SEISMIC EFFECTS 73 
periods range from about 4 seconds to over 8 seconds. 
The periods are not mixed up, for the same period will per- 
sist for many hours. In the second class the periods range 
from about 12 seconds to about 30 seconds. The movements 
are irregular, look like badly drawn sinusoidal curves, and the 
periods occur indiscriminately. These occur only on windy 
days. 
There seems to be little doubt that the second class is due 
to the gusts of local wind setting the ground and buildings 
into movement, for they start with the wind and cease as soon 
as the wind subsides. In my own experience the movements 
are not very pronounced until the speed of the wind is about 
20 miles per hour, and I should say that the movements 
tend to become more regular and of shorter period as the wind 
increases in speed. It has been found at Pulkowa that the 
amplitude is much reduced in an underground room, and that 
it is an advantage to prevent direct access of air to the sides 
of the piers. 
Microseisms of the first type present an interesting problem 
for solution. They are observed at quite inland stations and 
at considerable depths as well as at stations near the coast. 
A systematic comparison of observations has been undertaken 
by the International Association of Seismology, but results are 
not yet available. The main features, are, however, fairly 
definite. The longer periods are associated generally with larger 
amplitude. The longer period movements (8 seconds) come out 
strongly in stormy weather, but persist for many hours after 
all local wind has ceased, and then the period and amplitude 
usually gradually diminish until a normal period of from 4 
to 5 seconds prevails it may be for several days. Again 
the microseismic movement of this type is more pronounced 
in winter than in summer. Indeed there are often occasions 
in summer where the movement becomes imperceptible and 
this is rarely the case in winter. 
Plate 7A is a reproduction of a portion of a specimen 
record obtained at Pulkowa. It shows clearly a feature usually 
to be observed, that the amplitude rises to a maximum and 
then subsides, the maxima being at intervals of about I minute. 
