﻿F. 
  A. 
  Ferret 
  — 
  Vertical 
  Motion 
  Seismographs. 
  297 
  

  

  Art. 
  XXIX. 
  — 
  A 
  Method 
  of 
  Increasing 
  and 
  Controlling 
  the 
  

   Feriod 
  in 
  Vertical 
  Motion 
  Seismographs; 
  by 
  Frank 
  

   A. 
  Perret. 
  

  

  In 
  order 
  to 
  obtain 
  a 
  satisfactory 
  period 
  of 
  vibration 
  in 
  seis- 
  

   mographs 
  for 
  recording 
  vertical 
  motion, 
  the 
  designers 
  of 
  such 
  

   instruments 
  are 
  generally 
  under 
  the 
  necessity 
  of 
  employing, 
  

   for 
  the 
  suspension 
  of 
  the 
  weight, 
  a 
  long 
  and 
  sensitive 
  spring 
  

   under 
  strong 
  tension. 
  The 
  use 
  of 
  such 
  a 
  spring, 
  however, 
  

   introduces 
  a 
  very 
  serious 
  defect 
  in 
  the 
  practical 
  working 
  of 
  the 
  

   instrument, 
  viz. 
  a 
  lack 
  of 
  stability 
  in 
  the 
  position 
  of 
  rest 
  due 
  

   to 
  the 
  effect 
  upon 
  the 
  spring 
  of 
  variations 
  of 
  temperature. 
  

   The 
  result 
  is 
  a 
  continual 
  wandering 
  of 
  the 
  recording 
  lever 
  

   from 
  its 
  normally 
  central 
  position, 
  creating 
  a 
  difficulty 
  which 
  

   the 
  most 
  ingenious 
  of 
  compensating 
  devices 
  have 
  not, 
  so 
  far, 
  

   been 
  able 
  to 
  satisfactorily 
  obviate. 
  Furthermore, 
  it 
  is 
  prob- 
  

   able 
  that, 
  aside 
  from 
  the 
  effect 
  of 
  temperature 
  changes, 
  a 
  

   spring 
  of 
  such 
  length 
  and 
  in 
  so 
  delicate 
  a 
  condition 
  of 
  balance 
  

   will 
  always 
  be 
  more 
  or 
  less 
  subject 
  to 
  slight 
  alterations 
  which, 
  

   magnified 
  by 
  the 
  multiplying 
  levers, 
  cannot 
  fail 
  to 
  be 
  trouble- 
  

   some 
  — 
  the 
  difficulty 
  is 
  inherent 
  in 
  the 
  large 
  amount 
  of 
  spring 
  

   ordinarily 
  required 
  in 
  this 
  type 
  of 
  instrument. 
  

  

  It 
  occurred 
  to 
  the 
  writer 
  that 
  the 
  variations 
  of 
  a 
  magnetic 
  

   field 
  — 
  due 
  to 
  relative 
  motion 
  of 
  the 
  parts 
  of 
  an 
  instrument 
  in 
  

   action 
  — 
  might 
  be 
  utilized 
  as 
  a 
  counter 
  influence 
  to 
  the 
  other- 
  

   wise 
  brusque 
  action 
  of 
  a 
  coarse 
  and 
  stiff 
  spring, 
  thus 
  permit- 
  

   ting 
  the 
  use 
  of 
  one 
  so 
  short 
  and 
  robust 
  as 
  to 
  be 
  free 
  from 
  

   extreme 
  sensitiveness 
  to 
  temperature 
  and 
  other 
  variations. 
  

   From 
  materials 
  already 
  at 
  hand 
  in 
  the 
  laboratory 
  the 
  crude 
  

   apparatus 
  shown 
  in 
  the 
  figure 
  was 
  erected, 
  it 
  is 
  scarcely 
  neces- 
  

   sary 
  to 
  say 
  as 
  an 
  experimental 
  instrument 
  for 
  testing 
  the 
  

   principle, 
  and 
  not, 
  in 
  any 
  sense, 
  as 
  a 
  model 
  for 
  eventual 
  con- 
  

   struction. 
  

  

  A 
  horizontal 
  lever 
  of 
  20 
  c 
  ' 
  m 
  , 
  pivoted 
  at 
  one 
  end 
  and 
  carrying 
  

   at 
  the 
  other 
  a 
  weight 
  of 
  2*5 
  kilos, 
  is 
  supported, 
  as 
  shown, 
  by 
  a 
  

   stiff 
  spring. 
  The 
  period 
  of 
  vibration 
  is 
  less 
  than 
  half 
  a 
  second 
  

   and, 
  once 
  set 
  in 
  motion, 
  the 
  lever 
  continues 
  to 
  oscillate 
  for 
  

   more 
  than 
  a 
  minute, 
  thus 
  forming, 
  it 
  will 
  be 
  seen, 
  an 
  impossible 
  

   instrument 
  from 
  the 
  standpoint 
  of 
  modern 
  requirements. 
  

  

  If 
  two 
  armatures 
  are 
  now 
  mounted 
  upon 
  the 
  lever 
  and 
  their 
  

   relative 
  magnets 
  attached 
  to 
  the 
  frame 
  of 
  the 
  instrument 
  above 
  

   and 
  below, 
  as 
  shown 
  in 
  the 
  figure, 
  the 
  pull 
  of 
  the 
  upper 
  mag- 
  

   net 
  is 
  counterbalanced 
  by 
  the 
  downpull 
  of 
  the 
  lower 
  one 
  and, 
  

   statically, 
  the 
  entire 
  system 
  is 
  in 
  precisely 
  the 
  same 
  condition 
  

   as 
  before. 
  If 
  we 
  now 
  imagine 
  a 
  sudden 
  upward 
  movement, 
  

   the 
  weighted 
  end 
  of 
  the 
  lever, 
  by 
  its 
  inertia 
  and 
  mode 
  of 
  sus- 
  

  

  