232 REPORTS ON THE STATE OF SCIENCE, ETC. 
omission in the tabular part of all stations which recorded only L or M; 
those giving L waves are mentioned by name at the end of ‘ Additional 
Readings ’: this change was introduced in the Summary for 1930. 
(3) The material now available in this and the previous catalogue strongly 
confirms an earlier suggestion of Prof. Turner on the monthly frequency 
of earthquakes. A count of the total number of epicentres occurring per 
month over the whole period 1918-1930 gives a marked maximum in Septem- 
ber and some evidence of a secondary maximum in the spring; these two 
periods account for nine maxima in the thirteen years. Attention is directed 
to the only other months having a maximum, March, April, June, and 
August. This is well shown by Fig. 1, for which the abscissa is the month 
and the ordinate is the total number of earthquakes occurring in that month 
for the whole period. A related question is how pronounced this maximum 
is in any given year. This is shown in Fig. 2, where the upper complete 
line gives the number of earthquakes occurring in the ‘ Maximum Month,’ 
and the lower broken line the number of earthquakes occurring in the 
‘Minimum Month,’ each as a function of the year. The letters attached 
to each point denote what months in each of the years 1918 to 1930 are 
the ‘Maximum’ and the ‘Minimum.’ The vertical distance between 
the two lines is a measure of the departure from a uniform number of 
earthquakes per month throughout the year, so that 1923, the year of the 
frequently repeated earthquake in Japan of September 1, was a year with 
a very pronounced maximum, while 1921, 1922 and 1925 were years with 
very uniform distribution. 
(4) It is necessary to repeat some part of the explanation of the columns 
in the present Catalogue ; the former Catalogue for 1918-1924 gives the 
explanation and reasons more fully. 
The first column is the day of the month, given at the head of each group, 
with the To, of the shock in hours, minutes and seconds of G.M.T. from 
midnight. * a 
The second and third columns show the latitude (North +, South —) 
and longitude (East +-, West —) of the epicentre. 
The fourth column has the number of stations which have given 
recognisable observations of the shock ; this number indicates very roughly 
which are severe shocks observed at considerable distances, and which are 
only slight and local. The number in this column really represents the 
number of stations in the world that have sent records to Oxford and been 
used in the tabular part. These shocks, for which the preliminary wave P 
has been observed at a distance of at least 80° from the epicentre, are marked 
with an asterisk (*). The dagger (tT) in column four refers to notes collected 
at the end. Most of these notes show the cases of anomalous focal depth, 
expressed in fractions of the earth’s radius and measured from the normal 
focal depth as reference depth. 
“ The fifth column, headed ‘ Former Occasions,’ is, it is hoped, an addition 
of some value. It was left an open question for some years whether earth- 
quakes were apt to recur at precisely the same epicentre or merely in 
proximity to it; and accordingly independent determinations of epicentre 
were made for successive shocks in the same neighbourhood. But it 
gradually became apparent that the hypothesis of exact recurrence was 
often as good as any other, while the convenience of utilising the calculations 
of A and azimuth already made was considerable. Accordingly the habit 
of using old epicentres became gradually established ; and there is this 
to be said in favour of it, that those who doubt the validity of the implied 
hypothesis may be glad to have an easy reference to test cases.” 
“The sixth column, ‘ Minor Ents.,’ shows the number of observations rele- 
