S82 Dr. 0. Davison on the Diurnal 



Thus, in four records, the maximum-epoch o£ the semi- 

 diurnal period occurs at about 9 to 11 a.m. and p.m. ; and, in 

 two, at about 1J to 3 a.m. and P.M. In the latter, the 

 maximum-epoch of the diurnal period occurs at about mid- 

 night. 



Diurnal Periodicity in relation to Intensity. 



If the origin of seismic periodicity be due to causes which 

 precipitate, rather than produce, the occurrence of earth- 

 quakes, we should expect to find the periodicity more marked 

 with weak, than with strong, earthquakes. In many of the 

 records considered in this paper, it is possible to make a 

 rough classification of the earthquakes according to intensity. 

 The earthquakes in the diagrams of which a maximum can 

 be clearly distinguished are not of necessity strong, but 

 among them are included all strong earthquakes. Those 

 which contain no marked maximum are, as a rule, of much 

 less intensity. In Tables Til. and IV. the lines marked A 

 give the results for the maximum-epochs of the earthquakes 

 recorded, those marked B give the results for the initial 

 tremors when no maximum can be detected. 



The following results may be deduced from Table III. :■ — 



(i.) With one exception f Victoria, B.C.) the amplitude of 

 the diurnal period is much greater in the weak, than in the 

 strong, earthquakes. 



(ii.) In the strong earthquakes, the maximum-epoch occurs 

 about noon at Shide, Kew, Edinburgh, San Fernando, and 

 Gape of Good Hope ; about midnight at Toronto and Vic- 

 toria, B.C. ; and in the afternoon at Cordova. The midnight 

 maximum at Shide in summer may be accidental, as the 

 amplitude is not much in excess of the expectancy. 



(iii. J In the weak earthquakes, the maximum-epoch occurs 

 about noon at Shide, Kew, Toronto, Victoria, B.C., and 

 Cape of Good Hope ; about midnight at San Fernando and 

 Cordova ; at Edinburgh, it occurs about noon in winter and 

 shortly before midnight in summer. 



(iv.) There is thus an inversion of epoch in strong and 

 weak earthquakes at San Fernando, Toronto, and Victoria, 

 B.C. ; a partial inversion at Edinburgh and Cordova ; and a 

 general correspondence in epocli at Shide, Kew, and the 

 Cape of Good Hope. 



