DETERMINATION OF EPICENTRE AND FOCUS 69 
disturbance. He assumes as an approximate law for small 
depths 
9 
PN? 
y= (*) = sbaee = 1) 
v 
where 
r=(1-%) 
R)? 
and v, is the velocity at the surface, v the velocity at depth 4, 
and R the earth’s radius. Now Zoppritz’ results give 
d= falg cts | Sec. 
Oa 7 OO) kms See. 
and hence c= 3'529 while R= 6370 km. 
Integral expressions for the distance 4 and the time T 
from focus to station are then obtained and used to compute 
the following among other tables. 
Distance Time from focus to station in secs. Differences. 
4 km. h=1km. h = 10 km. h = 40 km. Tio — Ti- Ty - Ty. 
S. Ss. S. s. S. 
£ o°13 1°43 5°52 + 1°30 + 5°39 
50 7°35 6°99 8:80 — 0°34 + 1°47 
Too 13°33 13°90 14°80 + O13 + 0°97 
150 20°80 21°12 21°32 + 0°32 + 0°52 
200 27°07 27°57 28°00 — O'1o + 0°33 
250 34°80 34°65 34°76 = Ong — 0°04 
300 41°74 41°56 41°51 — 018 — 0°23 
350 48°71 48°58 48°42 — o'r3 — 0'29 
400 55°25 55°44 55°05 + o'19 moo 
450 62°44 62°29 61°80 — O15 — 0°64 
500 69°43 69°14 68°55 — 0°29 — 0°88 
The columns of differences suggest that some error of com- 
putation has crept into the numbers. 
The table on the following page is obtained on the simple 
hypothesis that the velocity is constant for any depth here 
considered and equal to 7°17 km. per second. 
Several points are suggested by a comparison of these 
tables. We notice that the point of inflexion on the time 
curve is so ill defined that it is useless for estimating Z. Further, 
anywhere between 200 and 400 km. is quite useless to attempt 
to discriminate between the two tables or for any value of % 
