68 MODERN SEISMOLOGY 
traverse OF would be OF /v, = 4 cosec e/v, where % is the depth 
of the focus and wv, the velocity of the disturbance at the sur- 
face. Thus for great distances we may pass to the corrected 
Fic, 13. 
curve by applying to the original point (¢, J), the corrections 6t, 
and:64 where 6¢=% cosec e/v,, 64=hcote. The corrections 
would, of course, differ for the P and S curves and e would be 
determined from the corresponding curve. 
This procedure is probably accurate enough for dis- 
tances >1,000 km., but entirely breaks down as we get 
close to the epicentre. In any case no correction can be at- 
tempted until Z is known, Thus we may now consider how, 
if at all, Z can be obtained by observation. 
It seems evident that only observations not far from the 
epicentre would be of much use for this purpose, but what I 
think one is hardly prepared for is the extreme closeness to 
the epicentre required, if we are to depend on the times of 
arrival of P for the determination of %. 
It is not often that data are available which make any 
attempt to determine the depth of focus worth while, but the 
occurrence of an earthquake in South Germany on 16 Nov- 
ember, I9I11, tempted several investigators to see what could 
be made out as to the depth. Galitzin (Nach. d. Seis. Comm. 
Petersburg, Bd. v. L3, 1912) went into the problem very 
carefully, but it is to be feared that the data finally proved to 
be too unsatisfactory to justify an elaborate analysis. 
Galitzin first attempts to take account of the influence of 
depth on the velocity of propagation of the longitudinal 
