﻿210 
  ON 
  THE 
  MAGNETIC 
  DIP 
  

  

  copper 
  y's, 
  which 
  can 
  be 
  raised 
  so 
  as 
  to 
  relieve 
  the 
  needle 
  from 
  the 
  

   agate 
  supports, 
  and 
  then 
  being 
  depressed, 
  restore 
  it 
  to 
  its 
  bearings 
  

   upon 
  them. 
  A 
  sensitive 
  level 
  is 
  attached 
  to 
  the 
  instrument, 
  which 
  is 
  

   levelled 
  by 
  three 
  foot 
  screws. 
  The 
  two 
  needles 
  which 
  accompany 
  

   the 
  instrument 
  are 
  in 
  the 
  form 
  of 
  very 
  acute 
  rhombs, 
  in 
  length 
  across 
  

   the 
  longer 
  diagonal 
  about 
  seven 
  and 
  a 
  half 
  inches, 
  in 
  breadth 
  across 
  the 
  

   shorter 
  diagonal 
  three-eighths 
  of 
  an 
  inch. 
  

  

  The 
  method 
  of 
  observation 
  usual 
  with 
  this 
  kind 
  of 
  needle 
  was 
  

   resorted 
  to. 
  The 
  eccentricity 
  of 
  the 
  axis 
  of 
  the 
  needle 
  in 
  relation 
  to 
  

   the 
  vertical 
  circle 
  on 
  which 
  the 
  readings 
  were 
  made, 
  was 
  corrected 
  

   by 
  readings 
  at 
  the 
  two 
  extremities 
  of 
  the 
  needle. 
  The 
  want 
  of 
  paral- 
  

   lelism 
  of 
  the 
  zero 
  line 
  and 
  level 
  was 
  corrected 
  by 
  turning 
  the 
  limb 
  

   180° 
  in 
  azimuth, 
  and 
  making 
  two 
  readings, 
  one 
  with 
  the 
  limb 
  direct, 
  

   the 
  other 
  with 
  it 
  reversed. 
  The 
  inclination 
  of 
  the 
  magnetic 
  axis 
  to 
  

   the 
  axis 
  of 
  figure, 
  was 
  corrected 
  by 
  turning 
  the 
  needle 
  in 
  the 
  y's 
  ; 
  and 
  

   the 
  error 
  resulting 
  from 
  the 
  centre 
  of 
  gravity 
  of 
  the 
  needle 
  being 
  out 
  

   of 
  the 
  axis, 
  by 
  inverting 
  the 
  poles 
  of 
  the 
  needle. 
  

  

  A 
  detailed 
  example 
  of 
  this 
  method 
  will 
  be 
  given 
  to 
  indicate 
  the 
  

   degree 
  of 
  accuracy 
  to 
  which 
  the 
  particular 
  instrument 
  in 
  question 
  en- 
  

   abled 
  us 
  to 
  carry 
  it 
  ; 
  no 
  special 
  interest 
  attaching 
  to 
  these 
  details, 
  they 
  

   are 
  suppressed 
  in 
  other 
  cases 
  : 
  care 
  has 
  been 
  taken 
  that 
  those 
  selected 
  

   are 
  not 
  culled 
  and 
  put 
  forward 
  on 
  account 
  of 
  any 
  peculiar 
  accordance 
  

   of 
  the 
  different 
  parts, 
  but 
  that 
  they 
  fairly 
  represent 
  the 
  series 
  of 
  ob- 
  

   servations. 
  

  

  MAGNETIC 
  DIP 
  AT 
  BALTIMORE, 
  MARYLAND. 
  

  

  The 
  dip 
  at 
  Baltimore 
  was 
  observed 
  by 
  one 
  of 
  us 
  when 
  on 
  a 
  visit 
  to 
  

   that 
  city 
  in 
  July 
  of 
  the 
  present 
  year. 
  The 
  place 
  of 
  observation 
  was 
  

   in 
  the 
  front 
  yard 
  of 
  one 
  of 
  the 
  dwellings 
  in 
  Holliday 
  street, 
  opposite 
  

   to 
  the 
  theatre 
  : 
  the 
  time 
  about 
  5 
  P.M. 
  All 
  the 
  circumstances 
  attend- 
  

   ing 
  the 
  observations 
  were 
  favourable. 
  

  

  The 
  latitude 
  and 
  longitude 
  of 
  Baltimore, 
  as 
  stated 
  in 
  the 
  American 
  

   Almanac 
  for 
  1835, 
  are 
  39° 
  17' 
  13" 
  N. 
  and 
  76° 
  37' 
  50" 
  W. 
  

  

  