ON SEISMOLOGICAL INVESTIGATIONS, 9D 
1908, September 3, 1909. The last date, however, only represents a 
period of partial quiescence. 
The next section of this Report, drawn up by Professor H. H., 
20 ~ 
Number of Earthquakes in a Group. 
| 
fe) 30 40 sO 60 
Intervals in days between centres of groups. Mean position 
of groups are indicated by circles. 1899-1908. 
Turner, shows that my determination of 443 days is somewhat too small. 
It should be about 452 days. 
Another series of periods of rest is separated by intervals of 402 
days. It contains the following dates: December 23, 1899, January 
30, 1901, March 8, 1902, April 14, 1903, May 20, 1904, June 28, 1905, 
August 4, 1906, September 10, 1907, October 16, 1908, November 23, 
1909. 
IX. Ona New Periodicity in Earthquake Frequency. 
By Professor H. H. Turner. 
The publication of the ‘ Catalogue of Destructive Earthquakes, a.p. 7 
to a.D. 1899,’ made it possible to inquire into possible periodicities ; and 
I therefore instituted such an inquiry for periodicities near fourteen 
months, the period of the free oscillation of the earth’s axis. It was 
soon noticed that there was a marked period near fifteen months, the 
best value for which is 104/7 months: so that twenty-one periods 
occupy twenty-six years very closely. The maierial used in tite first 
instance was from 1899 back to 1750, before which the records become 
very scanty; but more modern material subsequent to 1899 confirmed 
the result, and the old records from 1750 back to 1350, broken and 
incomplete though they are, still show the periodicity. 
The following table shows the values of the calculated coefficients 
for cos @ and sin @, expressed as percentages of the total number of 
earthquakes per month, with the adopted period 104/7 months. The 
grouping is easily effected by repeating the value for a single month at 
