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ON SEISMOLOGICAL INVESTIGATIONS. 285 
depth, but the results provisionally obtained for focal depth do collect them- 
selves roughly into three groups, which may possibly refer to foci at these 
critical surfaces; at any rate the intervals between the groups correspond to 
the intervals between the surfaces. The information about focal depth was 
entirely relative, and we had no means of judging the absolute focal depth. If, 
however, this identification is confirmed the missing constant is supplied. 
In a note to this effect (Geop. Sup. to Mon. Not. R.A.S., Jan. 1923) the times 
of arrival of [P] at the anticentre from foci actually on these surfaces are 
found to differ from the adopted tables by 
Group : ; F m3 II. rior 
Depth : : of OOS —0°036 —0°077 
Diff. from normal - +0°019 0°000 —0°041 
Time for [P] : - +14s. —3s. —39s. 
To Group I. we may assign the following :— 
dex «he © ° s. 
1916 Oct. 38 i 14°0S 74°5 W [P] =+16 
TOG) Oct.—20 «17 180 S 173°0 W =+16 
1917 June 13 6 30°2S 1777 W =+15 
oy ae May, 1 4,18 29°28 177°0 W =+13 
In the Jast case the solution printed in the Bulletin is probably in error, 
as is seen by detailed comparison with June 13, 1917, probably from the same 
focus : and a corrected solution will be given, subtracting 25s. from the adopted 
T,. To this group we may provisionally assign also 
1914 June 26 4 13°08 166°8 E [P]=+8 
Lot6 van, 1. 13 5°5 154°0 E (P]=+7 
To Group II. we assign the great majority of -earthquakes. Several cases 
where [P] can be well determined are collected in the paper in vol. i. No. 1, of 
the Geop. Sup.; its values are as follows :— 
+3s., —1s., —4s., —4s., —5s., —7s., —9s., —9s., —12s. (—17s.), 
but these cases have not been yet fully revised. 
To Group III. we assign the following :— 
d. h. c S Ss. 
1913 Noyv.10 21 18'0S 170°0 E [PP] =—26 
1914 . Feb. 26 4 1608S 61°0 W —42 
1915 Jan. 5 14 16°5S 168°5 EB —27 
1916 June 2l 21 1708S 57°0 W —48 
1916 Sept. 3 ¥f TOS 1550 E —28 
1917 April 21 0 37° 2 Ni 704 EK —26 
1918 Feb. 7 5 65N 127°0 B& ? 
1918 April10 2 440 N 131°0 E —57 
1918 May 22 6 1708 1775 W —42 
1918 May 25 19 31°08 91°0 W —15? 
It is admitted that at the present stage there is too much scattering within 
the group and too much overlap between groups; but a great deal of the 
material is still rough, and we may be content to await further developments. 
It will be seen that there is some appearance of local restriction in the groups. 
Thus the epicentres of Group I. are all south and west. But this may be an 
accident due to the distribution of observing stations. No case has been 
included at this stage unless there is a good determination of T, from stations 
near the epicentre and also of [P} from stations near the anticentre, and one or 
other of these may fail for want of observing stations (or of observations from 
them) in the appropriate neighbourhood. Thus the earthquake of 1918 January 
30d. 21h. at 47°.5 N. 129°.0 E. has some large residuals which suggest an abnormal 
_ focal depth, but the South American stations give us no information about [P], 
unless we accept a single observation at La Paz iP=+18m. 6s. as an observation 
of [P] with residual [—104s.], which is far too large to help us. It seems more 
likely that the observation refers to the P wave, with residual +31s. from 
adopted tables as printed. 
