the 
  

   free 
  magnetic 
  needle. 
  Since 
  that 
  time 
  my 
  efforts 
  have 
  been 
  

   directed 
  towards 
  obtaining 
  a 
  clear 
  conception 
  of 
  the 
  mode 
  of 
  

   action 
  of 
  the 
  forces 
  whose 
  relations 
  were 
  indicated 
  in 
  the 
  

   Bulletin, 
  and 
  I 
  am 
  now 
  prepared 
  to 
  add 
  a 
  note 
  as 
  a 
  further 
  

   preliminary 
  statement 
  of 
  the 
  progress 
  made 
  in 
  this 
  study. 
  

  

  On 
  re-examination 
  of 
  the 
  mode 
  of 
  analysis 
  already 
  published, 
  

   I 
  find 
  that 
  the 
  main 
  conception 
  is 
  not 
  to 
  be 
  modified 
  and 
  that 
  

   the 
  successive 
  steps 
  are 
  correct. 
  When 
  making 
  an 
  attempt 
  to 
  

   reduce 
  the 
  observations 
  by 
  means 
  of 
  this 
  treatment, 
  namely, 
  

   the 
  combination 
  of 
  current 
  functions 
  by 
  the 
  use 
  of 
  harmonics, 
  

   it 
  was 
  evident 
  that 
  a 
  very 
  complex 
  system 
  of 
  computation 
  

  

  * 
  I 
  am 
  unable 
  at 
  this 
  writing 
  to 
  refer 
  to 
  the 
  Illinois 
  reports, 
  and 
  therefore 
  can- 
  

   not 
  cite 
  the 
  exact 
  references 
  to 
  the 
  statements 
  therein 
  made. 
  R. 
  d. 
  s. 
  

  

  