1-42 R. .D. Oldham — Tidal Periodicity in Earthquakes of Assam. [No. 3, 



angles to it. The horizontal force attains its maximum along two 

 circles distant about 54° 44' from the zenith and nadir respectively, the 

 direction being towards the satellite in the former case and away from 

 it in the latter. If then the tidal stresses have any influence in deter- 

 mining the time of origin of earthquakes we should look for the effect 

 in connection with these circles. 



Both sun and moon, as is well known, vary their position in the 

 heavens, travelling alternately north and south of the equator, the sun 

 moving to about 23", and the moon to about 26 D , from it. From this it 

 follows that neither can ever be in the zenith of any spot distant more 

 than 26 3 from the earth's equator, that is in more than 26° of latitude 

 either north or south, and no spot situated outside those limits can ever 

 experience the maximum upward force. "Within those limits, at either 

 one or two periods in each year, when the declination of the sun and 

 the latitude of any given place are the same in amount and sign, the 

 maximum upward force, due to the sun, will be experienced at midday 

 and midnight; and similarly in each lunar month there will be either 

 one or two periods at which the maximum upward force will be ex- 

 perienced, when the moon is either overhead, mid-moon-day, or under- 

 foot, mid-moon-night. Outside the limits of the two 26° parallels, and 

 within them at all times when the declination of the sun or moon is 

 different in amount or sign from the latitude, the maximum upward 

 force will not be experienced, but, as the earth revolves on its axis, the 

 circles of maximum horizontal and downward force sweep over its sur- 

 face, aud pass any given place at an interval, before and after the meri- 

 dian passage of the satellite, which depends on the declination of the 

 satellite at the time and the latitude of the place. 



From these considerations it will be seen that, before discussing the 

 frequency of earthquakes with reference to the tidal stresses, it is 

 necessary to group them according to their place of origin, and then 

 see whether, for any one district, there is a connection between the 

 relative frequency of earthquakes and the times of passage, over the 

 epicentre, of the circles of maximum tidal force. 



One method of discovering whether there is any such connection 

 would be to calculate for each earthquake the exact time which 

 separated the time of its origin from that of the passage of each of the 

 circles of maximum tidal force, and then to classify the records accord- 

 ing to these intervals, and see whether there was any preponderance of 

 earthquakes at or about these times. The process would be a laborious 

 one, and, in view of the want of exact accuracy in the times, did not seem 

 worth going through, as a result within the limits of accuracy of the 

 records can be obtained in a simpler manner. 



