ON SEISMOLOGICAL INVESTIGATIONS. 273 
Depth of Focus. 
A list of cases of abnormal focus up to May 1918 was given in the last report. 
To these may now be added 
Grover I. (Hicu Focus). [P] 
i lw cnl aay 1 aia 5 ‘i i Depth s. 
1918 July 8 10 22 265N 91:2E —-010 + 6 
1918 Sept. 7 17 15 
1918 Sept. 8 0 9 
1918 Sept.l1l1 4 6 
1919 April 30 7 16 1958S 173-:0W —-015 + 7 
1919 May 6 19 40 45 60S 153:0E —030 +36 
1919 May 29 10 59 45 315N 1005 E —020 ? 
1919 Oct. 12 21 48 15 20S 1025E —-020 +12 
465N 1514E —-030 +20 
or eo or 
ance 
(Group II. represents the vast majority of earthquakes at normal depth.) 
Grovp III. (Dre Focus). [P] 
dicate) oper 83 ‘3 Md Depth 8. 
1918 Nov.18 18 41 45 z 
1918 Nov.23 22 57 rey 0S hg SB ai ea an 
1918 Dec. 14 18 39 15 1308S 1668E + -030 —25 
T9LS. Dec. .25), 4 LO». 21..,;;, 10 70S 153:0E +-070 ? 
1919 Jan. 1 
1919 Mar. 1 
1919 Mar. 2 
1919 Mar. 2 
1919 Mar. 9 
1919 Mar.13 14 16 55 858 124-5E (suggested) 65 
1919 Mar. 16 3 
cere) Mon 6 95N 1270E 4-015 ? 
1919 Aprill7 20 53 5 I145N 910W +-010 2 
1919 May 3 0 51 55 40:7N 1458E 4-005 —7 
1919 June 1 6 51 13 25-7N 1248E +-040 —15 
1919 Aug.18 16 55 25 
ge ate Te + anes of 170S 177-5W +-050 —34 
1919 Aug.31 17 20 34 1578 1673E 4-015 —13 
1919 Oct. 27 $8 40 48 1160S 69:5W +-040 —44 
1919 Nov. 6 7 13 10 I35N 590W +010 ? 
1919 Nov.20 14 11 38 130S 1668E +-040 —29 
In some of these cases the rule of insisting on the existence of evidence from both 
epicentral and anticentral stations has been relaxed ; for with accumulating evidence 
of variation in depth, it is natural to accept plain indications without too exacting 
conditions. But generally the double evidence is available. 
With regard to the position tentatively occupied in the last report, that there are 
three definite surfaces at which shocks occur, it will be seen that there are seven 
_ entries out of fifteen with depth either +-030 or +-040; there are two greater 
(+-050 and +-070), six less (+-020, +-015, +-015, +-010, +-010, and +-005), and 
one uncertain: all these below the normal, with a mean value near +-030. On the 
side above the normal the mean value — -025 is not far from any of the entries. The 
difference -055 between these extremes is comparable with Galitzin’s -060 between 
his uppermost and lowest surfaces, so that the hypothesis of identity may reasonably 
be retained for trial, though the three critical depths are clearly regions of maximum 
occurrence rather than definite surfaces. 
3.0 0 2058 1785W +-030  —30 
Ce iee rs NE TOE. ol Oat, IS 
43-78 77:0W -+-020 —9 
Periodicity. 
Further study of the 21-minute periodicity led, by a devious route, to the con- 
clusion that it is controlled by the Moon, and is of the nature of a tidal effect. The 
re-examination of the material (especially the series of 6,000 Jamaica earthquakes) 
from this point of view is not yet complete, and has involved more than one change 
of hypothesis in detail. The general idea that the meridian passage of the Moon on 
each day should be taken as the reference point, and that there are an exact number 
1924 1b 
