6 REPORTS ON THE STATE OF SCIENCE.—1918, 
had two stations, A and Z, at ends of a diameter of the earth, and the epi- 
centre E lay midway between them, so that EA=EZ=90°, for which 
the tables give S—P=658 seconds. We are suggesting that this ought 
to be 658s.—17s.—641 seconds. If we are right, then 641 seconds will 
be observed at both A and Z. In proceeding to determine the epicentre 
we may use A alone (for in practice A may not be a single station but the 
whole body of European stations), and using our erroneous tables we 
should put the epicentre only 86°-8 from A, and therefore 93°-2 from Z ; 
so that the observed 641 seconds at Z would be doubly in defect, since 
for 93°-2 the tables give 674 seconds. However we determine the 
epicentre (provided A, and A, are not too unequal), this double defect 
of 33s. or 34s. will be shared between A and Z. Two such antipodal 
stations are specially valuable because we cannot alter the sum of the 
two distances (A,+ A,) by changing the azimuth of the epicentre ; and 
even when the antipodal condition is not quite fulfilled (as, say, for Pulkovo 
and Riverview, which are about 140° apart) a change of azimuth makes 
very little change in A,+A,. For pairs of stations closer to the 
epicentre, the azimuth is often well determined by other stations; and 
unless they are very close to the epicentre the azimuth error only affects 
(A,+4,) to the second order. Hence we have a good check on the 
values of S—P. The following satisfactory instances were collected from 
the published results of 1913, 1914, 1915. They are worth giving in 
detail to show the good accordance. 
TABLE I. 
Pairs of Stations on opposite sides of the Epicentre. 
Ay Ay | Mean =P), & liroS=P)y Mean 
2 | -- aes | eee 2 2 : = t's 
2 ? = 8. | Ss. 8. 
60 6-9 6-4 +39 | —10 +14 | 
70 18-7 12'8 Ob Sa aT +4 | 
5-9 19-9 12:9 +22 + 6 +14 | 
59 20-4 13-2 +7 0 4 
7:8 20 0 13-9 +12 +26 +19 
99 | 19-4 14.6 +30 = | $14 
99 | 223 16-1 +4 Ee es ee 
18-6 23-1 20-8 oe en he ees —10 
14-8 30-0 22-4 11 set SO fee 
22:8 36:2 29:5 —3 —5 —A4 
26:3 368 | 316 —1 —4 —2 
20-8 53.1 37-0 + 8 =i — 6 
259 | 490 9 ||) “B74 a aa koe ee. aa ee 
26-2 18-4 523 a |! ae Ne ee 
40-2 695 =| B48 —32 4-15 — 8 
7-8 863 | 67-0 — 9 —21 -15. | 
47-2 87:0 67:1 —20 —10 =o, 
460 | 88-2 67:1 —20 —24 =e! 
58-6 791 =| 68:8 —27 —10 Sis 6 
50-3 885 | 69-4 — 8 —12 =O: > 
48-9 90-6 69:8 —13 —3 — 8 
65-6 75-1 70-3 —12 —-15 | —14 
68:5 ey aes Mee et —12 —10 —Il 
436 | 909 72-6 — 8 = —10 
63:9, eee 1726 —1 —18 —9 
66-1 316 | 738 +30 |o; aad een 
at ae hhes » 
oe 
