181 
1918-19.] The Propagation of Earthquake Waves. 
the margin of any one cap to the margin of the next following. The 
corresponding differences, SA, give the areas of these zones. The ratio 
SE/SA measures the average value over each zone of the amount of energy 
associated with unit area of that zone. 
2 a. 
20. 
E. *17 
|(1 Pens 20). 6 • 
A - sA 
|( l - COS 2a). 
8E/5A. 
Primary Wave. 
19 25 
26 29 
40 57 
47 56 
57 40 
65 32 
73 51 
91 36 
112 26 
120 8 
180 
97 28 
85 4 
60 36 
53 18 
49 16 
48 36 
48 6 
41 50 
36 2 
30 
0 
•543 ' 108 
•746 ' 203 
•799 053 
•827 '° 28 
•831 '° 04 
•834 ' 003 
•R73 ' 039 
•904 ' 031 
•933 '° 29 
i-ooo -° 67 
•°2S5 , 024 
•0525 -0705 
•123 9795 
'I 65 
■230 989 
f» -068 
55 :is 
'691 -Ofi 
' 751 -249 
1-000 
15-25 
4-5 
2-88 
1-26 
•431 
•064 
•044 
•255 
•175 
•483 
■269 
Secondary Wave. 
25 37 
41 4 
54 9 
76 27 
92 31 
110 20 
125 44 
180 
84 26 
59 20 
52 46 
49 48 
43 18 
37 0 
30 56 
0 
™ '206 
•so3 :°< 8 
£ : 
' 90 ° -029 
•Q9Q 
-071 
1*000 ' 
'049 
* 123 -084 
•207 ™ 
•383 
*522 l f 
‘ 674 -lift 
' 792 -208 
i-ooo 
11-2 
2-78 
•571 
•113 
•295 
•237 
•246 
•241 
Each value of ^E/^A may be very approximately associated with the 
middle point of the corresponding zone, thus giving a fairly accurate 
relationship between the epicentral distance and the energy incident on 
unit area at that distance. This relationship is given in the tables on 
p. 182, along with the corresponding values of <p. 
It will be seen at once that the value of $E/&A passes through a well- 
marked minimum in the neighbourhood of 65°. Is there any evidence of 
such a minimum in the seismograms due to a given earthquake as recorded 
at different stations ? Apparently not. In searching for it, however, we 
encounter the difficulty of comparing accurately, as regards the amplitude 
of displacement, the records obtained by quite different forms of instru- 
ment. Were all stations supplied, say, with the Galitzin seismographs, 
whose indications can be absolutely standardised, it might be possible to 
