A Note on the Earthquakes of the South African Table-land. 197 
Of course the diurnal oscillation of barometric pressure is super- 
imposed on these daily averages, accentuating the disturbances very 
largely. For example, the hourly pressures on Feb. 20, 1912, were : — 
Barometer. 
Feb. 20, IX 26-063 inches. 
X -061 
XI 053 
Noon -041 
XIII -028 
XIV -004 
XV. ...25-987 
XVI -967 
XVII -965 
XVIII -979 
XIX -997 
XX 26-019 
XXI -028 
XXII -051 
XXIII -059 
From which it appears that the two shocks of this day not only came 
when the barometer was at its lowest on the days' averages, but nearly at 
the time when it was lowest for the day itself. 
It would be unsafe to argue from these few cases that an earthquake 
can only occur in South Africa when there is some amount of barometric 
disturbance. But it is not unlikely that when other conditions (say the 
flow of underground water carrying away lime in solution, and so making 
the support of the earth's crust at the spot unstable) are favourable to 
earth-movement, the passage of a barometric disturbance across the 
continent may decide the time when the movement shall take place. 
The idea that barometric disturbances have some influence over earth- 
movements is by no means a new one. The fact, e.g., that the earth- 
quakes of China are most frequent in summer, Mr. N. F. Drake explains 
as due to the rapid and strong variations of atmospheric pressure, assisted 
by the heavy rain-storms, which occur in summer. [See the Bulletin of 
the Seismological Society of America for March, 1912.] 
