464 I)r. 0. Davison on the Diurnal 



varies from 0*75 to 0*82, and increases with the scientific 

 character of the record. Also, the value of d/n being small 

 for moderate shocks and approaching unity for severe ones, 

 M. de Montessus infers that earthquakes in reality occur just 

 as frequently by day as by night. 



The seventh class comprises the records from thirteen geo- 

 dynamic observatories in Italy, all obtained by means of con- 

 tinuously recording instruments. The value of d/n varies 

 from 0'50 for Corleone to 2*06 for Bologna and S. Luca, 

 being on an average 1*49. If all shocks are excluded but 

 those of intensity I. of the Rossi-Forel scale, the mean value 

 rises to 1*80. This seems to imply the existence of a true 

 diurnal period, but M. de Montessus interprets the inequality 

 otherwise, referring the more numerous slight shocks of the 

 day-time to "mouvements dus a Thomme, roulements de 

 voitures et de trains de chemins de fer, explosions de mines, 

 &c." The suggestion is a useful one and deserves careful 

 consideration. At the same time, it should be remarked that 

 the phenomena admit of another explanation, for we might 

 expect that slight earthquakes would be subject to periods of 

 greater amplitude than violent shocks *. With regard to the 

 non-instrumental records, however, M. de Montessus's analysis 

 leaves little doubt that the more frequent observation of earth- 

 quakes at night is due to the conditions being then more 

 favourable for the detection of weak tremors. 



3. Prof. Omori's investigation is based for the most part on 

 seismometric records. He makes use of the valuable Tokio 

 register from 1876 to 1893, that for all Japan from 1885 to 

 1890 (which is only in part a seismometric record), and 

 especially the lists of after-shocks at Kumamoto, Grifu and 

 Nagoya. and Chiran, during the thirteen or fourteen days 

 following the Kumamoto earthquake of 1889, the Mino-Owari 

 earthquake of 1891, and the Kagoshima earthquake of 1893. 

 The shocks are grouped in hourly, two-hourly, and six-hourly 

 intervals, and curves are drawn, not through the points 

 corresponding to the numbers so obtained, but by some 

 process of smoothing which is not explained. The periods 

 which are brought into prominence by this method are 24 

 hours, about 8 or 9 hours, and about 4 hours, in length. In 

 the case of the Mino-Owari earthquakes, while both the eight- 

 hourly and four-hourly periods are shown on the Gifu and 

 Nagoya curves, the former is more marked at Gifu and the 

 latter at Nagoya. The diurnal period for these two stations 

 ^appears to have its maximum about 1 a.m., for Tokio in the 

 eVening, and for all Japan early in the morning. 



4. The method adopted in this paper is that of harmonic 



* See Phil. Trans. 1893 A, pp. 1116-1120. 



