88 MODERN SEISMOLOGY 
“Writing 
f4ut+T ; ty+T . 
A-| y cos kt dt and B=) y sin kt at 
ty 1 
and forming R = (A? + B?)? 
the quantity R will, with increasing values of T, fluctuate 
about some mean value, which increases proportionally to T}, 
provided T is taken sufficiently large. 
“ Tf this theorem is taken in conjunction with the two follow- 
ing well-known propositions :— 
“(1) Ify=cos é, R will, apart from periodical terms increase 
proportionally to T. 
“(2) If y=cos AZ, A being different from %, the quantity R 
will fluctuate about a constant value, 
it is seen that we have means at our disposal to separate 
any true periodicity of a variable from among its irregular 
changes, provided we can extend the time limits sufficiently.” 
The method of applying this will be found in “ Camb. Phil. 
Trans.,” Vol. 18, 1900. 
I have referred to this problem specially, because statistics 
about earthquakes are rapidly increasing in number and ac- 
curacy, and the search for periodicity will again be taken up. 
It is desirable that the search should proceed on the lines in- 
dicated by Schuster. 
I understand that by application of this method, Prof. 
Turner (“ Brit. Assoc.,” 1912) finds evidence of a 452 day 
period of earthquake activity. The result is interesting as it 
is so near the Chandler period of precessional nutation, and 
here we may fitly close the volume with a quotation from 
Milne (“ Earthquakes,” 6th edition, 1913, p. 377): “ Speak- 
ing generally, so far as I know, neither tidal, barometric, 
thermometric, solar, lunar, or other epigene influences beyond 
those mentioned, show a relationship to the periodicity or 
frequency of megaseismic activity. Their frequency is ap- 
parently governed by activities of hypogene origin.” 
ABERDEEN: THE UNIVERSITY PRESS 
