Page:Proceedings of the Royal Society of London Vol 60.djvu/496

Rh (1) The anom alistic m onth (27*545 days). (2) The tropical m onth (27*322 days). (3) The synodic m onth (29*531 days). (4) The sidereal m onth (27*3228 days). (5) The nodical month (27*212 days).

Of these, the last two cannot be regarded as having any influence bn earthquake frequency, for the only conceivable effect is a tidal one, and the sidereal and nodical m onths have no necessary tidal relations. A t the same tim e the periods of the sidereal and tropical months are so nearly the same th a t they can hardly be discrim inated in the lapse of eight years. On the other hand, th e anom alistic m onth may show itself in earthquake frequency, since the moon in perigee has a greater tidal action th an when it is in apogee. Again, because of the m oon’s variation in declination, being now north of the E quator, now south, we m ay reasonably search for a tropical m onthly periodicity. A nd, finally, the synodic or common month may make itself apparent, there being possibly a g reater tidal stress w hen the moon is in syzygy (as in ordinary spring tides) than when the inoou is in quadrature (as in neap tides).

The earthquakes were accordingly tabulated according to these four m onths, whose periods differ app reciab ly ; the nodical m onth being also included. For, by analysing the statistics in term s of both the tropical and nodical m onths, we may be th e b etter able to draw conclusions as to the real existence of one or other periodicity. The relative daily frequencies, as finally reduced, are given in Table Y l, and the curves are shown in th e figure. As in the case of Table II, each of the tabulated num bers is the mean of five successive num bers, and is regarded as belonging to the time of the middle one of these five.

It should be m entioned—and the rem ark applies also to the former cases— that the num ber of earthquakes which really occurred during the last time interval was increased in the proper ra tio : so th at the frequency during this last interval was made comparable w ith the frequencies of the other intervals. It was interesting to find how admirably the number so obtained harmonised w ith its neighbours of the first and penultim ate interval.

In all cases the obvious aftershocks of any earthquake occurring on the same day were neglected. The 3000 aftershocks of the great disaster of October 28, 1891, were also left out.

The earthquakes on which the discussion is based num bered from 4725 to 4741, the num ber varying slightly for each m onthly period, since, at the beginning and end of the eight years’ interval, there were always a few, differing for the different months, which did not make up a complete period, and were, consequently, neglected.

Each series of numbers was analysed harmonically as far as the