1902.] R. D. Oldham — Tidal Periodicity in Earthquakes of Assam. 143 



We may assume that the epicentres of the earthquakes now under 

 consideration all lie in 26° N. Lat., without introducing any material 

 error, and, calculating for that latitude the time intervals, which elapse 

 between the meridian, passage of the satellite and the passage of the 

 tidal circles, we obtain, for extreme and mean values of declination the 

 intervals given in the tabular statement below, ' where h represents 

 the lower, and 12 h. the upper, meridian passage, or midnight and mid- 

 day in the case of the sun. 



I. — Table showing the times of passage of circles of maximum horizontal 

 and vertical Tide-producing force ; calculated for Lat. 26° N. 



Decl. 



Hor. force, 



Vert, force, 



Hor. force, 



Direct. 



Downward. 



Indirect. 





12 h. + 



Oh. + 



Oh. + 





h. m. 



h. m. 



h. m. 



26° N. 



4-15 



4-38 







9°N. 



3-31 



5-34 



2-14 



0° 



2-59 



6-0 



2-59 



9°S. 



2-14 



6-26 



3-31 



26° S. 



"~~~ 



7-22 



4-15 



From this table it is obvious that, if the total number of shocks is 

 divided into three groups, according to the position of the sun, the first 

 comprising those which occurred when the sun was more than 9° N"., 

 the second when its declination did not exceed 9° N. or S. and the 

 third when the declination was more than 9° S., then in the first group 

 the effect of the horizontal force must be looked for between 3| and 

 4 hours before and after midday, and within two hours on each side of 

 midnight ; in the second group the effect is to be looked for between 

 3 and 3| hours on either side of midnight and midday ; while in the 

 third the condition will be the same as in the first, with the substitu- 

 tion of midnight and midday. Moreover, as the effect may be due 

 rather to the rapidity of changes in the amount, than to the 

 actual amount, of the force exerted, the horizontal force may have 

 but small influence when the passage of the circles takes place at 

 less than two hours on either side of the meridian passage, that is 

 to say, when the intersection of the circles is oblique, and the rate and 

 range of change in the amount of force is less than when the passage 

 takes place at a greater time-interval than 2 hours from the meridian 

 passage. This, combined with the much greater length of time during 



1 The intervals are not exactly the same on either side of the meridian passage 

 oit account of the motion of the sun and moon in the heavens, but the inequality 

 is not sufficient to be of importance in this connection, 



