﻿298 
  F. 
  A. 
  Perret 
  — 
  Vertical 
  Motion 
  Seismographs. 
  

  

  pension, 
  tends 
  to 
  remain 
  at 
  rest. 
  But, 
  unfortunately, 
  this 
  

   involves 
  the 
  stretching 
  of 
  the 
  spring 
  by 
  which 
  means 
  the 
  

   weight 
  is 
  lifted 
  and 
  set 
  in 
  vibration, 
  and 
  it 
  is 
  precisely 
  for 
  the 
  

   minimizing 
  of 
  this 
  effect 
  that 
  so 
  long 
  and 
  sensitive 
  a 
  spring 
  

   must 
  ordinarily 
  be 
  provided. 
  

  

  In 
  the 
  present 
  case, 
  however, 
  it 
  will 
  be 
  seen 
  that 
  the 
  rela- 
  

   tive 
  motion 
  of 
  the 
  different 
  parts 
  of 
  the 
  instrument 
  has 
  altered 
  

  

  Fig. 
  1. 
  

  

  the 
  conditions 
  in 
  the 
  magnetic 
  field, 
  the 
  upper 
  magnet 
  reced- 
  

   ing 
  from 
  its 
  armature, 
  increasing 
  the 
  " 
  entrefer 
  " 
  and 
  decreas- 
  

   ing 
  the 
  upward 
  pull 
  on 
  the 
  lever 
  while, 
  per 
  contra, 
  the 
  lower 
  

   "entrefer" 
  has 
  diminished, 
  thus 
  increasing 
  the 
  downward 
  

   pull. 
  These 
  two 
  effects 
  combine, 
  therefore, 
  to 
  offset 
  and 
  

   counteract 
  the 
  increased 
  tension 
  of 
  the 
  spring 
  and 
  they 
  may 
  be 
  

   given 
  any 
  desired 
  value 
  in 
  relation 
  thereto. 
  In 
  the 
  present 
  

  

  