﻿F. 
  A. 
  Ferret 
  — 
  Vertical 
  Motion 
  Seismographs. 
  299 
  

  

  very 
  small 
  apparatus, 
  and 
  with 
  the 
  crudest 
  of 
  adjustments, 
  the 
  

   period 
  was 
  readily 
  increased 
  to 
  from 
  two 
  to 
  four 
  seconds 
  and 
  

   could 
  even 
  — 
  within 
  narrow 
  limits 
  of 
  movement 
  — 
  be 
  made 
  

   almost 
  absolute, 
  i. 
  e., 
  the 
  instrument 
  could 
  be 
  made 
  aperiodic. 
  

   The 
  motion 
  in 
  the 
  magnetic 
  field 
  produced 
  a 
  fair 
  amount 
  of 
  

   damping, 
  so 
  that 
  the 
  application 
  of 
  this 
  magnetic 
  principle 
  

   may 
  be 
  said 
  to 
  have 
  converted 
  an 
  impossible 
  instrument 
  into 
  

   one 
  which, 
  for 
  its 
  size, 
  might 
  be 
  considered 
  as 
  a 
  satisfactory 
  

   vertical 
  motion 
  seismograph. 
  

  

  It 
  will 
  be 
  seen 
  that 
  the 
  magnetic 
  apparatus 
  used 
  in 
  this 
  

   experiment 
  was 
  far 
  from 
  being 
  well 
  adapted 
  to 
  the 
  purpose. 
  

   The 
  closeness 
  together 
  of 
  the 
  poles 
  of 
  the 
  magnets 
  gives 
  an 
  

   exceedingly 
  restricted 
  external 
  field, 
  necessitating 
  the 
  locating 
  

   of 
  the 
  magnetic 
  control 
  near 
  to 
  the 
  pivoted 
  end 
  of 
  the 
  lever 
  

   instead 
  of 
  at 
  the 
  weighted 
  end, 
  where 
  its 
  efficiency 
  would 
  have 
  

   been 
  far 
  greater. 
  In 
  the 
  case 
  of 
  a 
  heavy-weight 
  seismograph 
  

   a 
  battery 
  of 
  long 
  field 
  magnets 
  is 
  indicated, 
  with 
  armatures 
  

   possibly 
  Y-shaped 
  to 
  ensure 
  diagonal 
  approach, 
  the 
  magnetic 
  

   system 
  to 
  be 
  mounted 
  on 
  an 
  outer 
  extension 
  from 
  the 
  weight 
  

   for 
  greater 
  leverage. 
  But 
  the 
  details 
  of 
  construction 
  must 
  be 
  

   left 
  to 
  the 
  manufacturer, 
  who 
  will 
  adapt 
  the 
  magnetic 
  aux- 
  

   iliary 
  to 
  the 
  design 
  of 
  the 
  instrument, 
  the 
  present 
  paper 
  being 
  

   merely 
  a 
  presentation 
  of 
  the 
  bare 
  principle. 
  It 
  need 
  scarcely 
  

   be 
  stated 
  that 
  the 
  magnets 
  should 
  be 
  of 
  the 
  best 
  material, 
  

   strongly 
  charged 
  and 
  then 
  ripened 
  down 
  to 
  the 
  point 
  of 
  perma- 
  

   nence, 
  and 
  that 
  their 
  mounting 
  should 
  be 
  provided 
  with 
  fine 
  

   screw 
  adjustments. 
  

  

  Anticipating 
  a 
  possible 
  criticism, 
  the 
  writer 
  would 
  state 
  that 
  

   he 
  cannot 
  believe 
  that 
  any 
  variations 
  of 
  terrestrial 
  magnetism 
  

   nor 
  action 
  of 
  telluric 
  currents 
  at 
  the 
  time 
  of 
  an 
  earthquake 
  

   could 
  adversely 
  affect 
  this 
  appliance. 
  Even 
  supposing 
  that 
  

   these 
  phenomena 
  had 
  the 
  power 
  to 
  momentarily 
  weaken 
  or 
  

   strengthen 
  such 
  powerful 
  magnetic 
  fields, 
  both 
  would 
  be 
  

   affected 
  alike 
  and 
  the 
  only 
  effect 
  upon 
  the 
  seismograph 
  would 
  

   be 
  a 
  difference 
  in 
  the 
  ratio 
  of 
  the 
  values 
  of 
  magnetic 
  control 
  

   and 
  spring 
  power. 
  But 
  the 
  failure 
  of 
  even 
  delicate 
  apparatus 
  

   designed 
  to 
  act 
  as 
  seismographs 
  by 
  magnetic 
  variation, 
  may 
  

   serve 
  as 
  a 
  sufficient 
  guarantee 
  of 
  the 
  integrity 
  of 
  powerful 
  

   fields. 
  

  

  We 
  have 
  so 
  far 
  considered 
  this 
  magnetic 
  control 
  as 
  applied 
  

   solely 
  to 
  seismographs 
  for 
  vertical 
  motion, 
  and 
  this 
  for 
  the 
  

   reason 
  that, 
  on 
  account 
  of 
  the 
  spring 
  suspension, 
  it 
  is 
  here 
  that 
  

   the 
  need 
  is 
  most 
  apparent. 
  But 
  it 
  is 
  conceivable 
  that, 
  in 
  many 
  

   cases, 
  a 
  horizontal 
  pendulum 
  would 
  be 
  the 
  better 
  for 
  a 
  more 
  

   positive 
  self-centering 
  factor 
  in 
  the 
  pendulum 
  per 
  se, 
  the 
  

   period 
  then 
  being 
  increased 
  to 
  any 
  desired 
  value 
  by 
  the 
  mag- 
  

   netic 
  control. 
  

  

  Am. 
  Jour. 
  Sci.— 
  Fourth 
  Series, 
  Vol. 
  XXXVI, 
  No. 
  213.— 
  September, 
  1913. 
  

   20 
  

  

  