of this chapter, then the value turns out to be about 9.2 seconds. 
Then from Table 19, the best estimate of the percentage error which 
would result from an incorrect upward extrapolation of the pressure 
record to the free surface is 6.9 per cent. 
Seiwell's results 
Publications by Seiwell [1949, 1950] and Seiwell and Wadsworth 
[1949] have claimed that a purely cyclic (or sinusoidal) component 
is present in wave records. Later the original interpretation was 
modified to include the presence of two or three cyclic components. 
The autocorrelation method is quite laborious, and the earlier con- 
clusions were based on one second lags for the first complete “oscil- 
lation" of the autocorrelation function followed by skipping some 
arbitrary number of lags and then finding another "cycle." For ex- 
ample, if the autocorrelation record shown in figure 38 were given 
for only the first 10 seconds followed by no data from 12 seconds to 
40 seconds and then by another cycle from 42 seconds to 52 seconds 
it might be very easy to conclude that one "cyclic" component was pre- 
sent. This conclusion is of course shown to be incorrect by the rest 
of the data. Once one cyclic component is found, then a little more 
detail in the autocorrelation leads to the hypothesis that several 
"cyclic" components are present. 
ED ES TE ES ES 
Tukey and Hamming [1949] have analyzed Seiwell's data, and al- 
though the autocorrelation function employed was normalized in a way 
which makes the values somewhat different from the correct procedure 
given in equation (10.30), the results are of interest here. The 
following paragraphs are quoted from Tukey and Hamming and figure 
98 
