230 REPORT—1899. 
with musical reverberations inside the world. For the present my opinion 
inclines to the latter, and I see in the earthquake followers the likeness 
of their parents. 
VI. Larthquake Precursors. 
The series of movements to which I now refer is the procession of 
vibrational groups which run before the main disturbance, with the 
smaller of which, under the name of preliminary tremors, we are already 
more or less familiar. These precursors have in several respects char- 
acters which are exactly the opposite to those of the earthquake followers. 
They have a definite commencement, and with large earthquakes group 
after group usually increases suddenly in amplitude and period. 
Another characteristic of the precursors is that whilst group after group 
may grow larger, they become more and more irregular in their contours. 
The first of the preliminary tremors, if they ever had any /frétillements 
have lost the same, whilst those which follow carry serrations which are 
marked. This observation, together with that of growth in amplitude, 
suggests the idea that each group of precursors starting from a common 
origin has reached an observing station by different routes: the first 
have come along the path of least time, and the latter, culminating in the 
shock, along paths continually approximating to that of free surface waves. 
Now and again we see in groups of preliminary tremors a likeness in 
contour and arrangement of what is to follow. Near to an origin they 
may have a duration of from 1 or 2 up to 10 or 20 seconds, and their 
period has been recorded at from } to ;), of a second. When they are 
preceded by a sound wave, we have evidence of a very much higher 
frequency. If these vibrations have travelled long distances and through 
our earth, most records indicate a period of 3 or 4seconds. Records from 
Rome have shown periods of less than half a second, but even these are 
probably much too large. My own records only indicate a slight switch- 
ing at the end of a light elastic boom, or that the same has been moved 
very rapidly to and fro relatively to its steady point. Until a steady 
point seismograph with extremely light multiplying indices or some other 
special form of apparatus has been employed as a recorder, our knowledge 
of this end of the seismic spectrum is not likely to increase. 
The last points connected with the earthquake precursors are the 
intervals of time which elapse between the arrival of the first tremor and 
the largest wave or waves corresponding to the originating impulse and 
the duration of the first series of preliminary tremors. As measured on 
seismograms for disturbances which have originated at different distances 
from the Isle of Wight Observing Station, these two intervals are given 
in the following table :— 
First P.T. to Duration of first 
+s Distance ; : 
Origin é Max. motion in roup of P.T.’s 
: in degrees minutes ‘ in eee 
Iceland . . . 5 Le 4or5 14 
Greece . ‘: ‘: 3 22° 6 3 
Tashkend : ‘ : 48° 14 8 
Hayti . , H : 62° 30 Up yo 
Japan . 3 ¢ : 84° 47 85 
Borneo . : ‘4 4 112° 55 6:0 
* his is dependent on a single observation, and may be too high. 
