1902.] R. D. Oldham — Tidal Periodicity in Eartliquakes of Assam. 141 



frequency, and instead of contenting myself with a mere record of the 

 relative frequency of the earthquakes, as has usually been done in the 

 past, I have made an attempt to see whether there is any trace of extra- 

 terrestrial influence in this frequency. 



As pointed out by me in a short note published in 1901 1 any effect 

 which the attraction of the sun and the moon may have, will be most 

 effectively, if not solely, exerted by the Tide-producing forces they set up, 

 and that, to trace the effect of these, it is not sufficient to merely tabu- 

 late earthquakes by the hours in which they occur. The time at which 

 the tide-producing forces reach their maximum depends on the declination 

 of the sun and the moon, that is to say it is subject to seasonal vari- 

 ations, and to determine whether these forces have any influence it is 

 necessary to classify the records, according to the position of the sun or 

 moon with reference to the equator, and then examine the frequency 

 to see whether there is any variation which can be coi'related with the 

 tidal forces. 



II. — Statement op the Problem. 



There is neither space nor occasion to recapitulate what is known 

 of the theory of the tides, but a brief account of the form of the tide- 

 producing influence of the attraction of the sun and the moon is desirable, 

 that the nature of the effect to be looked for may be clearly understood, 

 and the review will be simplified by the fact that we need not consider 

 the theory of the tides themselves, but merely of the stresses to which they 

 owe their origin. Omitting all reference to the why, it will be sufficient 

 to point out that the effect of the attraction of a satellite— and in this 

 connection the sun is regarded as a satellite equally with the moon — is 

 to produce a stress equivalent to an upward force at the spot which is at 

 any moment directly under the satellite, and at the antipodes of that spot. 

 Along the great circle half way between these two spots, separated from 

 each by 90° of arc, there is a force acting downwards towards the centre 

 of the earth, and equal in amount to one half of the upward force. At 

 spots between these two points and the great circle just referred to, 

 the stresses produced are equivalent to forces acting in directions away 

 from the vertical, and along a circle which is distant about 54° 44' 14" 

 from the spots where the satellite is in the zenith or nadir the force acts 

 horizontally. 



Now if we suppose the force exerted at any point to be resolved 

 into two separate forces, one acting vertically and the other horizontally, 

 then the vertical force attains its upward maximum where the satellite 

 is in the zenith or nadir, and its downward maximum along the great 

 circle intersecting the line joining these two points and lying at right 



l Geol. Mag. 4. Decade, viii, 449, (1901). 



