ON SEISMOLOGICAL INVESTIGATION. 201 
that between 18 and 19 hours the pendulum was swinging through a 
range of half a millimetre. On August 23 the motion was continuous 
for the whole twenty-four hours, and the extent of motion was 10 milli- 
metres. On days that are omitted, unless there are remarks to the 
contrary, the pendulum was at rest. Although the natural period of 
Fie. 5.—Tremor Storm and Deflection. 
Oct.17 1895 Cio tee ADS es re _4hrs. . 
the pendulum was 17 seconds, it will be noticed that sometimes its 
period exceeded 5 minutes, while periods of 15 minute are com- 
mon. Irregular and comparatively rapid swingings of the instrument 
are called tremors. Some of these are apparently due to the establish- 
ment of air currents within the case of the instruments, while others 
seem to have their origin in actual movements of the supporting pier. 
Fic. 6.—Pulsations at Shide. 
Ne, 
9.28.9.30.P.M ocT19"" 1895 10.30.P.M 
Pulsations are slow movements which are regular in the period and 
amplitude on the photogram, having an appearance like that produced by 
a tuning fork recording its vibrations on a moving smoked surface. 
These pulsations are referred to as such, or as waves. Often they are 
distinct from tremors, but at other times they lead up to tremor storms, 
and in such cases it becomes difficult to distinguish between pulsations 
and tremors. 
