AND SE^IT-ANNUAL SEISMIC PERIODS. 
1167 
to find any general rule in other cases, and it is possible that the materials are too 
imperfect to allow any law to be deduced. 
86. The amplitude of the semi-annual period ranges from '06 (Southern Hemisphere, 
Fuchs) to '79 (Mexico), the average value being '24. 
In fifteen cases the amplitude of the semi-annual period is greater than that of the 
annual period, and in three others the amplitudes are equal ; the amplitude of the 
annual period in the former cases ranging from '05 to '43, on an average being '19. 
It is worthy of notice that different catalogues for the same district generally agree 
in this respect; especially is this the case with the Balkan Peninsula, Zante, and the 
south-east of Europe, and in New Zealand. 
Moreover, eleven of these fifteen records include the following insular districts, 
which are among the most well-marked seismic regions in the world, namely, the 
Grecian Archipelago, Japan, the Malay Archipelago, New Zealand, and the West 
Indies. The average amplitude of^the annual period in these cases is '16, and that 
of the semi-annual period '24 ; that is, the average amplitude of the annual period is 
just half that of all the districts examined, while, in the case of the semi-annual 
period, the two average amplitudes are exactly the same."^ 
Origin of the Annual Seismic Period. 
87. The most probable theory of the origin of the majority of non-volcanic earth¬ 
quakes is that they are due to the impulsive friction occasioned by the slips which 
are the elements in the growth of faults. These slips are due, in the first instance, 
to endogenic causes, and, were there no external phenomena of a periodic nature to 
affect them, would take place equally at all times of the year. 
In nearly every part of the world, however, there is a well-marked annual 
barometric period, the maximum epoch over the land-areas being somewhat variable, 
but occurring as a rule during winter. The range, or double amplitude, is small, the 
maximum being about one inch, and in some districts less than one-tenth of an inch. 
The distortion of the earth’s surface by parallel waves of barometric elevation and 
depression has been investigated by Professor G. H. DARWiN.t The vertical displace¬ 
ment is 
gwli 
e cos T 
where the axis of x is vertically downwards and the axis of s horizontal and perpendi¬ 
cular to the barometric undulations, g gravity, v the modulus of rigidity, lo the 
* This phenomenon shows that the annual and semi-annual periods must have entii-elj different 
origins. 
t “ On Variations in the Vertical due to Elasticity of the Earth’s Surface,” ‘ Brit. Assoc. Report,’ 
1882, pp. 106-119; ‘Phil, Mag.,’ vol. 14, 1882, pp. 409-427. 
