On the Diurnal Periodicity of Earthquakes. 463 



velocity of electromagnetic waves are not greatly changed 

 (i, e, not at all within our experimental limits) by the Rontgen 

 radiation, and this applies alike together in air and in benzene. 



(5) A selenium cell composed of platinum electrodes and 

 highly purified selenium is affected by Rontgen radiation to 

 an extent which is comparable with the effect produced by 

 diffused daylight. 



(6) No permanent or temporary electromotive force is set 

 up in a selenium cell by the Rontgen radiation. 



XLYIII. On the Diurnal Periodicity of Earthquakes. By 

 Charles Davison. Sc.D., F.G.S., Mathematical Master at 

 King Edivard's High School, Birmingham*. 



1. VI^ITHIN the last seven years, two important memoirs 

 " have appeared dealing in part with the diurnal 



periodicity of earthquakes. In a paper published in 1889, 

 M. de Montessus de Ballore f considers the question from a 

 negative point of view, his object being to show that the 

 diurnal period is apparent rather than real. More recently, 

 in 1894, Prof. F. Omori :£, in a valuable investigation on the 

 after- shocks of earthquakes, points out that there are various 

 periodic fluctuations in their decline of frequency, three of the 

 periods being a day or less in length. I will first give a brief 

 summary of the methods and conclusions of these two writers 

 before proceeding with the immediate object of this paper, 

 which is to subject the records used by them, or similar 

 records, to the more rigid process of harmonic analysis. 



2. M. de Montessus's statistical inquiries are based on a 

 great catalogue of more than 45,000 earthquakes. The 

 separate entries being of unequal value, he divides them into 

 seven classes, according to the nature of the district and the 

 mode of record. The first six classes include all registers 

 obtained without instrumental aid, the seventh those of the 

 Italian geodynamic observatories. For every region of each 

 class he gives the total number of shocks during each hour of 

 the day. Representing by d the number of shocks occurring 

 in the twelve day-hours (6 a.m. to 6 p.m.), and by n the 

 number in the twelve night-hours (6 p.m. to 6 a.m.), he then 

 evaluates the ratio din for each region. For the first group, 

 excluding the fifth or volcanic series, the mean value of d/n 



* Communicated by the Author, with some alterations, after being- 

 read before the Royal Society on March 5, 1896. 



t " Etudes sur la repartition lioraire diurne-nocturne des S6ismes et leur 

 pretendue relation avec les culminations de la lune." Arch, des So. phi/s^ 

 et nat. vol. xxii. 1889, pp. 409-430 and tables. 



\ " On the After-shocks of Earthquakes." Journal of the Coll^ of 

 Science, Imp. Univ. Japan, vol. vii. 1894, pp. 111-200. 



