﻿300 
  F. 
  A. 
  Ferret 
  — 
  Vertical 
  Motion 
  Seismographs. 
  

  

  If 
  it 
  shall 
  be 
  found 
  possible 
  to 
  obtain 
  uniform 
  action 
  in 
  all 
  

   azimuths, 
  the 
  great 
  length 
  of 
  the 
  simple 
  pendulum, 
  now 
  

   requiring 
  a 
  tower, 
  might 
  be 
  reduced 
  to 
  the 
  dimensions 
  of 
  an 
  

   ordinary 
  building, 
  and 
  this 
  suggests 
  a 
  somewhat 
  different 
  

   application 
  in 
  the 
  case 
  of 
  inverted 
  pendulums, 
  viz. 
  to 
  employ 
  

   the 
  repulsive 
  action 
  of 
  like 
  magnetic 
  poles 
  — 
  uniformly 
  spaced 
  

   around 
  the 
  magnetized 
  pendulum 
  rod, 
  or 
  magnets 
  mounted 
  

   thereon 
  — 
  in 
  lieu 
  of 
  springs, 
  for 
  the 
  maintaining 
  of 
  this 
  in 
  its 
  

   central 
  position. 
  Such 
  an 
  instrument 
  — 
  especially 
  if 
  provided 
  

   with 
  optical 
  registration 
  — 
  would, 
  by 
  its 
  freedom 
  from 
  mechan- 
  

   ical 
  contact 
  with 
  the 
  static 
  mass, 
  closely 
  approach 
  the 
  ideal. 
  

  

  The 
  writer 
  has 
  not 
  tried 
  out 
  these 
  last 
  forms 
  experimentally 
  

   and 
  they 
  are 
  here 
  given 
  simply 
  as 
  constituting 
  a 
  natural 
  line 
  of 
  

   thought 
  from 
  the 
  first 
  idea. 
  

  

  Posillipo, 
  Naples, 
  June 
  9, 
  1913. 
  

  

  