312 Intelligence and Miscellaneous Articles, 



October. But for other regions it does not seem possible as yet 

 to deduce any law. The amplitudes of the semi-annual period 

 range from 0*06 (southern hemisphere) to 0*79 (Mexico), the 

 average value being 0*24. 



3. In fifteen cases the amplitude of the semi-annual period 

 exceeds that of the annual period. Eleven of these records include 

 the following insular districts, which are among the most well- 

 marked seismic regions in the world : — namely, the Grecian Archi- 

 pelago, Japan, the Malay Archipelago, New Zealand, and the "West 

 Indies. The average amplitude of the annual period in these 

 eleven cases is 0*16, and that of the semi-annual period 0*24; 

 *. e. the average amplitude of the annual period is just half that 

 for all the districts examined, while in the case of the semi-annual 

 period the average amplitudes are the same. 



Origin of the Annual Period. — In this, the concluding, section of 

 the paper, an attempt is made to show that the annual change in 

 barometric pressure may be the cause of the annual change in 

 seismic frequency. It would be difficult to prove that such a con- 

 nexion exists, but reasons are given which seem to render it in 

 some degree probable. 



1. The most probable cause of the origin of the majority of non- 

 volcanic earthquakes is the impulsive friction, due to slipping, of 

 the two rock-surfaces of a fault. Now, whatever be the causes of 

 seismic periodicity, it seems probable that they are merely auxiliary, 

 and determine the epoch when an earthquake shall take place, rather 

 than whether there shall be an earthquake at all. Prof. Gr. H. Darwin 

 has shown that the vertical displacement of the earth's surface by 

 parallel waves of barometric elevation and depression is not incon- 

 siderable, and that it diminishes at first very slowly as the depth 

 increases. Since the fault-slip which produces even a moderately 

 strong shock must be very small, and since the work to be done 

 in such a case is, not the compression of solid rock, but the slight 

 depression of a fractured mass whose support is nearly, but not 

 quite, withdrawn, the annual range of barometric pressure does not 

 seem incompetent to produce the effects observed. 



2. Comparisons between the dates of the maximum epochs of 

 the seismic and barometric annual periods are made in 31 of the 

 districts treated in this paper. The seismic maximum approxi- 

 mately coincides with the barometric maximum in 10 districts, 

 and follows it by about one month in 9, and by about two months 

 in 4, districts ; the other cases generally admitting of some ex- 

 planation. 



3. In several insular seismic districts, and especially in Japan 

 and New Zealand, the amplitude of the annual period is very 

 small ; and, if many of the earthquakes of these districts originate 

 beneath the sea, this should be the case ; for, in the course of a 

 year, as the barometric pressure changes, the sea will have time to 

 take up its equilibrium position, and thus the total pressure on the 

 sea-bottom will be unaltered. — Royal Society, June 15; Nature, 

 xlviii. p. 359. 



