470 Dr. C. Davison on the Diurnal 



catalogue the observatory at Rome was situated close to a 

 much-frequented street. At the various observatories of the 

 second order the arrangement of the recording instrument, 

 I am informed, was not then irreproachable. On the other 

 hand, at Rocca di Papa, Dr. Cancani tells me that the appa- 

 ratus could not be in any way influenced by accidental or 

 artificial movements on the ground outside. He does not, 

 however, attribute the movements of the tromometers to 

 seismic causes so much as to the action of the wind either 

 near to, or at a distance from, the observatory. 



This one case, therefore, being free from doubt so far 

 as artificial disturbances are concerned, it becomes un- 

 necessary to reject entirely the results obtained from the 

 other records, especially when the epochs of the principal 

 harmonic components agree so closely, as some of them do, 

 with those derived from the Rocca di Papa register. More- 

 over, while some variability might be expected in the epoch 

 of the diurnal period if it were due to natural causes, it is 

 difficult to understand how, according to M. de Montessus's 

 explanation, the epoch could ever occur near midnight. 



Summary of Results. 



11. The following conclusions may, I think, be drawn 

 from the results of the above analysis : — 



(1) The reality of the diurnal variation of earthquake- 

 frequency seems to be proved by the approximate agreement 

 in epoch (mean local time) of the first four components for 

 the whole year at Tokio and Manila, and for the winter and 

 summer halves of the year at Tokio. 



(2) In ordinary earthquakes there is in nearly every case 

 a marked diurnal period, the maximum generally occurring 

 between 10 a.m. and noon. The semi-diurnal period, though 

 less prominent, is also clearly marked, the maximum 

 occurring, as a rule, between 9 a.m. and noon and between 

 9 P.M. and midnight. Other minor harmonic components 

 are also occasionally important — the first maximum of the 

 eight-hour component probably occurring about 6.30 a.m. 

 and that of the six-hour component about 3 or 4 a.m. ; but in 

 these two epochs the results are not always concordant. 



(3) Though the materials are insufficient for any general 

 conclusion, a comparison of the results for Tokio and Rocca 

 di Papa seems to show that the slighter disturbances at the 

 latter place are subject to a more marked diurnal periodicity. 



(4) In the after-shocks of great earthquakes the diurnal 

 periodicity, as a rule, is strongly pronounced. The maximum 

 of the diurnal period occurs within a few hours after mid- 

 night, but the epochs of the other components are subject to 



