DETERMINATION OF EPICENTRE AND FOCUS 65 
ledge. When the distance 4 and the azimuth a have been 
determined at a station we may calculate the co-ordinates of 
the epicentre by means of the formule 
sin dg =cos J sin ds + sin A cos $s cos a 
and 
cos 4-sing sin dr 
cos (Ag — As) = cos dg COS bp 
where ds, As are the latitude and longitude of the station and 
zr, Az are the latitude and longitude of the epicentre. 
As an illustration of the accuracy obtained by the use of 
Galitzin’s seismographs, compare the independent determina- 
tions of the epicentre of the Monastir earthquake of 18 Febru- 
ary, I911, made at Pulkowa and Eskdalemuir. 
For Pulkowa ¢s=59 46’ N Ns = 30:19 E 
and the seismogram gave 4 = 2260 km.=20° 19’ 
and a=22 53 West of South. Hence for the epicentre 
dz = 405° N Az =20'1° E. 
For Eskdalemuir ds = 55° 19’ N No sma G 
and the seismogram gave 4= 2360 km. =21° 14’ 
and a=55 50’ East of South. Hence for the epicentre 
gn =406 N Ne 2030 E: 
As long as the first impulse is really sharp no trouble arises ; 
but with a small and gradual start, it is sometimes difficult to 
identify the corresponding movements on the horizontal and 
vertical seismograms, owing to a phase difference of the maxi- 
mum displacement. Thus instruments with the same funda- 
mental constants are required to remove this source of error of 
judg ment. 
It is clear that if the azimuths have been accurately 
determined at two stations the epicentre can be determined 
from these alone without reference to the determinations 
of distance (see Galitzin and {Walker, ‘“ Nature,” August, 
GUA\: 
The preceding example gives in this way 
dz =40'4 N Mo= BOs IE. 
for the epicentre, while the deduced distances from Pulkowa 
and Eskdalemuir are then 20° 18’ and 21° 26’ respectively. 
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