912 Dr. C. Davison on the Annual and 



at Kyoto, the reversal o£ the epoch of the annual period is 

 less clearly marked ; the epoch occurs in September with 

 the strong earthquakes and in May with the weak ones. 



(ii.) The earlier epoch o£ the semi-annual period usually 

 occurs in spring (March to May). In the destructive and 

 slight earthquakes recorded at Kyoto, the epochs of the 

 semi-annual period are reversed. 



(iii.) As Prof. Omori has pointed out, the maximum 

 seismic frequency occurs during the winter months in some 

 parts of Japan, and during the summer months in others. 

 The latter places are for the most part confined to the 

 eastern portion of the northern half of the country. The 

 smallness of the amplitude of the annual period for the 

 whole country is in part due to this opposition in epoch. 



(iv.) Submarine earthquakes with origins not more than 

 five or ten miles from the coast are subject to an annual 

 periodicity with the epoch nearly the same as in land-earth- 

 quakes, namely, in winter (Dec-Mar.) ; those with origins 

 at a distance of twenty or more miles have the epoch in 

 summer (June). 



Diurnal Periodicity of Ordinary Earthquakes, 



In considering the diurnal periodicity of earthquakes, 

 seismographic records only are considered, all others being 

 useless for the purpose, owing to the varying conditions of 

 observation which prevail throughout the day. The records 

 for the whole of Japan an 1 its eastern and western districts 

 may to a certain extent be incomplete. They include all 

 earthquakes in Prof. Milne's great catalogue in which the 

 time of occurrence is given in hours, minutes, and seconds. 

 There may, however, be other earthquakes recorded instru- 

 mentally in which the time is given in hours and minutes 

 only. 



For the diurnal periodicity of ordinary earthquakes, we 

 have the following results : — 



(i.) Though the amplitude of the diurnal period is in each 

 case not much above the value of -^/'(tt/ii), the close agree- 

 ment in epoch throughout is sufficient to establish the reality 

 of the period, with its maximum epoch shortly before or 

 about noon. 



(ii.) For the semi-diurnal period, the average amplitude 

 is one-half that for the diurnal period. The maximum epoch 

 occurs at about 8 or 9 a.m. and p.m. 



