Dr. C. Davison—British Earthquakes. 413 
‘37 of the average monthly number. The semi-annual period is less 
pronounced, and its reality is doubtful owing to the smallness of the 
amplitude, which is only ‘14. ‘he analysis gives the maximum 
epochs in the middle of May and November. 
The next Table (III) illustrates the hourly distribution of the 
earthquakes, those which are reported as occurring at the exact hours 
being divided equally between the hours before and after. 
TasuE IIT, 
As the shocks were not recorded instrumentally, it is useless 
applying the method of overlapping means to these figures. The 
variations in frequency are no doubt chiefly due to more favourable 
conditions existing at certain times of the day, as, for example, the 
early hours of the night (9 to 11) and the hour 1 to 2 a.m., when 
many persons lie awake after their first sleep. The increase of 
apparent frequency in these hours is manifest in every earthquake 
catalogue, but a feature which seems to be peculiar to British earth- 
quakes is the large number felt from 4 to 5 p.m. Of the sixteen 
earthquakes recorded during this hour, seven occurred on Sunday, 
while two others were strong shocks that could not have passed 
unnoticed at any time of the day. It is, therefore, probable that the 
number of slight shocks recorded in Great Britain would be almost 
doubled if all the hours of the day were as restful as those devoted 
to the modern institutions of the Sunday afternoon siesta and the 
5 o'clock tea. 
NATURE OF THE SHOCK. 
As a general rule, in British earthquakes the shock consists of 
a single series of vibrations, which increase rapidly in strength, until 
one or several of greater prominence are felt, after which they fade 
as rapidly away. The average duration of the shock in such cases 
is about four seconds, though, as in the Carnarvon earthquake of 1908, 
it may be as much as 6°7 seconds. In very slight earthquakes there 
are no prominent vibrations, and only a weak tremor is perceived; in 
slight earthquakes the shock often begins with a single prominent 
vibration, like the thud of a falling body, followed by a brief tremor 
as such a fall would produce in a building. In these earthquakes 
the average duration of the shock seldom exceeds two seconds and is 
often less. 
In a few earthquakes the shock consists of two parts, in each 
of which the vibrations increase to a maximum and then die away. 
The two series generally differ in intensity and duration, and 
occasionally in the period of their vibrations. As a rule, the interval 
between the two parts is one of absolute rest and quiet, but 
occasionally it is occupied by a weak tremor and sound which are 
