On Lunar Periodicities in Earthquake Frequency. 463 



(1) The anomalistic month (27'545 days). 



(2) The tropical month (27'322 days). 



(3) The synodic month (29'531 days). 



(4) The sidereal month (27'3228 days). 



(5) The nodical month (27'212 days). 



Of these, the last two cannot be regarded as having any 

 influence on earthquake frequency, for the only conceivable effect 

 is a tidal one, and the sidereal and nodical months have no necessary 

 tidal relations. At the same time the periods of the sidereal and tropical 

 months are so nearly the same that they can hardly be discriminated 

 in the lapse of eight years. On the other hand, the anomalistic 

 month may show itself in earthquake frequency, since the moon in. 

 perigee has a greater tidal action than when it is in apogee. 

 Again, because of the moon's variation in declination, being now 

 north of the Equator, now south, we may reasonably search for a 

 tropical monthly periodicity. And, finally, the synodic or common 

 month may make itself apparent, there being possibly a greater tidal 

 stress when the moon is in syzygy (as in ordinary spring tides) 

 than when the moon is in quadrature (as in neap tides). 



The earthquakes were accordingly tabulated according to these 

 four months, whose periods differ appreciably ; the nodical month 

 being also included. For, by analysing the statistics in terms of 

 both the tropical and nodical months, we may be the better 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 VI, and the curves are shown in the figure. 



As in the case of Table II, each of the tabulated numbers is the 

 mean of five successive numbers, and is regarded as belonging to the 

 time of the middle one of these five. 



It should be mentioned and the remark applies also to the former 

 cases that the number of earthquakes which really occurred during 

 the last time interval was increased in the proper ratio ; so that the 

 frequency during this last interval was made comparable with the 

 frequencies of the other intervals. It was interesting to find how 

 admirably the number so obtained harmonised with its neighbours of 

 the first and penultimate 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 numbered 

 from 4725 to 4741, the number varying slightly for each monthly 

 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 



