﻿152 
  Dr. 
  J. 
  G. 
  Gray 
  and 
  Mr. 
  A. 
  D. 
  Ross 
  on 
  an 
  

  

  the 
  main 
  portion 
  of 
  the 
  bed-plate, 
  it 
  is 
  formed 
  from 
  one 
  

   piece 
  of 
  wood, 
  the 
  two 
  lengths 
  being 
  set 
  accurately 
  at 
  right 
  

   angles 
  and 
  half 
  checked 
  into 
  one 
  another. 
  The 
  junction 
  is 
  

   made 
  rigid 
  by 
  means 
  of 
  glue 
  and 
  brass 
  screws. 
  A 
  channel 
  

   11 
  '5 
  cm. 
  broad 
  is 
  formed 
  over 
  the 
  entire 
  length 
  of 
  the 
  cross- 
  

   piece 
  by 
  means 
  of 
  two 
  mahogany 
  strips 
  which 
  are 
  square 
  in 
  

   section 
  and 
  fixed 
  parallel 
  to 
  the 
  edges 
  of 
  the 
  arms. 
  A 
  

   similar 
  channel 
  runs 
  down 
  the 
  main 
  length 
  of 
  the 
  bed-plate, 
  

   being 
  discontinued 
  where 
  it 
  is 
  crossed 
  by 
  the 
  channel 
  already 
  

   mentioned. 
  The 
  wooden 
  strips 
  forming 
  these 
  channels 
  are 
  

   permanently 
  fixed 
  by 
  glueing 
  and 
  by 
  brass 
  screws 
  driven 
  in 
  

   from 
  the 
  underside 
  of 
  the 
  base-board. 
  After 
  they 
  have 
  been 
  

   constructed 
  they 
  are 
  made 
  of 
  perfectly 
  uniform 
  width 
  by 
  

   sand-papering, 
  the 
  width 
  being 
  tested 
  from 
  time 
  to 
  time 
  

   during 
  the 
  process 
  by 
  means 
  of 
  a 
  wooden 
  gauge. 
  

  

  A 
  is 
  a 
  mahogany 
  box 
  consisting 
  of 
  bottom, 
  sides, 
  and 
  top, 
  

   the 
  ends 
  which 
  face 
  east 
  and 
  west 
  being 
  left 
  open. 
  Jn 
  the 
  

   bottom 
  is 
  a 
  slot 
  running 
  parallel 
  to 
  the 
  cross-piece 
  of 
  the 
  

   bed-plate. 
  A 
  brass 
  screw 
  projecting 
  upwards 
  from 
  the 
  base- 
  

   board 
  of 
  the 
  magnetometer 
  passes 
  through 
  this 
  slot 
  and 
  is 
  

   provided 
  with 
  a 
  brass 
  washer 
  and 
  locking-nut. 
  By 
  this 
  

   means 
  the 
  box 
  can 
  be 
  moved 
  through 
  a 
  small 
  distance 
  in 
  the 
  

   north 
  and 
  south 
  direction, 
  and 
  securely 
  clamped 
  in 
  position. 
  

   On 
  the 
  upper 
  surface 
  of 
  the 
  box 
  is 
  fastened 
  a 
  plate 
  of 
  glass 
  

   on 
  which 
  stands 
  the 
  magnetometer 
  proper. 
  This 
  part 
  of 
  the 
  

   instrument 
  is 
  also 
  constructed 
  of 
  mahogany. 
  A 
  wooden 
  

   pillar 
  20 
  cm. 
  in 
  height 
  has 
  a 
  narrow 
  hole 
  drilled 
  longitudi- 
  

   nally 
  down 
  through 
  it. 
  This 
  hole 
  terminates 
  in 
  a 
  small 
  cell 
  

   with 
  a 
  glass 
  window 
  in 
  front. 
  The 
  cell 
  is 
  just 
  large 
  enough 
  

   to 
  contain 
  the 
  mirror 
  of 
  the 
  magnetometer 
  — 
  a 
  concave 
  mirror, 
  

   1 
  cm. 
  in 
  diameter, 
  having 
  a 
  focal 
  length 
  of 
  50 
  cm. 
  The 
  

   mirror 
  has 
  attached 
  to 
  its 
  back 
  a 
  small 
  piece 
  of 
  magnetized 
  

   watch-spring 
  about 
  8 
  mm. 
  in 
  length. 
  The 
  needle 
  and 
  mirror 
  

   are 
  suspended 
  by 
  a 
  fine 
  qnartz 
  fibre 
  from 
  a 
  screw 
  at 
  the 
  top 
  

   of 
  the 
  upright 
  pillar 
  of 
  the 
  magnetometer. 
  By 
  means 
  of 
  

   this 
  screw, 
  the 
  axis 
  of 
  which 
  is 
  vertical, 
  all 
  torsion 
  can 
  be 
  

   removed 
  from 
  the 
  fibre 
  when 
  the 
  needle 
  is 
  hanging 
  in 
  its 
  

   equilibrium 
  position 
  ; 
  and 
  by 
  giving 
  the 
  screw 
  an 
  observed 
  

   number 
  of 
  complete 
  turns 
  a 
  determination 
  of 
  the 
  torsional 
  

   rigidity 
  of 
  the 
  fibre 
  can 
  be 
  made. 
  The 
  pillar 
  of 
  the 
  magneto- 
  

   meter 
  is 
  attached 
  to 
  a 
  circular 
  base 
  provided 
  with 
  three 
  

   small 
  brass 
  levelling-screws. 
  The 
  position 
  of 
  these 
  feet 
  en 
  

   the 
  glass 
  top 
  of 
  the 
  box- 
  stand 
  is 
  defined 
  by 
  the 
  hole, 
  slot, 
  and 
  

   plane 
  method. 
  

  

  A 
  A 
  (fig. 
  5) 
  is 
  the 
  magnetizing 
  solenoid. 
  Two 
  brass 
  tubes 
  

   4.5 
  cm. 
  in 
  length 
  are 
  connected 
  att 
  their 
  ends 
  by 
  brass 
  rings 
  

  

  