42 MODERN SEISMOLOGY 
we get 9=I110° nearly or 4=about 12,000 km. Now it has 
been observed that special difficulty attaches to the identifica- 
tion of S just when J is about 12,000 km. Thus with an 
earthquake in the northern Philippines which are about 11,000 
km. from this country S usually comes out very clearly, while 
in the case of an earthquake in the Caroline Islands about 
12,000 km. from us S is most indistinct and the tendency is to 
put it rather late. The result we have obtained throws some 
light on the matter. 
We may have a disturbance which starts as transversal 
with velocity V, along EA and then proceeds as longitudinal 
with velocity V, along AO. Here again we cannot expect 
any pronounced effect unless @ is greater than the value given 
bypcos!0/2\— VAIN. 
Lastly we have the disturbances that traverse the whole 
path with velocity V,. These start after S and culminate ina 
maximum when 
t= - sin g 
and this we may identify with Wiechert’s SR,. 
We must of course add to the cases indicated, the disturb- 
ances that travel to the station by the opposite side of the 
earth. They may be considered by the method already used, 
and we shall point out only the PR, which reaches the station 
by the longer path. It arrives when 
i= 4 cos 6/4 
and will thus be later than S unless 
A Vie 
sin 0/4 > Th 
The critical value is @=about 140° or 4=about 15,500 km. 
Thus we have here another critical value tending to indistinct- 
ness of the second phase S. 
There appears to be no reason why we should not also 
have diffracted effects in which @, is negative. 
Let us now consider the problem of regular reflexion when 
a disturbance of either type is incident at a point on the earth’s 
surface. 
