AND SEMI-ANNUAL SEISMIC PERIODS. 
1169 
89. Comparisons between the dates of the maximum epochs of the seismic and 
barometric annual periods can be made in thirty-one of the districts treated in this 
paper. The epoch of the seismic maximum approximately coincides with that of the 
barometric maximum in ten districts, and follows it by about one month in niue, and 
by about two months in four districts. In four others (Japan, Tokio, India, Cali¬ 
fornia), the seismic epoch precedes the barometric by about two months ; but, in the 
first two of these at least, this may be due to the inclusion of slight shocks having 
their maximum in summer. To the same cause, perhaps, may be due the four 
exceptional cases (Scandinavia and Iceland, Great Britain, the district round and 
including Vesuvius, and Sicily), though many of the Vesuvian and Sicilian earthquakes 
are obviously of volcanic origin, and are probably more frequent with a low barometric 
pressure. Thus, as a general rule, the epoch of the seismic maximum either coincides 
with that of the barometric maximum, or follow it by a month or two. 
If the epoch of the barometric or seismic maximum were constant in either hemi¬ 
sphere, this general agreement might not possess much significance, but it continues 
when the epochs vary. For example, in the southern hemisphere, we have the 
following dates :— 
District. 
Barometric maximum. 
Seismic maximum. 
South-east Australia . 
New Zealand .... 
Chili. 
Peru, Bolivia, and Quito. 
hlay aud J une 
April 
July 
July 
May 
March-May 
August 
July 
Again, while in both North and South America the maximum epochs of both 
periods fall in the winter months, in the Sandwich Islands (lat. 20° N) the epoch of 
the seismic maximum occurs in June, and that of the barometric maximum at 
Honolulu in April or May. 
90. Lastly, supposing the view here suggested to be correct, the annual variation in 
barometric pressure cannot have any influence on seismic frequency when the earth¬ 
quakes originate entirely beneath the sea ; for, as the barometric pressure changes, the 
sea will have time to take up its equilibrium position, and the total pressure on the 
sea-bottom will be unaltered. Hence, if some of the earthquakes felt in a district 
originate beneath the sea and some beneath the land, the amplitude of the annual 
period will be less than it would be if all were to originate underneath the land. 
Since some of the foci of the earthquakes felt in Japan and New Zealand are known 
to lie beneath the ocean, it seems not improbable that the small amplitude of the 
annual period in these two, as well as in other, insular districts may be thus 
accounted for. 
7 K 
MDCCCXOIII.—A. 
