1116 
MR. C. DAVISON ON THE ANNUAL 
Hungary, and Switzerland during the years 1865-1884, and in Italy during the years 
1865-1883. The total number is 2253. In the six summer months (April to September) 
the number of shocks felt during the night was 545, and during the day 358. In the 
six winter months (October to March) the number felt during the night was 821, and 
during the day 529. Thus, the ratio of the number of shocks observed during the 
night to that observed during the day was 1’52 in summer and 1'55 in winter. These 
ratios agree so closely that we may feel confident as to the reality of any marked 
annual period observable in these countries. 
Again, the annual period is well marked in several tropical districts where the 
length of the night is nearly constant throughout the year. And, also, as will 
presently appear, there is an annual period, though of slight amplitude, in earth¬ 
quakes which produce damage to buildings, and shocks of intensity so great could 
not possibly escape observation at any time of the night or day. 
Seismic Peeiodicity in relation to Intensity. 
14. If the causes of seismic periodicity are in the main auxiliary, as has been 
suggested above, it seems possible that the periodicity might be more marked in the 
case of very slight, than of very severe, earthquakes. An attempt is therefore made 
in the present section to classify earthquakes according to their intensity, so far as it 
is possible to do so with the incomplete materials at our disposal. For this purpose I 
have in the first place used Mallet’s well-known catalogue,* and have selected from 
the shocks recorded by him : (1) those which may be regarded as slight, and (2) 
those which are strong enough to damage buildings ; the latter class corresponding 
to intensities 8 to 10 of the Possi-Forel scale, and the former, probably, to intensities 
3 and 4. Both classes contain earthquakes felt in the northern hemisphere only. 
Slight Earthquakes .—The shocks included under this heading are those which, 
from Mallet’s description,! can have been hardly more than perceptible even at the 
places where the intensity was greatest. When two or more such shocks are felt 
within the same area on the same day, these have been omitted, for they may be 
regarded as equivalent to a single shock of greater intensity. And, for the same 
reason, I have excluded isolated slight shocks which are members of a long and 
continuous series of earthquakes of variable intensity. This has been done in four 
* “Catalogue of recorded Earthquakes,” ‘Brit. Assoc. Rep.,’ 1852, pp. 1-176; 1853, pp. 117-212; 
1854, pp. 1-326. 
t The following are the terms employed by Mallet which I have taken to characterize a slight 
shock:—A tremor, a trembling, a slight trembling, a slight tremor, a slight tremulous shock, a trembling 
shock, a very feeble shock, a feeble vibration, a very slight shock, a slight shock, a slight vibration, a 
slight earthquake, a scarcely perceptible shock, a slight vibratory shock, rather slight, of trifling 
importance, a feeble shock, a trifling shock, a little shock, a slight undulatory motion, a very slight 
subterranean commotion, a slight undulatory shock, a slight oscillating motion. 
