Rejports & Proceedings — Geological Society of London. 189 



This gave the first suggestion for grouping the records. The year 

 was divided into two halves by the equinoxes, and the day into two 

 halves, at six hours before or after noon, called day and night for 

 convenience, irrespective of the time of sunrise or sunset. The 

 result is given in the tabular statement below, the frequency being 

 -expressed as a ratio to the mean, of each group, taken as 100 : — 



Distribution of Shocks by Day and Night. 



Prom 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 in the winter, with an opposite variation 

 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 1897,' 

 stood ready for use. They show a variation identical in character 

 witli 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 respectively, 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 connexion may be better traced, dependent on the fact that the 

 prevailing effect of the vertical stress is in the direction of lightening 



^ Seismol. Journ. Japan, vol. iv, 1895. 



^ Mem. Geol. Surv. India, vol. xxxv, pt. ii, 1903. 



