﻿On 
  the 
  Frequency 
  of 
  Earthquakes. 
  505 
  

  

  being 
  expressed 
  as 
  a 
  ratio 
  to 
  the 
  mean, 
  of 
  each 
  group, 
  taken 
  

   as 
  100 
  :— 
  

  

  Distribution 
  of 
  Shocks 
  by 
  Day 
  and 
  Night. 
  

  

  Italy, 
  1891-1910. 
  Bay. 
  Night. 
  

  

  June-July 
  90 
  : 
  110 
  

  

  Summer 
  half 
  88 
  : 
  112 
  

  

  Wholeyear 
  84 
  : 
  116 
  

  

  Winter 
  half 
  81 
  : 
  119 
  

  

  December-January 
  77 
  : 
  123 
  

  

  Japan, 
  1885-1892. 
  Day. 
  Night. 
  

  

  Summerhalf 
  102 
  : 
  98 
  

  

  Wholeyear 
  97 
  : 
  103 
  

  

  Winterhalf 
  93 
  : 
  107 
  

  

  Assam 
  Aftershocks. 
  

  

  Summerhalf 
  113 
  : 
  87 
  

  

  Wholeyear 
  107 
  : 
  93 
  

  

  Winterhalf 
  101 
  : 
  99 
  

  

  From 
  this 
  statement 
  it 
  will 
  be 
  seen 
  that 
  the 
  mean 
  ratio 
  of 
  day 
  

   to 
  night 
  shocks 
  over 
  the 
  whole 
  period 
  is 
  represented 
  by 
  the 
  figures 
  

   84 
  : 
  116 
  ; 
  for 
  the 
  summer 
  half 
  of 
  the 
  year 
  they 
  become 
  88 
  : 
  112, 
  

   and 
  for 
  the 
  winter 
  half 
  81 
  : 
  119, 
  showing 
  that 
  during 
  the 
  day 
  the 
  

   shocks 
  are 
  somewhat 
  less 
  frequent 
  than 
  the 
  average 
  in 
  summer 
  

   and 
  somewhat 
  more 
  frequent 
  in 
  the 
  winter, 
  with 
  an 
  opposite 
  varia- 
  

   tion 
  during 
  the 
  night. 
  Taken 
  by 
  itself 
  this 
  difference 
  might 
  be 
  

   merely 
  fortuitous, 
  and 
  further 
  confirmation 
  is 
  required 
  : 
  this 
  can 
  

   be 
  got 
  in 
  two 
  ways. 
  In 
  the 
  first 
  place 
  by 
  comparison 
  with 
  other 
  

   records, 
  two 
  of 
  which, 
  Milne's 
  catalogue 
  of 
  Japanese 
  earthquakes 
  

   from 
  1885 
  to 
  1892 
  # 
  , 
  and 
  the 
  aftershocks 
  of 
  the 
  Indian 
  earthquake 
  

   of 
  1897t 
  stood 
  ready 
  for 
  use. 
  They 
  show 
  a 
  variation 
  identical 
  in 
  

   character 
  with 
  that 
  of 
  the 
  Italian 
  record. 
  A 
  second 
  test 
  depends 
  

   on 
  the 
  argument 
  that, 
  if 
  the 
  variation 
  is 
  in 
  any 
  way 
  seasonal, 
  the 
  

   divergence 
  should 
  be 
  increased 
  at 
  the 
  height 
  of 
  each 
  season 
  ; 
  the 
  

   figures 
  for 
  the 
  months 
  of 
  January-February 
  and 
  of 
  June- 
  July 
  

   were 
  taken 
  out, 
  as 
  representing 
  midwinter 
  and 
  midsummer 
  respec- 
  

   tively, 
  and 
  found 
  to 
  show 
  a 
  divergence 
  in 
  each 
  case 
  greater 
  than, 
  

   and 
  in 
  the 
  same 
  direction 
  as, 
  the 
  respective 
  half-years. 
  

  

  Taken 
  by 
  itself 
  the 
  variation, 
  as 
  between 
  any 
  pair 
  of 
  ratios, 
  is 
  as 
  

   likely 
  to 
  be 
  in 
  one 
  direction 
  as 
  in 
  the 
  other, 
  but 
  the 
  odds 
  against 
  

   a 
  complete 
  concordance 
  throughout 
  the 
  whole 
  series 
  is 
  31 
  to 
  1 
  ; 
  

   there 
  is, 
  therefore, 
  a 
  strong 
  presumption 
  that 
  the 
  variations 
  are 
  

   not 
  fortuitous, 
  but 
  due 
  to 
  some 
  common 
  cause 
  which 
  tends 
  to 
  

   increase 
  the 
  frequency 
  during 
  the 
  day 
  and 
  decrease 
  it 
  during 
  the 
  

   night 
  in 
  summer, 
  with 
  the 
  opposite 
  in 
  winter. 
  

  

  The 
  variation 
  in 
  the 
  frequency 
  of 
  earthquakes 
  may, 
  or 
  may 
  not, 
  

   be 
  connected 
  with 
  the 
  variation 
  in 
  the 
  gravitational 
  stresses 
  due 
  to 
  

   the 
  sun 
  ; 
  but 
  there 
  is 
  another 
  line 
  of 
  investigation 
  by 
  which 
  a 
  con- 
  

   nexion 
  may 
  be 
  better 
  traced, 
  dependent 
  on 
  the 
  fact 
  that 
  the 
  

   prevailing 
  effect 
  of 
  the 
  vertical 
  stress 
  is 
  in 
  the 
  direction 
  of 
  

   lightening 
  the 
  load, 
  and 
  the 
  prevailing 
  direction 
  of 
  the 
  horizontal 
  

   stress 
  between 
  east 
  and 
  south, 
  during 
  the 
  six 
  hours 
  before 
  the 
  

   meridian 
  passages 
  at 
  noon 
  and 
  midnight, 
  and 
  of 
  an 
  increase 
  in 
  the 
  

   * 
  Seismol. 
  Journ. 
  Japan, 
  vol. 
  iv. 
  (1895). 
  

   t 
  Mem. 
  Geol. 
  Surv. 
  India, 
  vol. 
  xxxv. 
  pt. 
  2 
  (1903). 
  

  

  