Page:Proceedings of the Royal Society of London Vol 60.djvu/495

462 Table IY .— The Coefficients c and a, the am plitudes and phase coefficients.

District. Cl. c2. ^3* c4. *1- a2. a3. (X4. i 94 *4 68-7 46 -9 16-6 7-85 9-95 2-08 3-43 2 - 5 29*9 36-5 32 -4 35-2 24 -2 4-33 0-88 2-95 6 18*4 20 -7 29 -9 14-6 21-8 11 -5 1-79 5-65 1 13 *0 16-4 3-17 8-98 6-7 7-9 1 -3 6-1 8 54 *2 45-8 10-0 6-17 20 -2 9-03 2-53 2-07 9—10 65 -0 56-9 53 -8 5-26 15-9 3-94 3-0 1 -92 11 42*8 91-5 32-5 26-0 3-55 4-19 6-91 1-53 12 245*0 233-0 111 -5 36-7 3-38 4-93 3-57 4-48 13 73-9 167-0 193 0 7-48 15-1 4-62 3-9 1-09 14—15 175-0 247-6 91-9 41 -5 11-2 2-78 2-79 1-75 All 10 3 17-9 10 -9 3-97 6-62 7-97 2-42 2-43

A comparison' of these tim es w ith the tim es of high w ater in the various districts failed to establish any relation. W e are forced to the conclusion th a t if there be any lunar-diurnal periodicity imposed upon earthquake frequency, it is the result of tidal stresses acting directly on the approxim ately rigid crust of the earth, and not indirectly th ro u g h the loading due to the ocean tides. Because of the com paratively great num ber of earthquakes the results for districts 6 and 7 are the m ost im portant. D uring the eight years under discussion, the shocks in district 6 occurred w ith norm al frequency. A ll were com paratively sm all; none were disastrous. On the other hand, the case of district 7 is altogether peculiar. In general, this is a com paratively quiet d istrict; b u t the great disaster of October 28,1891, was followed by a vast num ber of after-shocks. These show distinct daily and half-daily periodicities, the latte r having the greater am plitude. Thus, from district 6, w ith its 1432 earthquakes distributed w ith fair uniform ity over eight years of norm al activity, and from district 7 w ith its 3(332 earth ­ quakes, almost wholly included in a short fierce interval of fourteen months, we obtain very sim ilar evidence as to the existence of a lunar half-daily period in earthquake frequency. The results for “ All ” depend, in the main, upon the statistics for districts 6 and 7. The curious way in which the comparatively prom inent 1st harmonics of these two districts tend to cancel ope another, is a w arning of the danger of lum ping together statistics of different countries or different seismic areas in the search for possible periodicities. 3. The Lunar Monthly and Fortnightly Periodicities.— There are five distinct kinds of m onths recognised by astronomers, nam ely:—