﻿THE 
  UNFELT 
  EARTHQUAKE. 
  229 
  

  

  {3) 
  Consists 
  of 
  a 
  pendulum 
  of 
  im. 
  length 
  and 
  a 
  weight 
  of 
  10 
  kgm. 
  It 
  is 
  in= 
  

   tended 
  to 
  register 
  sensible 
  earthquake, 
  and 
  the 
  record, 
  which 
  is 
  started 
  by 
  (1), 
  only 
  

   lasts 
  for 
  one 
  minute. 
  

  

  (4J 
  A 
  simiar 
  instrument, 
  but 
  with 
  a 
  weight 
  of 
  20 
  kgm. 
  and 
  a 
  continuous 
  record. 
  

   The 
  recording 
  surface 
  moves 
  at 
  the 
  rate 
  of 
  10 
  cm. 
  per 
  hour, 
  and 
  on 
  it, 
  besides 
  the. 
  

   record 
  of 
  this 
  instrument, 
  the 
  time 
  at 
  which 
  (1) 
  closes 
  circuit 
  is 
  also 
  marked. 
  

  

  (5) 
  A 
  pair 
  of 
  delicate 
  spirit 
  levels, 
  set 
  N-S 
  and 
  E«W, 
  which 
  are 
  directly 
  

   observed 
  by 
  eye. 
  

  

  (6) 
  A 
  spiral 
  spring 
  of 
  7m. 
  length 
  supporting 
  a 
  weight 
  of 
  150 
  grammes. 
  

  

  (7) 
  Self-registering 
  geodynamic 
  levels. 
  These 
  consist 
  each 
  of 
  a 
  pair 
  of 
  cylin- 
  

   drical 
  receptacles 
  30 
  cm. 
  diameter 
  and 
  25 
  cm. 
  high, 
  connected 
  by 
  a 
  tube 
  some 
  2 
  m, 
  

   long 
  and 
  15 
  cm. 
  diameter. 
  There 
  are 
  two, 
  set 
  N-S 
  and 
  E-W, 
  and 
  the 
  movement 
  

   of 
  the 
  water 
  surface 
  is 
  recorded, 
  with 
  an 
  amplification 
  of 
  50 
  times, 
  on 
  a 
  surface 
  

   moving 
  11 
  mm. 
  per 
  hour. 
  

  

  (8) 
  A 
  pair 
  of 
  horizontal 
  pendula, 
  with 
  weight 
  of 
  12 
  kg. 
  recording 
  their 
  move- 
  

   ments 
  with 
  an 
  amplification 
  of 
  1 
  to 
  8 
  on 
  a 
  smoked 
  paper 
  moving 
  at 
  the 
  rate 
  of 
  

   30 
  cm. 
  per 
  hour. 
  The 
  period 
  of 
  vibration 
  of 
  the 
  N-S 
  pendulum 
  was 
  I3*5s., 
  and 
  of 
  

   the 
  E-W 
  one 
  i3"os., 
  on 
  the 
  12th 
  June. 
  

  

  (9) 
  A 
  similar 
  but 
  improved 
  instrument 
  with 
  three 
  pendula 
  of 
  a 
  weight 
  of 
  3kg* 
  

   oriented 
  at 
  6o° 
  to 
  each 
  other. 
  

  

  Of 
  these 
  instruments 
  Nos. 
  1 
  to 
  6 
  are 
  set 
  up 
  at 
  the 
  Grade 
  Sentinella, 
  Casamic- 
  

   cola, 
  and 
  7 
  to 
  9 
  at 
  the 
  Port 
  of 
  Ischia. 
  

  

  All 
  these 
  instruments 
  acted 
  on 
  the 
  12th 
  June. 
  No. 
  (1) 
  gave 
  the 
  

   alarm 
  at 
  uh. 
  rgm. 
  3s. 
  Greenwich 
  mean 
  time. 
  

  

  No. 
  (2) 
  shows 
  a 
  sudden 
  commencement 
  of 
  rapid 
  oscillations 
  at 
  uh. 
  

   17m. 
  13s., 
  which 
  give 
  place 
  to 
  slow 
  oscillations 
  of 
  a 
  complete 
  period 
  

   of 
  about 
  20s. 
  at 
  uh. 
  42m. 
  33s. 
  These 
  attain 
  their 
  maximum 
  at 
  

   about 
  uh. 
  47m., 
  whilst 
  for 
  a 
  couple 
  of 
  hours 
  the 
  diagram 
  shows 
  a 
  

   much 
  smaller 
  but 
  continuous 
  movement. 
  

  

  It 
  may 
  be 
  noticed 
  that 
  both 
  the 
  first 
  rapid 
  oscillations 
  and 
  the 
  

   subsequent 
  slower 
  ones 
  show 
  a 
  considerably 
  greater 
  amplitude 
  in 
  

   the 
  E-W 
  component 
  than 
  in 
  the 
  N-S 
  ; 
  specially 
  so 
  in 
  the 
  case 
  of 
  the 
  

   former. 
  On 
  the 
  copy 
  of 
  the 
  diagram 
  of 
  this 
  instrument, 
  for 
  which 
  I 
  

   am 
  indebted 
  to 
  Prof. 
  Grablowitz, 
  I 
  find 
  that 
  the 
  E-W 
  component 
  

   is 
  three 
  times 
  the 
  N-S 
  in 
  the 
  case 
  of 
  the 
  first, 
  and 
  one 
  and 
  a 
  half 
  

   times 
  in 
  the 
  case 
  of 
  the 
  second. 
  

  

  The 
  record 
  of 
  (3) 
  is 
  of 
  little 
  interest 
  as 
  it 
  is 
  limited 
  to 
  a 
  period 
  of 
  

   one 
  minute 
  only. 
  No. 
  (4) 
  shows 
  a 
  slight 
  thickening 
  of 
  the 
  line 
  of 
  the 
  

   E-W 
  component 
  for 
  a 
  few 
  minutes, 
  but 
  there 
  are 
  records 
  of 
  repeated 
  

   closures 
  of 
  the 
  circuit 
  by 
  (1) 
  for 
  nearly 
  an 
  hour. 
  

  

  ( 
  229 
  ) 
  

  

  