SURFACE REFLEXION OF EARTHQUAKE WAVES. 
383 
Table III. 
Epicentral 
distance. 
A in kilometres. 
For P. 
For S. 
e from time 
curve. 
e from time 
curve. 
e computed. 
e observed at 
Pulkovo. 
0 
0 
o 
22 
° 
o 
0 
500 
11 
23 
— 
12 
1,000 
21 
27 
— 
22 
1,500 
30 
32 
— 
30 
2,000 
37 
37 
— 
37 
2,500 
44 
42 
48 
43 
3,000 
49 
47 
44 
49 
3,500 
53 
52 
43 
54 
4,000 
57 
54 
42 
57 
4,500 
60 
58 
43 
60 
5,000 
63 
60 
44 
62 
5,500 
65 
62 
46 
63 
6,000 
65 
62 
48 
63 
6,500 
65 
63 
51 
63 
7,000 
65 
63 
54 
64 
7,500 
66 
63 
58 
64 
8,000 
66 
64 
62 
65 
8,500 
67 
64 
65 
65 
9,000 
67 
65 
67 
66 
9,500 
68 
66 
68 
67 
10,000 
69 
67 
70 
67 
10,500 
70 
67 
71 
68 
11,000 
70 
68 
72 
69 
11,500 
71 
69 
72 
70 
12,000 
72 
70 
73 
71 
12,500 
73 
71 
73 
72 
13,000 
74 
72 
74 
73 
the earth as a whole. The explanation will then have to be sought either in some 
structural peculiarity of the rocks composing the crust at Pulkovo or else in a 
modification of the general time curves. But until the required data are available 
we must *be content with recording a discrepancy that requires further examination 
in the future. 
The first impulse P which arrives by a path of minimum time must be regarded 
as the only part of a seismogram that arises from one definite cause. Immediately 
after P the seismogram is due to a variety of causes. Thus in discussing any 
subsequent pronounced effects that occur it is important to keep in view that they 
are superimposed on general disturbance which we do not for the moment seek to 
disentangle. 
The relative magnitude of a reflected longitudinal disturbance PB n that has been 
reflected n times at the surface to that of the first impulse P may be regarded as 
determined by two main circumstances : (l) As a disturbance proceeds outwards 
3 E 2 
