Periodicity of Earthquakes. 471 



wide variation. A special feature of after-shocks is the 

 prominence of the eight-hour and four-hour components. 

 After a year or two there is some return to ordinary con- 

 ditions ; but even when the average hourly number of 

 shocks is reduced to one-hundredth of that during the first 

 few days, the characteristics of after-shocks are still per- 

 ceptible. 



Origin of the Diurnal Periodicity of Earthquakes. 



12. The pressure of the atmosphere, either at rest or in 

 motion, has for some time been regarded by seismologists as 

 a cause of earth-tremors. Prof. Milne, who has made a 

 detailed analysis of tremors recorded at Tokio, concludes that 

 they are more frequent with a low than with a high baro- 

 meter, and with a high than with a low barometric gradient ; 

 also, that a majority of the tremors were produced by the 

 action of either local or distant winds upon the surface of the 

 earth, and possibly by their pressure against a neighbouring 

 mountain- range *. 



My object is now to inquire how far the larger disturbances 

 considered in this paper are subject to similar laws. If there 

 is any intimate relation between the diurnal variation of 

 earthquake-frequency on the one hand and that of barometric 

 pressure or wind-velocity on the other, it is evident that the 

 epochs of their respective harmonic components should not 

 differ widely ; since any distortion of the earth's surface 

 by changes in the distribution of atmospheric pressure must 

 be propagated, both along the surface and downwards, with 

 great velocity. 



The records of most value for the purposes of this com- 

 parison are : (1) those of ordinary earthquakes at Tokio, 

 Manila, and Rocca di Papa ; and (2) those of after-shocks at 

 Kumamoto, Grifu, Nagoya, Chiran, and Nemuro. 



13. Taking, first, the diurnal variation of barometric 

 pressure and seismic frequency, it will be seen that the only 

 approximate agreement in epoch is in the second and fourth 

 components at Tokio and the second at Rocca di Papa. The 

 epochs of the first component differ by as much as eight or 

 nine hours at both places. 



* " Earth-Tremors in Central Japan," Japan Seismol. Soc. Trans, 

 vol. xi. 1887, pp. 1-78, vol. xiii. 1890, pp. 7-19 ; « Earth-Tremors and 

 the Wind," Roy. Met. Soc. Journ. vol. xiv. 1888, pp. 6i-72. It should 

 be mentioned, however, that P. Camillo M. Melzi has arrived at different 

 conclusions from his analysis of the tromometric records at Florence. 

 See especially his memoir, " Nuove osservazioni sull' independenza dal 

 vento nei moti tromometrici nei pendoli isolati," Pontif. Accad. dei 

 Nuovi Lincei, Mem. vol. v. 1889, pp. 3-39. 



2 L2 



