1114 
MR. C. DAVISON ON THE ANNUAL 
11 . Defijiition of the Unit Earthquake. —The difficulty of deciding what constitutes 
an earthquake attends all work on seismic periodicity, and, unless some uniform and 
natural system is followed, our conclusions cannot possess much value. For example, 
every great earthquake is preceded and followed by a large number of minor or 
secondary shocks, which are no doubt intimately associated with the principal shock. 
Tn such a case, are we to count every shock that is recorded, however slight, or are 
we to regard the whole series as a single earthquake ? Again, the distinction 
between an earthquake and earth-tremor is a purely arbitrary one; Are we to 
include every earth-tremor recorded by a most delicate instrument, and rank it 
equally with the most destructive shock; or, if not, where are we to draw the line 
between the two ? 
It must be admitted that these are serious difficulties, but I do not think they 
would be met, as has been proposed, by taking the intensity of the shocks into 
account as well as their frequency. Even supposing it possible to estimate the 
intensity correctly, it may be doubted whether the results obtainable in the finite 
time at our disposal would possess much value, at any rate from the periodicity point 
of view. For, whatever be the causes of seismic periodicity, it seems highly probable 
that they are merely auxiliary, and determine the epoch when an earthquake shall 
take place rather than that there shall be an earthquake at all. The intensity is 
probably determined almost entirely by erogenic causes, the frequency partly by the 
same causes, and partly by certain others which are possibly variable and periodic in 
their action. 
I believe, then, that, in the present state of our knowledge, it is desirable to 
regard each shock felt and recorded as one earthquake. Moreover, an approximate 
uniformity in estimating the minimum intensity of an earthquake will be attained if 
we consider special districts of comparatively small size—countries, for example, like 
Italy and Switzerland, rather than continents or hemispheres. 
There is still room, however, for some arbitrariness of treatment, owing to the 
imperfection of nearly all our seismic records. The following principles have, there¬ 
fore, been adhered to throughout this paper : (l) whenever the entry in a catalogue 
is “ several shocks about this time,” or some equivalent expression, without any 
attempt being made to give the time of occurrence of each shock, I have always 
i-eckoned such as a single earthquake ; (2) when the shocks have been considered 
important enough to be entered separately, the time of occurrence of each being given, 
every such shock has been counted, provided the interval between any two or more 
was not less than five minutes ; and (3) shocks entered as doubtful and earthquake- 
sounds unaccompanied by any shocks have been omitted. 
12. Explanation of the Tables, Eigiires, &c. —Table I. contains the number of 
earthquakes for the different districts investigated during each half-month or month. 
The reduced six-monthly means obtained in discussing the annual period, as described 
