﻿OF 
  TERRESTRIAL, 
  MAGNETISM 
  AT 
  SEVERAL 
  PLACES, 
  ETC. 
  435 
  

  

  lations 
  at 
  the 
  temperature 
  t, 
  T' 
  the 
  time 
  of 
  making 
  the 
  same 
  number 
  

   at 
  the 
  temperature 
  f, 
  and 
  m 
  is 
  a 
  constant 
  to 
  be 
  determined 
  by 
  experi- 
  

   ment, 
  from 
  the 
  equation 
  

  

  T' 
  — 
  T 
  

   m 
  — 
  T' 
  (t' 
  — 
  t) 
  

  

  The 
  effect 
  of 
  the 
  expansion 
  of 
  the 
  needle 
  upon 
  its 
  moment 
  of 
  in- 
  

   ertia 
  is, 
  of 
  course, 
  too 
  minute 
  to 
  enter 
  into 
  the 
  discussion. 
  The 
  

   method 
  adopted 
  for 
  finding 
  this 
  coefficient 
  was 
  similar 
  to 
  that 
  de- 
  

   scribed 
  by 
  Captain 
  Sabine.* 
  The 
  observations 
  in 
  the 
  first 
  series 
  were 
  

   made 
  in 
  the 
  small 
  magnetic 
  observatory, 
  where 
  most 
  of 
  the 
  observa- 
  

   tions 
  at 
  Philadelphia 
  were 
  made. 
  

  

  In 
  the 
  First 
  Series 
  the 
  arrangements 
  for 
  observing 
  were 
  as 
  fol- 
  

   lows. 
  The 
  apparatus 
  for 
  oscillating 
  was 
  placed 
  in 
  a 
  large 
  wooden 
  

   vessel, 
  forming 
  a 
  considerable 
  inclosure 
  around 
  it. 
  The 
  temperature 
  

   of 
  the 
  inclosure, 
  and 
  of 
  course 
  of 
  the 
  apparatus, 
  was 
  lowered 
  by 
  filling 
  

   around 
  the 
  latter 
  with 
  ice, 
  the 
  melting 
  of 
  which 
  was 
  occasionally 
  pro- 
  

   moted 
  by 
  sprinkling 
  with 
  salt. 
  The 
  top 
  of 
  the 
  inclosure 
  was 
  covered, 
  

   except 
  only 
  a 
  sufficient 
  space 
  to 
  look 
  down 
  upon 
  the 
  northern 
  half 
  of 
  the 
  

   needle, 
  and 
  upon 
  the 
  thermometer 
  within 
  the 
  box. 
  Access 
  of 
  air, 
  and 
  

   radiation 
  from 
  the 
  sides 
  of 
  the 
  observatory, 
  were 
  thus, 
  in 
  a 
  great 
  mea- 
  

   sure, 
  cut 
  off. 
  A 
  local 
  dew 
  point 
  resulted 
  from 
  this 
  arrangement, 
  within 
  

   the 
  inclosure, 
  which 
  prevented 
  embarrassment 
  from 
  the 
  deposition 
  of 
  

   moisture 
  upon 
  the 
  needle, 
  or 
  upon 
  the 
  glass 
  cover 
  of 
  the 
  box. 
  

  

  The 
  needle 
  to 
  be 
  observed 
  having 
  been 
  allowed 
  to 
  remain 
  in 
  the 
  

   box 
  a 
  sufficient 
  length 
  of 
  time 
  to 
  acquire 
  the 
  temperature 
  of 
  the 
  me- 
  

   dium 
  within, 
  the 
  experiment 
  was 
  commenced. 
  The 
  temperature 
  of 
  

   the 
  box 
  was 
  noted, 
  by 
  the 
  inclosed 
  thermometer, 
  at 
  intervals 
  during 
  

   the 
  oscillations, 
  the 
  temperature 
  being 
  made 
  as 
  nearly 
  stationary 
  as 
  

   possible. 
  The 
  mean 
  temperature 
  thus 
  obtained 
  does 
  not, 
  of 
  course, 
  

   coincide 
  with 
  the 
  half 
  sum 
  of 
  the 
  temperatures 
  taken 
  at 
  the 
  beginning 
  

   and 
  end, 
  and 
  which 
  are 
  recorded 
  with 
  the 
  means 
  in 
  the 
  table 
  which 
  

   follows. 
  

  

  The 
  observation 
  of 
  the 
  passage 
  of 
  the 
  needle 
  over 
  the 
  meridian, 
  and 
  

   of 
  the 
  arcs 
  of 
  vibration, 
  had 
  been 
  rendered 
  easy 
  and 
  accurate 
  by 
  tracing 
  

  

  * 
  Brande's 
  Quarterly 
  Journal 
  of 
  Science, 
  Vol. 
  XXVIII. 
  p. 
  14, 
  &c. 
  

   VOL. 
  V. 
  5 
  M 
  

  

  