part 2] 



IX THE FREQUENCY OF EARTHQUAKES. 



101 



.after-shocks of the Mino-Owari earthquake of October 28th, 1891, 

 and of these three quarters fall in the winter half-year. As a result, 

 the mean number of shocks in the two-hour periods of the summer 

 half-year is only 255, a number much too small to give a good 

 average, with the result that the curve of frequency shows great 

 irregularity. This could be obviated by enlarging the groups by a 

 series of overlapping periods ; but the method seems less legitimate 

 than that of simply enlarging the periods to three hours each, by 

 which a series of numbers close to the minimum admissible is 

 obtained. The result of this tabulation is given in statement No. 3 

 ( p. 102), and this and the Italian record are shown graphically in 

 the accompanying diagram (PL XII). 



An examination of these curves shows that the variation in 

 frequency is distributed symmetrically with regard to the time 

 of meridian passage of the sun, but oppositely in each half-year. 

 In other words, it is represented, for each season, by a curve of 

 24-hour period, with its maximum and minimum, at the times of 

 meridian passage, superimposed on the general curve of variation 

 in frequency — this superimposed curve being similar in each half- 

 year, but with the maximum of one half coincident in time with the 

 minimum of the other. As the A'ariation in the gravitational 

 stresses set up by the sun follows precisely the same course, it is 

 not unnatural to suppose that the two may be connected as cause 

 and effect, and the suggestion obtains some support from the lunar 



I. Day and Night Frequency of Earthquakes. 



Number of Shocks. 



Itai.v, 1891-1910. 



June-July 



Summer half-year 



Whole year 



Winter half-year 



December-January 



Japan, 1835-1892. 



Summer half-j'ear 



Whole year 



Winter half-year 



Assam Aftershocks. 



Sun's declination over 9° X. .. 



Whole year 



Sun's declination over 9° S. . 



Shillong Seismograph Lunai 



Moon's declination over 9° N.. 



Whole record 



Moon's declination over 9° S 



Ratio. 











Day. 



Xight. 



Bail : 



Sight. 



791 



967 



90 



110 



2057 



2615 



88 



112 



3828 



5238 



84 



116 



1771 



2623 



81 



119 



583 



928 



77 



123 



1522 



1537 



98 



102 



3872 



4456 



97 



103 



2350 



2919 



93 



107 



1476 



1126 



113 



87 



3839 



3329 



107 



93 



1175 



1147 



101 



99 



260 



250 



102 



98 



632 



642 



99 



101 



191 



237 



90 



110 



i2 



