450 B. D. Oldham — The Periodicity of Earthquakes. 



This would be true if the frequency were such a function of the 

 zenith distance of the sun or moon that the one might be expected 

 to increase or decrease continuously with the other. This is, 

 however, by no means necessarily, or even probably, the case, for 

 if the attraction of the sun or the moon have any effect it is probably 

 through the strains set up by the tide-producing forces. 



Now these have three separate maxima distributed in two points 

 and three circles : at the extremities of the diameter pointing to the 

 sun or moon, as the case may be, the upward vertical tide-producing 

 force is at its maximum ; the maximum of the downward tide- 

 producing force lies along the great circle at right angles to this 

 diameter ; while the maximum horizontal tide-producing force lies 

 along the small circles half-way between. If, then, external attraction 

 is in any way the cause of earthquakes, we may look for the frequency 

 to have some relation to the times of passage of one or other of these 

 points or circles over the place where the earthquakes originated. 

 I am at present engaged in a discussion of the records of the after- 

 shocks of the great earthquake of 1897, with a view to seeing 

 whether any such relation can be traced ; the discussion is not yet 

 far enough advanced to have yielded any results, and the matter 

 would not have been referred to but that in the paper quoted above 

 the records are tabulated in a form which makes it possible, without 

 lengthy calculation, to roughly test the hypothesis that the frequency 

 of earthquakes is influenced by the tide-producing force generated 

 in the earth by the sun and moon. 



The three hundred earthquakes, recorded during a period of little 

 over a lunar month, are classified, according to time of occurrences, 

 and also with regard to the phases of the moon, as to whether the 

 earthquake occurred nearer to the syzygies or the quadratures. In 

 the table printed below the figure opposite to is the total recorded 



in the hour preceding and that following the lower culmination, 

 that opposite 2 being the total recorded between one and three 

 hours after the lower culmination, and so on. 



