ON SEISMOLOGICAL INVESTIGATION. 193 
mencement of the second phase of motion. For the commencement of 
other phases of motion, of which there may be several before the 
appearance of the largest waves (L.W.’s.), reference must be made to the 
seismogram. 
For entries in the first column all records should be fairly comparable. 
The entries in the second column are only comparable in those instances 
where I have been able to place the seismograms for the stations to which 
they refer side by side. Where this has been the case will be seen by 
reference to the reproductions of such seismograms. The accuracy of the 
determinations of the times given in the third column is dependent upon 
conditions which govern the accuracy of the entries in the second column. 
If a station reports a series of times for the first, second, third, &ec., sudden 
increases in range of motion, unless we have the seismograms before us it 
is by no means certain that these correspond to phases of movement 
which have been similarly numbered at a second station. 
The time entries for Potsdam are only given approximately. (See 
. 194.) 
a The first illustration of these three-column entries is Earthquake 
No. 182. 
Determination of Origins. 
The methods by which origins may be determined from time observa- 
tions are numerous.! The simplest, perhaps, is that of circles, and its 
application is as follows :—If the large waves of an earthquake reach 
stations B, C and D four, six and eight minutes after reaching station A, 
then when they reach A the wave fronts are respectively about 600, 900 
and 1,200 kms. distant from B, Cand D. Ona globe with B, C and D as 
centres I draw circles 600, 900 and 1,200 kms. radius. The centre of the 
circle, found by trial, which passes through A and touches the circles round 
_B, C and D, is the origin required. The assumption is that whilst the 
P.T.’s are propagated with variable velocities through the earth, the large 
waves traverse the surface of the earth with a velocity that is nearly con- 
stant. In this illustration I have assumed this velocity to be 2-5 kms. 
per second. 
The observations which support these assumptions are too numerous 
to require special reference. 
With times of arrival at only three stations we are left to decide 
between two possible centres. See Earthquake 252. 
In consequence of the want of sufficient records which are strictly 
comparable, no attempt has been made in the present report to determine 
erigins with any degree of accuracy. 
As an assistance in these determinations the times at which prelimin- 
ary tremors have been recorded and intervals by which they have outraced 
the large waves at various stations are not neglected, whilst the topo- 
graphical and geological character of the locality in which the origin 
is placed is often an indication as to whether the determinations are 
correct. 
Earthquakes, Nos. 133 and 134, September 20 and 21, 1897.* 
_ These earthquakes, which were separated from each other by an 
interval of about ten hours, evidently came from the same origin, and were 
' See ‘ Harthquakes,’ Int. Sci. Series, pp. 200-212. 
' * See British Association Report, 1898, p. 211, 
Eeo9. o 
