Periodicity of Earthquakes. 879 



of the periodicity is uncertain. In others, the period is very 

 distinct, the amplitude of the diurnal period at Trinidad and 

 Christchurch (N.Z.) being about 80 per cent, of the mean 

 hourly number of earthquakes. In the following' Tables I 

 have included only those results which seem to me free from 

 doubt. 



In Tables I. and II. the time at which the preliminary 

 tremors were first recorded is taken as the time of occurrence 

 of the earthquake. The maximum-epoch is thus retarded, 

 but by an amount that is certainly less than the time required 

 by the tremors to travel from the most distant origin. In 

 measuring the amplitude, the mean hourly number of earth- 

 quakes is taken as unity. In the column headed ^/ir/n, n is 

 the number of earthquakes included in the record. Unless 

 the amplitude obtained from isolated records exceeds the 

 value of s/tt/ii, the result, as Prof. A. Schuster has shown *, 

 must be regarded as doubtful. If, however, the epochs 

 obtained for the six winter months (October to March, in 

 the northern hemisphere) agree with those for the six summer 

 months (April to September), the doubt is lessened or 

 removed, even if the amplitudes should fall below the corre- 

 sponding values of 'sjirju. 



Diurnal Seismic Period, 



In Table I. are given the periodic elements deduced Irom 

 17 records divided into three groups. Those obtained from 

 the catalogue of local earthquakes at Tokyo are added for 

 comparison. It will be seen that, with one exception 

 (Baltimore), there is a close agreement in the epochs for the 

 winter and summer months. 



Thus, in nine records, the maximum-epoch of the diurnal 

 period occurs about noon ; in three, about midnight ; and in 

 five others (three of them in India) early in the afternoon. 

 At Baltimore,* the maximum-epoch for the winter months 

 occurs at 1 A.M., and for the summer months at 1^ p.m. 

 Other examples of this reversal will be given later. 



Semi-Diurnal Seismic Period. 

 The number of records giving satisfactory results for the 

 semi-diurnal period is small. In those which are excluded 

 here, the amplitude is usually close to the value of ^ir\n 

 and the series of six-hourly means vary somewhat irregularly. 

 The results obtained for local earthquakes at Tokyo are added 

 for comparison. (Table II.) 



* Roy. Soc. Proc. vol. lxi. pp. 455-465 (1897). 



