458 Prof. C. G. Knott. 



are unquestionably the most complete ever constructed for an earth- 

 quake-disturbed country. 



The discussion is really a working out of certain lines suggested in 

 a paper on " Earthquake Frequency," communicated by me in May, 

 1885, to the Seismological Society of Japan, and published in vol. 9 

 of the ' Transactions ' of that Society. In that paper I pointed out 

 the importance of subjecting earthquake statistics to some strict 

 form of mathematical analysis, and gave a simple arithmetical 

 process for separating the annual and semi-annual periods in earth- 

 quake frequency. The results then obtained have been fully corro- 

 borated by Dr. C. Davison in his paper " On the Annual and Semi- 

 annual Seismic Periods " (' Phil. Trans.,' vol. 184, 1893) ; and my 

 suggestion that the annual period is connected with barometric 

 pressure is also strongly supported by Dr. Ferd. Seidl in his pam- 

 phlet 'Die Beziehungen zwischen Erdbeben und Atmospharischen 

 Bewegungen' (Laibach, 1895). The semi-annual period, which was 

 first clearly brought into evidence in my earlier paper, does not 

 admit of a very ready explanation. 



In my paper of 1885 I also considered in some detail the various 

 tidal actions which might reasonably be supposed to have a determin- 

 ing influence on earthquake frequency. From lack of material it 

 was not possible at that time to make a satisfactory search for 

 lunar periodicities ; but the remarkable fulness of information con- 

 tained in Professor Milne's latest catalogues tempted me to under- 

 take the labour involved in (first) tabulating the statistics in terms 

 of lunar periods, and (second) analysing harmonically the tables so 

 prepared. 



2. The Lunar Daily and Half-daily Periods.. In one of the cata- 

 logues the earthquakes are classed according to district. Districts 

 1 to 6 lie on the N.E. and E. coasts of Japan, reckoning from the 

 north; districts 6 to 11 on the S. coast; and 12 to 15 on the W. 

 coast. Districts 6 and 7 are the most important, the former being 

 the region including Tokyo and Yokohama, and the latter the region 

 including Nagoya, which was the scene of the destructive earthquake 

 of October 28, 1891. The investigation into a possible lunar daily 

 period is conveniently based upon this classification into districts. 

 Had that not been done by Professor Milne the labour involved in 

 taking into account differences in local time would have been 

 enormous ; for, to compare the time of occurrence of a recorded 

 earthquake with the immediately preceding meridian passage of the 

 moon, it was necessary to apply corrections for longitude and local 

 time. 



The statistics for each district were, in the first instance, separated 

 out and tabulated according to time of occurrence, estimated in hours 

 after the immediately preceding passage of the moon. The method 



