ON REISMOLOGICAL INVESTIGATIONS. 



2rj9 



These figures sliow a fluctuation, with a slow increase. Comparing corre- 

 sponding pairs at two years' interval we get for the increases : — 



+ 22°, +27°, +65°, +25°, +8° : mean +29° 



Considering the nature of the material, these are wonderfully consistent. 

 Applying the appropriate correction (which represents, of course, error in 

 adopted period) we get : — • 



The range is thus about 180° ; a more exact value cannot be assigned at present. 

 The first column corresponds to March 1913, 1915, &c., and the third to March 

 1914, 1916, &c. 



The correction to period is about 29° per two years, which contain some 

 48,000 periods, and thus represents 0.000036m. per period. The value previously 

 adopted being 21.001512, we have as the corrected period 21.001548m. 



Using this period and allowing for the 6- and 24-monthly fluctuations of 

 maximum already mentioned, and using the table for latitude above given, the 

 numbers of earthquakes in each minute of the twenty-one for the years 1913-16 

 (taken together) and for 1917 January-September (incomplete, but with many 

 more determinations of epicentres) are as follows. The starting point has been 

 chosen so that the maximum falls in the middle of the series : — 



Minute 1913-16 1917 



Sum 



114 



81 



1917 



15 

 6 

 6 



8 

 7 

 6 

 9 



57 



The middle column is thus sensibly, though not extravagantly, greater than 

 the wings. But the special features of the variation are made clearer by 

 arranging the series in two halves which are symmetrical with respect to the 

 maximum. The number of minutes being odd, one item is left without a 

 partner, and may be doubled as a rough expedient : — 



Minutes . 



11 10 



12 13 14 15 



21 



1913-16 



Sum 



29 28 39 30 25 22 23 20 23 28 (22) 



1917 



The special feature is clearest in the line of totals, but can be recognised in 

 each of the others. The position assigned to the maximum by the harmonic 

 of the first order is on the left, between the minutes 11 and 12 ; but the largest 

 figures cccur symmetrically on either side of this point between minutes 9 and H 

 or 14 and 15, i.e. about 35m. away from the mid-point, the total separation 



