152 R. D. Oldham — Tidal Periodicity in Earthquakes of Assam. [No. % 



We have consequently the effect which was to be looked for if the 

 frequency of earthquakes is influenced, either by the amount of the 

 horizontal tide generating force, or by the rate of change of the tide 

 generating forces, and the fact that this effect becomes more marked 

 the larger the number of shocks — suitably distributed as regards time 

 of occurrence— which are taken into consideration, lends support to the 

 supposition that the apparent relation between cause and effect is a 



real one. 



Passing on to the last line, representing the night shocks when 

 the declination is north and the day shocks when it is south, that is 

 to say a time when the rate of variation of the tidal stresses is at 

 its lowest and less effect to be looked for, we find that the marked 

 maxima have disappeared, and that there is an almost equally distinct 

 increase in frequency about six o'clock, that is at a time corresponding 

 to the passage of the circles of maximum vertical force. This has the 

 appearance of indicating that the purely vertical stresses have less 

 influence than those which have a large element of horizontal stress, 

 and that the effect of the former only becomes apparent when 

 that of the latter becomes small. Too much stress must not, however, 

 be attached to this conjecture, as the number of shocks dealt with is 

 smaller than in any of the other combinations, and the possibility of 

 fortuitous irregularities in the curve more probable in a corresponding 

 degree, and besides this the effect here only lags half an hour behind 

 the presumed cause, while in the case of the 4| and J0| hour maxima 

 it lags l\ hours behind the presumed cause. 



It appears then that the tidal stresses have a distinct effect in 

 determining the time of origin of earthquakes, though their influence is 

 small in proportion to other causes, but at the same time it is necessary 

 to enter a caution that, though the facts in this case seem to support 

 the conclusion, they are far from proving it. For proof a more extended 

 series of observations are required, not only from Assam, but from other 

 stations also, and even in the record discussed in this paper there is 

 reason to doubt the correctness of the conclusion, inasmuch as the 

 effect found appears to be out of proportion to the cause invoked. 



When we consider that the maximum upward tidal force exerted by 

 the moon is only 1/8, 450,000 of gravity, that this corresponds very closely 

 to the difference in downward strain which would be produced by the 

 removal or replacement of half a grain on a one-ton weight, that the 

 maximum horizontal tide generating force is only three quarters of this, 

 and finally that the tide generating forces set up by the sun are a little 

 less than half of those set up by the moon, it is surprising that they should 

 have any effect at all. On the other hand when we consider that these 



