156 Scientific Intelligence. 



gether, 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: (l) 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) Professor 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 maxi- 

 mum 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. 



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 south- 

 ern, 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 sup- 

 posed 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. 



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 Southeast Australia, the maximum epoch gen- 

 erally falls either in February or March and August or Septem- 

 ber; in North America, as a rule, in March or April and Sep- 

 tember or October. But for other regions it does not seem 

 possible as yet to deduce any law. The amplitude of the semi- 

 annual period ranges from 0*06 (southern hemisphere) to - 79 

 (Mexico), the average value being 0*24. 



In fifteen cases, the amplitude of the semi-annual period 

 exceeds that of the annual period. Eleven of these records 



