Intelligence and Miscellaneous Articles. 311 



the augmenting factor 1*589, in order to obtain the correct value 

 of the ratio a x : a . The curve obtained by plotting these reduced 

 means thus gives special prominence to the annual period, by 

 eliminating the semi-annual period and all those which are frac- 

 tions of six months, and by diminishing the amplitudes of all other 

 periods with respect to that of the annual period. 



In investigating the semi-annual period, the numbers corre- 

 sponding to the first halves of January and July are added together, 

 and so on ; the rest of the method being the same as for the annual 

 period. The result gives special prominence to the semi-annual 

 period by eliminating the annual period, and by eliminating or 

 diminishing the amplitudes of all periods less than six months. 



Seismic Periodicity in relation to Intensity. — This discussion is 

 founded on : (1) lists compiled from Mallet's great catalogue, first, 

 of shocks which were so slight as to be just perceptible, and, 

 secondly, of those which were strong enough to damage buildings ; 

 (2) Prof. Milne's classification of the Japanese earthquakes of 1885 

 to 1889 according to the areas disturbed by them ; and (3) different 

 catalogues relating to the same district, it being obvious that two 

 such catalogues for the same time can only differ by the omission 

 or inclusion of slight shocks. 



The following results are obtained : — (1) In both periods the 

 amplitude is greater for slight than for strong shocks ; (2) there 

 appear to be two classes of slight shocks with an annual period, the 

 stronger having their maximum in winter, the weaker in summer ; 

 and (3) in the case of the semi-annual period, both strong and 

 slight shocks, as a rule, have nearly the same maximum epochs. 



Seismic Periodicity in relation to Geographical Position. — The 

 number of records examined is 62, 45 belonging to the northern 

 hemisphere, 14 to the southern, and 3 to equatorial countries. 



1. Annual Period. — In every district, and in all but five records 

 (which are obviously incomplete), there is a fairly well-marked 

 annual period. As a rule, different records for the same district 

 agree in giving the same, or nearly the same, maximum epoch. 

 Excluding, however, those which disagree in this respect, we have 

 left 34 records for the northern hemisphere, 9 for the southern, 

 and 2 for equatorial countries. In the northern hemisphere, 4 

 records give the maximum in November, 16 in December, and 6 in 

 January ; in the southern hemisphere, 2 in April, 2 in May, 3 in 

 July, and 2 in August ; the end of the month being supposed in 

 each case. As a rule, then, the maximum epoch occurs in winter 

 in both hemispheres. The amplitude of the annual period ranges 

 from 0-05 (New Zealand) to 0*67 (Sicily and Algeria), the average 

 of 57 records being 0*33. 



2. Semi-Annual Period. — Of the 62 records examined, only 3 fail 

 to show a semi-annual period, the cause of the failure in these 

 cases being no doubt the imperfection of the seismic record. In 

 New Zealand and South-east Australia the maximum epoch gene- 

 rally falls either in February or March and August or September; 

 in North America, as a rule, in March or April and September or 



