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XXI. On the Annual and Semi-Annual Seismic Periods. 
By Charles Davison, M.A., Mathematical Master at King Kdtvard’s High School, 
Birmingham. 
Communicated hy Professor J. H. Poynting, F.R.S. 
Received June 13,—Read June 15, 1893. 
1. In 1834 the existence of an annual period of earthquake frequency was pointed 
out by Merian, and his discovery has been fully confirmed by the woi-k of later 
seismologists. No result of equal importance was, however, added to our knowledge 
until fifty years afterwards, when a valuable memoir* was presented by Dr. C. G. Knott 
to the Seismological Society of Japan, As the present paper follows somewhat 
closely in the lines laid down by Dr, Knott, I will first give a short account of the 
results obtained by him, and in the next section will describe his method of 
investigation. 
One of the objects of Dr, Knott’s work w^as to ascertain whether any trace could 
be detected of a six-monthly, as well as of an annual, period. He examined for this 
purpose lists of earthquakes in Japan from 1872 to 1880 (Milne), Europe and the 
adjacent parts of Asia and Africa from 306 to 1843 (Perrey), the Grecian Archipelago 
from 1859 to 1873 (Schmidt), the East Indian Archipelago from 1873 to 1881 
(Bergsma), New Zealand from 1869 to 1879, and Chili from 1873 to 1881 (Vergara), 
The chief results obtained from this discussion are the following : (l) In five of the 
six districts selected there is a well-marked annual period, the maximum occurring 
about December or January in Europe, the Grecian Archipelago, and Japan, and 
between June and September in New Zealand and Cliili; i.e., during the winter 
season in every case : while in the East Indies, lying close to and on either side of 
the equator, no such period could be discerned, ( 2 ) With the exception again of the 
East Indies there seems to be in the earthquakes of each district a semi-annual period, 
the maxima occurring about January! and June or July in Europe, about March and 
September in the Grecian Archipelago, about June and November in Japan, about 
February and August in New Zealand, and about April (for the first maximum) in 
Chili, the second not being clearly defined. In the case of Europe, the semi-annual 
period is not very pronounced, and the smoothness of the curve representing it as 
* “Earthquake Frequency” [1884], ‘Japan, Seismol. Soc. Trans.,’ vol. 9, part 1, 1886, pp. 1-20. 
t These dates differ from those given hy Dr. Knott. The reason for the change is stated below in § 3. 
7 B 2 10,1.94. 
