1915-16.] The Ochil Earthquakes of the Years 1900-1914. 283 
8 were of intensity 5, 34 of intensity 4, 101 of intensity 3 or nearly so, while 
6 were earth-sounds without any accompanying tremor. The disturbed 
area is generally very small, rarely, if ever, exceeding 60 square miles. 
Periodicity . — The monthly distribution of the earthquakes is shown 
in Table I, the first half of the month being the first 14 days in February 
and the first 15 days in the other months. 
Table I. 
J. 
F. 
M. 
A. 
M. 
J . 
J. 
A. 
S. 
0 . 
N. 
D. 
First half . 
0 
10 
1 
9 
16 
1 
11 
1 
2 
7 
2 
2 
Second half 
15 
3 
12 
22 
13 
3 
14 
2 
10 
22 
1 
8 
Taking account of the slightly unequal lengths of the halves of the 
months, and using the method of six-monthly means for the annual period 
and of six-half-monthly means for the semi-annual period, it would appear 
that both periods are well marked. In the annual period, the maximum 
epoch is about the end of April and the amplitude 0 - 38 ; in the semi- 
annual period, the maximum epochs are in April and October and the 
amplitude 039.* 
For the successive hours of the day (counting from midnight), the 
numbers of earthquakes are 8, 8, 9, 8, 7, 8, 9, 8, 5, 9, 3, 0, 4, 8, 8, 8, 8, 14, 3, 
6, 4, 8, 9, 9. Thus, during the twelve night-hours (6 p.m. to 6 a.m.) there 
were 80 earthquakes; during the twelve day-hours (6 a.m. to 6 p.m.), 82. 
Nature of the Shock . — When the shock is of intensity 3, it consists as 
a rule of a series of tremors, the separate vibrations of which do not 
vary much in strength. Sometimes, however, only one vibration is felt; 
and, in a few cases, there is one prominent vibration, usually called a 
“ thud,” followed by slight tremulous motion, the effect being like that 
caused by the fall of a heavy weight on the floor, succeeded by the quiver- 
ing which such a fall would produce. When the intensity of the shock 
attains the degree 4 or 5, the thud, followed by a tremor, is the normal 
form, series of vibrations of nearly uniform strength being less frequently 
felt. Shocks of intensity 6 or 7 contain from two to four prominent 
vibrations, preceded and followed by tremulous motion. In a few cases, 
* If any number (n) of earthquakes were to occur at random, Dr Schuster has shown 
that the amplitude should exceed vX 71 "/^)* which in the present case is T3 (Boy. Soc. Proc ., 
vol. lxi, 1897, pp. 455-465). * Though the above figures for the amplitude are about three 
times this amount, it seems to me doubtful whether the results obtained above are of 
much value, owing to the marked occurrence of the earthquakes in a limited number of 
series. 
