ON SEISMOLOGICAL INVESTIGATION. 227 
it is only very large earthquakes which can be recorded with steady point 
seismographs at distances exceeding 300 miles. 
Directly we come to the other members in the list we are apparently 
dealing with the duration of earth tilting, and with regard to any par- 
ticular earthquake we may ask for information respecting the duration of 
the same near to its origin or at stations between this point and Central 
Italy, or in countries further afield. The information we have on these 
points is, however, scant, but such as exists is far too definite to be 
ignored, For example, Dr. Odone gives in his list the Japan earthquake 
of March 22, 1894, on which occasion the seismographs at Rocca di Papa 
and at Rome were respectively agitated for lh. 3m. and Lh. 20m. 
Because the duration of this earthquake as recorded by a bracket 
seismograph in Tokio was ten seconds, it must not be assumed that we 
have here an illustration of a seismic movement increasing in its dura- 
tion as it radiated. On this occasion, after feeling the first heavy move- 
ment, I went to my observatory and watched the boom of a horizontal 
pendulum follow very irregular heavings of the ground for some fifteen 
minutes, when I was joined by my colleague, Mr. C. D. West, and we 
continued to watch the erratic, fitful movements for lh. 47m. longer. 
We have in this instance—and others might be quoted—distinct 
evidence of earth movements near to their origin continuing for a very 
‘much longer period than they were observable at distant localities. What 
was noted in Europe were the earthquake precursors or preliminary 
tremors, the duration of which increases with distance from an origin, 
and, after that, the earthquake echoes with possible traces of waves 
which had travelled round the world in a direction opposite to that con- 
stituting the maximum phases in the seismograms, In Tokio, although 
the preliminary movements were of shorter duration than in Europe, the 
total duration of the disturbance in that city, on account of the great 
length of the concluding vibrations, seems to have exceeded that which 
was recorded in Italy. 
The shiverings of our world recur on the average every thirty minutes, 
but the heavy breathing or true ground swell does not happen more than 
once a week. Popularly they are both earthquakes, but they differ in 
their character, in their duration, and probably in their origin, and as 
they radiate, their life, as exhibited at stations farther and farther remote 
from their origin, rather than increasing becomes less. 
V. Earthquake Echoes. 
(This and the following Section are in part abstracted from Notes published in 
‘Nature,’ February 16 and March 1, 1899.) 
An earthquake disturbance as recorded at a station far removed from 
its origin shows that the main movement has two attendants, one which 
precedes and the other which follows. The first of these by its charac- 
teristics indicates what is to follow, whilst the latter in a very much 
more pronounced manner will often repeat at definite intervals but with 
decreasing intensity the prominent features of what has,passed. Inas- 
much as these latter rhythmical but decreasing impulses of the dying 
earthquake are more likely to result from reflection than from interference 
I have provisionally called them Echoes. 
When an earthquake is comparatively small, and has originated as a 
single effort at no great distance (one or two thousand miles) from the 
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