jR. D. Oldham — The Periodicity of Earthqualxes. 451 



Taking the solar times first, the sun's declination was south, and 

 increasing slowly throughout the period of the record ; the mean 

 value may be taken as 16° south and the mean latitude of the 

 origins of the earthquakes oO° north. Hence the sun was always 

 more than 60° from the zenith at its upper culmination, and the circle 

 of maximum horizontal tide-producing force nearest the sun never 

 reached the place of origin of the earthquakes. That furthest away 

 from the sun did, however, cross this, and an easy calculation shows 

 that, for a latitude of 50° north and a declination of 11° south, the 

 times of crossing would be about 2^ hours before and after midnight. 

 The observed maxima accord very fairly well with the times of 

 passage of the circle of maximum horizontal tide-pi'oducing force 

 due to the attraction of the sun. 



Turning to the moon, the problem is not so simple, for, instead 

 of preserving a fairly constant declination like the sun, the moon 

 ranged from its extreme northerly to its extreme southerly declina- 

 tion. Luckily, however, the syzygies happened to nearly coincide 

 with the extreme declinations, while the quadratures wex-e as close 

 to the times when the moon crossed the equator. Now when on 

 the equator the circles of maximum horizontal tide-producing force 

 would never reach 50° north latitude, and only when the declination 

 increased to 5° would one of them touch it, at the upper or lower 

 culmination as the case might be. Moreover, as the rate of change 

 has probably more effect than the amount of the foi'ce, and as this 

 rate of change would be small in the case of such tangential passage 

 of the circle, we might expect the efi'ect to be small, but so far as it 

 goes to show a slight tendency to maxima coincident with the upper 

 and lower culmination. 



As a matter of fact, there is no marked sign of periodicity except 

 a small increase in frequency about the upper and a decrease at the 

 lower culmination which may be accidental. 



At the syzygies the moon was near its maximum declination, and 

 for a declination of 25° one circle of maximum horizontal tide- 

 producing force would never touch latitude 50° north, while the 

 other would cross the place of origin at five hours before and after 

 the upper or lower culmination, according as the moon's declination 

 was north or south. As the declination decreased this interval 

 would decrease, but would not fall much below three hours before 

 the moon passed the half-way point between the syzygies and 

 quadratures ; and as the declination was north during half the 

 period of record and south during the other half, the result is 

 that we should expect to find no well-defined maxima, but a greater 

 number of earthquakes occurring more than three hours from the 

 culminations and fewer occurring within three hours of them. 

 Such is practically the case, and the accordance is as close as could 

 reasonably be expected from so limited a record. 



So far, then, from the tabulated results showing no indication of 

 any influence of the sun and the moon, they distinctly support the 

 hypothesis of a maximum frequency at the time of passage of the 

 circle of maximum horizontal tide-producing force. They are far 



