878 Dr. C. Davison on the Diurnal 



strains. It was therefore decided to load all the specimens 

 at such a rate that the rate of longitudinal straining on 

 surfaces -J- inch from the neutral surface was approximately 

 at the rate of 1 in 4000 per minute. 



The author wishes to thank the former Principal J. 0. M. 

 Grarnett, M.A., D.Sc.,for providing facilities for conducting 

 the research, and Professor Gerald Stoney, F.B.S., for the 

 interest he has shown throughout. 



XCVI. The Diurnal Periodicity of Earthquakes. 

 By Charles Davison, Sc.D* 



THE present paper may be regarded as a sequel to two 

 earlier papers dealing with the same subject. In 1896, 

 I applied the method of harmonic analysis to seismographic 

 records obtained in Japan, the Philippine Islands, and Italy f. 

 The results showed a di-tinct diurnal period, with its maximum- 

 epoch about noon, except in two of the nine Italian records 

 when the maximum-epoch occurred about midnight. In the 

 early after- shocks of Japanese earthquakes, the maximum- 

 epoch was also about or shortly after midnight. Returning 

 recently to the subject J, I was able to avail myself of more 

 numerous and extensive records of Japanese earthquakes, 

 and found, by the method of overlapping means §, that, in 

 ordinary Japanese earthquakes, the maximum-epoch of the 

 diurnal period occurred at or shortly before noon, and that 

 of the semi-diurnal period at 8 or 'J a.m. and p M. ; while, in 

 after-shocks, the epoch of the diurnal period was at first 

 about or shortly after midnight, returning later to the 

 neighbourhood of noon. 



Since the latter paper was written, I have applied the 

 same method to the registers collected by the Seismological 

 Committee of the British Association ||. As the instruments 

 providing these registers are capable of recording earthquakes 

 from very distant regions, it might be expected that, if the 

 maximum-epoch at every place were to occur at a constant 

 local time, the result would be a marked diminution in the 

 amplitude of the diurnal periodicity. In some registers this 

 may be the case ; the amplitude is so small, or the variations 

 in the twelve-hourly means are so irregular, that the existence 



* Communicated bv the Author. 

 t Phil. Mag. vol. xlii. pp. 463-476 (1896). 

 + Loc. cit. vol xli. pp. 903-916 (1921). 



§ Boll. Soc. Sism. Ital vol. iv. pp. 89-100 (1898). See also "A 

 Manual of Seismology " (Camb. Univ. Press, 1921), pp. 185-188. 

 || As a rule, the registers at my disposal end with the year 1912. 



