144 R. D. Oldham — Tidal Periodocity in Earthquakes of Assam, [No. 3, 



which the interval exceeds three hours, shows that in a general list of 

 all the shocks the effect must be looked for between 3 and 4 hours on 

 either side of midday and midnight. Further, as it is a common pheno- 

 menon in nature that the maximum of effect lags behind the maximum 

 of cause, it may be that the effect will not be found between 3 and 4 

 hours on either side of the meridian passages, but at some time after 

 that epoch. Another effect which may be looked for, which follows 

 from the consideration of the greater efficiency of the force when its 

 rate of variation is greater, is that we may expect the number of shocks 

 recorded during the day to be proportionately greater when the sun is 

 more than 9® N., that is during the summer, and the night shocks to be 

 proportionately more numerous during the winter, when the sun is more 

 than 9° South of the equator. 



There is another supposition which must also be tested, that the 

 effect, if any, of the tidal forces is not to be looked for in connection 

 with the times when they attain their maximum, but with the times at 

 which the rate of change, of amount and direction of the forces, is at 

 its maximum. For any particular place the rate of change always 

 reaches its maximum at 3 hours before and after the meridian passage, 

 but along a great circle, passing through the place of observation and 

 the place where the satellite is in the zenith, the maximum rate of 

 change is at 45° from the latter, and it will be useful to see what is 

 the time interval for different declinations at which a circle 45° distant 

 from this spot passes the place of observation. The result is given in 

 the following table. 



II. — Times of passage of circles of maxirnuni rate of change of the Tide- 

 'producing forces calculated for Lat. 26° JV". 



Decl. 



Direct. 



Indirect. 





12h + 



Oh + 





h. m. 



h. m. 



26°N. 



3-22 





9°N. 



2-56 



1-56 



0" 



2-33 



2-33 



9^S. 



1-56 



2-56 



26°S. 





3-22 



It must be distinctly understood that the times given in this table 

 are not those at which the rate of change is actually greatest, but those 

 at which the rate is greatest, as measured along a different circle to the 

 east and west one, along which the place of observation travels. In the 

 solitary case where this place and the satellite are both on the equator 

 the two agree, and in no other ; but the table is useful, for the closer the 



