﻿THE 
  UNFELT 
  EARTHQUAKE. 
  233 
  

  

  other 
  observations. 
  The 
  two 
  important 
  points 
  are 
  (1) 
  the 
  sudden 
  

   commencement 
  of 
  the 
  nearly 
  horizontal 
  movements 
  of 
  short 
  period, 
  

   and 
  their 
  subsequent 
  gradual 
  dying 
  out, 
  and 
  (2) 
  the 
  gradual 
  coming 
  

   in 
  of 
  the 
  slower 
  movements 
  accompanied 
  by 
  an 
  actual 
  tilting 
  of 
  the 
  

   surface, 
  in 
  which 
  the 
  horizontal 
  was 
  combined 
  with 
  vertical 
  motion. 
  

   These 
  increase 
  to 
  a 
  maximum 
  and 
  then 
  diminish 
  again, 
  but 
  continue 
  

   for 
  about 
  an 
  hour 
  and 
  a 
  half. 
  As 
  will 
  be 
  seen 
  when 
  the 
  records 
  as 
  a 
  

   whole 
  come 
  to 
  be 
  discussed, 
  Prof. 
  Grablowitz's 
  second 
  is 
  really 
  a 
  

   third 
  phase, 
  as 
  there 
  is 
  another 
  recognisable, 
  intermediate 
  in 
  time, 
  

   between 
  the 
  two 
  discussed 
  by 
  him. 
  

  

  Rome.— 
  R. 
  Osservatorio 
  Geodinamico 
  di 
  Rocca 
  di 
  Papa.— 
  

  

  I 
  am 
  indebted 
  to 
  Dr. 
  Cancani 
  for 
  photographs 
  of 
  the 
  records 
  

   obtained 
  in 
  this 
  observatory 
  from 
  (1) 
  a 
  horizontal 
  pendulum 
  instru- 
  

   ment 
  of 
  which 
  the 
  N-S 
  pendulum 
  has 
  a 
  period 
  of 
  14s. 
  and 
  the 
  E-W 
  

   of 
  1 
  2s. 
  ; 
  and 
  (2) 
  of 
  a 
  simple 
  pendulum 
  of 
  15 
  metres 
  length 
  and 
  a 
  

   bob 
  weighing 
  250 
  kilogrammes. 
  

  

  The 
  photograph 
  of 
  the 
  trace 
  of 
  the 
  vertical 
  pendulum 
  does 
  not 
  

   contain 
  the 
  commencement 
  or 
  the 
  end 
  of 
  the 
  disturbance. 
  Slower 
  

   undulations 
  with 
  an 
  increased 
  amplitude, 
  marking 
  the 
  advent 
  of 
  the 
  

   second 
  phase, 
  commence 
  about 
  nh. 
  25m. 
  58s. 
  and 
  the 
  long 
  undula- 
  

   tions 
  at 
  1 
  ih. 
  41m. 
  50s., 
  attaining 
  their 
  maximum 
  at 
  nh. 
  47 
  m. 
  8s. 
  

   As 
  at 
  Ischia, 
  the 
  largest 
  movement 
  is 
  in 
  the 
  E-W 
  direction. 
  

  

  A 
  copy 
  of 
  the 
  diagram 
  of 
  the 
  horizontal 
  pendula 
  is 
  reproduced 
  on 
  

   Plate 
  XLI. 
  From 
  this 
  it 
  will 
  be 
  seen 
  that 
  the 
  commencement 
  of 
  the 
  

   disturbance 
  is 
  very 
  distinct 
  on 
  the 
  E-W 
  component 
  at 
  nh. 
  17m. 
  40s. 
  

   The 
  pen 
  of 
  the 
  N-S 
  component 
  was 
  not 
  writing 
  properly 
  and 
  its 
  

   trace 
  is 
  very 
  faint 
  till 
  nh. 
  27m. 
  51s., 
  when 
  the 
  line 
  becomes 
  distinct 
  

   once 
  more 
  ; 
  in 
  spite 
  of 
  the 
  faintness 
  of 
  the 
  trace 
  the 
  commencement 
  

   of 
  the 
  disturbance 
  can 
  be 
  recognised. 
  The 
  second 
  phase 
  is 
  not 
  

   well 
  marked 
  by 
  this 
  instrument, 
  but 
  its 
  commencement 
  can 
  be 
  recog- 
  

   nised 
  by 
  an 
  increase 
  of 
  amplitude 
  and 
  more 
  open 
  character 
  of 
  the 
  

   waves 
  at 
  nh. 
  25m. 
  54s. 
  The 
  great 
  undulations 
  are 
  very 
  conspicuous 
  

   on 
  this 
  diagram, 
  commencing 
  at 
  nh. 
  41m. 
  40s. 
  and 
  attaining 
  their 
  

  

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