﻿584 
  

  

  Staff- 
  Commander 
  E. 
  W. 
  Creak 
  on 
  the 
  

  

  [June 
  17, 
  

  

  It 
  is 
  now 
  time 
  to 
  say 
  something 
  about 
  these 
  iron 
  masts. 
  The 
  bow- 
  

   sprit 
  and 
  foremast 
  are 
  so 
  far 
  removed 
  from 
  the 
  compasses 
  that 
  their 
  

   effect 
  may 
  be 
  regarded 
  as 
  zero. 
  The 
  mainmast 
  also 
  being 
  at 
  a 
  distance 
  

   of 
  62 
  feet 
  from 
  the 
  standard 
  compass, 
  its 
  red 
  and 
  blue 
  poles 
  must 
  

   neutralize 
  one 
  another. 
  At 
  the 
  steering-compasses 
  a 
  slight 
  repulsion 
  

   towards 
  the 
  stern 
  may 
  exist, 
  but 
  sufficiently 
  small 
  to 
  be 
  neglected. 
  

  

  When, 
  however, 
  we 
  consider 
  the 
  position 
  of 
  the 
  mizzenmast, 
  at 
  a 
  

   distance 
  of 
  only 
  8 
  feet 
  6 
  inches 
  from 
  all 
  compasses, 
  we 
  shall 
  find 
  that 
  it 
  

   has 
  produced 
  deviations 
  of 
  very 
  decided 
  amount 
  and 
  marked 
  character 
  ; 
  

   and 
  therefore 
  our 
  attention 
  may 
  be 
  confined 
  to 
  that 
  mast 
  alone. 
  

  

  This 
  mizzenmast, 
  which 
  is 
  82 
  feet 
  in 
  length 
  and 
  24 
  inches 
  in 
  diameter, 
  

   was 
  built 
  at 
  Chatham 
  of 
  the 
  " 
  best-best 
  " 
  iron, 
  half 
  an 
  inch 
  thick, 
  the 
  

   plates 
  overlapping 
  making 
  it 
  1 
  inch 
  thick 
  at 
  its 
  thickest 
  parts, 
  and 
  for 
  the 
  

   purposes 
  of 
  this 
  discussion 
  may 
  be 
  considered 
  a 
  hollow 
  cylinder. 
  The 
  

   effects 
  of 
  iron 
  in 
  that 
  form 
  have 
  been 
  already 
  treated 
  on 
  mathematically 
  

   by 
  the 
  late 
  Mr. 
  Archibald 
  Smith, 
  F.R.S., 
  in 
  the 
  Phil. 
  Trans. 
  Eoy. 
  Soc. 
  

   part 
  i. 
  1865, 
  pp. 
  317, 
  318. 
  

  

  Whilst 
  building, 
  the 
  mast 
  lay 
  in 
  a 
  horizontal 
  position, 
  the 
  direction 
  

   N. 
  43° 
  W. 
  and 
  S. 
  43° 
  E., 
  heel 
  towards 
  the 
  northward, 
  which 
  was 
  a 
  

   favourable 
  position 
  for 
  that 
  end 
  of 
  the 
  mast 
  to 
  receive 
  a 
  permanent 
  

   quantity 
  of 
  red 
  magnetism 
  as 
  the 
  riveting 
  was 
  proceeding. 
  When 
  the 
  

   mast 
  was 
  stepped 
  in 
  its 
  nearly 
  vertical 
  position 
  and 
  subjected 
  to 
  the 
  

   tremor 
  caused 
  by 
  the 
  rapid 
  revolutions 
  of 
  the 
  screw 
  and 
  engines, 
  red 
  

   magnetism 
  would 
  become 
  more 
  and 
  more 
  developed. 
  To 
  test 
  to 
  what 
  

   extent 
  this 
  was 
  accomplished 
  the 
  following 
  experiments 
  were 
  made. 
  

  

  At 
  15 
  feet 
  from 
  the 
  heel 
  on 
  the 
  upper 
  deck, 
  and 
  at 
  12 
  inches 
  from 
  

   the 
  mast, 
  a 
  small 
  compass 
  was 
  carried 
  round, 
  the 
  blue 
  pole 
  of 
  which 
  

   pointed 
  invariably 
  to 
  the 
  mast. 
  On 
  the 
  poop, 
  at 
  22^ 
  feet 
  from 
  the 
  heel 
  

   and 
  12 
  inches 
  from 
  the 
  mast, 
  the 
  blue 
  pole 
  of 
  the 
  compass 
  was 
  attracted 
  

   three 
  points. 
  At 
  9 
  J 
  feet 
  above 
  the 
  poop, 
  and 
  approaching 
  the 
  centre 
  of 
  

   the 
  mast, 
  there 
  was 
  no 
  attraction 
  ; 
  but 
  the 
  neutral 
  zone 
  was 
  very 
  limited 
  

   in 
  extent, 
  as 
  the 
  blue 
  magnetism 
  began 
  to 
  make 
  itself 
  felt 
  a 
  few 
  inches 
  

   further 
  up. 
  This 
  description 
  of 
  the 
  mizzenmast 
  shows 
  that 
  at 
  8 
  feet 
  

   6 
  inches 
  from 
  all 
  compasses 
  there 
  exists 
  a 
  vertical, 
  hollow, 
  cylindrical 
  

   magnet, 
  the 
  effects 
  of 
  which 
  are 
  clearly 
  demonstrated 
  by 
  the 
  following 
  

   coefficients 
  (p. 
  585). 
  

  

  According 
  to 
  the 
  custom 
  of 
  Her 
  Majesty's 
  navy 
  the 
  ships 
  were 
  swung 
  

   when 
  ready 
  for 
  sea, 
  to 
  ascertain 
  the 
  deviations 
  of 
  the 
  standard 
  and 
  

   steering-compasses. 
  The 
  vertical 
  force, 
  however, 
  on 
  board 
  the 
  'New- 
  

   castle' 
  was 
  not 
  observed, 
  as 
  in 
  wooden 
  ships 
  experience 
  has 
  shown 
  it 
  to 
  

   be 
  so 
  little 
  disturbed. 
  To 
  remedy 
  this 
  deficiency 
  I 
  must 
  now 
  invite 
  

   attention 
  to 
  another 
  wooden 
  frigate, 
  IT.M.S. 
  ' 
  Challenger.' 
  When 
  the 
  

   'Challenger' 
  was 
  about 
  to 
  start 
  on 
  her 
  present 
  voyage 
  from 
  Sheerness, 
  

   an 
  extended 
  series 
  of 
  observations 
  for 
  the 
  horizontal 
  and 
  vertical 
  forces 
  

   on 
  board 
  was 
  made, 
  in 
  order 
  to 
  prepare 
  for 
  the 
  correction 
  of 
  the 
  magnetic 
  

   observations 
  now 
  being 
  carried 
  out 
  in 
  her 
  at 
  sea. 
  

  

  