a SS LE 
— 
The autocorrelation function was not determined by the proced- 
ures given in Chapter 10 and equations (10.29) and (10.30). Such a 
computation is laborious, and instead the record was mechanically 
autocorrelated by the machine described by Seiwell [1950a]. Eighty 
lags of two seconds each were evaluated and the values were corrected 
so that they essentially correspond to the Q, of equation (10.30) 
apart from a constant factor. Reduction of % to unit value then 
yields the normalized autocorrelation function, and multiplication 
of each value by E which is known from direct computation of the 
PHmax 
standard deviation of the Gaussian distribution would then yield the 
non-normalized autocorrelation function. 
Since the record was taken in 30.5 feet of water, two second 
lags were used with the assurance that aliasing would be negligible. 
From considerations in Chapter 10, only about two per cent of the 
height of a 4 second elemental component would show up in the pressure 
record. 
The autocorrelation function given at the top of figure 38 shows 
several interesting features. It dies out in a few oscillations to 
low values after about 18 lags (or after 36 seconds). Then it re- 
combines to rise again to values near 0.15 after 26 lags (or after 52 
seconds). After 56 lags, the autocorrelation function dies down to 
small values and from then on it never amounts to anything substantial 
again rarely assuming values near 0.10. 
If there had been one pure sine wave component (or cyclic com- 
ponent) present in the record of an amplitude containing great enough 
85 
